Head&Neck Anatomy

Question 1

What are branches of the Vagus in the neck?

Describe the course of the vagus nerves in the mediastinum.

Answer 1

The vagus emerges through the middle compartment of the jugular foramen, runs straight down the neck within carotid sheath, between and behind carotid artery and jugular vein, and gives off the following branches in the neck:

• Small afferents to the meninges
• Auricular branch
• Carotid body branch (forms plexus w carotid sinus branch of glossopharyngeal)
• Pharygeal branch
• Superior laryngeal nerve - internal and external br.
  
  • large internal laryngeal nerve pierces thyrohyoid membrane to reach piriform recess, carrying sensory fibres for pharynx & larynx
  • small external laryngeal nerve runs close to superior thyroid vessels, outside the larynx, to supply cricothyroid and contribute to supply of inferior constrictor
• Cervical cardiac branches
• Right recurrent laryngeal nerve (Left recurrent laryngeal nerve is given off in superior mediastinum)
  
  • both nerves have cardiac branches & also supply trachea, oesophagus and cricopharyngeus part of inf constrictor, before passing under lower border of inf contrictor and entering pharynx & larynx to supply laryngeal muscles (except cricothyroid) and the laryngeal mucosa from the vocal folds downwards

In root of neck passes in front of subclavian artery & enters mediastinum to supply thoracic and abdominal viscera

Right vagus

• In contact with trachea during descent through thorax
• Medial (deep) to the azygous vein

Left vagus

• Held away from trachea by great branches from aortic arch - recurrent br. around Lig. Art.
• Crosses the arch medial (deep) to the left superior intercostal vein

Both descend posterior to lung roots, contributing to pulmonary plexuses, and pass onward to the oesophagus to become anterior and posterior vagi.

Question 2

Describe the course of the phrenic nerves.

Answer 2

• Arising principally from ant rami of C₄ (C₃, C₄, C₅) in the neck, the nerve passes down over the anterior scalene muscle (lateral to medial underneath prevertebral fascia), and across the dome of the pleura behind the subclavian vein to enter the thorax
• Both cross anterior to vagus and in the thorax run in front of the lung root
• Both in contact with mediastinal pleura throughout whole course
• Right
  
  • Related medially to venous structures throughout its thoracic course;
    
    • RBCV, SVC, RA, IVC
  • Passes through vena cava hiatus opening in diaphragm at level of T8
• Left
  
  • Related medially to arterial structures throughout its thoracic course
    
    • LCC, LSubC, Aortic arch, Left ventricle
  • Crosses arch lateral to superior intercostal vein
  • Pierces the diaphragm left of the pericardium.

About 2/3 of phrenic nerve fibres are motor to diaphragm; rest are sensory to diaphragm (except for the most peripheral parts which receive intercostal afferent fibres), and to the mediastinal pleura, the fibrous pericardium, the parietal layer of serous pericardium, and the central parts of the diaphragmatic pleura and peritoneum.

Question 3

Describe the external branch fo the superior laryngeal nerve and its position in relation to the superior thyroid artery.

Answer 3

The EBSLN is a branch of the superior laryngeal nerve from the vagus (smaller than the internal branch). Superior laryngeal nerve separates from vagus at base of skull & descends towards superior pole of thyroid along int carotid artery; at level of hyoid cornu, divides into 2 branches. Smaller external branch continues to travel along lateral surface of inf pharyngeal constrictor muscle & usu descends anteriorly & medially along w superior thyroid artery then enters & supplies cricothyroid which adducts vocal cords (int branch supplies mucosa above vocal cords)

The Cernea classifcation categorizes the nerve in relation to superior thyroid vessels and the upper edge of the superior thyroid pole into three types:

A type-1 nerve crosses the superior thyroid vessels more than 1 cm above the upper edge of the thyroid superior pole and occurs in 68 % of patients with small goiter and in 23 % of patients with large goiter.

A type-2A nerve crosses the vessels less than 1 cm above the upper edge of the superior pole and occurs in 18 % of patients with small goiter and 15 % of patients with large goiter.

A type-2B nerve crosses the superior thyroid pedicle below the upper border of the superior thyroid pole and occurs in 14 % of patients with small goiters and 54 % of patients with large goiters. Obviously, a type-2B nerve is most vulnerable to inadvertent injury.

Question 4

Describe the triangles of the neck. (JB)

Answer 4

• Anterior triangle
  
  • SCM, Midline, Mandible
• Submental
  
  • anterior digastric, hyoid bone, mandible
  • AJV, LNs
• Submandibular
  
  • Ant. digastric, post. digastric, mandible
  • Gland, facial artery & vein, XII
• Carotid
  
  • SCM, sup. omo, post. diagastric
  • Carotid bifurcation & branches of carotid except posterior auricular, IX, X, XII superior root ansa, LNs, ?IJV
• Muscular
  
  • Sup. omo, SCM, midline
  • Strap muscles, larynx, pharynx, trachea, oesophagus, thyroid, RLN, LNs
• Posterior triangle
  
  • SCM, Trapezius, clavicle
• Supraclavicular / subclavian
  
  • SCM, inf. omo, clavicle
  • Subclavian artery, scalenus, brachial plexus trunks, LNs
• Occipital
  
  • SCM, inf. omo, trapezius
  • Cervical plexus, upper brachial plexus, XI, LNs.

Question 5

How can you identify the facial nerve during surgical dissection?

Answer 5

1. Tragal pointer (cartilaginous pointer)
   
   • The nerve lies 1cm below and 1cm medial to this
2. The posterior belly of digastric
   
   • ​During parotidectomy, lateral retraction of SCM exposes PBD
   • This muscle v easy to identify by position (just deep to sternomastoid) and also be direction of muscle fibres that run towards mastoid tip
   • Facial nerve trunk lies approx 1cm above & parallel to upper border of digastric muscle near its insertion at mastoid tip
3. Tympaniomastoid suture line - palpable as a hard ridge deep to cartilaginous portion of external auditory canal; facial nerve emerges 5mm deep to its outer edge
4. Styloid process: lateral to
5. Stapedial artery: just deep to
6. Mastoid process also described as one of landmarks but lies deep to insertion of SCM hence mainly a palpatory landmark
7. Facial nerve trunk can also be identified by performing retrograde dissection whereby peripheral branches are traced back to reach main trunk

**If you can slide your finger from the tragal pointer onto the styloid process; the nerve must be under the pulp of your finger. The nerve runs in the plane of the digastric.

**Habib Rahman uses the pointer and the recess between the SCM and the posterior belly of digastric as his marker.

Question 6

Facial nerve

Answer 6

• Emerges through stylomastoid foramen, which lies posterior to styloid process
• Nerve crosses lateral to styloid process just above tendon of origin of stylohyoid muscle
• Key landmarks are:
  
  • Tragal pointer: facial nerve usu lies ~1cm deep and inferior to pointer
  • Tendon of posterior belly of digastric
    
    • During parotidectomy, lateral retraction of SCM exposes posterior belly of digastric
    • This muscle v easy to identify by the position (just deep to sternomastoid) and also by the direction of the muscle fibres that run towards the mastoid tip
    • Facial nerve trunk lies approx. 1cm above and parallel to upper border of digastric muscle near its insertion at mastoid tip
  • Tympanmastoid suture line – palpable as a hard ridge deep to the cartilaginous portion of the external auditory canal; facial nerve emerges 5mm deep to its outer edge
  • Styloid process: lateral to
  • Stapedial artery: just deep to
  • Mastoid process is also described as one of the landmarks but lies deep to insertion of SCM hence mainly a palpatory landmark
  • Facial nerve trunk can also be identified by performing retrograde dissection whereby peripheral branches are traced back to reach the main trunk
• Intracranial branches
  
  • greater petrosal nerve
  • nerve to stapedius
  • Corda tympani
• Distal to stylomastoid foramen:
  
  • posterior auricular nerve
  • posterior belly of digastric
  • stylohyoid
  • 5 major facial branches (in parotid gland):
    
    • temporal branch
    • zygomatic branch
    • buccal branch
    • marginal mandibular branch
    • cervical branch

Question 7

Describe the cervical fascia.

