The nosology of the spinal muscular atrophies.

In the last few years there has been an increase in interest among neurologists and geneticists in the spinal muscular atrophies. With the advent of several relatively sophisticated diagnostic procedures many patients once thought to be suffering from muscular dystrophy have in fact been found to have spinal muscular atrophy. The spinal muscular atrophies may be defined as a group of inherited diseases in which there is degeneration of the anterior horn cells (lower motor neurones) of the spinal cord and often the bulbar motor nuclei, but with no evidence of pyramidal tract involvement. When so defined, motorneurone disease (progressive bulbar palsy, progressive muscular atrophy, and amyotrophic lateral sclerosis) and its variants and the various forms of amyotrophy are excluded as are congenital abnormalities of the spinal cord, and traumatic, toxic, infective, and neoplastic causes of anterior horn cell degeneration. Early literature on the spinal muscular atrophies has been extensively reviewed by Wiesendanger (1962); Smith and Patel (1965); HausmanowaPetrusewicz (1970); and Namba, Aberfeld, and Grob (1970). The frequency of all forms of spinal muscular atrophy is at least 1 in 20,000 (Brandt, 1950a) and possibly very much more. The 'wobbler' mutant of mouse is a possible analogue (Duchen, Strich, and Falconer, 1968). The main clinical features of spinal muscular atrophy are wasting and weakness of muscles supplied by the affected anterior horn cells. Necropsy confirmation of degeneration and loss of anterior horn cells has been reported both in severe (Brandt, 1950a) and more benign (Kohn, 1968; Namba et al, 1970) forms of spinal muscular atrophy. It is essential to recognize the heterogeneity among the spinal muscular atrophies when giving a prognosis and for genetic counselling. Unfortunately early attempts to classify them were largely unsatisfactory for two reasons. Firstly, because such classifications were often based on clinical features