Autoimmune myopathy is a group of diseases where the immune system incorrectly targets and damages the body’s own muscle tissue. The term “myopathy” refers to conditions affecting muscle fibers, leading to weakness. In autoimmune myopathies, this impairment stems from a misdirected immune system, which normally defends against threats like viruses.
These conditions are acquired, not inherited, and develop later in life. The immune system’s attack on muscle fibers, and sometimes blood vessels within the muscles, causes inflammation and damage. This process disrupts the function of skeletal muscles, which are responsible for voluntary movements, resulting in progressive weakness that can impact daily activities.
Types of Autoimmune Myopathy
Several primary conditions fall under the umbrella of autoimmune myopathy. Dermatomyositis is a type that involves both muscle weakness and characteristic skin rashes, caused by inflammation of the muscles and the blood vessels under the skin. The presence of specific rashes helps to distinguish it.
Polymyositis also causes muscle inflammation and weakness but without the skin manifestations seen in dermatomyositis. It affects skeletal muscles on both sides of the body and typically develops progressively, as inflammation directly damages muscle fibers.
Inclusion body myositis (IBM) also causes progressive muscle weakness but often affects distal muscles, such as those in the forearms, hands, and lower legs. The progression of IBM is slower than that of other inflammatory myopathies. Under a microscope, its muscle cells show unique features, including vacuoles and protein clumps that give the condition its name.
Necrotizing autoimmune myopathy (NAM) is a rarer and more severe form characterized by the rapid death (necrosis) of muscle tissue, leading to a swift onset of weakness. This condition is strongly associated with specific autoantibodies, such as anti-SRP and anti-HMGCR, the latter of which can be triggered by statin medications.
Common Symptoms and Signs
The most prevalent symptom is progressive muscle weakness that begins in the proximal muscles—those closest to the body’s trunk, such as the hips, thighs, and shoulders. This weakness is usually symmetrical, affecting both sides of the body and making activities like climbing stairs or getting up from a chair challenging.
Many individuals also experience significant fatigue and muscle pain, known as myalgia. The constant inflammatory process can lead to a pervasive sense of exhaustion that is not relieved by rest, and muscle pain can range from a dull ache to sharp pains.
In some forms, the muscles involved in swallowing and breathing can be affected. Difficulty swallowing, or dysphagia, occurs when throat and esophagus muscles weaken, creating a risk of choking. When respiratory muscles are impacted, it can lead to shortness of breath.
Specific skin signs are unique to dermatomyositis. A heliotrope rash, a purplish discoloration on the eyelids, is a key indicator, as are Gottron’s papules—reddish, scaly bumps that appear over the knuckles, elbows, and knees.
The Diagnostic Process
Confirming a diagnosis of autoimmune myopathy involves a multi-step process to rule out other conditions and identify specific markers of the disease.
- Blood tests check for elevated levels of muscle enzymes, particularly creatine kinase (CK), which are released into the bloodstream when muscle fibers are damaged. Blood is also screened for specific autoantibodies associated with different types of myopathy, like anti-Jo-1, anti-SRP, and anti-HMGCR.
- An electromyography (EMG) assesses the health of muscles and nerves. This test involves inserting a small needle electrode into the muscle to measure its electrical activity. An EMG can detect abnormal electrical patterns that indicate muscle damage, helping to distinguish it from nerve disorders.
- Magnetic resonance imaging (MRI) is a non-invasive imaging technique used to visualize muscle inflammation. An MRI can highlight areas of swelling and damage within large muscle groups, helping doctors pinpoint the best location for a muscle biopsy.
- A muscle biopsy is a conclusive diagnostic tool where a small piece of muscle tissue is surgically removed and examined under a microscope. This allows a pathologist to see direct evidence of inflammation or cell death (necrosis) and can reveal the inclusion bodies that define inclusion body myositis.
Managing the Condition
The primary goal of treatment is to manage the condition by reducing inflammation, suppressing the immune system, and improving function. Treatment plans are tailored to the specific type of myopathy and symptom severity. Corticosteroid medications, such as prednisone, are powerful anti-inflammatory drugs that can quickly reduce muscle inflammation.
When corticosteroids are not sufficient or their long-term side effects are a concern, other immunosuppressant drugs may be used. These medications work by tamping down the overactive immune response to prevent further muscle damage.
For some individuals, particularly those with necrotizing autoimmune myopathy or dermatomyositis, intravenous immunoglobulin (IVIg) therapy can be an effective treatment. IVIg is a blood product administered intravenously that contains antibodies from healthy donors, and it is thought to work by modulating the immune system.
Physical therapy is a key component of managing autoimmune myopathy. A carefully designed exercise program can help improve muscle strength and flexibility. A physical therapist guides patients through safe and effective exercises to maintain mobility and perform daily activities more easily.