Myositis is a group of autoimmune conditions in which the body’s immune system attacks its own muscle tissue, causing progressive weakness. The three main types are dermatomyositis, polymyositis, and inclusion body myositis (often called IBM). If you searched “body myositis,” you were most likely looking for inclusion body myositis, the most common acquired muscle disease in adults over 50 and the one that tends to be the most stubborn to treat.
Types of Inflammatory Myositis
All forms of myositis share a core feature: inflammatory cells infiltrate skeletal muscle and damage the fibers, leading to weakness. But each subtype behaves differently.
- Dermatomyositis causes muscle weakness alongside distinctive skin changes, including a violet-colored rash on the eyelids and knuckles. It can affect adults or children.
- Polymyositis produces symmetrical weakness in the muscles closest to the trunk, like the shoulders and hips, without the skin involvement seen in dermatomyositis.
- Inclusion body myositis (IBM) develops slowly, typically after age 45, and targets a unique pattern of muscles. Unlike the other two types, IBM responds poorly to standard immunosuppressive treatment.
What Inclusion Body Myositis Feels Like
IBM creeps in gradually. Early on, the weakness is subtle enough that most people chalk it up to aging. The clinical hallmarks are weakness and wasting of the quadriceps (front of the thigh) and the deep finger flexors, the muscles that let you grip a jar lid or pinch your fingertips together. Weakness in the distal finger flexors is often the earliest finding.
Over time, these changes show up in everyday life in specific ways:
- Difficulty climbing stairs or standing up from a chair (quadriceps weakness)
- Knees buckling unexpectedly, leading to falls
- Slower walking speed and tripping over uneven surfaces due to foot drop
- Reduced grip strength, making it hard to open bottles or turn keys
- Trouble reaching overhead shelves or combing your hair
- Difficulty lifting your head off a pillow when neck muscles are involved
Swallowing difficulty is another common problem. When the throat muscles weaken, food or liquid can slip into the airway, sometimes without any obvious coughing or choking. Over time, this “silent aspiration” raises the risk of lung infections. A speech-language pathologist can evaluate swallowing safety and recommend changes to food textures or positioning during meals.
What Happens Inside the Muscle
IBM involves two overlapping processes: autoimmune attack and degeneration. Immune cells, particularly a type of white blood cell called CD8+ T-cells, surround and invade muscle fibers that aren’t yet dead. IBM muscle shows the most intense adaptive immune response of any inflammatory myopathy, with highly specific T-cell and B-cell activity directed at the muscle fibers.
At the same time, the muscle fibers accumulate abnormal protein deposits. Under a microscope, biopsied muscle shows characteristic “rimmed vacuoles,” tiny holes in the fiber lined with displaced nuclear proteins. Amyloid, the same type of misfolded protein found in Alzheimer’s disease, deposits within these damaged fibers. A protein called TDP-43, which normally stays inside the cell nucleus, leaks into the surrounding muscle tissue. This combination of immune destruction and protein buildup is what makes IBM so difficult to treat: blocking the immune system alone doesn’t address the degenerative side of the disease.
How Myositis Is Diagnosed
Diagnosis typically starts with a blood test for creatine kinase (CK), an enzyme that leaks from damaged muscle cells. In a healthy person, CK ranges from about 22 to 200 IU/L. In active myositis, levels rise significantly, though in IBM the elevation is often more modest than in polymyositis or dermatomyositis.
Blood tests can also detect myositis-specific autoantibodies. Anti-Jo-1 is the most common antibody in polymyositis and dermatomyositis, and its presence nearly excludes IBM. Anti-Mi-2 antibodies are most tightly linked to dermatomyositis. These antibodies help doctors distinguish between subtypes and anticipate complications.
A muscle biopsy remains the gold standard for confirming IBM. The pathologist looks for endomysial inflammation (immune cells within the muscle tissue), rimmed vacuoles, and clusters of shrinking muscle fibers. MRI can also show patterns of muscle inflammation and fatty replacement that point toward a specific diagnosis before biopsy.
Lung and Other Systemic Complications
Polymyositis and dermatomyositis can affect more than just muscles. The most significant extra-muscular complication is interstitial lung disease (ILD), a form of scarring in the lungs that causes progressive shortness of breath. A meta-analysis covering two decades of data found that roughly 41% of people with polymyositis or dermatomyositis develop ILD. The prevalence is highest in Asian populations (about 50%) and lower in Europe and the Americas (23 to 26%). Certain autoantibodies, particularly the anti-synthetase group that includes anti-Jo-1, signal a higher risk for lung involvement.
IBM, by contrast, primarily stays in the muscles. Lung disease is uncommon in IBM, but swallowing problems and the resulting aspiration risk remain a serious concern throughout the disease course.
Treatment for Polymyositis and Dermatomyositis
High-dose corticosteroids are still the first-line treatment for polymyositis and dermatomyositis. Most doctors combine steroids with a second immunosuppressive drug early on, both to control the disease more effectively and to reduce the long-term side effects of steroids, like bone thinning, weight gain, and elevated blood sugar.
Common second-line options include drugs that broadly suppress immune activity. One widely used agent is given orally, starting at a low dose and gradually increasing, and has shown particular benefit for patients with lung involvement. For more resistant cases, targeted therapies that deplete specific immune cells have shown promise, especially in patients carrying certain autoantibodies. A therapy that blocks T-cell activation has been tested in small trials for treatment-resistant disease with encouraging early results.
Side effects vary by drug but can include nausea, increased infection risk, low white blood cell counts, and liver stress. Regular blood monitoring is standard during treatment.
Why IBM Is Harder to Treat
IBM does not respond reliably to corticosteroids or conventional immunosuppressive drugs. This is the major frustration of the disease. Because IBM involves both immune-driven damage and protein accumulation within muscle fibers, shutting down the immune response alone fails to halt progression. No medication has been proven to stop or reverse IBM’s slow decline. Treatment currently focuses on maintaining function for as long as possible through exercise and adaptive strategies.
Exercise as a Core Strategy
For decades, doctors worried that exercise might worsen muscle inflammation. That fear was based on studies of healthy athletes doing extreme activities like marathons, not patients with myositis doing moderate training. The evidence now clearly supports exercise as safe in all types and stages of myositis, and researchers increasingly describe it as a form of medicine for the condition.
In polymyositis and dermatomyositis, intensive endurance exercise has been shown to reduce inflammation, promote muscle growth, and improve mitochondrial function compared to not exercising. The recommendation is to start at low intensity under a physical therapist’s guidance and gradually increase the load over time.
For IBM specifically, regular exercise helps maintain function even though it can’t reverse the underlying disease. In one study, patients who did stationary cycling and knee extension exercises three days a week for a year preserved strength better than inactive patients. Blood-flow restricted resistance training, a technique where a cuff partially limits circulation during light lifting, helped maintain quadriceps strength compared to a control group that lost an average of 9.2% of quadriceps strength over just three months of inactivity. Targeted hand exercise programs are also being developed to address the grip weakness that makes daily tasks so difficult in IBM.
The core message across all forms of myositis is that inactivity accelerates decline. An individually adapted, progressively challenging exercise program is one of the most reliable tools available for preserving independence.