Answer 7

• The cervical fascia surrounds the muscles, bones, vessels, and viscera of the neck and is divided broadly into superficial and deep components.
• The superficial fascia is simply the subcutaneous tissue of the neck which is continuous with platysma.
• The deep cervical fascia has three components; deep, middle, and superficial, which define a series of cylindrical compartments that extend longitudinally from the base of skull to the mediastinum.
  
  • The investing layer begins at the nuchal line and extends anteriorly to the hyoid bone, wrapping around Trap. and SCM, as well as the parotid, the SMG, and the strap muscles. Above it attaches to the skull base at the origins of sternomastoid and trapezius muscles and to the lower border of the mandible
  • The middle or pretracheal fascia encloses the cervical viscera including the pharynx, esophagus, larynx, trachea, thyroid, and parathyroid glands.
  • The deep or prevertebral fascia arises from the nuchal ligament and encloses the vertebral column and muscles of the spine. It is split into two layers: the alar fascia anteriorly and the prevertebral fascia posteriorly. Prevertebral fascia thus forms the floor of a radical neck dissection. Important not to breach it in a neck dissection as the cervical plexus and trunks of brachial plexus lie deep to it. Large cutaneous branches of these cervical plexus pierce the prevertebral fascia adn must be divided as neck dissection proceeds

All three layers of the deep cervical fascia contribute to the carotid sheath, which forms a neurovascular compartment that encloses the carotid artery, the internal jugular vein, and the vagus nerve.

Question 8

Describe the anatomical relations of the sternocleidomastoid muscle.

Answer 8

The SCM arises from two heads; the front of manubrium sterni and the superior suface of the clavicle. It ascends obliquely to insert into the outer surface of the mastoid process and anterior part of the superior nuchal line.

• The EJV courses down SCM from the base of the ear lobe to the midpoint of the clavicle (where it joins the SCV).
• Emerging from the mid-point of the posterior border of SCM (Erb’s point) is the great auricular nerve (ascends towards ear) and the transverse cutaneous nerve (transverse). The lesser occipital nerve runs superiorly along its posterior border and the supraclavicular nerve and its branches run inferiorly roughly along its posterior border.
• CN XI emerges from the posterior border (within 2cm of Erb’s point 90% of the time) and courses towards the lower third of trapezius.
• The lower part of the parotid overlies the upper part of the gland.
• SCM overlies the carotid sheath, access to this is gained by incising the investing fascia of the neck, which envelops SCM.
• Supplied by CN XI, functions to rotate the head level whilst working in conjunction with the contralateral side.

Question 9

Describe the anatomical relations of the posterior belly of the digastric muscle. Why is this worth knowing?

Answer 9

The diagstric has two bellies; an anterior belly from the back of the mandible, and a posterior belly passing towards the medial side of the mastoid. The sling draws the mid-point onto the hyoid.

The posterior belly crosses the accessory nerve, the TP of the atlas, the IJV, CN XII, and the external and internal carotid arteries. The submandibular gland covers the anterior portion of the posterior belly.

During radical neck dissection, tissue superficial to the posterior belly of digastric can be confidently divided.

Question 10

Describe the cervical lymph node stations/levels.

Include reference to key landmarks that distinguish levels.

Why are the jugulodigastric, jugulo-omohyoid, and Delphian nodes of relevance?

What are zones of the neck for trauma?

Answer 10

Key landmarks are the hyoid bone and cricoid cartilage for clinical horizontal planes, or the carotid bifurcation and omohyoid for surgical horizontal planes, and for vertical planes the lateral border of sternohyoid and posterior border of SCM.

• Level I: body of mandible, hyoid, anterior belly of contralateral digastric muscle, posterior belly of ipsilateral digastric muscle, stylohyoid muscle
  
  • Level Ia: submental triangle (ant bellies digastric & hyoid)
  • Level Ib: submandibular triangle (body of mandible, ant & post bellies of digastric)
• Level II: upper int jugular chain surrounding IJ & SAN incl JG node which is most common site for oral cancer mets
  
  • skull base, level of hyoid/carotid bifurcation, border of stylohyoid/posterior border of SMG, posterior border of SCM
  • divided by accessory nerve into IIa (anterior) and IIb (posterior)
• Level III: middle int jugular chain
  
  • inf border hyoid/carotid bifurcation, inf border cricoid/omohyoid, lateral border of sternohyoid, posterior border SCM
• Level IV: lower int jugular chain
  
  • inf border cricoid/omohyoid, level of clavicle, posterior border SCM, lateral border sternohyoid
• Level V: posterior triangle
  
  • skull base at apex of convergence of SCM and trapezius, middle third of clavicle, posterior border SCM, anterior border trapezius
  • divided by level of inf border cricoid/omohyoid into Va superiorly, Vb inferiorly
• Level VI: central (anterior) compartment
  
  • inf border hyoid to superior border manubrium
  • posteriorly trachea medially & prevertebral space laterally
  • laterally lateral border sternohyoid or medial border both common carotids
• Level VII: superior mediastinal lymph nodes - extension of paratracheal chain below suprasternal notch but above level of brachiocephalic artery
  
  • superior border manubrium to superior border arch of aorta
  • laterally common carotid artery on left, brachiocephalic artery on right

Lesions of the tonsil often drain directly to the jugulodigastric node. Lesions of the tongue often drain directly to the jugulo-omohyoid node. The Delphian node drains the thyroid and larynx and confers poor prognosis if affected by metastases.

Trauma zones: sternal notch to cricoid (zone 1), cricoid to angle of mandible, angle of mandible to base of skull (zone 3)

Question 11

What should be taken into account when locating an incision around the jawline?

Answer 11

1. The marginal mandibular branch of the facial nerve
   
   • Usually exits parotid near angle of mandible and forms a lazy recurve crossing the mandible
   • Rarely lower than 2.5cm from the lower border of mandible
2. The cervical branch of the facial nerve
   
   • Runs deep to platysma downwards 1-2cm behind the angle of the mandible
   • Divides into anterior and posterior branches 2cm below angle of mandible
   • Anterior division curves forwards and upwards so supply muscles of the corner of the mouth

Therefore, if incisions are made 2cm or more behind the angle of the mandible and 4cm or more below it and continued through platysma, neither branch will be affected.

Question 12

Describe the anatomy of the parotid gland and its anatomical relations

Answer 12

• The parotid gland is a salivary gland that produces serous saliva, with only a few scattered mucous acini
• It is a large irregular lobulated gland that extends from the zygomatic arch to the upper neck and from the pre-auricular region to the cheek.
• It overlaps the anterior borders of SCM, PBD, and masseter. Posteriorly it extends below EAM onto mastoid process.
• In its transverse section it is wedge-shaped, occupying the space between the mastoid & styloid process of the temporal bone and the ramus of the mandible reaching close to the lateral wall of the oro-pharynx.
  
  • superficial portion of gland sits on masseter & extends posteriorly
  • deep portion sits behind ramus of mandible, in front of mastoid process, to sit lateral to styloid process & posterior to medial pterygoid

Surfaces

• lateral (superficial) surface = covered by skin & superficial fascia
  
  • investing layer of deep cervical fascia splits to envelop gland
  • overlying gland = SMAS (superficial muscular aponeurotic system), which is continuous above w temporoparietal fascia & frontalis, and below w platysma
  • great auricular nerve supplies fascia superficial and deep to parotid gland and transmits pain caused by stretching of fascial envelope when acute enlargement of gland occurs as in mumps
• anteromedial surface = grooved by posterior border of mandibular ramus & is related to masseter & medial pterygoid which are attached to the ramus
  
  • gland also wrapped around capsule of TMJ
  • parotid duct & facial nerve branches emerge from anteromedial surface & run forwards deep to ant border
  • terminal branches of ext carotid artery (superficial temporal & maxillary) leave this surface further back
• posteromedial surface = in contact w mastoid process w its attached SCM and posterior belly of digastric muscle
  
  • more medially, styloid process and its attached muscles (stylohyoid, stylopharyngeus, styloglossus) separate the gland from the carotid sheath and IJV/ICA; external carotid artery enters gland through lower part of this surface
  • facial nerve trunk, or its temporofacial & cervicofacial divisions, enter gland between mastoid & styloid processes

Within the gland

• superficial to deep: facial nerve, retromandibular vein, ECA (nerve, vein, artery)
• facial nerve: divides gland into superficial & deep portions
  
