Motor neuron disease: classification and nomenclature.

The classification and nomenclature of motor neuron disease, whether sporadic or familial, is confused. For example, both the sporadic and familial motor neuron diseases are phenotypically heterogeneous and, in familial ALS, phenotypic heterogeneity correlates only weakly with different underlying mutations in the SOD1 gene. We propose a classification which is based on underlying causative mechanisms, where these are known, but which also recognizes different clinical phenotypes when the cause is unknown. This classification is flexible, and allows reattribution of clinical syndromes when their causation is understood. Currently uncertain associations--for example, a possible association of ALS with cancer--are given tentative recognition in this classification. In addition, this new classification recognizes geographical clustering and descriptions of unusual motor neuron disorder phenotypes of unknown origin in different parts of the world.