THE FINE STRUCTURE OF THE SPINDLE ABNORMALITY IN MYOTONIC DYSTROPHY

The fine structure of the intrafusal muscle fibres, and of their innervation, has been studied in three muscle spindles found in muscle biopsies from two patients with myotonic dystrophy. The poles of the intrafusal muscle fibres were longitudinally fragmented. The fragments were arranged in up to twenty clusters, each bounded by a single layer of basement membrane. Degenerative and regenerative changes were observed in the fibre fragments. The most abnormal fragments were situated at the periphery of the clusters and there was some evidence that these degenerate fragments were extruded from the clusters, into the periaxial space. Many fibre fragments were anucleate. Only six motor endplates were found, despite a search of the spindle poles by serial section. All were simplified in form. The fine structure of the primary and secondary sensory innervation was normal, and the equatorial regions of the intrafusal fibres were not fragmented. It is suggested that these observations are consistent with the hypothesis that the characteristic spindle abnormality in myotonic dystrophy is due to fragmentation of the poles of the intrafusal muscle fibres, due to mechanical stresses imposed on abnormal muscle fibres by myotonia or myotonic after‐discharges, and that this, as a continuing process, leads to denervation of the individual fragments, and to the observed degenerative changes, at a time when the extrafusal muscle fibres may appear relatively mildly affected.