To estimate the minimum prevalence of adult hereditary ataxias (HA) and spastic paraplegias (HSP) in Eastern Quebec and to evaluate the proportion of associated mutations in identified genes.

We conducted a descriptive cross-sectional study of patients who met clinical criteria for the diagnosis of HA (n = 241) and HSP (n = 115) in the East of the Quebec province between January 2007 and July 2019. The primary outcome was the prevalence per 100,000 persons with a 95% confidence interval (CI). The secondary outcome was the frequency of mutations identified by targeted next-generation sequencing (NGS) approach. Minimum carrier frequency for identified variants was calculated based on allele frequency values and the Hardy–Weinberg (HW) equation.

The minimum prevalence of HA in Eastern Quebec was estimated at 6.47/100 000 [95% CI; 6.44–6.51]; divided into 3.73/100 000 for autosomal recessive (AR) ataxias and 2.67/100 000 for autosomal dominant (AD) ataxias. The minimum prevalence of HSP was 4.17/100 000 [95% CI; 4.14–4.2]; with 2.05/100 000 for AD-HSP and 2.12/100 000 for AR-HSP. In total, 52.4% of patients had a confirmed genetic diagnosis. AR cerebellar ataxia type 1 (2.67/100 000) and AD spastic paraplegia SPG4 (1.18/100 000) were the most prevalent disorders identified. Mutations were identified in 23 genes and molecular alterations in 7 trinucleotides repeats expansion; the most common mutations were c.15705–12 A > G in SYNE1 and c.1529C > T (p.A510V) in SPG7.

We described the minimum prevalence of genetically defined adult HA and HSP in Eastern Quebec. This study provides a framework for international comparisons and service planning.