Definition:

Facioscapulohumeral muscular dystrophy (FSHD) is a usually autosomal dominant inherited form of muscular dystrophy. At disease onset, typically in the second decade of life, FSHD is characterized by initially restricted weakness of shoulder and facial muscles. With progression, the lower extremities, both distal and proximal, become involved. FSHD is caused by a reduction in the copy number of the D4Z4 macrosatellite repeat. However, the reduction of D4Z4 copy number is not sufficient by itself to cause FSHD. A number of epigenetic events appear to affect the severity of the disease, its rate of progression, and the distribution of muscle weakness.