  • trunk of facial nerve enters gland between mastoid & styloid process after exiting stylomandibular foramen
• retromandibular vein: formed within parotid by confluence of superficial temporal and maxillary veins; emerges from lower part (pole) of gland and divides into:
  
  • anterior branch - joins facial vein to form common facial veein that crosses carotid to enter IJV
  • posterior branch which joins posterior auricular vein to form EJV
• external carotid artery
  
  • enters after passing deep to PBD (at this point gives rise to posterior auricular artery)
  • divides within substance of gland into superficial tmeporal and maxillary arteries
• lymph nodes of preauricular (parotid) group lie on or deep to fascial capsule of parotid, as well as within the gland
• NB PBD passes superficial to & protects almost all structures passing between submandibular triangle superiorly & carotid triangle inferiorly - includes IJV and ICA in carotid sheath, last 4 cranial nerves and ECA)
  
  • but retromandibular vein or its branches, cervical branch of facial nerve & greater auricular nerve pass superficial to posterior belly of digastric

Parotid duct

• ~5cm long, passes forward across masseter & turns around its anterior border to pass through the buccal fat pad & pierce buccinators
• palpable on clenched masseter muscle
• duct opens on mucous membrane of cheek opposite second upper molar tooth; pierces buccinator further back & runs forward beneath mucous membrane to its orifice
• an accessory parotid gland usu lies on masseter between duct and zygomatic arch
  
  • several small ducts open from it & into parotid duct
• blood supply: posterior auricular and superficial temporal arteries, branches of external carotid artery
• venous drainage: retromandibular vein, formed by unification of superficial temporal and maxillary veins
• lymphatic drainage:
  
  • within superficial condensation
  • preauricular or parotid lymph nodes
  • ultimately drain to deep cervical chain
• innervation: separate card

Question 13

Describe the nervous control of parotid secretion

Answer 13

• Receives sensory and autonomic innervation
  
  • autonomic innervation controls rate of saliva production
• sensory innervation supplied by auriculotemporal nerve, a branch of mandibular nerve (V3)
• innervation to overlying skin is from great auricular nerve (C2,3 roots); divides into anterior and posterior division within dense fascia covering parotid gland - anterior supplies skin over parotid and ear lobe; posterior supplies retroauricular
• parasympathetic innervation
  
  • fibres that mediate parotid salivation in repsonse to a meal arise from inferior salivary nucleus in medulla
  • these preganglionic fibres are carried by the glossopharyngeal nerve and its tympanic branch, via the lesser petrosal nerve, to the otic ganglion
  • postganglionic secretory fibres then reach parotid via auriculotemporal nerve (branch of mandibular nerve V3)
• ​sympathetic innervation
  
  • ​originates from superior cervical ganglion, part of paravertebral chain
  • fibres from this ganglion travel along external carotid artery to reach parotid gland
  • increased activity of SNS inhibits saliva secretion, via vasoconstriction

Question 14

Describe the anatomy of the recurrent laryngeal nerves.

Answer 14

The right recurrent laryngeal nerve originates from the vagus nerve in the neck and recurs under the subclavian artery. From here it takes an ascending, oblique course, towards the inferior constrictor in the tracheo-oesophageal groove. The right RLN is rarely (1 in 200) non-recurrent in the context of an abnormal (retroesophageal) origin of the subclavian artery.

The left recurrent laryngeal nerve is a branch of the left vagus in the superior mediastinum. This recurs under the arch of the aorta just distal to the ligamentum arteriosum. It ascends in the tracheo-oesophageal groove. The left nerve has a more vertical course than the right

Both nerves dive under the inferior constrictor to enter larynx just above cricothyroid joint; may split into anterior (motor) and posterior branches prior to disappearing. The only constant relationships of the nerves are that they are intimately associated with the ITA (50% through on the right, next most common anterior; 50% posterior on the left, or through) and they are always adjacent to the tubercle of Zuckerkandl when present - in >90%, RLN runs posteiror to tubercle, either in TOG (~60%), posterior (24%) or lateral (5%) to the trachea

In both nerves, most constant portion is last 1-2cm

• can be found in Simon’s triangle (ITA sup, CCA lat, oesoph med)
• most constant relationship if present = TOZ; here it lies between ligament of Berry & TOZ just adjacent to superior PT gland

Supply the muscles of the larynx except cricothyroid & sensation from vocal cords down

Question 15

Describe the anatomy of the parathyroid glands

Answer 15

• Usually 4 glands; ~10% have more, ~5% have fewer
• 40mg in weight. Approximately 5x3mm in size
• Located on posterior aspect of lateral lobes of thyroid; 80% symmetrically
• Inferior glands more variable in position due to embryological descent; complete by 11 weeks.
• Superior glands
  
  • usu within pretracheal fascial capsule
  • usu (80%) posterior and superior to RLN, just above (1cm) and medial to junction of RLN and ITA, at level of cricoid cartilage/1st tracheal ring (roughly junction of upper 1/3 and lower 2/3 of thyroid posteriorly)
  • other locations
    
    • 14% behind superior pole
    • 3% adjacent to ITA, behind tubercle of Zuckercandl
    • 1% retropharyngeal or retro-oesophageal
    • 0.8% above superior pole of thyroid
    • 0.2% intrathyroid
    • undescended gland - pharyngeal wall, parapharyngeal space, neurovascular structures in common carotid sheath
• inferior glands
  
  • usually anterior to RLN along posterolateral aspect of inferior pole of thyroid (46%), 1cm caudal to junction of RLN and ITA
  • less often subcapsular
  • other locations
    
    • 26% thyrothymic ligament
    • 17% anterolateral to inferior thyroid pole
    • 6% just below intersection of RLN and ITA
    • 2% intrathymic
    • 2% anterior mediastinum (adenoma will descend into anterior mediastinum with thymus)
    • 0.2% intrathyroid
    • undescended gland - in carotid sheath structures, can be as superior as skull base/submandibular region
• The parathyroids derive blood supply from the ITA, with collateral circulation from superior thyroid arteries, thyroid ima artery and laryngeal, tracheal and oesophageal arteries
• PT veins drain itno thyroid plexus of veins
• lymphatics drain w thyroid lymphatics into deep cervical LNs and paratracheal LNs
• extensive nerve supply derived from thyroid branches of cervical (sympathetic) ganglia
  
  • note tehse are vasomotor, not secretomotor - endocrine secretion of PTH controlled hormonally

Question 16

What is the anatomical extent of ectopia with regard to the parathyroid glands?

Answer 16

• The superior parathyroid gland (para IV) is usually located above and lateral to the intersection of the ITA and RLN (80%)
• A truly ectopic para IV may be found anywhere within the 4th arch derivatives including the
  
  • pharyngeal wall
  • parapharyngeal space
  • neurovascular structures in the carotid sheath
• The inferior parathyroid gland (para III) is usually found either behind the inferior thyroid pole (46%) or within the thyrothymic ligament (26%).
• A truly ectopic gland may be found anywhere within the 3^rd arch derivatives including the:
  
  • thyroid, carotid sheath, thyrothymic ligament, anterior mediastinum, and aorto-pulmonary window.
• The embryological 3^rd branchial arch vessels fuse with the aorta to become the internal carotid artery, so an undescended lower gland may be found as superiorly as the skull base.

