The superior oblique muscle and its disorders.

The superior oblique muscle with its innervation by the trochlear nerve has several unusual features, and it plays a role in neurology disproportionate to its size. If the muscle were to contract by itself it would intort the eye and turn it downwards and outwards, but under normal circumstances it does not act in isolation but in combination with other extraocular muscles. One of its roles is to depress the eye when the eye is adducted, in which position the inferior rectus loses mechanical advantage. It follows that patients who develop a superior oblique palsy commonly complain of vertical diplopia on looking downwards, for example when reading or descending stairs. Image separation and hypertropia of the affected eye worsen when the patient’s gaze is directed away from the affected eye or when the head is tilted towards the affected eye. Preserved fourth nerve function in patients with a third nerve palsy can be detected by getting the patient to attempt downgaze. If the fourth nerve is intact the eye will depress slightly and intort, which can be identified by …