Osteology Of The Base Of The Skull, The Orbit And The Ear Flashcards
Sutures
Fibrous joints between several individual bones in the base of skull
Cranial fossae
3 distinct depressions at the base of the skull from above
How many cranial fossae are there
3
What are the 3 cranial fossae
Anterior cranial fossa
Middle cranial fossa
Posterior cranial fossa
Cranial foramina
Small holes in the cranial fossa that allow the nerves, arteries and veins to pass in and out of the skull
Which lobes rest in the anterior cranial fossa
Frontal lobe
How many bones form the anterior cranial fossa
3- orbital part of the frontal bone
Cribriform plate and the crista galli of the ethmoid bone
Lesser wings of the sphenoid bone
What is the Foramen located in the anterior cranial fossa
Cribriform plate- transmits olfactory fibres that allow our sense of smell
Orbital part of the frontal bone
2 rounded elevations are the spherical cavities of the bony orbits (where the eyes are located)
Cribriform plate
Has many small holes for the passage of olfactory nerves
Crista galli
Vertical protrusion in the centre of the Cribriform plate
Lesser wings of the sphenoid bone
The sphenoid bone has smaller (lesser) superior wings, a body in its centre and larger (greater) inferior wings
How many bones for the middle cranial fossa
2- petrous and squamous parts of the temporal bone
Greater wing and body of the sphenoid bone
Number of Foramen in the middle cranial fossa
6
6 Foramen in middle cranial fossa
Optic canal
Superior orbital fissure
Foramen rotundum
Foramen ovale
Foramen lacerum
Foramen spinosum
Petrous part of the temporal bone
Very hard and bulbous inferior and medial part of the temporal bone
Inner and middle ear cavities located inside it
Where are the inner and middle ear cavities located
Inside the petrous part of the temporal bone
Squamous part of the temporal bone
Flat, lateral part of the bone
Sella turcica
Where the pituitary gland is located
Body of sphenoid bine
Body of the sphenoid bone
Includes a small rounded cavity - pituitary fossa
Also known as sella turcica- where pituitary gland is located
Optic canal
Transmits the optic nerve into the bony orbit
Superior orbital fissure
Transmits several nerves that provide motors innervation (oculomotor, trochlear and abducens nerves) and sensation (ophthalmic branch of trigeminal nerve) to the orbital region
Foramen rotundum
Transmits the maxillary branch of the trigeminal nerve
Foramen ovale
Transmits the mandibular branch of the trigeminal nerve
Foramen lacerum
The internal carotid artery exits the carotid canal through this Foramen to enter the skull
Foramen spinosum
Transmits the middle meningeal artery
Which parts of the brain rest in the posterior cranial fossa
Occipital lobes
Cerebellum
Brainstem
Which bones form the posterior cranial fossa
Occipital bone
Part of the petrous part of the temporal hone
Number of foramina in the posterior cranial fossa
4
4 foramina in posterior cranial fossa
Internal auditory meatus
Jugular Foramen
Hypoglossal canal
Foramen magnum
Internal auditory meatus
Transmit the vestibulocochlear and facial nerves into the inner ear cavity
Jugular Foramen
Transmits the Glossopharyngeal, vagus and accessory nerves and the internal jugular vein
Hypoglossal canal
Transmits the Hypoglossal nerve
Foramen magnum
Allows central nervous system fibres to leave the skull and become the spinal cord
Head injuries
A traumatic injury to the head may result in a fracture of the skull. If this occurs, there may be numerous consequences including:
• The brain itself could be directly damaged by the force.
• The fracture could extend through some of the foramina and damage the structures passing through them.
• The dura and arachnoid meninges may be damaged which could cause CSF to leak out. Clinically, this may be suspected if a clear liquid is seen to be leaking from the patient’s nose or ears after a head injury.
• Significant bleeding may occur from the fractured bone or due to damage to intracranial arteries, veins or dural venous sinuses.
Pterion
The pterion is an area of the skull often referred to as the ‘temple’ and it is located just lateral and posterior to the eyebrow. It is a shallow depression where four bones of the skull converge: the frontal, temporal, sphenoid and parietal bones. Because of this, it is considered to be the weakest part of the skull and prone to fracture if struck. Unfortunately, the middle meningeal artery lies immediately behind the pterion, therefore traumatic injuries to this area may cause an extradural haemorrhage.
