Background: Although mentioned in most series, "pure" autosomal dominant cerebellar ataxias, except spinocerebellar ataxia type 6, are difficult to differentiate on clinical grounds.
Objective: To describe Portuguese families with a peculiar pure form of dominant ataxia that, to our knowledge, has never been documented before and in which cerebellar signs are preceded by spasmodic cough.
Patients: Through a population-based survey of hereditary ataxias in Portugal, we identified 19 patients in 6 families with this particular disorder.
Results: The majority of patients had a pure late-onset ataxia with a benign evolution. In all of the families, attacks of spasmodic coughing preceded ataxia for 1 to 3 decades and were a reliable marker of the disease. In Portugal, this form of ataxia accounts for 2.7% of all of the dominant ataxias.
Conclusions: The families that we describe shared some relevant clinical and imagiological features with spinocerebellar ataxia type 5 and the recently described spinocerebellar ataxia type 20, allelic to spinocerebellar ataxia type 5. Spinocerebellar ataxia types 5 and 20 could be different phenotypic expressions of the same molecular disorder. The association of a dominant ataxia with spasmodic cough is rare but probably underdiagnosed.