In those with 8 or fewer repeats, symptoms are more likely than in those with 9 - 10 repeats, leading to diagnosis regardless of an additional SMCHD1 mutation.The apparent frequency of FSHD1/FSHD2 cases in the 9 - 10 repeat range, combined with the FSHD2-like methylation pattern, suggest the 9 - 10 repeat size to be an overlap zone between FSHD1 and FSDH2. Pathophysiology
Molecular
As of 2020, there seems to be a consensus that aberrant expression of DUX4 in muscle is the cause of FSHD. DUX4 is expressed in extremely small amounts, detectable in 1 out of every 1000 immature muscle cells (myoblast), which appears to increase after myoblast maturation, in part because the cells fuse as they mature, and a single nucleus expressing DUX4 can provide DUX4 protein to neighboring nuclei from fused cells.It remains an area of active research how DUX4 protein causes muscle damage. DUX4 protein is a transcription factor that regulates many other genes. Some of these genes are involved in apoptosis, such as p53, p21, MYC, and β-catenin, and indeed it seems that DUX4 protein makes muscle cells more prone to apoptosis. Other DUX4 protein-regulated genes are involved in oxidative stress, and indeed it seems that DUX4 expression lowers muscle cell tolerance of oxidative stress. Variation in the ability of individual muscles to handle oxidative stress could partially explain the muscle involvement patterns of FSHD.
Another form of scapular fixation, although not commonly done in FSHD, is tendon transfer, which involves surgically rearranging the attachments of muscles to bone. Examples include pectoralis major transfer and the Eden-Lange procedure. Various other surgical reconstructions have been described. Upper eyelid gold implants have been used for those unable to close their eyes. Drooping lower lip has been addressed with plastic surgery. Select cases of foot drop can be surgically corrected with tendon transfer, in which the tibialis posterior muscle is repurposed as a tibialis anterior muscle, a version of this being called the Bridle procedure. Severe scoliosis caused by FSHD can be corrected with spinal fusion; however, since scoliosis is a compensatory change in response to muscle weakness, correction of spinal alignment can result in further impaired muscle function. Scapular winging management

Prognosis
Genetics partially predicts prognosis. Those with large D4Z4 repeat deletions (with a remaining D4Z4 repeat array size of 10-20 kbp, or 1-4 repeats) are more likely to have severe and early disease, as well as non-muscular symptoms. Those who have the genetic mutations of both FSHD1 and FSHD2 are more likely to have severe disease. It has also been observed that disease manifestation is milder when a prominent family history is present, as opposed to a new mutation. In some large families, 30% of those with the mutation do not show symptoms, and 30% of those with symptoms do not progress beyond facial and shoulder weakness.