Extra-ocular Eye Muscles, Actions and Diplopia

Question 1

What is the difference between extrinsic and intrinsic muscles of the eye?

Answer 1

Extrinsic muscles: muscles of the eyelid, extra-ocular muscles that move the eyeball

Intrinsic muscles of the eye: Muscles of the iris (dilator and constrictor of the pupil), ciliary muscle controls thickness of the lens

Question 2

Which cranial nerves supply the muscles that move the eyeball?

Answer 2

CN III -Oculomotor
CN IV -Trochlear
CN VI -Abducens

Question 3

What muscles move the eyeball?

Answer 3

Superior, inferior, medial and lateral rectus
Superior and inferior oblique.

They all have attachments to the sclera and four muscles (the recti) arise from a common tendinous ring

Question 4

Which cranial nerves innervate which muscles?

Answer 4

Oculomotor nerve innervates all the extra ocular muscles of the eye except:

Lateral rectus - Abducens nerve (CN 6)
Superior Oblique - Trochlear nerve (CN 4)

Question 5

How do you maintain the primary resting gaze?

Answer 5

Equal and opposite pull of all extra ocular muscles.

Each muscle has an antagonist of its movement but during resting gaze their actions are balanced to allow for a forward gaze.

Question 6

What does superior rectus do?

Answer 6

Elevate
Slightly intort (internally rotate)
Slightly  adduct (pull eye medially)

Question 7

What does inferior rectus do?

Answer 7

Depress 
Slightly extorts (externally rotates)
Slightly adducts (pulleys medially)

Question 8

What does superior oblique do?

Answer 8

Intort (internally rotate)
Depress 
Slightly abducts (pull eye laterally)

Question 9

What does inferior oblique do?

Answer 9

Extorts (externally rotates)
Elevate
Slightly abduct (pulls eye laterally)

Question 10

How does understanding the anatomical actions of the extra-ocular muscles help make sense of abnormalities of gaze?

Answer 10

If a muscle is weakened, its ‘influence’ / pull on the eyeball is lost so the other muscle actions are no longer antagonised (balanced).

This means that the resting position of the eyeball changes.
Strabismus = squint

This can be congenital or acquired (cranial nerve lesions).

Question 11

How can you test the actions of medial and lateral rectus?

Answer 11

Abduction (lateral) and adduction (medial) of the eye.

Question 12

Why can we not elevate and depress the eye to test the other muscles?

Answer 12

Because each of these actions requires more than one muscle.

Elevation = superior rectus and inferior oblique 
Depression = Inferior rectus and superior oblique

This means that muscles haven’t been isolated so you the other one could be compensating.

Question 13

How do you isolate and test muscles that elevate and depress the eye?

Answer 13

Abduct and adduct the eye first to isolate the muscles then elevate and depress. (Make a H shape and get patient to follow finger)

Abduction and Elevation = Superior rectus

Abduction and depression = Inferior rectus

Adduction and elevation = Inferior oblique

Adduction and depression = Superior oblique

Question 14

What cranial nerve is most commonly affected by raised inter cranial pressure?

Answer 14

CN VI (Abducens). This innervates the lateral rectus so patient would present with the eye medial.

Question 15

Other than raised intercranial pressure, what can affect the eye?

Answer 15

Vascular disease (microvascular complications) from diabetes and hypertension

Question 16

How would a patient with an oculomotor nerve palsy present?

Answer 16

“Down and out”

Because there is unopposed action of lateral rectus and superior oblique.

Question 17

In what circumstances is the pupil involved / not involved in a CN III lesion?

Answer 17

Pupil involved - compressive lesion (raised ICP, tumour, aneurysm)

Pupil spared: Vasculopathic (microvascular) lesion e.g. diabetes / hypertension

Question 18

How does a patient with a CN IV lesion present?

Answer 18

Extorted, slightly elevate and adducted.
Because only function fo superior oblique is lost.

The patient will compensate the the slight extortion by tilting the head slightly. The abnormality in the gaze can be very subtle and often missed.

The patient will have worsening diplopia especially when looking medially and down.

Question 19

How does a patient with a CN VI palsy present?

Answer 19

Unable to abduct the eye on affected side.
Unapposed pull of medial rectus.

Patient will report diplopia made worse on horizontal gaze.

Question 20

What is the most likely cause for a CN III, IV, VI lesion?

Answer 20

Vasculopathic.

Patient will otherwise be asymptomatic (appart from CM signs / symptoms)

These lesions will usually self-resolve in a few months.

Question 21

In a CN III, IV, VI lesion, what things in the history make it more concerning?

Answer 21

Headache +/- vomiting -as could suggest +ICP (tumour or haemorrhage)

Recent head injury

PUPIL INVOLVEMENT - CN III