Congenital myopathy-2C (CMYO2C) is an autosomal dominant disorder of the skeletal muscle characterized by severe congenital weakness usually resulting in death from respiratory failure in the first year or so of life. Patients present at birth with hypotonia, lack of antigravity movements, poor head control, and difficulties feeding or breathing, often requiring tube-feeding and mechanical ventilation. Decreased fetal movements may be observed in some cases. Of the patients with congenital myopathy caused by mutation in the ACTA1 gene, about 90% carry heterozygous mutations that are usually de novo and cause the severe infantile phenotype. Some patients with heterozygous mutations have a more typical and milder disease course with delayed motor development and proximal muscle weakness, but are able to achieve independent ambulation (CMYO2A; 161800). The severity of the disease most likely depends on the detrimental effect of the mutation, although there are probably additional modifying factors (Ryan et al., 2001; Laing et al., 2009; Sanoudou and Beggs, 2001; Agrawal et al., 2004; Nowak et al., 2013; Sewry et al., 2019; Laitila and Wallgren-Pettersson, 2021). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Bethlem myopathy-1 (BTHLM1) is a congenital muscular dystrophy characterized by proximal muscle weakness and a combination of distal and proximal flexion joint contractures. The age at onset is highly variable, ranging from infancy to adulthood, and there is intrafamilial variability. Muscle biopsy may show myopathic and dystrophic features; serum creatine kinase is elevated. The progression is slow and ambulation is usually retained into adulthood (summary by Butterfield et al., 2013; Scacheri et al., 2002). For a discussion of genetic heterogeneity of Bethlem myopathy, see BTHLM1A (158810).