The trochlear nerve is the smallest cranial nerve by number of axons and the only cranial nerve to exit from the dorsal brainstem, making it the most vulnerable to stretching from raised intracranial pressure or tentorial herniation. It supplies only the superior oblique muscle, which intorts and depresses the eye (especially useful for looking down while descending stairs). Trochlear nerve palsy produces a characteristic head tilt toward the opposite shoulder to compensate for the vertical diplopia.
| Origin | Trochlear nucleus in the dorsal midbrain periaqueductal grey at the level of the inferior colliculus — the only cranial nerve to exit from the dorsal brainstem |
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Trochlear nerve palsy produces a hypertropia (the affected eye higher) that worsens on gaze toward the opposite side and on ipsilateral head tilt — these three characteristics form the Parks-Bielschowsky three-step test that isolates the superior oblique as the paretic muscle. The compensatory head tilt away from the affected eye is the classic posture of congenital trochlear nerve palsy that may have been present for years before diagnosis.
Superior oblique paralysis producing vertical diplopia with compensatory contralateral head tilt, managed with prism glasses for mild cases and superior oblique tuck surgery for significant deviation.
