Spasmodic torticollis: clinical and biologic features and their implications for focal dystonia.

A previous investigation of IST in Rochester, Minnesota, from 1960 through 1979 yielded an incidence of 1.1/100,000 person-years. The rate in women was almost three times higher than that in men. Thyroid disorders, other endocrine disorders, right-hand preference, symptomatic ovarian cysts, other disorders of the female genito-reproductive system, emotional depression, cerebral aneurysm, and family history of diabetes mellitus were common. In 347 cases of IST personally evaluated (151 seen between 1969-1971, a sample of 49 seen in 1978, and 147 other cases evaluated in 1985) the female-to-male ratio was 1.6:1. The mean age at onset was 43.5 years for women and 42 years for men, and the distributions of age at onset were virtually identical for the two sexes. Thyroid disorder was prevalent in all three referral patients samples. The increased frequency of thyroid disorder was due to a high overall frequency in female patients, 28.4% (95% confidence interval, 22.3-34.4%). Thyroid disorders were 4.5 times more frequent in women with IST than in men with IST. In the control population of 61 patients with peripheral sensory neuropathy, in which sex distribution and mean age at onset of symptoms were comparable to those in IST patients, the incidence of thyroid disorder overall was only 8.2% (in women, 6.6%). Only 12 of the 347 referral IST patients were non-right-handers. Five of these 12 were female patients (2.3% of the 215 women; 95% confidence interval, 0.3-4.3%), significantly below the anticipated frequency (10%) or the observed frequency (10% and 15.16%) in two attempted control populations. Light eyes seem to be unusually common in IST patients, as are personal and family histories of essential tremor. These results may reflect shared biologic risk factors which relate age, gender or estradiol/testosterone function, cerebral anatomic and chemical lateralization, endocrinopathy including elevated thyrotropin levels, immune status, emotional depression, common cells of origin in the neural crest, race, and familial occurrence to IST and other focal dystonias and essential tremor.