The underestimation of familial ALS and counseling patients with sporadic ALS

Abstract

Amyotrophic lateral sclerosis (ALS), the most common degenerative disease of the adult motor neuron system, occurs sporadically in 90% to 95% of cases. This implies no affected relative and no association with 1 of the 3 western Pacific foci.1 While “any affected relative,” the opposite of “no affected relative,” might be considered the definition for familial ALS (fALS), 2 or more remotely related members of an extended family might have sporadic ALS (sALS) due to chance alone. Conversely, a case of sALS might be a case of fALS, not recognized as such because of an incomplete family history or incomplete penetrance in ancestors. Because most forms of fALS are autosomal dominant, making the distinction between fALS and sALS has profound implications in counseling relatives of patients with sALS. Discussions of clinical definitions of fALS have focused on 2 questions: the smallest number of affected relatives required to define fALS (2 or more?) and the degree of relatedness (first, second, any?).2