Clinical and morphometric analysis of biopsied biceps brachii muscles in adult-onset chronic proximal spinal muscular atrophy. With special reference to intramuscular nerves.

Abstract

In a group of 16 cases with adult-onset chronic proximal spinal muscular atrophy, we performed a morphometric analysis of 88 intramuscular nerves in biopsied biceps brachii muscles and correlated this analysis with clinical and histochemical parameters. The total number of large and small myelinated fibres in all fascicles was reduced to 71%, 23% and 83% of control values, respectively. The percentage reduction of large myelinated fibres for each case was significantly correlated with duration of illness, biceps muscle power and histochemical atrophy factors. Histograms of large intramuscular nerve fascicles showed unimodal distributions and shifts to the left with single peak increases at 1 micron. The densities of small myelinated fibres in large fascicles correlated with counts of 'enclosed' muscle fibres in ATPase preparations. Compared with amyotrophic lateral sclerosis, a lesser reduction of large myelinated fibres, but a greater increase in small myelinated fibres, was noted in adult-onset chronic proximal spinal muscular atrophy. These findings imply that there is a less marked loss of myelinated nerve fibres with more effective reinnervation in adult-onset chronic proximal spinal muscular atrophy than in amyotrophic lateral sclerosis.