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Congenital fiber-type disproportion myopathy
Congenital fiber type disproportion myopathy (CFTDM) is a rare type of myopathy characterized by hypotonia and mild to severe generalized muscle weakness present at birth or within the first year of life. Prevalence is unknown, incidence is estimated to be inferior than 1:50,000 live births. 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. Ophthalmoplegia and bulbar weakness can be seen. Tendon reflexes are often decreased or absent. Approximately 30% of patients have mild to severe respiratory involvement and feeding difficulties. Contractures (ankles, fingers, hips, elbows, knees) and spinal deformities (scoliosis, kyphoscoliosis, lordosis) occur in approximately 25% of affected children. Congenital hip dislocation and talipes equinovarus may also be present. In rare cases, cardiac involvement, cognitive impairment and cryptorchidism may occur. Ophthalmoplegia, ptosis, and facial and/or bulbar weakness with severe limb/respiratory weakness predict a poor prognosis. 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 fibres, which are either normal or hypertrophied. CFTDM can be inherited in autosomal recessive, autosomal dominant, or X-linked manner. Causative mutations have been identified in two genes, ACTA1 and SEPN1. Diagnosis is based on a combination of clinical presentation and morphologic features observed on skeletal muscle histology. Molecular testing is clinically available for both genes. Prenatal testing is feasible. Differential diagnoses include other congenital myopathies (X-linked myotubular myopathy, multiminicore disease, nemaline myopathy) and neuromuscular disorders (congenital muscular dystrophy, Emery-Dreifuss muscular dystrophy) (see these terms). Management is directed towards treating the muscular weakness and contractures (through physical and occupational therapy, exercise, stretching), respiratory problems (breathing exercises, chest physiotherapy, ventilatory support), feeding difficulties (gavage or gastrostomy feedings). Regular orthopedic monitoring is needed and corrective surgery may be necessary. Over time, CFTDM becomes static in more than 90% of patients or even shows improvement and, rarely, is slowly progressive.
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