To report a case of unilateral trochlear nerve schwannoma in a patient without neurofibromatosis.

A 58-year-old male presented with acute onset of diplopia which developed 10 days prior. Alternate prism cover test, ductions and versions and Bielschowsky three-step test were compatible with left superior oblique muscle palsy. High-resolution magnetic resonance imaging showed a 6-mm-sized lobulated mass in the cisternal segment of the left trochlear nerve passing lateral to the brainstem. An additional thin-section gadolinium-enhanced orbit magnetic resonance imaging showed definite enhancement in the entire portion of the lobulated mass, compatible with a trochlear nerve schwannoma. Diplopia was managed conservatively with prism glasses and regular follow-up examinations were recommended without further treatment.

A trochlear nerve tumor should be considered in adults who develop diplopia associated with acquired superior oblique muscle palsy.