Which artery lies immediately behind the pterion
Middle meningeal
Craniosynostosis
The sutures (joints between the bones) of the skull do not completely fuse until a child is around two years old. This allows the brain to increase in size as the child grows quickly in infancy. If certain sutures of the skull fuse together too early, as the brain continues to grow it will cause the skull to become misshapen and this is called craniosynostosis. The skull may be elongated in the longitudinal, transverse or oblique planes, depending on which sutures fuse before they are supposed to.
At what age do the sutures in the skull fuse
2 years old
Burr holes and craniotomies
If there is a build-up of pressure within the fixed confines of the skull, this must be relieved, or the brain will eventually be compressed which can lead to death. Common causes of a build of pressure in this way may include intracranial bleeding (such as an extradural haemorrhage) or a brain tumour. To relieve the pressure quickly, a small hole (about 10-15 mm diameter) can be drilled into the skull. This allows the brain to expand enough to relieve the pressure, or it can be used to directly drain the bleeding that’s causing the pressure build-up. To perform surgery on the brain, a larger hole may be needed. This is called a craniotomy, and a circular piece of the skull is removed. This may be replaced later, or a prosthetic implant may be used to close the craniotomy instead.
Which 4 bones meet at the pterion
Frontal
Parietal
Temporal
Sphenoid
Shape of the bony orbits
Cones with a broad opening at the front, tempering to a narrow part at the back
Which bones form the bony orbits
Larger frontal, sphenoid, zygomatic and maxillary bones
Smaller ethmoid and lacrimal bones
Structures found in the orbits
Eye
Extraocular muscle
Nerves
Fat
Lacrimal gland
Number of foramina at back of bony orbit
3
3 foramina at back of bony orbit
Optic canal
Superior orbital fissure
Inferior orbital fissure
Lacrimal gland
Found in the superior lateral part of the orbit
Produces tears to lubricate the anterior surface of the eye
Number of extraocular muscles
7
LR6SO4
Lateral rectus- cranial nerve 6
Superior oblique - cranial nerve 4
Rest if the extraocular muscles- cranial nerve 3
Elevation
Look up
Depression
Look down
Adduction
Look medially
Abduction
Look laterally
Extorsion
To rotate the eye so the top of the eye rotates laterally
Intorsion
To rotate the eye so the top of the eye rotates medially
Conjugate eye movements
Eyes perform different movements to look to the same place
Eg to look left= left eye will abduct and right eye will adduct
7 extraocular muscles
Levator palpebrae superioris
Superior rectus
Inferior rectus
Medial rectus
Lateral rectus
Superior oblique
Inferior oblique
Nerve supply of Levator palpebrae superioris
Oculomotor nerve (CN III)
Nerve supply of superior rectus
Oculomotor nerve (CN III)
Nerve supply of inferior rectus
Oculomotor nerve (CN III)
Nerve supply of medial rectus
Oculomotor nerve (CN III)
Nerve supply of inferior oblique
Oculomotor nerve (CN III)
Nerve supply of lateral rectus
Abducens nerve (CN VI)
Nerve supply of superior oblique
Trochlear nerve (CN IV)
Action of the Levator palpebrae superioris
Elevate the superior eyelid
Action of the superior rectus
Elevate
Intort
Adduct
Action of the inferior rectus
Depress
Extort
Adduct
Action of the medial rectus
Adduct
Action of the lateral rectus
Abduct
Action of the superior oblique
Intort
Depress
Abduct
Action of the inferior oblique
Extort
Elevate
Abduct
Findings if non-functional Levator palpebrae superioris
Ptosis (drooping eyelid)
Findings if non-functional superior rectus
Unable to elevate
Findings if non-functional inferior rectus
Unable to depress
Findings if non-functional medial rectus
Unable to adduct
Findings if non-functional inferior oblique
Unable to elevate if eye is adducted
Findings if non-functional lateral rectus
Unable to abduct
Findings if non-functional superior oblique
Unable to depress if eye is adducted
Common tendinous ring (annulus of Zinn)
The four recti extraocular muscles originate at the back of the orbit on a fibrous ring
Superior and inferior rectus
Main function: elevate or depress the eye
Secondary effects: intorsion, extorsion or adduction
Trochlea
A fibrous sling which the superior oblique passes through located in the superior and medial corner of the orbit
Action and location of the superior oblique
Originates at the back of the eye
Passes through the trochlea
Inserts onto the top of the eye
Intorsion as pulls the top of the eye medially causing it to rotate
What help maintains steady vision when looking up or down
Oblique muscles are able to Intort and extort to counter the secondary effects of extorsion and Intorsion caused by the inferior and superior recti muscles
