Most patients have had the classic bursts of smallamplitude, high-frequency rotary oscillations causing oscillopsia, defined as intermittent uniocular microtremor [8}. Others, however, have had a slower, larger-amplitude intorsional movement causing vertical and torsional diplopia. We have termed this movement macrorota y deviation (MRD). MRD has occurred most often in association with the more classic IUM C6-8, 10, 111, but in a few patients it has been the only or predominant eye movement abnormality C2, 7, 121. Other associated motility problems have included intermittent overaction of the ipsilateral superior oblique muscle 110-121, ipsilateral superior oblique paresis ( 7 , 101, and, in our patient, an intermittent Brown's syndrome.We suspect that all of the grossly visible movements (MRD, intermittent overaction of the superior oblique, and intermittent superior oblique tendon sheath syndrome) represent the same rotary entasia expressed differently when assessed by varying techniques. An alternate-cover test would reveal an overaction of the superior oblique muscle; an apparent Brown's syndrome would be diagnosed if the motor anomaly occurred during attempted gaze inward and upward; and the MRD itself would be observed only during direct observation.The cause of IUM and MRD is unclear. Their frequent concurrence implies a continuum of motor anomalies. Electromyographs of the two types of movements are similar, with evidence of chronic denervation, supporting the idea that both are manifestations of superior oblique myokymia [7, 91. Kommerell's patient with MRD had a more stable rate of firing than others with IUM, suggesting that the firing pattern of the abnormal motor units may determine the clinical picture E91. All electromyographic evidence to date supports Hoyt's original suggestion that SOM is primarily a nuclear disorder C7-91.Possibly pertinent to SOM is a report by Baker and Bertoz, who evaluated oblique nystagmus induced by vestibular lesions in alert "encephale isole" cats C11.Using intra-and extracellular recordings from inferior and superior oblique motor neurons, they noted a "rhythmic oscillation" in the electrical discharge from the oblique motor neurons immediately after a sudden decrease in the stimulation rate. The occurrence of such a "rhythmic oscillation" immediately after a sudden decrease in the stimulation rate could be the physiological basis for the clinical observation that the uniocular microtremor is most easily provoked by moving the eye into and then out of the field of action of the superior oblique muscle.The authors wish to thank Dr J. Lawton Smith for referring his patients to us for follow-up.
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