Superior oblique myokymia and gaze direction

This is a patient with left SOM. Since the primary action of the SO muscle is incycloduction, with each contraction, the top pole of the left eye can be seen moving toward the right ear. This is not nystagmus because it is excitation of the SO muscle that initiates the movements (followed by a slower relaxation phase) rather than the slow phase being the culprit (as in jerk or pendular nystagmus). Since the secondary action of the SO muscle is depression, this is best appreciated in adduction where the vertical action of the muscle is maximal. These movements were purely monocular in the left eye, without right eye involvement.