Question 17

Describe the soft tissue spaces of the neck

Answer 17

1. The submandibular space
   
   • Between floor of mouth/mylohyoid and investing layer of fascia between hyoid bone & mandible
   • Divided into sublingual (under mucous membrane of floor of mouth) and submyelohyoid by the myelohyoid muscle which communicate with each other
   • Primary space involved in Ludwig’s angina
   • Spreads backwards into…
2. The parapharyngeal space
   
   • Located in the lateral aspect of the neck and shaped like an inverted cone, with its base at the skull and its apex at the hyoid bone
   • Divided by styloid process and muscles into anterior (nothing) and posterior (CN IX, X, XI, XII) compartments.
   • Communicates medially with…
3. The retropharnygeal space
   
   • The retropharyngeal space is bound anteriorly by the constrictor muscles of the neck and posteriorly by the alar layer of the deep cervical fascia.
4. The danger space
   
   • Between alar and pre-vertebral space
   • Potential space all the way down to mediastinum
5. The pre-vertebral space
   
   • Infection in this space can dissect all the way down the spinal column
6. Pretracheal, peritonsillar, and parotid spaces bound by investing fascia.
7. Superficial space = superficial to investing fascia

Question 18

Accessory nerve

Answer 18

• spinal portion arises from C1-C5/6 spinal roots; enters cranial cavity via foramen magnum
• cranial and spinal components unite in posterior cranial fossa and nerve exits via jugular foramen with CN IX and X; occupies middle compartment, just lateral to vagus, with which it shares a meningeal sleeve
• all fibres of its cranial root leave the nerve in a branch which joins vagus
• CN XI then crosses IJV usually laterally, as it lies in front of the TP of atlas
• passes deep to styloid process and posterior belly of digastric, where it is crossed by occipital artery
• with the upper sternocleidomastoid branch of the occipital artery it reaches the sterncleidomastoid, supplies it and enters its deep surface
• emerges from SCM ~1/3rd the way down, typically 1cm above Erb’s point, and crosses posteiror triangle to pass into trapezius at the junction of the lower and middle third of the muscle, to supply it as its main motor nerve.

If spinal accessory nerve is injured, results in shoulder drooping, trapezius atrophy & weakness of shoulder abduction. Injury to spinal accessory nerve in posterior triangle won’t affect function of SCM as branches to this would have already have been given off. Note muscular branches from C3 & C4 in cervical plexus to trapezius are mainly proprioceptive but occasionally whole of trapezius isn’t paralysed when accessory nerve is damaged as some of the cervical fibres may be motor.

Question 19

Describe the posterior triangle of the neck.

Answer 19

• Overview: 3 nerves (+ phrenic), 4 arteries, 3 veins
• Boundaries:
  
  • anterior = posterior border SCM
  • posterior = anterior border of trapezius
  • inferior = middle 1/3 of clavicle
  • apex = union of SCM & trapezius at superior nuchal line of occipital bone
  • floor = prevertebral fascia over splenius capitis, levator scapulae, scalenus medius +/- scalenus anterior
  • roof = investing layer of deep fascia
  • further sub-divided into a supraclavicular triangle and an occipital triangle by the inferior belly of omohyoid.
• Contents
  
  • nerves & plexuses - spinal accessory nerve, branches of cervical plexus (greater auricular, lesser occipital, transverse cervical, suprascapular), trunks of brachial plexus
    
    • (phrenic nerve is deep to floor/paravertebral fascia, runs on scalenus anterior)
  • arteries - 3rd part of subclavian artery, transverse cervical artery, suprascapular artery, occipital artery
  • veins - transverse cervical, suprascapular, terminal EJV
  • lymph nodes - occipital, supraclavicular
  • muscles - inferior belly omohyoid, anterior & middel scalene, levator scapulae, splenius capitus

The supraclavicular triangle contains the subclavian artery, scalenus muscles, brachial plexus trunks, and lymph nodes.

The occipital triangle contains piercing branches of the cervical plexus, upper brachial plexus, CN XI, and lymph nodes.

Question 20

Describe the supra- and infra-hyoid muscles and their nerve supply.

Answer 20

Sternohyoid, sternothyroid, thyrohyoid and omohyoid make up the infra-hyoid muscles. These so-called “strap muscles” are supplied by nerve roots C1-3 via ansa cervicalis, except for thyrohyoid which derives nerve supply from C1 fibres hitchhiking on the Hypoglossal nerve. Innervation of sternohyoid and sternothyroid muscles is usually double & innervation pattern can be variable; in spite of this, the innervation of the muscles proximally usu occurs above level of lower border of thyroid cartilage & innervation of muscles distally is usu close to level of suprasternal notch - so if strap muscles and/or descending limb of ansa cervicalis are to be divided, best to do it midway between lower border of thyroid cartilage and suprasternal notch

The supra-hyoid muscles include mylohyoid, anterior belly of digastric (both supplied by nerve to mylohyoid off V3), posterior belly of digastric, stylohyoid (both supplied by facial N) and geniohyoid (supplied by C1 fibres hitchiking on Hypglossal nerve.

Question 21

Which nerves are at risk during dissection of the carotid bifurcation?

Answer 21

The hypoglossal and vagus nerves are at risk during this procedure, and the glossopharyngeal nerve is at risk when dissection is extended superiorly.

The hypoglossal nerve crosses both the ICA and ECA deep to the facial vein. It is “pinned down” by the occipital br. of the ECA. The vagus nerve descends between the ICA and IJV. The glossopharyngeal nerve is more cephaled; it deep to the posterior auricular br. of the ECA. It is more at risk when there is a “high-bifurcation”. Injury to the spinal accessory nerve is rare but reported.

Question 22

Internal carotid artery

Answer 22

• The common carotid divides into the internal and external carotid arteries at the level of the upper border of the thyroid cartilage, approximately at C4.
• Carotid sinus = slight bulge at commencement of ICA; here arterial wall is thin & its contained baroreceptors are supplied by glossopharyngeal & vagus nerves, which mediate BP impulses to medullary centres
• Carotid body = small structure lying behind bifurcation of CCA, or between its branches, from which it receives 2 or 3 v small glomic arteries - its cells are chemoreceptors concerned w resp reflexus, innervated by glossopharyngeal & vagus nerves
• The internal carotid ascends into the cranium via the carotid canal where it terminates into the anterior and middle cerebral arteries
  
  • importantly it gives of the ophthalmic branch in the carotid canal; embolisation of this vessel manifests as amaurosis-fugax.
  • has no branches in neck
• relationships
  
  • lateral to ext carotid at its origin but soon passes up posteriorly to a medial & deeper level
  • posterior = sympathetic trunk (outside carotid sheath), pharyngeal veins & superior laryngeal branch of vagus
  • medial = ascending pharyngeal artery
  • lateral = IJV w vagus deeply placed between artery & vein
  • superficially
    
    • near its origin: crossed by lingual & facial veins, occipital artery & hypoglossal nerve
    • superior root of ansa cervicalis runs downwards along it, embedded in carotid sheath
    • at higher level, crossed by posterior belly of digastric, stylohyoid and posterior auricular artery, & the structures that separate it from the ext carotid
      
      • deep part of gland and its fascia
      • styloid process & its continuation the stylohyoid ligament
      • styloglossus
      • pharyngeal structures - stylopharyngeus muscle, glossopharyngeal nerve, pharyngeal branch of vagus
• surface marking = along a line from bifurcation of CCA to head of mandible
• exposed in neck by an incision along ant border of SCM
  
  • SCM retracted backwards, facial & lingual veins divided, carotid sheath incised
  • must safeguard hypoglossal nerve which may require division of lower SCM branch of occipital artery
  • emergencce of branches from ECA ensures its differentiation from int carotid

Question 23

Describe the anatomy of the internal jugular vein

Answer 23

• Overview
  
  • IJV emerges from the jugular bulb at the posterior aspect of the jugular foramen
  • At first behind ICA, passes down to become lateral to ICA then CAA, within lateral part of carotid sheath w vagus deeply placed between vein & arteries then the CCA with the vagus lying in-between
  • Terminal part of vein lies deep to triangular interval between sternal & clavicualr heads of SCM
  • Joins subclavian to form BCV behind sternal end of clavicle
• Tributaries
  
  • inf petrosal sinus = first, just below base of skull
  • pharyngeal, lingual, facial, superior and middle thyroid veins
  • lingual & superior thyroid may join facial vein & other variations possible
• Relationships
  
  • initially posterior to ICA, lies on TP of atlas, crossed by accessory nerve
  • passes down to become lateral to ICA then CCA, within carotid sheath
  • deep cervical nodes within sheath are closely adjacent to vein throughout its course
  • posteriorly
    
    • cervical plexus lying on levator scapulae & scalenus medius
    • phrenic nerve on scalenus anterior
    • thoracic duct crosses behind left vein at level of C7 vertebra
  • laterally
    
    • inferior root of ansa cervicalis curls round its lateral border, to unite w superior root (from hypoglossal nerve) at a variable level in front of the vein
  • anteriorly
    
