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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| Course | Fibres decussate in the superior medullary velum and emerge from the dorsal midbrain just below the inferior colliculus, giving the longest intracranial course of any cranial nerve; the nerve curves around the cerebral peduncle, runs in the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure above (outside) the common tendinous ring to supply the superior oblique |
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Trochlear nerve palsy is the commonest cause of isolated vertical diplopia. The affected eye sits higher (hypertropia), worse on looking toward the opposite side and on tilting the head toward the affected side; these features form the Parks-Bielschowsky three-step test. Patients tilt the head away from the affected side to compensate, a posture often present for years in congenital cases, which are the most common type. Acquired palsy is most often due to head trauma, because the thin nerve is stretched where it leaves the dorsal midbrain, and in older adults to microvascular ischaemia (diabetes, hypertension), which usually recovers within months. Because the fibres cross before leaving the brainstem, a lesion of the trochlear nucleus affects the opposite superior oblique.
Superior oblique weakness causes vertical diplopia with a compensatory head tilt away from the affected side, treated with prisms for mild cases and strabismus surgery for larger deviations.
Severe head injury can damage both nerves where they cross at the dorsal midbrain, causing bilateral superior oblique weakness with marked torsional double vision, especially on looking down.
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