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- Muscular dystrophies (MDs) are a heterogeneous group of disorders characterized by a slow degeneration of muscle with consequent weakness and contracture deformity. Cardiac muscle can be involved in some forms.
- MDs with childhood onset can be divided into five groups:
- Dystrophinopathies (i.e., Duchenne MD [DMD], Becker MD [BMD])
- Limb-girdle MD (LGMD)
- Congenital MD (CMD)
- Facioscapulohumeral MD (FSH-MD)
- Emery-Dreifuss MD (EDMD)
- Types of MDs can be differentiated by their clinical features (i.e., pattern of muscle weakness, joint contractures), age of onset, genetic test results, and/or muscle biopsy.
- DMD: 1 per 3,500 boys (most common)
- BMD: 1 per 30,000 boys
- LGMD (childhood onset): 5 to 10 per million
- CMD (all types): 1 to 10 per 100,000
- FSH-MD: 1 per 20,000
- EDMD: 1 per 300,000
Genetic testing is clinically available for most MDs:
- Dystrophinopathies (DMD/BMD): X linked
- DMD exon duplication/deletion in 70% cases
- DMD point mutation in almost 30% cases
- New mutations of the dystrophin gene are common, and hence, most cases have no affected relatives despite X-linked recessive inheritance. New mutations in the dystrophin gene are found frequently in the mothers of affected boys.
- LGMD: Most childhood-onset LGMDs are autosomal recessive.
- Sarcoglycanopathies (LGMD2C–F) make up roughly 70% of childhood-onset LGMD.
- LGMD2I (FKRP): 5% childhood-onset LGMD
- CMD: most autosomal recessive (12 genes)
- Nonsyndromic (LAMA2, COL6A1 to COL6A3)
- Syndromic (e.g., POMT1, POMGT1, FKRP)
- FSH-MD: autosomal dominant (D4Z4 deletion)
- EDMD: X linked (EMD or FHL1 mutations) or autosomal dominant (LMNA mutation)
- Deficient or defective muscle fiber proteins causing fiber dysfunction and/or increased membrane fragility
- Muscle biopsy: increased variability in muscle fiber size (i.e., degenerating, regenerating, and necrotic fibers), split muscle fibers and increased internal nuclei, fibrosis. Immunohistochemistry may note decreased/absent sarcolemmal proteins (e.g., DMD, LGMD, CMD).