Why is FSHD Frequently Misdiagnosed as Polymyositis?

Because facioscapulohumeral dystrophy (FSHD) is a genetic muscle disease, many people assume it always looks fundamentally different from autoimmune diseases like polymyositis. However, FSHD is uniquely deceptive: the genetic mechanism underlying the disease triggers a strong inflammatory response in the muscles ^[1]. When doctors investigate a patient’s unexplained muscle weakness using standard tests like muscle biopsies and MRIs, they often see this active inflammation. Because prominent muscle inflammation is the hallmark of inflammatory myopathies like polymyositis, physicians logically—but incorrectly—diagnose an autoimmune condition and prescribe corticosteroids, which are ultimately ineffective ^[2][3].

This frustrating diagnostic detour is a well-documented phenomenon, especially in late-onset FSHD or cases where there is no obvious family history ^[3].

The Biopsy Trap: Inflammatory Infiltrates

When a neurologist suspects a muscle disease, they frequently order a muscle biopsy. In polymyositis, the biopsy reveals immune cells invading the muscle tissue.

However, muscle biopsies in FSHD often show strikingly similar inflammatory changes ^[1]. The genetic driver of FSHD, the abnormal expression of the DUX4 gene, activates a cascade of immune and inflammatory genes within the muscle ^[1][4]. This causes immune cells—including macrophages, T-cells, and even eosinophils—to infiltrate the muscle tissue ^[5][6]. If a pathologist examines the tissue and sees these immune cells, they may interpret the sample as an inflammatory myopathy (like polymyositis) rather than a muscular dystrophy ^[5].

The MRI Illusion: Muscle Edema

Magnetic resonance imaging (MRI) is another common diagnostic tool that can blur the lines between FSHD and polymyositis.

When a muscle is actively inflamed or injured, it retains water. This excess water appears bright on a specific type of MRI sequence known as STIR (Short Tau Inversion Recovery). In active polymyositis, STIR sequences light up brightly due to widespread muscle edema (swelling).

In FSHD, the disease does not progress uniformly. Instead, individual muscles undergo phases of active destruction before eventually being replaced by fat ^[7]. During this active destruction phase, MRI STIR sequences frequently show significant muscle edema and inflammation ^[8][7]. Because this swelling looks identical to the active inflammation seen in autoimmune diseases, it heavily reinforces a misdiagnosis of polymyositis ^[7].

Why Corticosteroids Fail (And When to Stop Them)

Polymyositis is an autoimmune disorder where the immune system attacks the body’s own muscles. The standard treatment involves powerful immunosuppressants like corticosteroids (e.g., prednisone) to stop the attack.

When FSHD is misdiagnosed as polymyositis, patients are frequently prescribed high doses of steroids. However, the inflammation in FSHD is not the primary cause of the disease; it is a secondary reaction to the toxicity of the DUX4 gene ^[1]. Corticosteroids cannot shut off the DUX4 gene, so the underlying muscle destruction continues despite the medication ^[2].

Furthermore, long-term use of high-dose steroids can cause a condition called steroid-induced myopathy, creating additional muscle weakness that only complicates the clinical picture.

Important Safety Warning: If you suspect you have been misdiagnosed and are currently taking corticosteroids, do not stop taking your medication suddenly. Stopping steroids cold turkey can cause a life-threatening condition called adrenal crisis. You must work with your doctor to create a safe, gradual tapering plan.

Reaching the Correct Diagnosis

Correcting the misdiagnosis usually happens when the clinical picture simply stops fitting polymyositis. Red flags that prompt a doctor to reconsider the diagnosis and suspect FSHD include:

• Lack of response to steroids: The muscle weakness continues to worsen despite aggressive immunosuppressive therapy ^[3].
• Asymmetric weakness: Polymyositis typically affects both sides of the body equally (symmetric) and focuses on the proximal muscles (hips and thighs). FSHD is notoriously asymmetric—one shoulder or arm may be significantly weaker than the other.
• Scapular winging: A defining feature of FSHD is weakness in the shoulder blades, causing them to protrude (winging) and making it difficult to lift the arms above the head.
• Facial weakness: Difficulty whistling, sipping from a straw, or sleeping with your eyes slightly open are classic signs of FSHD that are absent in polymyositis.
• Genetic Testing: A definitive diagnosis of FSHD is ultimately made through specialized genetic testing, which is usually a simple blood test or cheek swab. This test looks for the specific genetic triggers (like a contraction in the D4Z4 DNA region or mutations in a gene called SMCHD1) that turn on the toxic DUX4 gene ^[9][10]. Results from these specialized tests can sometimes take a few weeks to come back.

Quick Comparison: Polymyositis vs. FSHD

If you spent months or years on an ineffective steroid regimen for polymyositis before finding out you have FSHD, your experience is unfortunately shared by many in the FSHD community. The overlapping inflammatory features make FSHD one of the most challenging muscular dystrophies to diagnose accurately without genetic testing ^[2][11]. If you are experiencing a lack of response to standard treatments for an inflammatory myopathy, it may be time to seek a second opinion from a specialized neuromuscular clinic or a Muscular Dystrophy Association (MDA) care center.