| A rare genetic congenital non-dystrophic myopathy characterized by neonatal or infantile-onset hypotonia and mild to severe generalized muscle weakness. Limb weakness may be greatest in the limb girdle and proximal limb muscles, but weakness is never solely distal. Facial weakness is often present, resulting in a long face, high-arched palate, and tented upper lip. Histologically, there is a characteristic (but not specific) reduction in the caliber of type 1 muscle fibers. Type 1 muscle fibers are predominant compared to type 2 fibers, which are either normal or hypertrophied. Causative mutations have been identified more frequently in 4 genes, ACTA1 (1q42.13), RYR1 (19q13.2), TPM3 (1q21.3), and SELENON (1p36.11). For the majority of cases the pattern of inheritance is either autosomal recessive or autosomal dominant. X-linked inheritance has been reported. |
