| OCR Text |
Show ]. Clin. Neura-aphtha/mal. 3: 131-132,1983. Superior Oblique Myokymia A Misnomer LENORE A. BREEN, M.D. LUDWIG GUTMANN, M.D. JACK E. RIGGS, M.D. Abstract Clinical examination of a 26-year-old, white man with episodic vertical diplopia and oscillopsia led to the diagnosis of "superior oblique myokymia." The dissimilarity between the observed superior oblique movements and myokymia as defined in other nervous system disorders is striking. Therefore, we suggest that superior oblique microtremor may be a better term to describe these superior oblique movements. In 1970, Hoyt and Keane described five patients with intermittent vertical diplopia and oscillopsia. I Examination of these patients revealed fine, rapid intorsional movements of one eye. They suggested the alternate terms "superior oblique myokymia" or "intermittent uniocular microtremor," to describe these movements. Superior oblique myokymia has become the popular term. Susac et aI. stressed the benign clinical course of this entity and, reported its beneficial treatment with Tegretol.- Electrophysiological studies of the intorsional movements have failed to define the underlying mechanism. I. :J. 4 Additionally, those cases studied by superior oblique electromyography failed to demonstrat~ the characteristic discharges seen in myokymia.'" 6 We evaluated recently a 26-year-old man with episodic vertical diplopia and oscillopsia. Examination revealed the characteristic monocular intorsional movements of the eye seen in superior oblique myokymia. The clinical features, however, did not satisfy the accepted definition of the term myokymia. A less specific term would be more appropriate until the underlying pathophysiology is better delineated. Case Report A 26-year-old, white man presented with an 8month history of vertical diplopia and oscillopsia. His symptoms were intermittent, occurring 1-10 From the Departments of Neurology (LAB, LG, JER) and Ophthalmology (LAB), West Virginia University School of Medicine, Morgantown, West Virginia. June 1983 minutes several times a day or disappearing completely for several days. He noted the onset of perceptable left eye movements and symptoms with reading, left head tilt, or chin elevation. His diplopia was usually relieved by adopting a right head tilt or depressing the chin placing the eye in an upgaze position. Lying down in bed or placing pressure with his finger over the left trochlear notch also alleviated his symptoms. Fatigue, ethanol, and caffeine had no effect on the frequency of his symptoms. He had no previous history of eye disease, strabismus, or other neurological dysfunction. In high school football games, he suffered three significant closed head injuries with loss of consciousness and amnesia. None required hospitalization. The family history was negative for neurologic disease. Physical examination, including a detailed neurological examination, was normal except for the abnormal extraocular movements of the left eye. Cover testing revealed an intermittent right hypertropia worse in gaze down and right. The left eye had episodic fine, rapid intorsional movements lasting seconds to minutes. The amplitude of the excursions was variable. These movements could be initiated or relieved by all the previously mentioned maneuvers. The eye could also remain at rest for several minutes duration between episodes. Hyperventilation did not incite these ocular movements. The remainder of the ophthalmic examination was normal. The patient's symptoms have totally disappeared after 2 months of Tegretol therapy (200 mg t.i.d.). Discussion Myokymia is a well-defined, clinical phenomenon with charact;~istic, clinical, and electrophysiological features."-' Clinically, myokymia is manifested by continuous, involuntary, undulating, fine contractions of numerous motor units of a muscle. This may be associated with a mild sustained contraction of the involved muscle. ElectrophysiologICally, bnef tetanic bursts of motor unit potentials recurring in a rhythmic or semirhythmic fashion several times per second are seen in myokymia. These tetaniC bursts appear as doublets, triplets, or 131 Superior Oblique Myokymia multiplets. Their frequency varies from one myokymic discharge to another. The discharges in different motor units are not synchronized with each other and are not under voluntary control. Myokymia may occur in a variety of clinical situations, but is most commonly seen in facial muscles in multiple sclerosis;' Guillian-Barre syndrome,1; and pontine tumors. 