A case of congenital neuromuscular disease with uniform type I fibers, abnormal mitochondrial network and jagged Z-line.

Histological, histochemical and ultrastructural studies of muscle biopsy in a case of congenital neuromuscular disease revealed unusual findings consisting of muscle fibers uniformity which were all type I and of small diameter, jagged Z-line and abnormally developed transverse network of mitochondria. E.M.G. examination demonstrated a myopathic pattern, but mitochondrial changes are quite different from those reported in mitochondrial myopathies and jagged Z-line seems poorly correlated with Z-line streaming present in denervation atrophy, target fibers, core-like lesions or other Z-line abnormalities of the nemaline myopathy. On the other hand type I histochemical uniformity seems more likely related to some dysfunction of the neuronal mechanisms that control both the fiber type differentiation and other trophic influences. It also suggests that myogenic E.M.G. pattern might actually be pseudo-myopathic and due to a reduction of the cross sectional area of the individual muscle fibers composing the motor unit.