CN IV is the thinnest cranial nerve and has the longest intracranial course, making it uniquely vulnerable to head trauma — closed head injuries frequently produce CN IV palsy from the nerve's stretch around the tentorial edge. Superior oblique palsy produces vertical diplopia worst on downgaze and contralateral gaze with a compensatory head tilt away from the affected side (to maintain fusion).
| Origin | Trochlear nucleus in the midbrain (inferior colliculus level) — the only cranial nerve that decussates completely and exits from the dorsal brainstem |
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CN IV palsy produces the classic triad: vertical diplopia, worsened on looking down and toward the contralateral side, with compensatory contralateral head tilt. The Parks-Bielschowsky three-step test isolates the paretic muscle. Post-traumatic CN IV palsy resolves spontaneously in 50-75% within 6 months. Surgical superior oblique tucking or inferior oblique weakening corrects persistent diplopia.
Superior oblique paralysis from head injury producing vertical diplopia managed with observation for 6 months and inferior oblique weakening for persistent cases.
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