7 "Superior oblique myokymia" is characterized by intermittent spontaneous rhythmic contractions of the superior oblique muscle presenting as rapid and small amplitude intortions of one eye. These movements can be inhibited by the patient's looking up or tilting-the-head maneuvers. They are frequently precipitated and/or accentuated by having the patient gaze in the direction of the superior oblique function. The physiological basis for this phenomenon is not known. Electromyographic studies of the superior oblique muscle in two patients with this syndrome have documented prolonged bursts of motor unit potentials lasting up to 10 seconds. I. 3 In one patient, this coincided with the diplopia and was inhibited during the off-saccade period.3 No rhythmicity of recurring bursts, a feature of myokymia, was reported. Clinically, myokymia would likely produce a sustained mild contraction of the superior oblique muscle. Furthermore, the patient should not be able to inhibit these movements nor should they disappear for periods of time if myokymia was responsible for this abnormal ocular movement. The clinical and electrophysiological evidence fail to establish myokymia as the basis for this syndrome. A more likely explanation is that tetanic bursts of action potentials may arise spontaneously in trochlear neurons or axons in an area of previous injury or inflammation. These spontaneous bursts would result in the repetitive discharge of motor units in the superior oblique muscle producing the observed abnormal ocular movements. This would be analogous to the phenomenon of hemifacial spasm in the facial nerve in which repetitive action potentials arise spontaneously (ectopic generator) and spread to other axons transaxonally (ephatic transmission) in the area of nerve alteration.s An alternative explanation is that the syndrome reflects recurrent contraction fasciculation of one or more large motor units in the superior oblique muscle. Contraction fasciculations are rhythmic twitches observed during a weak contraction and represent the repetitive firing of a motor unit. They are often noticeable in patients with motor neuron disease (e.g., old childhood spinal muscular atrophy), where there has been enlargement of motor units due to axonal degeneration and subsequent reinnervation by peripheral sprouting. These con- 132 traction fasciculations may produce the rhythmic pseudotremor seen in patients with motor neuron disease (also termed minipolymyoclonus).9.10 Therefore, a loss of some or many trochlear axons, with subsequent peripheral sprouting of remaining axons, might result in the development of a few large motor units in the superior oblique muscle. In turn, this might result in loss of fine voluntary control of the superior oblique muscle producing a microtremor of the eye. However, this explanation fails to account for the intermittent character of these abnormal eye movements. In conclusion, the term superior oblique myokymia seems inappropriate since myokymia is unlikely to be the pathophysiologic mechanism responsible for this abnormal ocular movement. Until there is a better understanding of the pathophysiological mechanism responsible for this phenomenon, superior oblique microtremor (Hoyt and Keane] suggested intermittent uniocular microtremor as an alternative term) may be a more appropriate label. References 1. Hoyt, W.F., and Keane, J,R.: Superior oblique myokymia: Report and discussion on five cases of benign intermittent uniocular microtremor. Arch. Ophthalmol. 84: 461-467, 1970. 2. Susac, J.O., Smith, J.L., and Schatz, N.}.: Superior oblique myokymia. Arch. Neurol. 29: 432-434,1973. 3. Kommerell, G., and Schaubele, G.: Superior oblique myokymia: An electromyographical analysis. Trans. Ophthalmol. Soc. u.K. 100: 504-506, 1980. 4. Herzan, V., Korner, F., Kommerell, G., and Friedel, B.: Disorders of Ocular Motility. Neuro-physiological and Clinical Aspects, G. Kommerell, Ed. Bergmann, Munich, 1978, pp. 81-90. 5. Gutmann, L., Thompson, H.G., and Martin, }.D.: Transient facial myokymia. ].A.M.A. 209: 389-391, 1969. 6. Mateer, }.E., Gutmann, L., and McComas, C.F.: Myokymia in Guillain-Barre syndrome. Neurology. 33: 374-376, 1983. 7. Waybright, E.A., Gutmann, L., and Chou, S.M.: Facial myokymia: Pathological features. Arch. Neural. 36: 244-245, 1979. 8. Auger. R.G. Hemifacial spasm: Clinical and electrophysiologic observations. Neurology 29: 1261-1272, 1979. 9. Denny-Brown, D., and Pennybacker, J.B.: Fibrillation and fasciculation of voluntary muscle. Brain 61: 311-334, 1934. lO. Fredericks, E.}., and Russman, B.5.: Bedside evaluation of large motor unit in childhood spinal muscular atrophy. Neurology 29: 398-400, 1979. Write for reprints to: Lenore A. Breen, M.D., Department of Neurology, West Virginia University Medical Center, Morgantown, West Virginia 26506. Journal of Clinical Neuro-ophthalmology |
