The trochlear nucleus is the small motor nucleus of cranial nerve IV in the lower midbrain. Its fibres are unusual: they cross completely to the opposite side and leave the back of the brainstem, so each nucleus controls the superior oblique muscle of the opposite eye.
| Location | Lower midbrain at the level of the inferior colliculus, in front of the periaqueductal grey, embedded in the medial longitudinal fasciculus |
|---|---|
| Parts |
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| Connections |
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| Blood Supply | Paramedian perforators of the basilar artery; Superior cerebellar artery |
Controls the superior oblique muscle of the opposite eye, which turns the eye down and in and rotates it inward.
A lesion of the trochlear nucleus causes superior oblique weakness in the opposite eye, while a lesion of the nerve after it crosses causes weakness of the same-side eye. Patients report vertical double vision worse when looking down, such as reading or walking downstairs, and often tilt the head away from the affected eye. Head trauma is a common cause of fourth nerve palsy because the crossing fibres are exposed.
Weakness of the superior oblique causing vertical and torsional double vision, especially on downgaze, often after head injury.
Damage to both crossing trochlear fibres in the anterior medullary velum from closed head injury, causing severe torsional double vision.
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