Introduction
Polymyositis is a chronic autoimmune disorder in which the immune system attacks and destroys muscles close to the trunk of the body, causing myopathy (muscle disease).¹
In polymyositis, types of immune system cells called CD8 cells and macrophages become unnecessarily activated. Once active, they attack the endomysium, a thin layer of connective tissue that surrounds each fibre within a muscle.¹
Polymyositis can cause prolonged muscle weakness in the shoulder, pelvis, and neck muscles - causing difficulty with everyday movements, such as getting up from the chair. However, polymyositis can also cause nonspecific symptoms such as low appetite, low-grade fever and weight loss.¹,²
Polymyositis is a type of idiopathic immunological myositis (IIM). Various studies have found that less than 5% of myositis cases are polymyositis, making it a rare disease.³
This disease generally has a poor prognosis, reducing patients’ quality of life and having a mortality rate of 10%. Therefore, adequate diagnosis and treatment in the early stages of the disease prevent complications and optimise each patient’s prognosis.¹
Diagnosis of polymyositis
The diagnosis of polymyositis is a diagnosis of exclusion; that is, it can only be diagnosed if the diagnostic criteria for other idiopathic inflammatory myopathies are not met. The first step is to diagnose idiopathic inflammatory myopathy in general, and then rule out specific types until only one remains.
A comprehensive evaluation is required to diagnose the entire range of myopathies, including polymyositis. The diagnostic evaluation includes:
Taking a medical history/examination
When you first see a doctor, they will ask questions about your medical history (and that of your family), any medications you may be taking, and any symptoms you are experiencing. Noteworthy symptoms include:
- Muscle weakness or fatigue
- Skin rash
- Muscle atrophy
- Weakness
Investigations
Next, doctors will look at the levels of different enzymes present in your blood serum. These enzymes include creatine kinase (CK), lactase dehydrogenase (LD), aspartate transaminase (AST), alanine transaminase (ALT) and aldolase. This may involve various tests, including:
- Taking biopsies from symptomatic muscles
- Magnetic resonance imaging (MRI) of symptomatic muscles
- Antibodies (including myositis-specific and myositis-associated autoantibodies)
- Electromyograms
Further studies
Polymyositis mainly affects the muscular system; however, it can also affect other organs such as the lungs, gastrointestinal tract, heart, and joints. That is why it is recommended that more studies be carried out, regardless of your symptoms. These additional studies include:²
- High-resolution computerised tomography of the lung
- Pulmonary (lung) function test
- Radiography or video radiography of the oesophagus and pharynx
- Electrocardiogram (ECG) and echocardiogram
Different classification criteria have been created to facilitate diagnosis. One that has proven useful and effective is the European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) Classification criteria.⁴
EULAR/ACR classification criteria
This EULAR/ACR allows doctors to differentiate between possible, probable and definitive cases of polymyositis. The criteria involves noting and/or evaluating the severity of:⁴
- The age at which the patient’s symptoms appeared
- The location and extent of muscle weakness
- Skin manifestations
- Dysphagia (difficulty swallowing) or esophageal dysmotility
- The presence of antibodies associated with myositis, such as Anti-Jo-1
- Elevated inflammation markers such as CK, LDH, AST and ALT
- Any inflammation in biopsied muscle
Differential diagnoses
A differential diagnosis allows doctors to rule out other myopathies with similar or overlapping symptoms. The most common myopathies that need to be excluded are:⁴
- Dermatomyositis
- Immune-mediated necrotising myopathy
- Inclusion bodysuit myositis
- Overlap myositis (where a patient has more than one type of autoimmune disease)
The European League Against Rheumatism (EULAR/ACR) Classification criteria can help doctors make differential diagnoses. They consider:⁴
- The presence of skin lesions such as Heliotrope rash, Gottron's papules, and Gottron's sign
- Other clinical features
- Results of any muscle biopsies taken
We will now discuss some of the diseases commonly mistaken for polymyositis.
Dermatomyositis
Dermatomyositis is a type of IIM which is characterised by muscle weakness associated with skin lesions. These lesions are often accompanied by skin changes like raised patches known as Gottron's papules and a purplish rash called heliotrope rash. Other visible skin changes might involve increased sensitivity to sunlight, patchy skin discolouration, widespread redness, small bumps on the palms, and hair loss across the scalp. Dermatomyositis can also cause nail issues like rough cuticles and redness around the nails.⁵
Necrotising autoimmune myopathy
This is another type of IIM which is characterised by the presence of dead cells in the muscle biopsy. It also causes weight loss and loss of muscle mass in the affected muscles.⁵
Inclusion bodysuit myositis
It is a type of myopathy characterised by progressive muscle weakness. It usually occurs in people over 50 and in men. It mainly affects the muscles involved in flexing or extending the knee and bending the fingers.⁵
Overlap myositis
Overlap myositis refers to the presence of two or more systemic rheumatological or autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, scleroderma, and skeletal muscle inflammation.⁵
Treatment approaches
Both pharmacological (drug) and physical therapy are vital in treating polymyositis. The severity of the disease, response to treatment, and the organ(s) affected by disease are taken into account when designing a treatment program.
