Why Was I Initially Diagnosed With Muscular Dystrophy?

Immune-mediated necrotizing myopathy (IMNM) is an acquired autoimmune disease, whereas limb-girdle muscular dystrophy (LGMD) refers to a group of inherited genetic disorders ^[1]. However, IMNM can initially resemble a muscular dystrophy, which is why some patients receive a suspected diagnosis of LGMD before their autoimmune condition is identified ^[2]. Because muscular dystrophies are well-known causes of this specific pattern of weakness, doctors often investigate genetic causes early in the diagnostic process ^[1].

Why They Look So Similar

Both IMNM and LGMD primarily affect the proximal muscles—the large muscles closest to the center of your body, such as those in your hips, thighs, and shoulders ^[2]. This causes shared symptoms like difficulty climbing stairs, struggling to stand up from a low chair, or trouble lifting your arms overhead.

Additionally, both conditions cause creatine kinase (CK) levels to become markedly elevated. CK is an enzyme that leaks into the bloodstream when muscle tissue is damaged. In both IMNM and LGMD, CK levels can be very high, sometimes exceeding 1,000 U/L ^[2].

While IMNM usually progresses rapidly over weeks or months, some cases—particularly those associated with the anti-HMGCR autoantibody—can progress very slowly over many years ^[1][3]. This slow, gradual decline strongly mimics the timeline typically seen in genetic muscular dystrophies, further complicating the initial diagnosis ^[4][5].

Clues That May Suggest IMNM

As the disease progresses or more specialized tests are performed, certain features might raise a doctor’s suspicion for an autoimmune myopathy, though they are not definitive proof on their own:

• Unique Weakness Patterns: While both diseases affect the hips and shoulders, IMNM can sometimes cause significant weakness in the neck muscles and difficulty swallowing (dysphagia) ^[6]. (Safety Note: Any new or worsening swallowing difficulty, choking, or breathing trouble requires prompt medical evaluation.)
• Muscle Pain: Severe muscle aching (myalgia) is sometimes reported in IMNM ^[6], although various muscular dystrophies can also involve cramps or pain.
• Electrical Muscle Tests (EMG): An electromyogram (EMG) measures the electrical activity of muscles. While an EMG can confirm a muscle disease (a myopathy) by showing abnormal electrical patterns, these findings overlap heavily between IMNM and LGMD and usually cannot establish the distinction by themselves ^[7].

Putting the Puzzle Together

Because clinical symptoms overlap, specialists use a combination of tools to clarify the diagnosis:

1. Autoantibody Blood Tests: Blood panels can look for anti-HMGCR or anti-SRP autoantibodies (misdirected immune proteins that target the body’s own muscle tissue) ^[8][5]. When present alongside typical symptoms, these strongly support an IMNM diagnosis ^[9]. However, some patients have seronegative IMNM (meaning no antibodies are found), so a negative test does not completely rule out the disease ^[10].
2. Muscle Biopsy: A small sample of muscle tissue examined under a microscope can provide important clues. IMNM biopsies typically show extensive muscle fiber death (necrosis) and regeneration, often with relatively sparse inflammation ^[11][5]. However, this is not a perfect separator, as genetic dystrophies can also feature necrosis and regeneration, and biopsies require expert interpretation by a neuromuscular pathologist ^[6].
3. Genetic Testing: Comprehensive genetic testing can identify mutated genes that cause LGMD. However, a negative genetic panel does not definitively rule out genetic disease, as tests can miss variants or find variants of uncertain significance ^[1]. If a genetic panel is unrevealing, doctors will often consider autoimmune testing in parallel, guided by the whole clinical picture ^[12][13].

Why Getting it Right Matters

The distinction between a genetic muscular dystrophy and an autoimmune myopathy is critical because the management approaches are different. Inherited muscular dystrophies generally require a long-term supportive management approach and are not treated with immunosuppression.

IMNM, on the other hand, is driven by the immune system and is potentially treatable with immunomodulatory therapies (drugs that suppress or modulate the immune system) ^[14]. The response to treatment varies; some patients see significant improvements in muscle strength and reductions in CK levels, while others may require prolonged treatment, face relapse risks, or experience residual muscle weakness ^[4]. Recognizing IMNM early ensures patients can access these therapies and specialized rehabilitation.