fourth nerve palsy
Also called trochlear nerve palsy, superior oblique palsy, CN IV palsy, CN 4 palsy, superior oblique paresis
Fourth nerve palsy is weakness of the superior oblique, the only muscle the trochlear nerve supplies. It is the commonest cause of acquired vertical double vision and the most frequently diagnosed vertical strabismus.2,3
The affected eye sits higher than its fellow and is rotated outward at the top.1,5 The hypertropia increases on gaze toward the opposite side and on tilt of the head toward the same side, and depression is limited when the eye is turned inward.1,5
The patient reports vertical, tilted double vision that is worse looking down, and holds the head tilted toward the opposite shoulder with the chin down.1 Trauma is the commonest acquired cause, and about three quarters of cases are congenital and present in adulthood when they decompensate.2,3
More detail
Losing the superior oblique removes intorsion, depression in adduction and some abduction.1,5 The unopposed inferior oblique overacts, so the deviation is often greatest in that muscle's field.1,5 The three-step test separates a fourth nerve palsy from the other causes of a vertical deviation.3 A large vertical fusional reserve is the clue to a long-standing deviation, which can decompensate during pregnancy.3 After a head injury a bilateral palsy must be excluded, and it shows as a right hypertropia in left gaze and a left hypertropia in right gaze.1,2 The excyclotorsion can be measured with double Maddox rods.4
Sources (5)
- Kanski, Clinical Ophthalmology (2024), ch. 19
- Yanoff and Duker, Ophthalmology (2023), ch. 9
- Pickwell's Binocular Vision Anomalies, ch. 17
- Liu, Volpe and Galetta, Neuro-Ophthalmology, ch. 15
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 20
Reviewed October 2026