1b Visual System Defects

Question 1

What is seen in a patient with 3rd cranial nerve palsy?

Answer 1

Down and out eye deviation
Ptosis
Dilated Pupil

Question 2

Explain why cranial nerve palsy (CNIII) results in the symptoms seen?

Answer 2

Down and Out Eye: Unopposed actions of the Lateral Rectus and Superior Oblique muscles which are innervated by cranial nerves 6 and 4

Complete ptosis - due to failure of the levator palepbral superioris, when stimulated = lifting of the eyelid, and since it is innervated by CNIII it is now non functioning

Question 3

What causes dilation of the pupil?

Answer 3

Either overactivation of sympathetic
Underactivation of parasympathetic

Question 4

What innervation does the oculomotor nerve control?

Answer 4

Parasympathetic to the eye

Question 5

What are the two categories of lesions to the oculomotor nerve?

Answer 5

Medical
Surgical

Question 6

What are medical lesions?

Answer 6

Typically affect the vasculature of the nerve and not pupil - pupil sparing as the parasympathetic fibres run to the outer portion of the eye

Question 7

What are some causes of medical lesions?

Answer 7

Hypertension
Microvascular effects of diabetes

Question 8

What usually causes surgical lesions?

Answer 8

Posterior communicating artery aneurysm - compresses the outer portion of the nerve fibres = where the parasympathetic fibres run

Question 9

Where does the CNIII run in relation to the circle of willis?

Answer 9

Between the posterior cerebral and superior cerebellar artery

Question 10

What is Pilocarpine and how does it work?

Answer 10

Muscarinic receptor agonist which acts on the M3 receptors in the iris sphincter muscle = results in contraction of the ciliary muscles and therefore leads to constriction of the pupil = independant from the parasympathetic response