2.12 The Larynx

Question 1

The Function and Anatomy of the Larynx

Answer 1

The larynx has a crucial role in protecting the airway from contamination. It does
this by invoking what is one of the most powerful physiological reflexes, and one to
which every anaesthetist who has managed intractable laryngospasm will attest.

The larynx has also evolved into an organ of phonation.

The larynx extends from the base of the tongue above, to the trachea below, and in
the adult male it lies opposite the third to sixth cervical vertebrae. In the adult female
and in children it lies higher.

The larynx comprises a number of articulating cartilages which are joined by
ligaments and which are subject to the action of various muscles that move these
cartilages in relation to each other.

Question 2

Cartilaginous Framework

Answer 2

The cartilaginous framework comprises the thyroid, cricoid and arytenoid cartilages.
(The smaller corniculate and cuneiform cartilages contribute little to this structure.)

The thyroid cartilage comprises two quadrilateral laminae which are fused anteriorly
to form the laryngeal prominence. It articulates inferiorly with the cricoid. The
thyroid notch lies at the level of C4.

The cricoid cartilage is a continuous ring with a narrow anterior arch and a deeper
posterior lamina. It articulates on each side with the inferior cornu of the thyroid
cartilage and with the base of the arytenoid cartilage.

Each of the paired arytenoid cartilages is pyramidal in shape. The smooth concave
base articulates with the cricoid cartilage. The lateral angle, or muscular process,
projects backwards, while the anterior angle, or vocal process, projects forwards.

The apex articulates with the corniculate cartilage.

The two corniculate cartilages are small nodules which are sometimes fused with the
arytenoids and which lie in the posterior aryepiglottic folds of mucous membrane.

The two cuneiform cartilages lie anterior to the corniculate cartilages, also within the aryepiglottic fold.

Question 3

Ligaments

Answer 3

There are a number of intrinsic and extrinsic ligaments.

Those of anaesthetic interest include the
thyrohyoid membrane,
which joins the upper border of the thyroid cartilage to the hyoid bone,

and the cricothyroid ligament between the
cricoid and thyroid cartilages.

Question 4

Ligaments

Answer 4

There are a number of intrinsic and extrinsic ligaments.

Those of anaesthetic interest include the
thyrohyoid membrane,
which joins the upper border of the thyroid cartilage to the hyoid bone,

and the cricothyroid ligament between the
cricoid and thyroid cartilages.

Question 5

Vocal cords

Answer 5

The vocal cords (also known as the vocal folds) are opalescent folds of mucous
membrane which extend from the arytenoid cartilages
as far as the middle of the angle of the thyroid cartilage.

The vestibular folds, or false cords,
lie lateral to the cords and
comprise thicker folds of mucous membrane which
also extend from the thyroid cartilage to the arytenoids.

Question 6

Laryngeal Muscles

Answer 6

There are a number of extrinsic and intrinsic muscles of the larynx.

The extrinsic muscles
(the sternothyroid, the thyrohyoid and the inferior constrictor of the pharynx)
attach the larynx to adjacent structures.

The intrinsic muscles are of more immediate interest to the
anaesthetist because they control the opening of the cords
during inspiration, the closure of the cords and laryngeal inlet during swallowing,
and the tension of the cords during speech.

Question 7

Intrinsic vocal cord muscles and movement

Answer 7

1. Abduction:
   Abduction of the cords is performed by the posterior cricoarytenoidmuscles.
2. Adduction:
   Adduction of the cords is performed by the lateral cricoarytenoids and
   the unpaired interarytenoid muscle.
3. Tensors:
   the main tensors of the vocal cords are the cricothyroid muscles.
4. Relaxors:
   the main relaxors of the vocal cords are the thyroarytenoid muscles.

5.

Question 8

Abduction

Answer 8

Posterior cricoarytenoid

Question 9

adduction

Answer 9

Lateral Cricroarytenoid
unpaired interarytenoid

Question 10

Innervation

Answer 10

All the muscles of the larynx,
with one exception,

are innervated by the recurrent laryngeal nerve.

The exception is the cricothyroid muscle, which is
supplied by the external branch of the superior laryngeal nerve

Question 11

Factors Affecting the Ease of Laryngoscopy

Answer 11

You will be aware that anaesthetists have long sought a test
or a combination of tests that have a high
sensitivity and specificity for predicting difficult intubation.

None has yet been found.

The simplest means of classifying the degree of difficulty is by
using the c+l

Question 12

Cormack and Lehane classification.

Answer 12

Cormack and Lehane classification.

(This describes the best view that is
obtained at laryngoscopy:
grade I – full view,
grade II – posterior part of the glottis
only, grade III – epiglottis only,
grade IV – soft palate only.)

Question 13

How is the larynx visualised

Answer 13

The larynx can be seen directly only if there is a single direct plane of view.

This means that the three axes of the
oral cavity, the pharynx and the larynx must be brought into alignment.

In practice this is done by opening the mouth wide, flexing
the neck, extending the head at the atlanto-occipital joint

and lifting the base of the tongue and epiglottis upwards and forwards.

Question 14

Predicting difficulty

Answer 14

Any factor which impedes this alignment will make direct laryngoscopy and intubation more difficult.

Such factors include limited (<4 cm) mouth opening,

prominent upper incisors,

maxillary prognathism and the

inability to protrude the lower incisors in front of the upper,
limited neck mobility with restricted extension
(thyromental distance of <6.5 cm),

and a high anterior larynx.

Question 15

Obesity?

Answer 15

Obesity is often cited as a factor,
but studies in patients undergoing bariatric surgery have demonstrated no
difference in laryngoscopic view between the morbidly obese and those patients of
normal body habitus.

This is not surprising, as obesity per se, and for that matter pregnancy,
do not in themselves impede the ability to obtain a single axis plane of
view between the incisors and the glottis.

Question 16

Radiographs?

Answer 16

Many other predictors of difficulty have been described,
such as the radiological assessment of the atlanto-occipital gap,
the C1–C2 gap and the anterior-posterior depth of the mandible.

These are of limited clinical use, as such radiographs are rarely available (or sought).

Question 17

It is important to be able to recognize structures that are seen at laryngoscopy.

Answer 17

Beyond the elevated epiglottis are the false and the true vocal cords.

Posteriorly are the arytenoid cartilages
(together with the bulges of the
corniculate and cuneiform cartilages).

Between the cords is the laryngeal inlet,
or rima glottidis, beyond

which may be visible the upper rings of the trachea
(Figure 2.4).

Question 18

Arytenoids + Intubation

Cricoarytenoids

Answer 18

The arytenoids can be dislocated or subluxed during tracheal intubation

or laryngeal mask insertion.

This will interfere with the function of some of the intrinsic muscles
and may compromise the airway.

The cricoarytenoid joint may also be affected by
systemic inflammatory arthropathies,

particularly rheumatoid arthritis and by the
tissue changes associated with acromegaly.

Question 19

Other anatomical relevant features

Answer 19

The anatomy of the cricoid cartilage is relevant both for rapid sequence induction of
anaesthesia, and also for emergency access to the airway.
It is also important to be able to recognize the airway signs of injury to the recurrent
laryngeal nerve as described in the next section.