Facioscapulohumeral muscular dystrophy (FSHD) typically presents with slowly progressive weakness of the facial muscles, scapular winging and foot drop. The severity is highly variable within and between families.
The diagnosis of FSHD is established by the identification of a pathogenic contraction of the D4Z4 repeat array of chromosome 4q35. The genetic structure involved has a number of repeated sequences. Greater than 12 repeat units is considered normal. If an individual has ten or 11 repeat units, mild disease may occur or there may be so-called non penetrance. The individual has the contraction but no clinical disease symptoms. Individuals with <9 repeats typically have more severe disease, with earlier onset. But the variation between individuals can be marked. Further, females are less likely to have severe disease than males. Individuals with a large contraction of D4Z4 (1-3 repeats) have a higher probability of earlier-onset disease and more rapid progression than those with smaller contractions of the D4Z4 locus. Although such broad correlation exists in individuals with FSHD in general, the range is wide and cannot be used to predict severity or age of onset for individuals.