Accommodation
The eye must be able to focus light to varying amounts depending on how far away the object is that is being visualised
Mechanism of accommodation
The eye adjusts the thickness of the lens
Thicker lens= greater refraction of light —> near objects
Thinner lens= less refraction of light —> far away objects
Thinner lens
Less refraction of light
Far away objects
Thicker lens
More refraction of light
Near objects
Which muscles are responsible for adjusting the thickness of the lens
Ciliary muscles
Which nerve innervates the ciliary muscles
Parasympathetic fibres of the oculomotor nerve (CN III)
Which nerve innervates the constrictor pupillae
Parasympathetic fibres of the oculomotor nerve (CN III)
How is the retina protected from over exposure to light
Constrictor pupillae in iris constricts = circular muscle
Which nerves innervate the dilator pupillae
Sympathetic fibres that originate from the sympathetic chain and enter the skull alongside the internal carotid artery
How is the iris dilated when it is dark
Dilator pupillae will dilate - radial muscle
Pupillary light reflex
Responsible for automatically adjusting the amount of light entering the eye
Afferent nerve of pupillary light reflex
Optic nerve - carries information about amount of light detected on retina to the midbrain
Synapse between the afferent and efferent nerves of the pupillary light reflex
Edinger-Westphal nucleus in midbrain
Efferent nerve of the pupillary light reflex
Oculomotor nerve - initiates constriction of the constrictor pupillae
Direct pupillary response
Constriction of pupil with light shone directly into it
Consensual pupillary response
Constriction of the pupil with light not shone into it
Causes of consensual pupillary response
Connection between the left and right Edinger-Westphal nuclei
Function of lacrimal gland
Produces tears to lubricate and moisten the surface of the eye
Location of the lacrimal gland
Superior and lateral corner of the orbit
Location of lacrimal ducts
Inferior and medial corner of the orbit
Pathway of tears
Secreted by lacrimal gland
Flow across the eye into lacrimal ducts
Drain into nasal cavity via nasolacrimal duct
What drains tears into the nasal cavity
Nasolacrimal duct
Which nerve innervates the lacrimal gland
Parasympathetic fibres of the facial nerve (CN VII)
Orbital wall fractures
Direct traumatic blows to the eye (as may be sustained during a fist fight) may fracture the walls of the orbit. These are known as ‘blow out’ fractures and a fracture of the inferior orbital wall is most common. The inferior rectus muscle can become trapped inside the fracture, tethering the eye in position and patients will be unable to look up.
Cataracts
A cataract is a common ocular condition characterised by clouding of the lens that is responsible for focusing light onto the retina. Cataracts develop slowly and painlessly, and whilst they cannot be prevented, they are relatively easily treated surgically, by removing the affected lens and replacing it with a new and clear intraocular lens.
Oculomotor nerve palsy
If the oculomotor nerve is not functioning on one side, it produces a very characteristic set of findings on clinical examination. The affected eye will rest in a ‘down-and-out’ position (depressed and abducted). This is because the lateral rectus and superior oblique muscles are unopposed so pull the eye into that position at rest. The affected eye’s pupil will also be dilated due to loss of parasympathetic nerve supply to the constrictor pupillae, leaving dilator pupillae unopposed. Finally, due to the loss of motor nerve supply to levator palpebrae superioris, the eyelid will droop. This is known as ptosis. Additionally, on asking the patient to look left and right, the affected side will be unable to adduct.
Abducens nerve palsy
If the abducens nerve is not functioning on one side, the affected eye will be unable to abduct as the lateral rectus muscle is no longer working. The lateral rectus muscle may be overpowered by the medial rectus, which is still functional, to pull the eye medially at rest causing strabismus (sometime referred to as a ‘squint’).
How do clinicians test the function of the trochlear nerve
To isolate the superior oblique muscle and ensure the patient is moving their eye in such a way that only the superior oblique can perform that movement, a clinician observes if the patient can depress an adducted eye. Whilst adducted, the inferior rectus, which is normally responsible for depression, is kinked in such a way that it is rendered weak and unable to depress the eye. Because of the orientation of the fibres of the superior oblique and where it attaches to the top of the eye, it becomes a powerful depressor of the eye in the adducted position.
