Polymyositis (PM)

Polymyositis (PM) is an autoimmune condition that causes chronic muscle weakness and inflammation. It can affect people of any age.

Polymyositis is a type of myositis – a group of conditions that cause muscle inflammation and weakness. These conditions are also known as idiopathic inflammatory myopathies.

As our understanding of myositis grows, polymyositis is now considered a rare diagnosis. It’s thought that many people who were previously diagnosed with polymyositis may have antisynthetase syndrome (ASyS), immune-mediated necrotising myopathy (IMNM), or muscle inflammation linked to another autoimmune disease (sometimes called overlap myositis, OM). We don’t discuss these conditions here.

Muscle weakness

Muscle weakness is the main symptom of polymyositis, and it usually affects the muscles in the torso, shoulders, upper arms, thighs, and buttocks. Muscle weakness usually develops slowly and may become noticeable after a few weeks or months. People affected may find it hard to stand from a seated position or from the floor, climb stairs, or raise their arms above their head.

They may also have some difficulty swallowing (dysphagia) if the oesophageal (food pipe) muscles are affected, or breathing problems if the chest muscles are affected. The heart muscle can also be affected. This can cause abnormalities in the heartbeat or reduce the heart’s ability to pump.

Non-muscle symptoms are seen in other forms of myositis, but not usually in polymyositis. These include a skin rash, lung inflammation, joint pain or swelling, or a colour change in the fingers and toes due to cold.

Pain

Some people may experience muscle pain, especially if symptoms start quickly. This can cause aching, discomfort, or mild tenderness in the affected muscles.

The cause of polymyositis is unclear, but it’s believed to be an autoimmune condition. This means the immune system, which normally protects the body, mistakenly attacks healthy tissues.

Polymyositis is typically not considered to be an inherited condition, but some people may inherit genes that make them more likely to develop an autoimmune condition.

A new diagnosis of polymyositis is now rare. However, some people may still have a diagnosis of polymyositis if they were diagnosed before changes in understanding.

Specialists use a clinical examination and one or more tests to make a diagnosis. This may include blood tests, a magnetic resonance imaging (MRI) scan, an electromyography (EMG) test, or sometimes a muscle biopsy. A blood test can look for high levels of creatine kinase (CK), which is a sign of muscle damage. There are no specific antibodies for polymyositis.

For more information, see our diagnosis page.

Access to a healthcare team

People with polymyositis should have access to a specialist team to assess their condition and discuss treatment. Usually, the lead professional will be a neurologist in a specialist neuromuscular clinic, or a rheumatologist. A neurologist is a doctor who specialises in conditions that affect the muscles and nervous system, and a rheumatologist is a doctor who specialises in inflammatory conditions that affect the joints and muscles. If you’re not in contact with a specialist doctor, speak to your GP about getting access to one.

Medications

• Steroids – Polymyositis is often first treated with corticosteroid tablets, such as prednisolone. Steroids reduce the activity of the immune system. Prednisolone is usually started at a high dose and can be given as tablets, injection, or by infusion. The dose is then gradually lowered based on the person’s response and improvement in symptoms. Not everyone with polymyositis needs steroids and if the condition is mild, disease-modifying anti-rheumatic drugs (DMARDs) alone may be enough. In the long term, steroids can cause side effects such as increased risk of infections, high blood pressure, fragile skin which bruises easily, weight gain, cataracts, and weakened bones (osteoporosis). Regular monitoring and additional medications can help manage these risks but the best way to avoid side effects is to gradually reduce the dose, aiming to eventually come off steroid treatment or to take the lowest required dose. Do not stop taking steroids suddenly, as this is dangerous and has serious health risks. It’s important to carry a Steroid Emergency Card, as it provides important information for healthcare professionals in an emergency. The card can be obtained from a GP, community pharmacy, or specialist hospital team.
• Disease-modifying anti-rheumatic drugs (DMARDs) – DMARDs, such as methotrexate, azathioprine, and mycophenolate are used for long-term management of polymyositis. These drugs also reduce the activity of the immune system and are generally safer than long-term steroid use. However, they can take weeks to months to start working effectively, which is why steroids may be needed at first. DMARDs require regular blood tests to monitor side effects and to make sure the treatment is working effectively.
• Biological treatments – In severe cases where symptoms don’t improve with steroids and traditional DMARDs, biological therapies, such as rituximab (through intravenous infusion) or abatacept (through subcutaneous injection or intravenous infusion), may be used. Biological treatments target more specific parts of the immune system than DMARDs. Biological treatments can only be prescribed by a specialist.
• Immunoglobulins (IVIG) – IVIG is another option for people with severe polymyositis. It is an infusion usually given over three to five days. The infusion contains antibodies extracted from donated blood.

Exercise and physiotherapy

Regular exercise is highly recommended for people with polymyositis. Exercise can help improve wellbeing, maintain muscle strength, and support a healthy heart. A physiotherapist is a healthcare professional who helps manage symptoms through movement, exercise, and manual therapy. They can assess a person’s needs and put together a suitable exercise plan. It’s important to exercise safely and allow time to fully recover before doing more exercise. For advice for adults, see exercising with a muscle wasting condition.