Congenital Myopathy With Cap-Like Structures and Nemaline Rods: Case Report and Literature Review
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BACKGROUND: Cap myopathy is a rare congenital myopathy characterized by cap structures located at the periphery of the muscle fiber. Cap structures consist of disarranged thin filaments with enlarged Z discs. The clinical presentation and natural history of cap myopathy is variable and overlaps with other congenital myopathies. METHODS: We describe a 10-year-old boy with cap myopathy and contrast him with 20 other individuals reported in the literature. RESULTS: Our patient presented at birth with hypotonia and weakness and subsequently developed respiratory failure in infancy. He is ambulatory but has increasing fatigue and requires a wheelchair by midafternoon. His muscle biopsy at 3 months revealed a nemaline myopathy and secondary fiber-type disproportion with type 1 hypotrophy and predominance. A repeat muscle biopsy at age 6 years revealed numerous peripherally located cap-like structures containing nemaline rods and exhibited a spectrum of Z-disk and myofibrillar abnormalities. Molecular genetic testing was performed for NEB, TPM2, TPM3, ACTA1, TNNT1, SEPN1, SMN1, DMPK, FSHMD1A, and mtDNA. A known pathogenic mutation, c.1152+1G>A, and a previously unreported variant, c.1782+4_1782+5delAG, were detected in NEB. CONCLUSION: Our patient has a more severe phenotype than most reported patients and is the first patient with cap myopathy to have a mutation in NEB. Our case supports the identification of cap myopathy as a congenital myopathy with significant overlapping features with nemaline myopathies and further elucidates the phenotype of this disease.
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