    • ansa cervicalis lies on front of it & gives branches to infrahyoid muscles; usually embedded within anterior wall of carotid sheath & classified as one of its contents
    • lower part overlaid by sloping SCM
    • low down omohyoid tendon crosses it
• Surface marking = along a line from lobule of ear to sternal end of clavicle, between the 2 heads of SCM
• For puncture, approached through centre of triangle formed by the 2 heads of the SCM and clavicle
  
  • needle directed caudally, parallel to sagittal plane, at 40 degrees posterior angle with coronal plane, entering vein at 4-5cm depth

Question 24

Describe the anatomy of the submandibular gland

Answer 24

• The submandibular gland is a salivary gland located in the digastric triangle of the neck, bounded by the anterior and posterior bellies of digastric and the ramus of the mandible. It is a mixed mucinous-serous gland which pre-disposes it to stone formation relative to the parotid (serous).
• It has superficial and deep parts, which are continuous with one another around the posterior aspect of myelohyoid. It drains its secretions via Wharton’s duct, which runs between the hyoglossus and myelohyoid to drain into sub-lingual papillae.
• The gland is crossed superficially by both the facial artery and facial vein, which often have to be taken above and below the gland during excision.
• The cervical branch of the facial nerve and to a lesser degree the marginal mandibular nerve, are at risk during the superficial dissection of the gland; incisions based 2cm behind and 4cm below the angle of the mandible with capsular dissection of the gland will spare these nerves.
• At the deep aspect of the gland the hypoglossal nerve is at risk below and the lingual nerve at risk above, again capsular dissection spares these.

Question 25

Describe the embryology of the thyroid and its importance clinically.

Answer 25

• thyroid originates as an epithelial proliferation in floor of pharynx at around 3wks gestation, at a point later indicated by the foramen caecum
• subsequently descends in front of the pharyngeal gut as a bilobed diverticulum, passing in front of the hyoid bone and laryngeal cartilages
• reaches its final position in front of the 2nd to 4th tracheal cartilages in 7th wk
• by then it has acquired a small median isthmus and 2 lateral lobes
• during this migration, the thyroid remains connected to tongue by a narrrow canal, thyroglossal fuct
  
  • becomes a solid structure in 5th wk, and obliterates & atrophies by weeks 9-10
  • obliteration leaves foramen caecum at base of tongue superiorly and, when present, pyramidal lobe inferiorly
  • failure to obliterate results in epithelial lined cysts & can also be associated w ectopic thyroid tissue
• lateral contribution from ultimobranchial bodies, which originate from 4th & 5th pharyngeal pouches, fuse to median portion of thyroid in 5th wk during embryological descent
  
  • give rise to calcitonin-secreting parafollicular C cells
  • Ce cells traditionally thought to originate from primordial cells of neural crest before infiltrating ultimobranchial bodies but now neural crest origin is in doubt
• microscopic thyroid follicles first appear as lateral lobes develop
• in 3rd month, follicular cells demonstrate iodine trapping & thyroid hormone secretion begins
• separate small masses of accessory thyroid tissue not uncommonly found near hyoid, in tongue, in superior mediastinum or anywhere along path of descent of thyroglossal duct

The thyroid develops from the median and paired lateral thyroid anlages at week 3.

The thyroglossal duct begins at the foramen caecum in the midline between the anterior two thirds and posterior one third of the developing tongue. This passes inferiorly in front of the hyoid bone, behind which it forms a recurrent loop.

The thyroid gland buds from the distal end of the thyroglossal duct, which itself may give rise to the pyramidal lobe.

Remnants of the tract may encyst (thyroglossal duct cyst) or persist as a tract. Rupture results in thyroglossal fistula.

Resection requires complete excision, including the middle segment of the hyoid bone.

Question 26

Describe the embryology of the salivary glands.

Answer 26

Salivary glands develop from the oral cavity in the 6/7^th - 10^th week.

The parotid appears first as a groove in the ectoderm of the mouth pit - forms a tunnel, from the blind end of which cells proliferate to form the gland.

The submandibular gland buds from the floor of the primitive mouth and grows on the lateral aspects of the tongue.

Question 27

Describe the anatomy of the subclavian arteries.

Answer 27

• the right is a branch of the brachiocephalic trunk off the aorta (rarely, has its own origin off the aorta from the diverticulum of Kommerell; lusorian artery). The left is the posterior-most branch off the aortic arch.
• first part = medial to scalenus ant
  
  • arches over suprapleural membrane & impresses groove on apex of lung
  • RLN recurves under right artery while thoracic duct loops over left
  • 3 branches: vertebral artery, thyrocervical trunk(suprascapular, transverse cervical, inferior thyroid artery, continues as ascending cervical), internal thoracic
  • Phrenic nerve runs down vertically across scalenus anterior & anterior to its internal thoracic branch, behind subclavian vein
• second part = behind scalenus anterior
  
  • only branch = costocervical trunk which passes back across suprapleural membrane towards neck of first rib (divides there into descending branch, superior intercostal artery & ascending branch, deep cervical artery)
    
    • superior intercostal artery enters thorax across neck of first rib
    • deep cervical artery passes backwards between transverse process of C7 vertebra & neck of first rib to run upwards behind cervical transverse processes
• third part = lateral to scalenus anterior, behind prevertebral fascia in PT
  
  • dorsal scapular artery

Surface marking = along a line arching upwards from SCJ to middle of clavicle & ~2cm above it

Surgical approach = can be exposed by dividing clavicular head of SCM from clavicle then detaching scalenus anterior from first rib, taking particular care not to damage phrenic nerve

Question 28

Subclavian vein

Answer 28

• anterior to scalenus anterior
• lies in a groove on first rib & due to slope of rib lies at a lower level than insertion of scalenus anterior
• runs medially & joins IJV at medial border of scalenus anterior to form brachiocephalic vein
  
  • thoracic duct on left and right lymph duct on right enter the angle of confluence of the two veins
• catheterisation
  
  • usu approach = infraclavicular, from a point 2cm below midpoint of clavicle along a line that passes behind clavicle towards jugular notch of sternum
  • needle pierces clavipectoral fascia and enters vein just behind fascia
  • pneumothorax due to puncture of pleura and lung, and puncture of the subclavian artery are complications of this procedure

Question 29

Common carotid artery

Answer 29

• left CC arises from arch of aorta - lies in front of subclavian artery up to SCJ where the two diverge
• right BCT bifurcates behind SCJ into CC and SC arteries
• CC gives off no branches prior to bifurcation
• Bifurcates at level of upper border of lamina of thyroid cartilage (upper border C4 vertebra) into int & ext carotids; or higher near tip of greater horn of hyoid (C3)
• Carotid sinus = slight dilatation at terminal portion of artery, which includes commencement of int carotid artery
• Relationships
  
  • lies in medial part of carotid sheath w IJV lateral and vagus nerve deep between the two
  • sheath overlapped superficially by infrahyoid muscles & SCM
  • medial to sheath = trachea and oesophagus; and higher, larynx and pharynx
  • posterior to sheath
    
    • sympathetic trunk is behind CCA & outside sheath
    • inferior thyroid artery crosses from thyrocervical trunk to thyroid gland - posterior to vagus and sympathetic chain
• Surface marking = along vertical line from SJC to level of upper border of thyroid cartilage
• Surgically expose by retraction of lower part of SCM backwards & incising carotid sheath; middle thyroid vein is divided between ligatures

Question 30

External carotid artery

Answer 30

• The external carotid starts against sidewall of pharynx anteromedial to int carotid
• ascends in front of int carotid deep to posterior belly of digastric & stylohoid, above which it pierces the deep lamina of parotid fascia & enters gland
• divides within gland behind neck of mandible into terminal branches maxillary and superficial temporal arteries
• in parotid gland separated from int carotid by:
  
  • deep part of gland and its fascia
  • styloid process and its continuation of stylohyoid ligament
  • styloglossus
  • the ‘pharyngeal’ structures: stylopharyngeus muscle, glossopharyngeal nerve & pharyngeal branch of vagus
• at commencement of artery, IJV lies lateral, but higher up is posterior and deep to artery
• facial vein crosses artery, w hypoglossal nerve between
• except at its commencement lies in front of ant border of SCM
• terminates as the superficial temporal and maxillary arteries within the parotid gland substance
• prior to that it gives off; the ascending pharyngeal branch medially, the superior thyroid, lingual, and facial arteries anteriorly, and the posterior auricular and occipital branches posteriorly. The occipital branch pins down the hypoglossal nerve as it winds superficial to both vessels, and deep to the facial vein.
• surface marking: along a a line from bifurcation of common carotid passing up behind angle of mandible to a point immediately in front of tragus of ear
• surgical approach: can expose in front of upper part of SCM before it enters the parotid gland by ligating the facial vein; hypoglossal nerve which crosses ext and int carotids superficially must not be damaged

