What is amyotrophic lateral sclerosis prevalence?

Objective: To assess amyotrophic lateral sclerosis (ALS) prevalence and to analyze how this estimate vary according to the historical depth of data collection. Methods: Data from the PARALS register have been used. Crude prevalence ratio was estimated on 31 December 2015 for the period 2015–2013 and then repeated extending the time interval by 3 years each time. For each time interval, prevalence ratio was calculated globally and stratified by sex, age at diagnosis, and phenotype. Prevalence was also calculated considering patients who underwent tracheostomy during the same study period. Results: Prevalence ratios increased proportionally to the length of the time period considered, ranging from 6 (95% CI 5.3–6.7) for a 3-year period to 12.1 (95% CI 11.1–13.1) per 100,000 population for a 21-year period. Prevalence ratio increase was inversely proportional to age at diagnosis, being null in the >85 years class and maximal in the 25–35 age class (+1700%). Among phenotypes, predominant UMN showed the highest increase (from 0.5, 95% CI 0.3–0.8, to 2.1, 95% CI 1.7 − 2.6, +320%). Discussion: Because of the variability of ALS survival, prevalence ratio strongly depends on the length of the follow-up period. A 12-year period should be sufficient to get a reliable estimate of ALS prevalence including long-survival patients.