Polymyositis (PM)

Overview

Polymyositis (PM) is an autoimmune condition that causes ongoing muscle inflammation and weakness, and can start at any age. It belongs to a family of conditions known as the idiopathic inflammatory myopathies, or myositis for short. A true PM diagnosis is now considered rare — with better testing, many people who would once have been labelled with PM are now recognised as having a related but distinct condition, such as antisynthetase syndrome, immune-mediated necrotising myopathy, or overlap myositis.

Symptoms

The main symptom is muscle weakness, typically centred on the torso, shoulders, upper arms, thighs and buttocks, developing gradually over weeks or months. This makes standing up from a chair or the floor, climbing stairs, and lifting the arms above the head progressively harder. If the muscles of the gullet are affected, swallowing becomes difficult; if the chest muscles are involved, breathing can be affected too. Less commonly, the heart muscle is involved, which can disturb the heartbeat or reduce the heart's pumping efficiency.

Unlike some other forms of myositis, PM doesn't typically come with a skin rash, lung inflammation, joint pain and swelling, or the finger and toe colour changes triggered by cold. Some people do experience muscle pain — aching, discomfort or mild tenderness — particularly when the weakness has come on quickly.

Cause

The exact trigger isn't fully understood, but PM is believed to be autoimmune — the immune system mistakenly attacking healthy muscle tissue. It isn't generally passed down through families, though some people may inherit a broader genetic tendency toward autoimmune conditions.

Getting a diagnosis

Because a fresh PM diagnosis is now uncommon, most cases seen today were diagnosed some years ago. Specialists build a picture using a clinical examination alongside blood tests, MRI scans, EMG (electrical muscle testing) and sometimes a muscle biopsy. Blood tests often show a raised creatine kinase (CK) level, a marker of muscle damage — though, unlike some related conditions, there's no specific antibody test for PM itself.

Management and outlook

Access to a specialist — a neurologist at a neuromuscular clinic, or a rheumatologist — makes a real difference to care. Treatment usually starts with the steroid prednisolone to calm the immune system, at a higher dose initially and then gradually reduced. For milder disease, a disease-modifying anti-rheumatic drug (DMARD) such as methotrexate, azathioprine or mycophenolate may be used alone. When steroids and DMARDs together aren't enough, biological therapies (rituximab or abatacept) or intravenous immunoglobulin (IVIG) are options.

Regular exercise is strongly encouraged — it supports general wellbeing, helps maintain muscle strength, and benefits heart health. A physiotherapist can put together a safe, appropriately paced exercise plan that allows for full recovery between sessions.