[Ocular neuromyotonia]

Ocular neuromyotonia is a rare ocular motility disorder characterized by involuntary contractions of one or several ocular motor muscles. In this report a typical case is presented.

A 61-year-old female patient was referred with persistent diplopia despite previous surgical treatment for right 6 (th) nerve palsy. The palsy was caused by a meningeoma of the petrous bone, which had been incompletely resected. Subsequently, the patient had received radiation therapy. Strabismological and neuroophthalmological examinations were performed. The results coincide with data collected in our department from three other patients with ocular neuromyotonia (2 cranial nerve VI, 1 cranial nerve IV), over the past three years.

The patient showed orthophoria at distance (5 m) and an exophoria of 6 degrees at near vision. The abduction and adduction of the right eye were restricted to 40 degrees and 35 degrees , respectively. After several seconds of eccentric gaze to the right, the right eye remained in an abducted position of approximately 25 degrees . The gaze to the left led to a retraction instead of an adduction of the right eye. The spasm of the lateral rectus muscle resolved after some seconds, returning to the aforementioned right eye motility. Under oral treatment with carbamazepine, initially 200 mg, later increasing to 400 mg per day, the symptoms improved significantly, did not resolve entirely, however.

These typical findings permit the diagnosis of ocular neuromyotonia. The characteristic symptoms of ocular neuromyotonia and the typical history of a previous intracranial tumor, treated neurosurgically with adjuvant radiotherapy, lead to the hypothesis that ephaptic transmission in the cranial nerve is the underlying pathophysiological mechanism in the development of ocular neuromyotonia. Hereby, efferent impulses from non-twitch motoneurons could activate neighbouring axons, and spread both peripherally and centrally. Beside other mechanisms discussed, an involvement of proprioceptive elements and their reafference is also a possible cause for the prolonged muscle contraction.