Myositis is a term used to describe inflammation of the muscles. It leads to muscle weakness, and often muscle pain and fatigue, that can affect a person's ability to carry out everyday activities such as climbing stairs, standing up from a chair or lifting objects.
In most cases, myositis is caused by the immune system mistakenly attacking healthy muscle tissue. This group of conditions is known as the idiopathic inflammatory myopathies and includes polymyositis, dermatomyositis, immune-mediated necrotising myopathy and inclusion body myositis. Some forms also affect the skin or other organs, including the lungs.
Without appropriate assessment and treatment, myositis can lead to progressive muscle weakness, difficulty swallowing or breathing, and reduced mobility and independence.
What is myositis?
Myositis refers to a group of rare autoimmune conditions, collectively known as idiopathic inflammatory myopathies, in which the immune system attacks the muscles, causing inflammation, weakness and, in some cases, muscle damage.
The main subtypes include polymyositis, dermatomyositis, immune-mediated necrotising myopathy and inclusion body myositis. Each has a distinct pattern of muscle involvement, associated antibodies and, in some cases, characteristic skin changes.
Myositis can occur on its own or alongside other autoimmune conditions such as lupus, systemic sclerosis or Sjogren's syndrome, in which case it is referred to as overlap myositis. It can develop at any age, although certain subtypes are more common in particular age groups; inclusion body myositis, for example, typically affects people over the age of 50.
What are the symptoms of myositis?
The main feature of myositis is muscle weakness, which most commonly affects the large muscles closest to the trunk, such as those of the hips, thighs, shoulders and upper arms. This is known as proximal muscle weakness.
Symptoms of myositis may include:
- Progressive weakness in the shoulders, upper arms, hips or thighs
- Difficulty rising from a chair, climbing stairs or lifting objects above the head
- Muscle aching or tenderness
- Persistent fatigue
- Difficulty swallowing (dysphagia) if the muscles of the throat are affected
- Shortness of breath, if the muscles involved in breathing or the lungs are affected
- Skin changes in dermatomyositis, including a purple or dusky rash on the eyelids (heliotrope rash), raised scaly patches over the knuckles (Gottron's papules), or a rash on the chest, back or hands
- Joint pain and swelling
- In inclusion body myositis, weakness that also affects the muscles below the knee or the small muscles of the hands, and is often asymmetrical
Symptoms typically develop gradually over weeks to months, although some forms of myositis can progress more rapidly.
When to Seek Medical Advice?
You should seek medical assessment if you notice unexplained or progressive muscle weakness, particularly if you are finding it increasingly difficult to climb stairs, rise from a seated position or lift your arms above your head. Weakness accompanied by a new skin rash, difficulty swallowing or shortness of breath should be assessed promptly.
Early diagnosis and treatment can help to limit muscle damage, reduce the risk of complications affecting the lungs, heart or swallowing, and improve your long-term function and quality of life.
What are the causes of myositis?
Myositis is an autoimmune condition, meaning the immune system mistakenly targets the body's own muscle tissue, and in some subtypes the skin and blood vessels, causing inflammation and damage. The exact trigger is not fully understood, but it is thought to result from a combination of genetic susceptibility and environmental factors.
Factors and associations linked to myositis include:
- Genetic predisposition, including certain immune system gene variants
- Environmental triggers, such as viral infections or ultraviolet light exposure, particularly in dermatomyositis
- Specific myositis-related autoantibodies, which are associated with particular patterns of muscle, skin, lung or joint involvement
- Co-existing autoimmune or connective tissue disease, such as lupus, systemic sclerosis or Sjogren's syndrome (overlap myositis)
- An association with underlying malignancy, particularly in adults diagnosed with dermatomyositis, which is why cancer screening is an important part of assessment
- Certain medications, including statins, which can rarely trigger an immune-mediated necrotising myopathy
How is myositis diagnosed?
Diagnosis is based on a combination of clinical assessment, blood tests, imaging and, in many cases, a muscle biopsy. Because myositis can affect the skin, lungs and heart as well as the muscles, a thorough assessment is important to establish the subtype and extent of the condition.
Dr Nuttall will begin with a detailed review of your symptoms, the pattern and speed of onset of any weakness, skin changes, medication history and family history, together with a physical examination assessing muscle strength.
Investigations for myositis may include:
- Blood tests to measure muscle enzymes, including creatine kinase (CK), which is typically raised when muscle is inflamed or damaged
- Myositis-specific and myositis-associated antibody testing, to help identify the subtype and any risk of lung or other organ involvement
- Electromyography (EMG), which assesses the electrical activity of muscles and can help distinguish myositis from other neuromuscular conditions
- MRI of the affected muscles, to identify areas of inflammation and guide biopsy
- Muscle biopsy, which remains the most definitive way of confirming the diagnosis and identifying the specific subtype of myositis
- Pulmonary function tests and CT imaging of the chest, if lung involvement is suspected
- Age-appropriate cancer screening, particularly in adults newly diagnosed with dermatomyositis
How to treat myositis?
Treatment is aimed at reducing inflammation, restoring muscle strength and preventing complications affecting the lungs, heart or swallowing. The approach is tailored to the specific subtype of myositis, its severity and any associated organ involvement.
Treatment options may include:
- Corticosteroids, usually given at a higher dose initially and then gradually reduced, to control inflammation
- Steroid-sparing immunosuppressant medications, such as methotrexate, azathioprine or mycophenolate mofetil, used to maintain control of the condition and reduce long-term steroid use
- Intravenous immunoglobulin (IVIG), which may be used in more severe or treatment-resistant disease, particularly in dermatomyositis
- Biologic therapies, such as rituximab, for cases that do not respond adequately to standard treatment
- Physiotherapy, to help maintain and rebuild muscle strength and function, and to support safe exercise as inflammation improves
- Sun protection and skin-directed treatment for the rash associated with dermatomyositis
- Monitoring and management of any associated lung, heart or swallowing involvement
- Investigation and treatment of any underlying malignancy, where relevant
Dr Nuttall will tailor treatment to control inflammation, restore muscle strength, and support your long-term function and quality of life.