Question 31

RLN injury

Answer 31

Nerve injury:

• damage to one RLN: some weakening of voice but as cricothyroid muscle is unaffected (if EBSLN hasn’t also been damaged) the cord lies close to the midlien and opposite cord can cross midline to compensate for the injury
• bilateral RLN paralysis leads to both cords initially being in semi-adducted position, so that there is usu not early resp difficulty
  
  • but w time, cords tend to move towards midline so that voice improves but resp difficulty develops & a trahce is usu required
  • damage to both RLN & EBSLN on same side leads to cord assuming the cadaveric position of mid adduction, so there is hoarseness of voice & inability to cough

Question 32

Recurrent laryngeal nerve aberrations

Answer 32

• right nerve non-recurrent in 0.5% or 1 in 200, assoc w 4th branchial arch anomaly involving right subclavian artery which takes off from distal aortic arch and crosses behind oesophagus (arteria lusoria)
• left rare - associated with more significant vascular anomalies

Question 33

Cervical plexus

Answer 33

• lies in series w brachial plexus, on scalenus medius, behind prevertebral fascia
• deep to and halfway up SCM (doesn’t lie in posterior triangle)
• formed by ant rami (divisions) of cervical spinal nerves C1-4, after each has received a grey ramus communicans from superior cervical ganglion
• muscular branches
  
  • deep to sensory branches
  • phrenic nerve (ant rami C3-5)
  • supply to geniohyoid and thyrohyoid (C1 fibres via hypoglossal nerve)
  • ansa cervicalis (C1-3) gives off 4 muscular branches - superior and inferior belly of omohyoid, sternohyoid, sternothyroid
  • other: rectus capitis anterior and lateralis, longus capitis, prevertebral muscles & SCM, levator scapulae, trapezius & scalenus medius, middle and ant scalene muscles
• sensory branches
  
  • enter skin at middle of posterior border of SCM - Erb’s point
    
    • greater auricular nerve (C2,3)
    • transverse cervical nerve (C2,3)
    • lesser occipital nerve (C2)
    • supraclavicular nerves (C3,4)

So the cervical plexus contributes:

• Meningeal branches to the posterior cranial fossa
• Muscular branches to the prevertebral muscles, SCM, trapezius, the hypoglossal nerve, and the inferior root of ansa cervicalis so the strap muscles also.
• Cutaneous branches to the scalp, face, and chest in the form of the lesser occipital nerve, the great auricular nerve, the transverse cervical nerve, and the supraclavicular nerve.

Question 34

Pharyngeal clefts and Branchial cyst aetiology

Answer 34

• branchial cysts form due to incomplete involution of branchial cleft structures
• around 4th wk gestation, neural crest cells migrate into future head & neck region, where the 6 pairs of pharyngeal arches begin to develop
• mesoderm is covered externally by ectoderm and internally lined by endoderm
• 5 pharyngeal arches, with the arches separated by depressions known on clefts on the ectodermal surface and corresponding pouches on the endodermal surface, yielding 4 pharyngeal clefts
• only one contributes to definitive structure of embryo - dorsal part of first cleft penetrates underlying mesenchyme & gives rise to EAM
• 2nd arch develops caudally & then covers the 3rd & 4th arches which lose contact w outside
• these buried clefts become ectoderm-lined cavities that normally involute completely by 7 weeks of gestation; if they don’t involute or incompletely involute, these pathological remnants will form cysts, sinuses or fistulae in predictable locations according to their branchial cleft of origin (fistulas occur when remnants of 2nd, 3rd or 4th clefts ermain in contact w the surface by a narrow canal
• lined w stratified squamous epithelium & may contain keratinous debris inside the cyst

JB:

First branchial remnants are typically located in the front or back of the ear or in the upper neck near the mandible. Fistulas typically course through the parotid and facial nerve and end in the external auditory canal.

Second branchial cleft remnants are the most common. The external ostium of these tracts is usually in the anterior middle third of SCM. Typically the fistula courses through platysma, ascends along the carotid sheath and passes between the bifurcation to course behind the posterior belly of digastric and into the tonsillar fossa. Multiple step-ladder counter incisions are often required.

Third branchial remnants are rare and usually do not have fistulas or deep sinuses, they typically present as a mass in the suprasternal or clavicular region.

Question 35

Which branches of the facial nerve are at risk during a submandibular gland excision?

Answer 35

• mandibular branch
  
  • exits from parotid gland near angle of andible & curves downwards and forwards deep to platysma before curving upwards to cross mandible close to facial artery
  • damage causes depression of corner of mouth, flattening lower lip & drooling
• cervical branch
  
  • runs deep to platysma in downwards direction ~1-2cm behind angle of mandible
  • 2cm below angle it divides into posterior and anterior divisions
  • damage to anterior division causes some drooping of corner of mouth but generally improves bc not only supply
• incisions made 2cm or more behind the angle of mandible & 3-4cm below it through platysma will spare both branches

Question 36

How do the facial artery and vein relate to the SMG?

Answer 36

• facial artery and vein and submental artery and vein lie superficially
• facial vein descends from inner angle of eye crossing border of mandible 2cm in front of angle, deep to platysma on external SMG
  
  • runs along external surface of SMG and receives submental vein, forming the common facial vein whcih then continues on to drain into the IJV
  • submental vein passes backwards across gland to enter facial vein
• facial artery (from carotid) passes upwards deep to digastric to loop over posterior belly in sigmoid fashion before arching over posterior portion of SMG
  
  • can be identified at posteiror aspect of SMG
  • closely related to mandibular branch of facial nerve

Question 37

Relationship of lingual nerve and hypoglossal nerve to SMG

Answer 37

• Lie deep to SM gland – LN superiorly, HN inferiorly, on the hyoglossus muscle
• A small secretorimotor, parasympathetic nerve branch leaves lingual nerve to enter submandibular ganglion & then gland
• Hypoglossal nerve curves forwards over ECA at commencement of facial artery & courses medial to posterior belly of digastric usu lying below submandibular gland
• A vessel always accompanies a nerve branch off the lingual nerve, and must be cauterized & divided to allow mobilization of the gland w preservation of lingual nerve
• Lingual nerve may be at risk when dealing w SM duct as it crosses it, but otherwise these structures should be free from dissection plane

Question 38

What is the nodal drainage from the skin of the face?

Answer 38

• Mid face: follows facial vessels to facial, submental, submandibular nodes
  
  • medial eyelid & ext nose - follows facial vein to level Ib
  • lower lip, chin, tongue tip, gingiva ?Ia
• Lateral face & frontotemporal scalp: to parotid - 20-30 parotid nodes, extraglandular & intraglandular; all lie lateral to posteiror facial vein but relationship to facial nerve variable
• Parietal & occipital scalp:
  
  • parietal drains away from scalp vertex anteriorly to parotid & posteriorly to mastoid nodes
  • occipital to mastoid only
• Ear variable
  
  • tragus & helical root to parotid
  • remainder of helix, lobule to retroauricular nodes

Question 39

What drains to each nodal level of the neck?

