The spectrum of multiple sclerosis.

Multiple sclerosis (MS) is the most common cause of chronic neurologic disability in young people. Genetic susceptibility and unidentified environmental triggers appear to be necessary in order to result in disease. MS is an extraordinarily complex trait with evidence of heterogeneity at clinical, pathologic, and therapeutic levels. Recent studies have not resolved the important question whether at a mechanistic level MS is a single disease with a wide spectrum of clinical expression, or whether it encompasses a group of separate diseases that share certain pathologic final common pathways. This question is important not only for helping to understand the causes of MS but also for designing and applying better treatments.