Polymyositis (PM)

Characteristics

Polymyositis, meaning “inflammation of many muscles,” develops over weeks to months and affects muscles but not the skin. The inflammation is caused by immune cells infiltrating between muscle fibers or fiber bundles. This condition primarily affects muscles in the upper arms and legs, causing weakness and sometimes pain, usually symmetrically on both sides of the body.

There are three subtypes of PM:

• Non-specific myositis (NSM): The most common subtype. Muscle biopsies reveal inflammation similar to dermatomyositis (DM) but without skin involvement.
• Immune-mediated necrotizing myopathy (IMNM): The second most common subtype, characterized by muscle fiber necrosis with little or no inflammation in biopsies. Blood creatine kinase (CK) levels are often significantly elevated.
• Myositis associated with antisynthetase syndrome (ASS): The least common subtype, associated with additional symptoms such as “mechanics’ hands” (skin changes), interstitial lung disease (ILD), arthritis, and Raynaud’s phenomenon (white fingers caused by cold-induced vessel constriction).

Prevalence and Disease Course

PM affects about 10 in 100,000 people and occurs only in adults, never in children. Early symptoms include muscle weakness, occasional muscle pain, general malaise, and sometimes fever. The muscles of the pelvis, upper legs, shoulders, and upper arms are typically affected first. This can manifest as difficulty climbing stairs, cycling, or lifting heavy objects. Weakness in throat and swallowing muscles can cause difficulties swallowing or choking.

The severity of symptoms ranges from mild, with minimal complaints, to severe weakness that spreads to other muscles. Pain may occur during muscle use, such as climbing stairs, and some individuals experience joint pain.

Cause

PM is thought to be caused by an abnormal immune response. Normally, the immune system uses antibodies and immune cells to target and eliminate foreign substances. In PM, the immune system mistakenly attacks muscle fibers or blood vessels within muscles. This autoimmune reaction leads to muscle inflammation and damage, similar to dermatomyositis.

Treatment

PM can be treated with medications, primarily aimed at suppressing the immune response:

1. First-line treatment: Prednisone, often combined with another immunosuppressive drug like methotrexate or azathioprine, depending on disease severity.
2. Second-line treatment: If initial treatments are ineffective, stronger therapies such as rituximab or intravenous immunoglobulins (IVIG) may be used.

Although these treatments can help, their effectiveness is not universally guaranteed. Some patients respond quickly, while others experience prolonged recovery or long-term limitations. Relapse occurs in one-third to half of patients within two to five years of treatment. However, the risk of relapse decreases significantly if no recurrence occurs within five years. Relapses are managed with renewed medication therapy.

Prognosis

PM’s progression and response to treatment vary widely. While some patients regain significant function, others face persistent challenges. Early diagnosis and tailored treatment plans can help improve outcomes and manage relapses effectively.