Answer 39

• level Ia: floor of mouth, anterior oral tongue, lower lip
• level Ib: oral cavity, anterior nasal cavity, soft tissues of midface, SMG
• level II: nasal & oral cavities, nasopharynx, oropharynx, hypopharynx, larynx, major salivary glands
• level III: oral cavity, nasopharynx, oropharynx, hypopharynx, larynx
• level IV: hypopharynx, larynx, trachea, cervical oesophagus, thyroid
• level V: nasopharynx, oropharynx, thyroid, skin of posterior scalp, neck
• level VI: lower lip, chin, tip of tongue, anteiror floor of mouth, thyroid, larynx, hypopharynx, oesophagus
• VII:

Question 40

Anterior triangle

Answer 40

• boundaries
  
  • posterolateral: anterior border SCM
  • superior: lower border of anterior half of mandible
  • inferior: vertical median line from symphysis menti to suprasternal notch
  • roof: investing layer of deep cervical fascia, platysma
• triangular divisions:
  
  • submental: hyoid bone, midline, anterior belly of digastric
  • submandibular: two bellies of digastric & lower border of mandible
  • carotid: anterior border of SCM, posterior belly digastric, superior belly omohyoid
  • muscular: anterior border of SCM, superior belly omohyoid, anterior vertical median line
  • submental: hyoid bone, midline, anterior belly digastric
• nerve supply
  
  • platysma supplied by cervical branch of facial nerve
  • sensation by transverse cervical nerve (C2,3)
  • muscular innervations elsewhere
• contents
  
  • muscles
    
    • 4 suprahyoid (stylohyoid, digastric, geniohyoid, mylohyoid)
    • 4 infrahyoid/strap muscles (omohyoid, sternothyroid, thyrohyoid, sternohyoid)
    • platysma
  • vessels
    
    • common carotid before bifurcation at C4, and branches of external
      
      • superior thyroid
      • ascending pharyngeal
      • lingual
      • fascial
    • mylohyoid artery
    • int & ext carotid
    • IJV
    • facial vein
  • nerves: vagus, hypoglossal, ansa cervicalis, mylohyoid
  • other: hyoid, larynx, thyroid, parathyroids, trachea, oesophagus, submandibular gland, lymph nodes
    *

Question 41

Pharyngeal arches, pouches & clefts - overview

Answer 41

• pharyngeal arches are first seen during 4th wk of development
• initially consists of bars of mesenchymal tissue covered externally by ectoderm and internally lined by endoderm
• the horseshoe-shaped arches are separated by depressions known as clefts on ectodermal surface & corresponding pouches on endodermal surface
  
  • NB 5th arch rudimentary & only 4 clefts visible
  • 4th & 5th pouches share common opening into lumen of pharynx
• in addition to mesenchyme derived from paraxial and lateral plate mesoderm, core of each arch receives subtantial numbers of neural crest cells, which migrate into the arches to contribute to skeletal components of face
• so each pharyngeal arch has: a central bar of cartilage, muscle which differentiates from mesoderm around it, an artery, and a cranial nerve

Question 42

Pharyngeal arches

Answer 42

1. (mandibular arch)

• muscles: muscles of mastication (temporalis, masseter, pterygoids), ABD, mylohyoid, tensor tympani, tensor veli palatine, mucous membrane & glands (but not muscle) of ant 2/3 of tongue
• skeletal: premaxilla, maxilla, zygomatic bone, part of temporal bone, Meckel’s cartilage and from its dorsal end incus & malleus, mandible formed by membranous ossification of mesenchymal tissue surrounding Meckel’s, sphenomandibular ligament
• nerve: trigeminal nerve (part of V2 and V3)
• artery: maxillary artery, ext carotid artery

2. (hyoid arch)

• muscles: muscles of facial expression incl buccinators, platysma, stapedius, stylohyoid, PBD, auricular
• skeletal: stapes, styloid process, lesser horn and superior part of body of hyoid, stylohyoid ligament
• nerve: facial nerve (VII)
• artery: stapedial artery, hyoid artery

3.

• muscles: stylopharyngeus
• skeletal: hyoid bone (greater horn and inf part of body)
• nerve: glossopharyngeal nerve (IX)
• artery: common carotid, internal carotid

4.

• muscles: cricothyroid muscle, all intrinsic muscles of soft palate (incl levator veli palatine) except tensor veli palatine
• skeletal: thyroid cartilage, epiglottic cartilage
• nerve: vagus nerve (X), superior laryngeal nerve
• artery: right 4th aortic arch (subclavian artery), left 4th aortic arch (aortic arch)

6.

• muscles: all intrinsic muscles of larynx except cricothyroid muscle
• skeletal: cricoid cartilage, arytenoid cartilages, corniculate cartilage, cuneiform cartilages
• nerve: vagus nerve (X), recurrent laryngeal
• artery: right 6th aortic arch (pulmonary artery), left 6th aortic arch (pulmonary artery and ductus arteriosus)

Question 43

Pharyngeal pouches

Answer 43

• except for 1st, each pouch grows laterally into dorsal & ventral diverticulum
  
  • 1st pouch = only one in which endoderm remains in close apposition to ectoderm of corresponding cleft, namely at tympanic membrane where mesoderm separating them is minimal
  • in other pouches, ectoderm & endoderm are minimally separated
• 1st pouch
  
  • primitive tympanic/middle ear cavity, and Eustachian tube
  • aids in formation of tympanic membrane
• 2nd pouch
  
  • assists with tympanic cavity
  • palatine tonsil/tonsillar fossa
• 3rd pouch
  
  • from epithelium of dorsal wing: inf PT gland (differentiates in 5th wk)
  • ventrally, thymic rudiment (medulla; whereas lymphocytes of cortex migrate from bone marrow)
• 4th pouch
  
  • from epithelium of dorsal wing, sup PT gland (when loses contact w wall of pharynx, attaches itself to dorsal surface of migrating thyroid)
• 5th pouch
  
  • usu considered to be part of 4th pouch
  • ultimobranchial body which is later incorporated into thyroid gland - gives rise to parafollicular C cells of thyroid which produce calcitonin

Question 44

Pharyngeal clefts

Answer 44

*

Question 45

Thyroid anatomy

Question 46

Larynx*

Question 47

Trachea*

Question 48

Hypoglossal nerve

Answer 48

• motor nerve to all muscles of tongue except palatoglossus
• emerges from hypoglossal canal, picks up a branch from anterior ramus of C1 then spirals behind inferior ganglion of vagus to emerge & crosses behind ICA to lie between ICA and IJV, passing through the carotid sheath
• then descends on carotid sheath, deep to styloid muscles and the posterior belly of digastric
• is crossed by occipital artery and its sternomastoid branch and curves forward superficial to both carotid arteries and the loop of the lingual artery just above the tip of the greater horn of the hyoid bone, and passes upwards on hyoglossus, deep to mylohyoid, where the nerve lies inferior to the deep portion of the submandibular gland
• thus the nerve is superficial to both int and ext carotid arteries, crosses teh loop of the lingual artery and passes upwards on hyoglossus, deep to mylohyoid, where the nerve lies inferior to the deep portion of the submandibular gland
• contains C1 nerve fibres which have ‘hitched a ride’ and some of these are given off as it crosses the ext carotid artery, as the upper root of the ansa cervicalis and the remainder as the nerves to thyrohyoid and geniohyoid
• the nerve exits behind the IJV in ~10% of cases and occasionally can cross at a lower level, a fact which is important to remember when mobilising the carotid bifurcation
• hypoglossal nerve is at risk of damage during neck dissection at the stage where the IJV is being separted from carotid arteries just superior to the bifurcation; this is at a level where metastatic nodes are common

Question 49

Lingual nerve

Answer 49

• a branch of the posterior division of the mandibular branch of the trigeminal nerve; supplies sensation to floor of mouth, side of gums & anterior 2/3 of tongue
  
  • latter is from fibres of chorda tympani which joins lingual nerve early in its course
• appears below lateral pterygoid and passes forwards and downwards on the medial pterygoid
• then comes into contact w mandible, where there is a shallow groove below and medial to the third molar, just above the posterior end of the mylohyoid line; this groove separates the attachments of the pterygomandibular raphe above and mylohyoid muscle below
• curves forwards on hyoglossus muscle above deep part of submandibular gland and passes in a spiral manner on lateral side of submandibular duct, underneath it and upwards on the medial side
• a small secretomotor, parasympathetic nerve branch leaves lingual nerve to enter submandibular ganglion & then gland
• gives off a gingival branch which supplies all the lingual gum and mucous membrane of the floor of the mouth
• during clearance of the submandibular triangle, traction on the gland and its duct pulls the lingual nerve inferiorly; care not to injure lingual nerve which springs away as soon as duct has been divided
• a vessel always accompanies a nerve branch of the lingual nerve, and must be cauterised & divided to allow mobilisation of the gland w preservation of lingual nerve

