Trochlear nerve palsies, or fourth nerve palsies, result in weakness of the superior oblique muscle.
Vertical diplopia and ipsilateral hypertropia in the absence of ptosis, combined with a head tilt away from the affected side, are strongly suggestive of trochlear nerve palsy. Characteristically, patients will have problems reading or walking down stairs. The Parks-Bielschowsky 3-step test is useful to identify patterns of ocular motility, specifically vertically-acting extra-ocular muscles.
Trochlear nerve palsy can be unilateral or bilateral, congenital or acquired.
- thought to be congenital and benign
- old photographs can be examined for compensatory head posture
- frequently causes bilateral lesions
- particularly vulnerable at the anterior medullary velum, through impact with the tentorial edge
- assumed etiology in context of systemic vascular risk factors (e.g. diabetes mellitus), in the absence of congenital onset
- intracranial masses, e.g. cerebral aneurysm, tumor
- multiple sclerosis
Neuroimaging is not required routinely for an isolated non-progressive trochlear nerve palsy but should be considered if no improvement is seen. MRI is superior to CT for evaluation of trochlear nerve palsy.
Treatment and prognosis
Treatment for the underlying process if identified:
- vascular workup should be considered
- lumbar puncture if considering meningitic processes
Any non-resolving trochlear nerve palsy should be referred to ophthalmology for specialist opinion
Some patients with microvascular or traumatic trochlear nerve palsy have some improvement 3-9 months after presentation
- restrictive vertical strabismus
- other paretic vertical strabismus, e.g. CN III
- myasthenia gravis
- ocular tilt deviation and skew deviation
- characteristic head tilt to the opposite side of the palsy; review old photographs to determine chronicity of observation
- can occur with relatively minor head trauma due to anatomical vulnerability
- 1. Bowling B. Kanski's Clinical Ophthalmology. (2015) ISBN: 9780702055720