Extra-ocular Eye Muscles, Actions and Diplopia Flashcards
What is the difference between extrinsic and intrinsic muscles of the eye?
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
Which cranial nerves supply the muscles that move the eyeball?
CN III -Oculomotor
CN IV -Trochlear
CN VI -Abducens
What muscles move the eyeball?
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
Which cranial nerves innervate which muscles?
Oculomotor nerve innervates all the extra ocular muscles of the eye except:
Lateral rectus - Abducens nerve (CN 6)
Superior Oblique - Trochlear nerve (CN 4)
How do you maintain the primary resting gaze?
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.
What does superior rectus do?
Elevate Slightly intort (internally rotate) Slightly adduct (pull eye medially)
What does inferior rectus do?
Depress Slightly extorts (externally rotates) Slightly adducts (pulleys medially)
What does superior oblique do?
Intort (internally rotate) Depress Slightly abducts (pull eye laterally)
What does inferior oblique do?
Extorts (externally rotates) Elevate Slightly abduct (pulls eye laterally)
How does understanding the anatomical actions of the extra-ocular muscles help make sense of abnormalities of gaze?
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).
How can you test the actions of medial and lateral rectus?
Abduction (lateral) and adduction (medial) of the eye.
Why can we not elevate and depress the eye to test the other muscles?
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.
How do you isolate and test muscles that elevate and depress the eye?
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
What cranial nerve is most commonly affected by raised inter cranial pressure?
CN VI (Abducens). This innervates the lateral rectus so patient would present with the eye medial.
Other than raised intercranial pressure, what can affect the eye?
Vascular disease (microvascular complications) from diabetes and hypertension
How would a patient with an oculomotor nerve palsy present?
“Down and out”
Because there is unopposed action of lateral rectus and superior oblique.
In what circumstances is the pupil involved / not involved in a CN III lesion?
Pupil involved - compressive lesion (raised ICP, tumour, aneurysm)
Pupil spared: Vasculopathic (microvascular) lesion e.g. diabetes / hypertension
How does a patient with a CN IV lesion present?
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.
How does a patient with a CN VI palsy present?
Unable to abduct the eye on affected side.
Unapposed pull of medial rectus.
Patient will report diplopia made worse on horizontal gaze.
What is the most likely cause for a CN III, IV, VI lesion?
Vasculopathic.
Patient will otherwise be asymptomatic (appart from CM signs / symptoms)
These lesions will usually self-resolve in a few months.
In a CN III, IV, VI lesion, what things in the history make it more concerning?
Headache +/- vomiting -as could suggest +ICP (tumour or haemorrhage)
Recent head injury
PUPIL INVOLVEMENT - CN III