Medications
Medications used to treat polymyositis include:
1. Corticosteroids
When starting therapy, it is recommended to start with oral glucocorticoids at high doses and then regulate it. A complication of its chronic use can cause adverse effects and long-term complications - which often result in the drugs not correctly controlling the disease.⁵,⁶
2. Immunosuppressants
Immunosuppressants (drugs that reduce the activity of immune cells and the immune system) are often recommended to address the negative side effects of corticosteroid treatment.
Immunosuppressants such as methotrexate or azathioprine are often initially prescribed alongside glucocorticoids. Azathioprine can be taken by most people, including those with liver disease, those who regularly drink alcohol, and those who are pregnant. If the initial treatment does not produce an adequate response, other immunosuppressants, such as mycophenolate, tacrolimus, or cyclosporine, can be used.
3. Intravenous immunoglobulin (IVIG)
Intravenous immunoglobulin is recommended in cases where muscle inflammation does not respond to treatment and/or is severe.
Physical therapy
It is recommended that patients start exercising regularly in the early stages of the disease, guided by a physical therapist. This helps maintain muscle strength and functionality and can even reduce inflammation in the affected muscles.⁶
Long-term monitoring and follow-up
Patients with polymyositis are at risk of a variety of complications, such as bone fractures, cancer, cardiac (heart) issues, inability to swallow, and mental health diseases. This is why long-term monitoring and follow-up visits are vital.
Bone fractures
Fractures occur primarily in women, with the risk increasing with age. An advanced bone health assessment is recommended for these patients.
Cancer
Patients are often at an increased risk of cancer in the year following their diagnosis and treatment. Preventive screening is recommended if they present with any of the following risk factors:⁷
- Male sex
- Does not respond well to treatment
- Fast disease onset/development
- Disease onset at an older age
- Difficulty passing food
- Skin necrosis (dead skin)
- Laboratory tests with positive anti-TIF1 and anti-NXP2 antibodies
Screening should be performed using CT scans of the thorax, pelvis, and abdomen.⁷
Cardiac issues
Patients with polymyositis are at an increased risk of cardiovascular diseases such as hypertension, diabetes, coronary artery disease, and dyslipidemia. Therefore, patients must undergo a regular cardiovascular risk assessment.⁷
Inability to swallow
Around 70% of people with advanced polymyositis have problems swallowing. This can cause them to regurgitate food, which is a common cause of aspiration pneumonia.¹
Summary
Polymyositis is a chronic autoimmune disorder that affects the muscles in and around the trunk of the body. It is characterised by activating immune system cells that attack the endomysium, a thin layer of connective tissue around muscle fibres. Polymyositis can cause persistent and worsening muscle weakness in the shoulders, pelvis, and neck and nonspecific symptoms such as lack of appetite, mild fever, and weight loss. Although it used to be more common, polymyositis is now considered a rare disease.
Polymyositis is diagnosed by excluding all other similar diseases. This requires a variety of tests, including blood tests, muscle biopsies, and imaging tests. Classification criteria, such as the European League Against Rheumatism (EULAR) criteria have been developed to aid diagnosis. Treatment includes medications such as corticosteroids, immunosuppressants, and physical therapy to improve muscle strength and function. Long-term follow-up is crucial because of the increased risk of complications such as fractures, cancer, heart problems, and difficulty swallowing.
References
- Sarwar A, Dydyk AM, Jatwani S. Polymyositis. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 [cited 2024 May 11]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK563129/
- Lundberg IE, Miller FW, Tjärnlund A, Bottai M. Diagnosis and classification of Idiopathic Inflammatory Myopathies. J. Intern. Med. 2016;280(1):39–51.
- Loarce-Martos J, Lilleker JB, Parker M, McHugh N, Chinoy H. Polymyositis: is there anything left? A retrospective diagnostic review from a tertiary myositis centre. Rheumatology (Oxford). 2021;60(7):3398–403.
- Lundberg IE, Tjärnlund A, Bottai M, Werth VP, Pilkington C, de Visser M, et al. EULAR/ACR Classification Criteria for Adult and Juvenile Idiopathic Inflammatory Myopathies and their Major Subgroups. Ann. Rheum. Dis. 2017;76(12):1955–64.
- Ashton C, Paramalingam S, Stevenson B, Brusch A, Needham M. Idiopathic inflammatory myopathies: a review. Intern. Med. J. 2021;51(6):845–52.
- Lundberg IE, Fujimoto M, Vencovsky J, Aggarwal R, Holmqvist M, Christopher-Stine L, et al. Idiopathic inflammatory myopathies. Nat. Rev. Dis. Primers. 2021;7(1):1–22.
- Oldroyd AGS, Lilleker JB, Amin T, Aragon O, Bechman K, Cuthbert V, et al. British Society for Rheumatology guideline on management of paediatric, adolescent and adult patients with idiopathic inflammatory myopathy. Rheumatology (Oxford). 2022;61(5):1760–8.