FSHD has been classified into two types: FSHD1A and FSHD1B. The symptoms are the same; the difference between the types is in their genetic locus.
Facioscapulohumeral muscular dystrophy 1A (FSHD1A), also known as chromosome 4 linked facioscapulohumeral muscular dystrophy, is by far the most common. FSHD1A is associated by genetic testing with the deletion of 3.3-kb repeats from a chromosomal tandem repeat called D4Z4 located near the end of chromosome 4 at the 4q35-qter location. The D4Z4 region is a polymorphic variable number tandem repeat (VNTR) array consisting of 3.3 kb units. Unaffected individuals have a chromosome 4 D4Z4 array that has a span of 11 to 150 contiguous units. In individuals with FSHD1A, the chromosome 4 D4Z4 repeat array is contracted to a range between 1 to 10 contiguous units. There is a rough and inverse relationship between clinical severity and the number of repeats; patients with the fewest repeats typically have the most severe symptoms.
Facioscapulohumeral muscular dystrophy 1B (FSHD1B), also known as non-chromosome 4 linked facioscapulohumeral muscular dystrophy, is much less prevalent than FSHD1A. It may be caused by different genes from FSHD1A, located on the same (despite its title) or different chromosomes.
Infantile FSHD or IFSHD is a more severe form of FSHD1A and FSHD1B that has recently been categorized as a subtype of FSHD1A and FSHD1B. What makes the disease more severe has not yet been determined. Hearing loss (high frequency bilateral sensorineural), vision problems (Coats’ Disease and retinal telangiectasias), and seizures have been documented in IFSHD. Hearing loss is often more severe in IFSHD; if not detected and treated early it can interfere with learning and cognitive development.