Supplementary Material for: Epidemiology of Amyotrophic Lateral Sclerosis in Western and Northern Finland

Introduction: The aims of this study were to define the incidence and prevalence of amyotrophic lateral sclerosis (ALS) in two north-western regions in Finland and to assess clinical ALS phenotypes in these areas.

Methods: We conducted a retrospective epidemiologic study by using hospital discharge registers in the regions of Central Ostrobothnia (population 68 158 in 2019) and Northern Ostrobothnia (population 412 830). All patients diagnosed with ALS during 2010–2019 and living in either region were included in the incidence study. The prevalence day was December 31, 2019. All ALS diagnoses were retrospectively re-evaluated and the clinical phenotype data reviewed and reassessed.

Results: In total, 214 ALS patients were identified. The age-adjusted 10-year incidence of ALS was 5.4/100 000 person-years in Central Ostrobothnia and 4.6/100 000 person-years in Northern Ostrobothnia. The age-adjusted prevalence rates were 13.1 and 14.6/100 000, respectively. The mean survival after the diagnosis was 16.8 months. Frontotemporal dementia (FTD) was identified in 15% of all patients. ALS-FTD was relatively more common among patients with bulbar or respiratory onset ALS (25%) than among those with limb-onset ALS (8%). Approximately 13% of the ALS patients had a positive family history for ALS. Genetic testing had been performed in 53 % of all cases and the most tested mutations were C9orf72 hexanucleotide repeat expansion (32%) and D90A-SOD1 (40%). C9orf72 repeat expansion was detected in 8% and a D90A-SOD1 mutation in 6% of all cases, that is 26% and 14% of all tested cases, respectively.

Conclusion: The incidence and prevalence rates of ALS in Finland are among the highest in the world. ALS-FTD seems to be more common among patients with bulbar or respiratory onset ALS than among those with spinal-onset disease. Cognitive evaluation of ALS patients and offering a possibility to genetic testing should be systematic in clinical practise.