Extraocular Muscles
Six extraocular muscles move the eye, supplied by three cranial nerves. One rule summarises the innervation: LR6, SO4, all others 3.
✦ The Mnemonic
"LR Six, Superior Four, All Others Third"
CN VI → Lateral Rectus · CN IV → Superior Oblique · CN III → all four remaining muscles
Clinical Breakdown
CN III palsy: eye 'down and out' (unopposed SO + LR), ptosis. If parasympathetic fibres compressed: fixed dilated pupil. CN IV palsy: most common cause of vertical diplopia; patients tilt head away from the affected side to compensate for extorsion. CN VI palsy: convergent squint, esotropia, horizontal diplopia.
The superior oblique passes through the trochlea (fibrocartilaginous pulley, medial orbital wall) before inserting on the posterior-superior globe. CN IV has the longest intracranial course — most vulnerable in closed head injury.
Internuclear ophthalmoplegia (INO): MLF lesion disconnects CN VI nucleus from the contralateral CN III nucleus. On horizontal gaze, the ipsilateral eye fails to adduct; contralateral eye shows nystagmus. Bilateral INO in a young adult = MS until proven otherwise.
⭐ Clinical Pearl
Bilateral INO in a young adult is essentially pathognomonic of multiple sclerosis. The MLF connects the ipsilateral CN VI nucleus to the contralateral CN III nucleus — a lesion here produces adduction failure on the ipsilateral side with contralateral abducting nystagmus.