Walsh & Hoyt: Superior Oblique Myokymia (Superior Oblique Microtremor)

Superior oblique myokymia (SOM)was first described by Duane in 1906, but clinicians became generally aware of the disorder following the description by Hoyt and Keane in 1970. Typical symptoms are monocular blurring of vision or tremulous sensations in the eye. Patients typically experience brief episodes of vertical or torsional diplopia, vertical or torsional oscillopsia, or both. Attacks usually last less than 10 seconds but may occur many times a day. The attacks may be spontaneous or be brought on by looking downward, by tilting the head toward the side of the affected eye, or by blinking. Most patients with SOM have no underlying disease, although cases have been reported following trochlear nerve palsy, after mild head trauma, in the setting of MS, after brain stem stroke, in a patient with a dural arteriovenous fistula, and in patients with cerebellar tumor. The eye movements of SOM are often difficult to appreciate on gross examination but are usually apparent with the ophthalmoscope or slit-lamp biomicroscope. They consist of spasms of cyclotorsional and vertical movements. Measurement of the movements of SOM using the magnetic search coil technique reveal an initial intorsion and depression of the affected eye, followed by irregular oscillations of small amplitude. The frequency of these oscillations is variable. Some resemble jerk nystagmus, with frequencies of 2 to 6 Hz, but superimposed upon these oscillations are low-amplitude, irregular oscillations with frequencies ranging up to 50 Hz