Facioscapulohumeral muscular dystrophy (FSHD) is a genetic muscle disease that causes progressive weakness, starting in the face, shoulders, and upper arms. It affects roughly one million people worldwide, making it one of the most common forms of muscular dystrophy. The name describes the three muscle groups hit first: the face (facio), the shoulder blades (scapulo), and the upper arms (humeral).
What Causes FSHD
FSHD traces back to a single problem: a gene called DUX4 that should be silenced in adult muscle cells gets switched on instead. DUX4 is normally active only during very early embryonic development. In people with FSHD, the packaging of DNA near this gene loosens up, allowing muscle cells to produce a protein they were never meant to make. That protein is toxic to skeletal muscle. It triggers cell death and activates developmental programs that have no business running in mature tissue.
The gene sits within a repeating stretch of DNA called D4Z4, located near the tip of chromosome 4. Healthy individuals typically carry 11 to 100 copies of this repeat. In FSHD, contractions shorten the array to between 1 and 10 copies. Fewer copies means less tightly packed DNA, which means less silencing of DUX4. But the shortened repeat alone isn’t enough. The person also needs a specific version of chromosome 4 (called the 4qA haplotype) that contains the genetic signal allowing the DUX4 protein to actually be produced in a stable form. Without that signal, even a shortened repeat array won’t cause disease.
FSHD1 vs. FSHD2
About 95% of people with FSHD have the type 1 form (FSHD1), caused by the D4Z4 repeat contraction described above. It follows an autosomal dominant inheritance pattern, meaning a single copy of the shortened array from one parent is enough to cause disease.
The remaining 5% have FSHD2, which works differently. These individuals don’t have an extremely short repeat array. Instead, they carry mutations in genes that normally help keep DUX4 silenced. The most common culprit is a gene called SMCHD1, found on chromosome 18, which accounts for roughly 85% of FSHD2 cases. FSHD2 requires two things to come together: the permissive 4qA chromosome and the SMCHD1 mutation. This makes it a digenic condition, needing variants in two separate genes. Clinically, FSHD1 and FSHD2 look identical. The distinction matters for genetic counseling and family testing but not for day-to-day symptoms.
Which Muscles Are Affected
FSHD follows a distinctive top-down pattern. Facial weakness often comes first: difficulty whistling, blowing up a balloon, or drinking through a straw. Smiling may look asymmetric, and the eyes may not close fully during sleep. Many people don’t recognize these early signs because the changes develop gradually.
From there, weakness moves to the muscles that stabilize the shoulder blades. This produces one of FSHD’s hallmark signs: scapular winging, where the shoulder blades stick out prominently from the back. Raising your arms overhead becomes difficult because the shoulder blades can’t anchor properly against the rib cage. The upper arms weaken next, particularly the biceps and triceps, while the forearms and hands are often spared.
Over time, weakness can spread to the muscles of the trunk and lower legs. Foot drop, caused by weakness in the muscles that lift the front of the foot, is common and can cause tripping. The weakness is often noticeably asymmetric, affecting one side of the body more than the other. This lopsidedness is unusual among muscular dystrophies and can be a clue during diagnosis.
Symptoms Beyond Muscle Weakness
FSHD isn’t purely a muscle disease. In one study, high-frequency hearing loss was found in 64% of patients, though it tends to be mild enough that many people don’t notice it without formal testing. Retinal blood vessel abnormalities showed up in about half of patients who underwent detailed eye imaging, though vision-threatening complications are uncommon in adult-onset cases. These extra-muscular features are more frequent and more clinically significant in people with early-onset disease.
Early-Onset FSHD
Most people with FSHD first notice symptoms between ages 15 and 30, but an estimated 7% to 15% develop the early-onset form, defined as facial weakness before age 5 and shoulder weakness before age 10. Early onset isn’t simply the same disease caught sooner. Research comparing early-onset patients to classic-onset patients matched for disease duration found that the early-onset group had significantly more severe weakness, with an average clinical severity score of 11 compared to 9 in the duration-matched group.
Wheelchair dependency reached 57% in the early-onset group, compared to 30% in duration-matched classic-onset patients. Hearing loss, spinal deformities, and breathing difficulties were all more common. This suggests that something about having fewer D4Z4 repeats (which correlates with earlier onset) drives faster and more widespread damage, not just a longer window for damage to accumulate.
How FSHD Is Diagnosed
Diagnosis starts with the clinical picture: facial, shoulder blade, and foot dorsiflexor weakness without involvement of eye movement or swallowing muscles. A family history consistent with autosomal dominant inheritance strengthens suspicion, though about 10% to 30% of cases arise from new mutations with no family history.
Genetic testing confirms the diagnosis. For FSHD1, the test measures the size of the D4Z4 repeat array on chromosome 4, traditionally using a technique called Southern blot analysis. Newer methods like optical genome mapping are also used. The test checks both the repeat length and the chromosome 4 haplotype. For FSHD2, additional testing looks for reduced DNA methylation at the D4Z4 region and mutations in SMCHD1 or, rarely, other genes involved in chromatin silencing. Standard genetic panels for muscular dystrophy may miss FSHD because the repeat contraction requires specialized testing that differs from typical gene sequencing.
Progression and Long-Term Outlook
FSHD progresses slowly in most people, but the variability is enormous. Some individuals have such mild involvement that they’re diagnosed only after a more severely affected family member brings attention to the condition. Others lose the ability to walk. About 20% of people with FSHD become wheelchair dependent, typically after age 50. The disease does not directly shorten life expectancy in most cases, though respiratory complications can become relevant in severe early-onset forms.
Progression can be unpredictable. Periods of relative stability may alternate with phases of more noticeable decline. Pregnancy has been noted to worsen symptoms in some women, though this isn’t universal.
Treatment and Management
No medication currently slows or stops FSHD. The most advanced drug candidate in clinical trials, losmapimod (which targets a stress-signaling pathway in muscle cells), showed some promising functional improvements in early-phase trials but failed to demonstrate clear clinical benefit in a larger phase 3 trial. This outcome highlighted how challenging drug development remains for this disease, partly because progression is so slow and variable that measuring meaningful change over a trial period is difficult.
Management today centers on maintaining function and quality of life. Physical therapy plays a key role, particularly exercises that strengthen muscles still functioning well. Aerobic exercise at moderate intensity is generally considered safe and beneficial, though aggressive resistance training of severely weakened muscles is not recommended.
For people with significant scapular winging who still have good deltoid strength, a surgical option called scapulothoracic arthrodesis can meaningfully improve shoulder function. The procedure fuses the shoulder blade to the ribs, providing a stable base for the deltoid to work against. The best candidates are those who can raise their arm to about 90 degrees, have full deltoid function, and have severe winging. After surgery, the shoulder is immobilized for about 10 weeks, followed by gradual rehabilitation. By six months, more intensive strengthening and stretching can begin. For people whose deltoid is also weakened, the surgery offers limited benefit.
Ankle-foot orthoses (braces) help compensate for foot drop, reducing tripping and improving walking efficiency. Hearing should be tested periodically, particularly in those diagnosed young, and eye exams can catch retinal vascular changes early in severe cases.