Fourth cranial (trochlear) nerve palsy

[Julian]

Fourth cranial nerve palsy impairs the superior oblique muscle, causing paresis of vertical gaze, mainly in adduction.
(See also Overview of Neuro-ophthalmologic and Cranial Nerve Disorders.)
Fourth cranial (trochlear) nerve palsy is often idiopathic. Few causes have been identified. Causes include the following:

Closed head injury (common), which may cause unilateral or bilateral palsies
Infarction due to small-vessel disease (eg, in diabetes)

Rarely, this palsy results from aneurysms, tumors (eg, tentorial meningioma, pinealoma), or multiple sclerosis.
Fourth cranial nerve palsy may affect one or both eyes. Because the superior oblique muscle is paretic, the eyes do not adduct normally. Patients see double images, one above and slightly to the side of the other; thus, going down stairs, which requires looking down and inward, is difficult. However, tilting the head to the side opposite the palsied muscle can compensate and eliminate the double images.
Examination may detect subtle impaired ocular motility, causing symptoms but not signs. CT or MRI may be done to identify the cause.
Oculomotor exercises or prism glasses may help restore concordant vision. If the palsy does not resolve, surgery, mainly for congenital strabismus, may eventually be needed.