Question 50

Glossopharyngeal nerve

Answer 50

• emerges from anterior part of jugular foramen on lateral side of inferior petrosal sinus
• makes a deep notch in inferior border of petrous bone & here its inferior gangion bulges the nerve
• the ganglion contains the cell bodies of most sensory fibres in the nerve
• nerve passes down on lateral aspect of ICA and curves forward round lateral side of stylopharyngeus
• passes deep to ECA and continues forward deep to hyoglossus to reach tongue
• tympanic branch (Jacobson’s nerve) - leaves at jugular fossa, enters temporal bone to supply middle ear, mastoid air cells and bony part of auditory tube w sensory fibres
  
  • in this branch are also parasympathetic fibres from the inferior salivary nucleus; they run through tympanic plexus on the promontary and continue in lesser petrosal nerve, which leaves middle ear thorugh its roof and runs along floor of middle cranial fossa to foramen ovale, through which it passes to reach otic ganglion; parasympathetic fibres relay in the otic and other small glands of the vestibule of the mouth
• motor branch to stylopharyngeus = given off as the nerve spirals round posterior border of that muscle
• carotid sinus nerve = main supply to carotid sinus and carotid body (baroreceptors and chemoreceptors)
• one or more pharyngeal branches join pharyngeal plexus on middle constrictor muscle
  
  • they pierce the muscle and supply the mucous membrane of oropharynx w common sensation and (a few) taste fibres
• tonsillar branch = supplies mucous membrane over most of (palatine) tonsil
• lingual branch = supplies posterior 1/3 of tongue w sensory fibres (common sensation and taste) and secretomotor fibres to glands of posterior third; these relay in small ganglia in mucous membrane

Question 51

Internal carotid artery

Answer 51

• arises at bifurcation of common carotid and continues upwards within carotid sheath
• carotid sinus = slight bulge at commencement of ICA; here arterial wall is thin and its contained baroreceptors are supplied by glossopharyngeal and vagus nerves, which mediate BP impulses to medullary centre
• carotid body = small structure lying between bifurcation of CCA, or between its branches, from which it receives 2 or 3 v small glomic arteries
  
  • its cells are chemoreceptors concerned w respiratory reflexes, and are innervated by glossopharyngeal and vagus nerves
  • carotid body tumours form a swelling at anterior border of SCM at level of carotid birfurcation, and exhibit transmitted pulsation from the arteries
• has no branches and passes straight up in carotid sheath, beside pharynx, to carotid canal in base of skull
• relationships
  
  • ​lateral to ext carotid at its origin but soon passes up posteriorly to a medial & deeper level
  • posterior = sympathetic trunk (outside carotid sheath), pharyngeal veins and superior laryngeal branch of vagus
  • medial = ascending pharyngeal artery
  • lateral = IJV w vagus deeply placed between artery and vein
  • superficially
    
    • near its origin: crossed by lingual and facial veins, occipital artery and hypoglossal nerve
    • superior root of ansa cervicalis runs downwards along it, embedded in carotid sheath
    • at a higher level: crossed by posterior belly of digastric and stylohyoid and the posterior auricular artery, and by the structures that separate it from ext carotid
      
      • deep part of gland and its fascia
      • styloid process and its continuation the stylohyoid ligament
      • styloglossus
      • the ‘pharyngeal’ structures: stylopharyngeus muscle, glossopharyngeal nerve & pharyngeal branch of vagus
• surface marking
  
  • along a line from bifurcation of CCA to head of mandible
• surgical approach
  
  • exposed in neck by an incision along ant border of SCM
  • SCM retracted backwards, facial and lingual veins divided between ligatures, carotid sheath incised
  • must safeguard hypoglossal nerve which may require division of lower SCM branch of occipital artery
  • emergency of branches from ECA ensures its differentiation from int carotid

Question 52

Scalene muscles

Answer 52

Descend from transverse processes of first 6 or 7 cervical vertebrae to insert into first two ribs.

Posterior scalene muscle arises from C5 and C6 and is not easily separated from middle scalene muscle, except that it passes beyond first rib to insert into second rib.

Middle scalene is largest of the three, arising from C6 or C7 vertebra and passing down to insert into first rib behind subclavian artery and brachial plexus

Anterior scalene arises from cervical transverse processes of C3-6 and inserts into first rib in front of subclavian artery but behind subclavian vein. Lateral border of SCM approximates the lateral border of sclaenus anteiro rmuscle, which is also where brachial plexus and subclavian artery emerge from behind the anterior scalene muscle. The thyrocervical trunk of subclavian artery typically arises alongside medial border of scalenus anterior and two of its branches run transversely in front of it

Scalene muscles are supplied by branches of cervical nerve roots.

Question 53

Ansa cervicalis

Answer 53

• lies on front of IJV and gives branches to infrahyoid muscles
• usu embedded within ant wall of carotid sheath and classified as one of its contents
• formed by union of superior and inferior roots
• superior root = branch of hypoglossal nerve containing only C1 fibres, which have hitch-hiked along hypoglossal nerve
  
  • runs down on front of ICA and CCA, giving a branch to superior belly of omohyoid
• inferior root = formed by union of a branch each from C2 and C3 ant rami in cervical plexus
  
  • single root formed spirals from behind around IJV and runs down to join superior root in a wide loop over lower part of vein, from which branches arise for the 4 infrahyoid muscls
  • sometimes inferior root passes forwards between IJV & ICA

The nerve loop which supplies the strap muscles of the neck; upper root comes from fibres from C1 nerve root which travel w the hypoglossal nerve and exit from it as the hypoglossal curves forwards around the occipital artery

Upper root then runs down on anterior aspect of internal carotid artery to join lower root at a level just above the crossing of omohyoid muscle.

Lower root usu comes from C2, 3 nerve roots and it passes down on lateral surface of int jugular vein before inclining forward to join upper root.

Question 54

What are the differences between the right and left RLNs?

What anatomic variants do you need to be aware of?

Where is the RLN most commonly injured?

Answer 54

• Left recurs around aortic arch, distal to ligamentum arteriosum and right around right SCA
• Left overall runs more posterior and has a more directly vertical course than right (enters neck more medially than right)
  
  • uncommonly non-recurrent
  • more commonly passes posterior to branches of ITA
• Right overall runs more anterior and approaches TOG more obliquely
  
  • More commonly non-recurrent (0.5-1.5%, associated with retrooesophageal subclavian artery), more commonly passes through branches of ITA (?or ant)
• Both common relationships:
  
  • In >90% cases, runs posterior to TOZ, either in TOG (~60%), posterior (24%) or lateral to trachea (5%) – only in small no of cases does it pass anterior
  • Passes under LOB (some through or over)
  • Last 1-2cm most constant – juxtaposed between lateral side of lig of Berry & medial side of TZ, plastered in place by an overlying fascia containing tertiary branches of ITA
    
    • also most prone to injury here due to difficulty freeing it from structures enveloping it – nerve essentially tethered as it dives beneath cricothyroid & can be stretched by overly vigorous dissection
    • nerve may be contained within ligament of Berry which may further exacerbate this issue/lead to traction injury w medial retraction of thyroid
  • Enter larynx at level of cricothyroid articulation passing deep to inferior constrictor
  • May branch prior to entry (70%) – anterior branches most important (motor)
  • Superior parathyroids are posterior to RLN and inferior parathyroids anterior

What anatomic variants do you need to be aware of for the recurrent laryngeal?

• Non-recurrent right laryngeal nerve 0.5-1.5% - arises directly from vagus & courses medially into larynx following course of STA or ITA
  
  • Occurs in setting of arterial anomalies, most commonly aberrant RSCA arises as a separate branch from aortic arch distal to LSCA and passes from L to R, posterior to oesophagus
• Can have right RLN and non-recurrent laryngeal nerve – normally join in a position behind lower border of thyroid
• Non-recurrent left laryngeal nerve – quite rare, assoc w more extensive and less common arch/great vessel anomalies than non-recurrent right laryngeal nerve

Where is the RLN most commonly injured?

• Closely related to ITA but now so well known that injury to nerve here unlikely
• Perhaps in more danger adjacent to medial surface of upper pole of thyroid gland – thyroid attached to trachea by quite dense fascia here and depending on relationship to thyroid nerve can be bound up in this fascia