Trochlear nerve palsy

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Trochlear nerve palsies, or fourth nerve palsies, result in weakness of the superior oblique muscle.

Clinical presentation

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.

Pathology

Trochlear nerve palsy can be unilateral or bilateral, congenital or acquired.

Etiology
  • idiopathic
    • common
    • thought to be congenital and benign
    • old photographs can be examined for compensatory head posture
  • trauma
    • frequently causes bilateral lesions
    • particularly vulnerable at the anterior medullary velum, through impact with the tentorial edge
  • microvascular
    • 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

Radiographic features 

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

Differential diagnosis

  • restrictive vertical strabismus
  • other paretic vertical strabismus, e.g. CN III
  • myasthenia gravis
  • ocular tilt deviation and skew deviation

Practical points

  • 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

See also