Hemifacial spasm in Rochester and Olmsted County, Minnesota, 1960 to 1984. The incidence of hemifacial spasm in residents of Olmsted County, Minnesota, was studied by reviewing the medical records of patients residing in the community between 1960 and 1984. The average annual incidence rate was 0.74 per 100,000 in men and 0.81 per 100,000 in women, age-adjusted to the 1970 US white population. The average prevalence rate was 7.4 per 100,000 population in men and 14.5 per 100,000 in women. The incidence and prevalence rates were highest in those from 40 to 79 years of age. 