Voice tremor in monozygotic twins

Sirs, The familial inheritance of essential tremor (ET) is well established. However, a family history is reported in percentages that range from as low as 17.4% to as high as 100% [1–3]. In non-familial ET or sporadic ET, a possible role of environmental factors in the pathogenesis of this disease has been proposed [1–3]. In the last 5 years, two studies showed a higher concordance for probable of definite ET in monozygotic (60–93%) than in dizygotic twins (27–29%), suggesting a genetic cause for ET [4,5]. Recently, Contarino et al. [6] reported two monozygotic twins with orthostatic tremor, although they also presented a mild postural arm tremor, suggesting a diagnosis of ET. We report two monozygotic twins that developed voice tremor. The patients were female monozygotic twins aged 68 years. They had neither brothers, sisters nor cousins and their parents were not consanguineous. They did not report previous family history of tremor or other movement disorders. They did not have other previous diseases and were not exposed to any drug before the onset of voice tremor. The patients grew up together, had lived always together in the city of Madrid, and had worked as administrative staff until retirement. The first twin developed lingual and voice tremor at the age of 65 and the second developed voice tremor at the age of 67. Neurological examination of the first twin showed lingual and voice tremor, mild postural tremor of the left 5th finger and buccolingual choreiform dyskinesia. No other abnormalities, including parkinsonian and cerebellar signs, were obtained during the neurological examination. Buccolingual dyskinesia had developed 1 month before our first evaluation and was probably related with the use of sulpiride that was prescribed 3 months ago to treat paroxysmal positional vertigo. Following sulpiride withdrawal, oral dyskinesia improved gradually and disappeared 3 months later. Neurological examination of the second twin was normal, except for the presence of voice tremor. Voice tremor caused them some functional interference with speech and severe impairment to sing (both had been amateur opera singers in their early years). The patients underwent a videotaped tremor examination that showed absence of tremor affecting limbs (with the exception of a mild postural tremor in the left fifth finger in the first twin), head and jaw, presence of voice tremor while speaking and singing, and presence in one of them of lingual tremor and oral choreiform dyskinesia. Laboratory studies including thyroid hormone levels and brain MRI were normal. We describe two monozygotic twins affected, by lingual and voice tremor the first one and isolated voice tremor the other one, without presence of any significant tremor in the upper limbs or in other locations. For this reason, they fulfilled the TRIG diagnostic criteria for possible ET [7]. Although our patients had no family history of tremor and had lived and worked always in the same environment, the development of voice tremor in both monozygotic twins with 2 years of difference in the onset of symptoms could suggest a genetic cause. One twin had also a buccolingual dyskinesia, which was probably iatrogenically induced by sulpiride and did resolve after withdrawal of this drug. Sulpiride was one of the offending drugs in nine of 72 cases with drug-induced tardive buccolinguomasticatory syndrome (in two cases, it was the only drug implicated) [8]. Despite the previously reported high concordance rates of ET in monozygotic twins [4,5], we have not found any previous clinical description of voice tremor in twins.