A Long-term Prospective Study of the Natural Course of Sporadic Adult-Onset Lower Motor Neuron Syndromes

Objective To determine the natural course of sporadic adult-onset lower motor neuron syndrome in a long-term prospective study of patients with the syndrome. Design Inception cohort with a follow-up of 72 months. Setting Three university hospitals in the Netherlands (referral centers for neuromuscular diseases). Patients Thirty-two patients were classified as having the following phenotypes according to previously defined criteria: progressive muscular atrophy (PMA; 10 patients), segmental distal muscular atrophy (8 patients), and segmental proximal muscular atrophy (14 patients). A disease duration of at least 4 years was chosen to exclude most patients with amyotrophic lateral sclerosis (ALS). Main Outcome Measures Muscle strength, functional impairment, and respiratory function were assessed at 0, 6, 12, 18, and approximately 72 months. Results The diagnosis had to be changed to ALS in 3 patients (classified at inclusion as PMA in 2 patients and segmental proximal muscular atrophy in 1) owing to the development of upper motor neuron signs in 2 patients and familial ALS in 1. The remaining 8 patients with PMA showed further deterioration, and the other 24 patients remained more or less stable during long-term follow-up. Respiratory insufficiency developed in 6 of the 11 patients with ALS or PMA, 5 of whom died. Conclusions Patients with lower motor neuron syndromes and a disease duration of at least 4 years usually have a favorable prognosis if muscle involvement has a segmental distribution. In patients with a generalized phenotype, progression is relentlessly progressive and eventually leads to death due to respiratory insufficiency.