Superior Oblique Myokymia

Excerpt

Superior oblique myokymia (SOM) is defined as "monocular, high-frequency, low-amplitude, torsional, and involuntary contractions of the superior oblique muscle that result in oscillopsia and diplopia." It is a monocular condition, first described by Duane in 1906 as "unilateral rotary nystagmus." His patient described seeing an object that rose vertically out of the other object and appeared to dance up and down. The term "superior oblique myokymia" was introduced in 1970 by Hoyt and Keane. The condition is now known as superior oblique myokymia of Hoyt and Keane, although the historically more accurate term would be Duane's uniocular micro-tremor. a term that was popular in the 1970s.

History: On the 10th of May, 1906, Alexander Duane saw a 24-year-old woman who complained that over the previous few months, she had episodes of double vision where "the objects appear to dance up and down." On examining the patient, Duane noted intermittent nystagmus, which was limited to the left eye and there were "rapidly repeated, short, quick, to-and-fro oscillations, sometimes mixed (vertical and rotary), at other times purely rotary." He termed it "unilateral rotary nystagmus.

Little progress was made in understanding this condition until 1970, when William Hoyt and James Keane described five patients with similar "hyperactive eye movements" and were the first to suggest that the primary problem was localized to the superior oblique muscle or its nerve. They termed this condition "superior oblique myokymia." Dr. Hoyt noted that the episodes in healthy patients were spontaneous and unilateral with oscillations or fluttering, which resulted in the patients experiencing brief torsional and/or vertical double vision. They noted that the attacks could occur at any time and multiple times a day and could resolve for years and then return. Dr. Hoyt also noted the rapid frequency (12 to 15 Hz) and low amplitude (1 to 2 degrees ) of the vertical and incyclotorsional movements.

He also pointed out the best way to examine for superior oblique myokymia: examining the patient under the slit-lamp and picking out a conjunctival vessel on the eye, which should be watched to detect the intermittent incyclotorsion. This remains the best way to examine these patients. Drs Hoyt and Keane hypothesized that the etiology of SOM included partial denervation of the muscle due to trochlear nerve injury or a pathologic alteration of the membrane threshold of neurons in the trochlear nucleus. They compared it to facial myokymia.

Based on his observations, Dr. Hoyt performed a superior oblique tenotomy and ipsilateral recession of the inferior oblique muscle with a resolution of the symptoms. As many patients experience long periods of remission, he suggested that most patients should be treated just with an assurance that this was a benign condition. He also noted that surgical weakening of the superior oblique was not recommended as the resultant persistent double vision in downgaze after such a procedure was more problematic than the initial intermittent oscillopsia.