Is Kugelberg-Welander spinal muscular atrophy a fetal defect?

Twelve children who had developed Kugelberg-Welander (K-W) spinal muscular atrophy (SMA) before the age of six years were investigated. Electrophysiological, histochemical, ultrastructural, and biochemical studies demonstrated features of immature muscle fibers suggesting a fetal defect as in the Werdnig-Hoffmann (W-H) form of SMA. Comparison with known patterns of human myogenesis and of experimental denervation of immature muscle suggested that the defect in K-W SMA probably takes place in fetal life at a stage later than in W-H SMA. In contrast to W-H SMA, a considerable percentage of mature fibers of normal structure and diameter were present. The immature fibers impair the normal development of muscle cells by preventing the increase in number of mature fibers and causing an overloading of the normal fibers with consequent hypertrophy and eventually destructive changes. This is the pattern in K-W disease that distinguishes the juvenile form of SMA from W-H disease.