Testing eye movements
The do this, a clinician may ask the patient to follow their finger as they draw the shape of the letter ‘H’ in front of the patient. The clinician observes the patient’s eyes to ensure they are moving together and asks the patient if they are experiencing any diplopia (double vision).
The four recti muscles are relatively straightforward to test as they are functioning if the eyes can elevate, depress, abduct and adduct.
Effect of opiates on pupillary response
Cause pupillary constriction in higher doses
Effect of sympathomimetics on pupillary response
Pupillary dilation
Examples of opiates
Morphine
Heroine (diamorphine)
Examples of sympathomimetics
Ecstasy (MDMA)
Cocaine
Head injuries and pupillary response
If a patient suffers a significant head injury and has bleeding inside the skull (for example, an extradural haemorrhage), intracranial pressure will rise. This may lead to compression of the oculomotor nerve on one or both sides. If compressed, the oculomotor nerve may be unable to function properly and an early sign of this is dilation of the ipsilateral pupil. This is another reason why clinicians often check the size of a patient’s pupils during clinical examination of unconscious patients. A fixed and dilated pupil is a concerning sign.
Most external part of the ear
Pinna
What makes up the outer ear
Pinna, ear canal and tympanic membrane
Pinna
Shaped to gather sound waves and direct them into the ear canal
Ear canal
Directs sound waves towards the tympanic membrane
Tympanic membrane
Vibrates and transmits sound waves deeper into the ear towards the cochlea
What does the middle ear cavity contain
The ossicles
Eustachian tube
Tensor tympani muscle
Stapedius muscle
Ossicles
3 of the smallest bones in the body
They carry the sound waves to the oval window which conveys it into the cochlea
3 ossicles
Malleus
Incus
Stapes
Malleus locatiom
First ossicle resting against the tympanic membrane
Malleus
Shaped like a hammer
‘Handle’ connects to tympanic membrane
‘Head’ connects to incus
Location of incus
Second ossicle
Which ossicle can be seen during otoscopy
Malleus
Stapes location
Third ossicle
Stapes function
Receives sound wave vibrations from the incus and transmits them onto the oval window which marks the boundary between the middle and inner ear cavity
Oval window
Marks boundary between the middle and inner ear cavity
Location of the superior opening of the Eustachian tube
Superior opening of the auditory tube in the middle ear cavity
Location of inferior opening of the auditory tube
Posterior and inferior part of the nasal cavity
Function of the Eustachian tube
Maintains equal air pressure on both sides of the tympanic membrane by providing a connection of airflow between the external environment and middle ear cavity via the nasal cavity
Tensor tympani muscle
Inserts on the malleus and when it contracts, increases tension in the tympanic membrane (reducing how much it can vibrate)
Innervation of the tensor tympani muscle
Mandibular branch of the trigeminal nerve (CN V)
Stapedius muscle
Inserts in the stapes and when it contracts, dampens the vibrations of the stapes
Innervation of the Stapedius muscle
Facial nerve (CN VII)
What does the inner ear cavity include
Bony labyrinth
Cochlea
Vestibular system
Bony labyrinth
A network of small bony passages within the petrous part of the temporal bone of the skull
Which 2 main organs does the bony labyrinth contain
Cochlea
Vestibular system
Cochlea
As sound waves and vibrations travel through fluid in the cochlea, they are converted into electrical impulses which are passed via the cochlear nerve to the auditory cortex - allowing us to perceive sound
Round window
Located near to the oval window and bulges in and out to allow the fluid in the cochlea to move
Vestibular system consists of
Semicircular canals
Utricle
Saccule
Vestibular system
Contains fluid, which flows when we move our head
Movement detected by specialised cells that cause them to produce electrical impulses
Passed along vestibular nerve towards parts of the brain eg cerebellum, thalamus, certain cranial nerve nuclei
Number of semicircular canals in vestibular system
3
Number of semicircular canals in the vestibular system
3
Function of semicircular canals
Perceive mivement
Arrangement of semicircular canals
3 positioned perpendicular to each other in three dimensions
Function of utricle and saccule
Perceive linear acceleration
Which nerve carries impulses from the cochlea and vestibular system
Vestibulocochlear nerve (CN VIII)
Where does the vestibulocochlear nerve pass through
The internal auditory meatus towards the nuclei in the pons
Oculocephalic reflex
Maintain a fixed gaze whilst moving our head due to connection between the vestibulocochlear nerve and oculomotor, trochlear and abducens nerves
Vertigo
Vertigo is the symptom of being able to perceive movement when there is none. This is commonly perceived and recreated if you spin around several times and then stop suddenly. The perception of the world continuing to spin around you, or the floor moving unevenly beneath you, is vertigo. There are various causes of vertigo, but a common presentation to clinical practice is a disorder of the vestibular system. Inflammation, infections, endo/perilymph disorders or cancers of the vestibular system or nerves may cause vertigo.
