Friedreich's ataxia: a descriptive epidemiological study in an Italian population.

All the cases of Friedreich's ataxia (FA) diagnosed between 1945 through 1984 among residents of a defined area of northwestern Italy were ascertained (N = 59). Cases were diagnosed according to the criteria of the "Quebec Cooperative Study on Friedreich's Ataxia (QCSFA)" with minor modifications. We identified 39 families with 47 probands and 12 secondary cases. Therefore ascertainment probability was 80%. Male to female ratio was 1:1. Pedigrees were compatible with autosomal recessive inheritance. Segregation ratio was 0.28 with both Weinberg's method and the "singles" method (under incomplete ascertainment). Point prevalence ratio was 1.2/100,000 population. Birth incidence rate was 1/36,000 live births. Gene frequency was estimated to be 1/191. The ratio of first-cousin marriages observed among parents of FA patients (3%) was lower than expected from Dahlberg's formula (8%). This finding is not compatible with the hypothesis of genetic heterogeneity for FA.