Aim: To report a new case of a little-known congenital myopathy. Cap myopathy is a rare congenital disease caused by an alteration in the structure of the fibre, with disorganised myofibrils at the edges. Since it was first described, only a few cases have been reported in the literature.
Case report: We describe the case of a 16-year-old patient with a history of neonatal hypotonia and psychomotor retardation. At the age of 4 years, the patient presented myopathic facies with atrophied pectoral muscles, winged scapula and lumbar hyperlordosis. A myopathic pattern was observed in the electromyogram. A muscle biopsy showed a very marked predominance of type I fibres, atrophy in part of the population of this type and 20% of them had cap-shaped subsarcolemmal accumulations, which were intensely positive with DPNH and SDH; in the ultrastructural study they correspond to disorganised peripheral myofibrils with preservation of the Z band and the absence of A bands. These alterations are similar to those described in what is known as cap disease or cap myopathy. Today, the patient has mild proximal tetraparesis and moderate restrictive respiratory failure.