Vestibular Schwannoma
Also known as an acoustic neuroma, this is a benign tumour of the Schwann cells surrounding the vestibulocochlear nerve. As it grows, it gradually leads to symptoms of unilateral hearing loss, tinnitus, a feeling of fullness in the ear and vertigo. If it grows large enough, it will start to compress the other cranial nerves that leave the brainstem around the same position. This position is known as the cerebellopontine angle, and other cranial nerves leaving the brainstem here are the trigeminal (CN V) and facial (CN VII) nerves.
Otitis media
Otitis media is the term for inflammation within the middle ear cavity. It is most often caused by a simple viral infection and is particularly common in young children. When a patient is suffering from an upper respiratory tract infection, the inflammation can spread throughout the upper respiratory tract from the nose and pharynx to the middle ear cavity via the auditory tube. This can lead to accumulation of inflammatory fluid and pus in the middle ear cavity, impairing conduction of sound waves along the ossicles.
The auditory tube is relatively narrow in young children and therefore less effective at draining the middle ear cavity. Because of this, a build of pus behind the tympanic membrane can increase pressure significantly which can be painful. If the inflammation continues, the tympanic membrane may rupture due to the pressure. This may relieve the pain and allow drainage of the pus. The tympanic membrane will usually later heal on its own. However, more serious cases of otitis media can spread deeper into the ear and cause inflammation of the cochlea or vestibular system, mastoid process of the temporal bone or the meninges.
What happens when our ears pop
pressure can be felt when descending in an aeroplane from a significant height, or when diving underwater. As you descend deeper, air (or water) pressure outside the tympanic membrane increases and causes the membrane to bulge inwards. To counter this, air is allowed to pass up through the auditory tube to increase air pressure on the inside of the tympanic membrane. Sometimes, if the pressure correction occurs suddenly, you can feel a ‘pop’ in your ear.
- List the cranial nerves which pass through the following foramina:
a. Superior orbital fissure
Oculomotor (III)
Trochlear (IV)
Ophthalmic (V1)
Abducens (VI)
- List the cranial nerves which pass through the following foramina:
b. Jugular foramen
Glossopharyngeal (IX)
Vagus (X)
Accessory (XI)
- List the cranial nerves which pass through the following foramina:
c. Internal acoustic meatus
Facial (VII)
Vestibulocochlear (VIII)
- Within which bone is the sella turcica situated?
Sphenoid bone
What structure is found within the sella turcica?
Pituitary gland
- Which four bones unite to form the pterion?
Parietal bone
Squamous part of temporal bone
Greater wing of sphenoid bone
Frontal bone
Which artery is located immediately deep to the pterion? Which kind of intracranial haemorrhage may be caused by a disruption of this artery?
Middle meningeal artery
Extradural haemorrhage
- Interruption to which nerve(s) would cause miosis (constricted pupil), ptosis and a lack of sweating on the affected side of the face?
Ascending sympathetic fibres in the sympathetic trunk - Horner’s syndrome
- How could you test the function of the trochlear nerve? What exactly would you ask your patient to do to achieve this?
Ask the patient to look down and to the opposite side if the eye that you’re testing i.e., if testing the left eye, ask the patient to look down and to the right
This ensures the eye is adducted and depressed
The superior oblique muscle is the only muscle able to fully depress an adducted eye
- A fracture of the roof of the maxillary sinus would mean which wall of the orbit is fractured? Which extraocular muscle may be impinged in this fracture? How would this appear on examination?
Inferior wall of the orbit
Inferior rectus muscle
Patient would be unable to look upwards
- What is the order of the ossicles from tympanic membrane to oval window?
Malleus
Incus
Stapes
- What happens when we feel our ears ‘pop’?
Equalisation of pressure in either side of the tympanic membrane
Where are the middle and inner ear cavities located
Petrous part of the temporal bone
Which nerve travels through the middle ear cavity and branches to supply the tongue
Facial nerve
