Expansion
LR6 SO4 rest 3
Mnemonic
LR6 SO4, all the rest are 3:
- LR6 - lateral rectus, abducens (VI)
- SO4 - superior oblique, trochlear (IV)
- The remainder, and levator palpebrae superioris, by the oculomotor nerve (III)
The trochlear nerve is unique three times over: it is the only cranial nerve to emerge from the dorsal surface of the brainstem, the only one to decussate completely, and it has the longest intracranial course, which makes it the most vulnerable to head injury.
Testing follows the anatomy: ask the patient to look down and in to isolate superior oblique, and look for a compensatory head tilt away from the affected side.
Expansion
- LR6 - Lateral rectus supplied by CN VI
- SO4 - Superior oblique supplied by CN IV
- Rest 3 - all other extraocular muscles supplied by CN III