Cardinal features of superior oblique myokymia: An infrared oculography study

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Question: What are the cardinal features of superior oblique myokymia?
Answer: There are 6 cardinal features of eye movement abnormality in superior oblique myokymia including:
1) involuntary intorsion and torsional oscillations;
2) episodic events lasting seconds;
3) worsening with infraduction and abduction positions that require activation of the superior oblique, and improvement with supraduction and adduction positions where the superior oblique is not activated;
4) overshooting of saccades on infraduction;
5) extorsion and diminished oscillations that were unmasked upon removal of a visual target, consistent with underlying weakness; and
6) improvement with membrane stabilizers used to treat neuropathic conditions. These features localized the lesion to the trochlear nerve, fascicle, or nucleus but not to the superior oblique muscle or neuromuscular junction.

Cardinal features of superior oblique myokymia: An infrared oculography study
 2017 Jun 23;7:115-119. doi: 10.1016/j.ajoc.2017.06.018. eCollection 2017 Sep.
Cardinal features of superior oblique myokymia: An infrared oculography study.
Thinda S1Chen YR2Liao YJ3Abstract
PURPOSE: Superior oblique myokymia (SOM) is a rare eye movement disorder characterized by unilateral oscillopsia and binocular diplopia. Our study aimed to better understand SOM using infrared oculography.
METHODS: We examined and recorded five patients with SOM.
RESULTS: Binocular infrared oculography showed that in primary gaze, all patients exhibited torsional oscillations, which worsened in infraduction and abduction and improved in supraduction and adduction. Saccades showed increased downward saccade amplitudes but normal peak velocities. During fixation in primary gaze, removal of target led to extorsion and supraduction, unmasking underlying superior oblique weakness.
CONCLUSIONS AND IMPORTANCE: Our data suggest both weakness and activity-dependent hyperactivity of the trochlear motor unit, supporting a model of injury followed by aberrant regeneration.

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