Fourth nerve palsy

The trochlear nerve directly innervates the superior oblique muscle and is one of three cranial nerves responsible for controlling eye movement. Superior oblique palsy is one of the most common etiologies of a newly acquired vertical diplopia. It is often characterized by its specific deviation pattern on motility examination, in which the affected eye develops a hypertropia, and the overall deviation worsens on contralateral horizontal gaze and ipsilateral head tilt. In cases of suspected fourth nerve palsy, it is also important to consider other differential diagnoses of vertical strabismus including bilateral fourth nerve palsies, skew deviation, myasthenia gravis, and thyroid eye disease. Neuroimaging studies may identify the etiology of a suspected palsy. The treatment options for the diplopia associated with a fourth nerve palsy include prisms, strabismus surgery, and occlusive therapy. Each of these options has its own benefits and limitations.