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Psychiatrist, Neuropsychiatry and Headache Clinic, Delhi, India
Staff Psychiatrist and Service Chief, Department of Psychiatry, Herrick Memorial Hospital, 500 East Pottawatamie Street, Tecumseh, Michigan 49286-2073, fax 517-423-9433, bobduggal{at}yahoo.com
Published Online, October 17, 2006. www.theannals.com, DOI 10.1345/aph.1H086
Case Report. An 8-year-old boy with no history of neurologic illness or previous exposure to psychotropic drugs was diagnosed with ADHD, inattentive type, according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition, text revision.2 The boy showed no response to psychotherapy over 6 months; therefore, the parents consented to a trial of methylphenidate 5 mg/day. Although the patient showed improvement in his attention and concentration, on day 7 of methylphenidate therapy, the parents noted that he had developed involuntary, rapid, fine, and rhythmic vertical movements of the jaw and lips; there were no associated tongue movements or any other signs of extrapyramidal symptoms such as rigidity, tremor, or bradykinesia. Neurologic examination revealed no focal deficits; results of a computed tomography scan of the head were normal. The patient scored 5 (most severe) on the Abnormal Involuntary Movement Scale.
Methylphenidate was discontinued; within 2 days, the patient showed complete resolution of the involuntary movements. He was not taking any concurrent medications and there was no evidence for any underlying illness, either of which could have explained the occurrence of these movements. According to the Naranjo probability scale, the association of rabbit syndrome with methylphenidate was probable in this patient.3 He was not treated again with methylphenidate or any other medication.
Discussion. Although methylphenidate has been associated with dyskinetic choreoathetoid movements,4 this is the first report associating methylphenidate with rabbit syndrome, per se. The temporal relationship between methylphenidate initiation and appearance of rabbit syndrome and the disappearance of symptoms upon discontinuation of the drug makes methylphenidate the most likely etiologic agent in our patient. Movements described in this case were typical for rabbit syndrome, whereas tardive dyskinesia is associated with irregular, slower, choreic movements with concurrent tongue dyskinesias.5 Tics were excluded because the movements were rhythmic and were not experienced as irresistible. The true incidence of rabbit syndrome with methylphenidate is difficult to estimate. One study showed that approximately 9% of children with ADHD developed tics or dyskinesias while taking stimulant medication6; another report cited the incidence of tics with stimulants (mostly transient) at 15-30%.7 However, tic disorders and ADHD are known to occur comorbidly.
The pathophysiologic mechanism of rabbit syndrome is poorly understood. Reports of antipsychotic-induced rabbit syndrome and its response to anticholinergic agents favor a dopamine-acetylcholine imbalance hypothesis5; however, reports of antidepressant-induced rabbit syndrome suggest an imbalance in serotonin-dopamine interactions.8 More research is needed to determine a mechanism that explains the occurrence of rabbit syndrome when drugs with apparently opposite pharmacodynamics such as methylphenidate, a dopaminergic agent, and antipsychotics, which are dopamine antagonists, are used.
References
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