Inflammatory Myopathy (Myositis)

How inflammatory myopathies work

In myositis, the immune system targets muscle tissue. In dermatomyositis, it damages small blood vessels in muscle and skin. In necrotizing myopathy, antibodies cause muscle fibers to die. In inclusion body myositis, inflammation and degeneration occur together. The common result is muscle weakness, sometimes with pain, plus effects on other organs.

Types of inflammatory myopathy

Symptoms

Muscle symptoms

• weakness of hip and shoulder muscles: difficulty climbing stairs, getting up from low chairs or the floor, lifting arms to wash hair
• neck weakness, difficulty lifting the head
• muscle aches or tenderness in some people
• fatigue
• in inclusion body myositis: weak grip, frequent falls from knee buckling

Symptoms beyond muscle

• skin: heliotrope rash, Gottron papules, sun-sensitive rash, cracked fingertips, calcium deposits in children
• lungs: dry cough and breathlessness from interstitial lung disease, which can be the main problem
• swallowing: difficulty swallowing, choking, nasal voice
• joints: arthritis
• heart: rhythm problems or inflammation, uncommon
• Raynaud's phenomenon
• general: fever and weight loss

Causes and risk factors

The cause is unknown. Contributing factors include genetic susceptibility, environmental triggers such as sunlight (dermatomyositis), viral infections, certain medicines (statins in HMGCR myopathy, immune checkpoint inhibitors for cancer) and, in dermatomyositis, an underlying cancer.

Diagnosis

• Creatine kinase (CK): a muscle enzyme, usually raised, sometimes very high in necrotizing myopathy.
• Myositis-specific and myositis-associated antibodies: help classify the type and predict lung disease, cancer risk and response to treatment.
• MRI of muscles: shows inflammation and guides where to biopsy.
• Electromyography (EMG): shows a muscle pattern of damage.
• Muscle biopsy: confirms inflammation and distinguishes types. Skin biopsy may help in dermatomyositis.
• Lung tests: high-resolution CT and lung function tests.
• Heart tests: ECG, troponin and echocardiogram.
• Swallowing studies if needed.
• Cancer screening in dermatomyositis and some other types, typically CT scans and age-appropriate screening, as cancer risk is raised for about 3 years around diagnosis.

Doctors also exclude other causes of weakness: statin-induced myopathy without antibodies, thyroid disease, muscular dystrophies, metabolic myopathies, myasthenia gravis and medication effects.

Treatment

• Corticosteroids (prednisone, sometimes IV methylprednisolone pulses) as the initial treatment for most types.
• Steroid-sparing drugs: methotrexate, azathioprine or mycophenolate, started early to allow steroids to be reduced.
• IVIG: effective for dermatomyositis (FDA approved) and necrotizing myopathy, and helpful for swallowing problems.
• Rituximab, tacrolimus or cyclophosphamide for resistant disease or significant lung involvement.
• Skin care for dermatomyositis: strict sun protection, hydroxychloroquine and topical treatments.
• Inclusion body myositis: no proven drug treatment. Management focuses on exercise, assistive devices and swallowing support.
• Exercise and physical therapy: safe and beneficial, improving strength and function once inflammation is controlled.
• Bone protection while on steroids, and infection prevention with vaccines.

Outlook

With treatment, most people with dermatomyositis, antisynthetase syndrome and necrotizing myopathy regain much of their strength, though recovery takes months and many need long-term treatment. Lung disease and associated cancer are the main factors affecting survival. Inclusion body myositis progresses slowly over years, and most people eventually need walking aids.

Frequently asked questions

What is inflammatory myopathy?

Inflammatory myopathy, or myositis, is a group of autoimmune diseases in which the immune system attacks muscle, causing weakness, usually of the hips, thighs, shoulders and neck. Some types also affect the skin, lungs, joints, heart or swallowing.

What are the types of myositis?

The main types are dermatomyositis, immune-mediated necrotizing myopathy, antisynthetase syndrome, overlap myositis, polymyositis and inclusion body myositis. They differ in symptoms, antibodies, associated conditions and response to treatment.

What is the difference between dermatomyositis and polymyositis?

Both cause muscle weakness, but dermatomyositis also causes a characteristic skin rash, such as a purple rash on the eyelids or red bumps over the knuckles, and carries a higher cancer risk in adults. Polymyositis affects muscles without a rash.

Can statins cause myositis?

Statins commonly cause mild muscle aches that go away when the drug is stopped. Rarely, they trigger an autoimmune necrotizing myopathy with anti-HMGCR antibodies, in which weakness continues or worsens after stopping the statin and needs immune treatment.

Is myositis curable?

Some people achieve long-term remission and can stop treatment, especially with dermatomyositis. Others need ongoing treatment to control disease. Inclusion body myositis does not respond to current immune treatments, and management focuses on maintaining function.