Polymyositis is an idiopathic inflammatory myopathy characterized by symmetrical, proximal muscle weakness, elevated skeletal muscle enzyme levels, and characteristic electromyography, and muscle biopsy findings.
Polymyositis and dermatomyositis have many shared clinical features but have unique features on biopsy.
Polymyositis is more common in women than in men.
It usually affects adults older than 20 years, especially those aged 45-60 years, and rarely affects children.
Pathophysiology of Polymyositis
Polymyositis is an immune-mediated syndrome and is most commonly associated with other systemic autoimmune diseases. It can occur alone or in association with viral infections, malignancies, or connective-tissue disorders.
T-cell–mediated cytotoxic process directed against unidentified muscle antigens is thought to be responsible.
Though the factors that trigger the disease are not clear, HIV viruses, HTLV and coxsackie viruses have been implicated. Other viruses are
- Hepatitis B
Many drugs are known to cause myopathy. Some of them are hydroxychloroquine, D-penicillamine, hydralazine, procainamide, phenytoin, and angiotensin-converting enzyme (ACE) inhibitors.
An autoimmune response results in two kinds of antibodies
Myositis-associated antibodies which are shared with other autoimmune diseases.
Antibodies that are unique to myositis or myositis-specific antibodies.
Myositis Specific Antibodies are usually against 3 distinct groups of proteins:
- Aminoacyl–transfer ribonucleic acid (tRNA) synthetases
- Nuclear Mi-2 protein
- Components of the signal-recognition particle (SRP).
Anti–Jo-1 antibodies are also seen in dermatomyositis and are rare in children. The presence of anti-Jo-1 antibodies defines a distinct group of polymyositis – patients with interstitial lung disease, arthritis, and fever.
Anti-Mi-2 antibodies are specific for dermatomyositis.
Patients with anti-SRP antibodies have acute polymyositis with cardiac involvement, a poor prognosis, and a poor response to therapy.
Myositis-associated antibodies are seen in 20-50% of patients and are commonly encountered in other connective tissue diseases.
The anti-PM/Scl autoantibodies are found in polymyositis overlapping with scleroderma.
Anti-Ku antibodies are found in patients with myositis overlapping with other connective tissue diseases.
Antibodies directed against snRNP myositis and in patients with connective tissue–disease overlap syndrome
Antibodies to Ro/SSA 60 kD, Ro/SSA 52 kD, and La/SSB – Sjogren syndrome and systemic lupus erythematosus.
Associated Systemic Involvement
- Aspiration pneumonia [due to pharyngeal and esophageal weakness]
- Interstitial lung disease
- Interstitial pneumonitis
- Bronchiolitis obliterans organizing pneumonia, and pulmonary capillaritis are known to occur.
- Unusual and signifies a bad prognosis.
- Rhythm disturbances
- Conduction defects
- Congestive heart failure
- Pulmonary hypertension
- Myocarditis can occur.
- Arthralgias/or arthritis
- Usually symmetrical and involves the knees, wrists, and hands
- Overlap syndromes
Symptoms may include the following:
- Nasal regurgitation
- Reflux esophagitis
- Abdominal bloating
Tubular necrosis due to rhabdomyolysis
Symptoms of polymyositis gradually develop over a period of 3-6 months and the patient may not seek medical advice initially.
Patients usually present with symmetrical, proximal muscle weakness in the upper and lower limbs. There may be a weakness of neck flexors.
Muscle weakness may fluctuate from week to week or from month to month. Patients with polymyositis may experience exertional dyspnea secondary to weakness of chest wall muscles and diaphragmatic muscles.
Fine motor movements involving distal muscles [buttoning a shirt, sewing, knitting, or writing] are affected only late in the disease.
About 30% of the patients may have difficulty in swallowing secondary to oropharyngeal and esophageal involvement. It is considered a poor prognostic sign. Dysphonia is also a poor prognostic sign but is much less common.
Ocular, facial and bulbar muscle weakness is extremely rare.
Patient may have systemic signs like morning stiffness, fatigue, anorexia. Fever and weight loss may be present. Other connective tissue disorders should be ruled out.
There is muscle weakness on examination. Sensory examination findings are normal. Ocular muscles remain normal even in advanced, untreated cases. Facial muscles remain normal except in rare advanced cases.
When the patient is first seen, many of the muscles of the trunk, shoulders, hips, upper arms, and thighs are usually involved.
In case of respiratory muscle weakness, dyspnea may be present.
Lung examination findings may show dry crackles indicating interstitial lung disease, such as dry inspiratory crackles in the lung bases.
The tendon reflexes are usually preserved.
- Amyotrophic Lateral Sclerosis in Physical Medicine and Rehabilitation
- Hyperthyroidism and Thyrotoxicosis
- Iatrogenic Cushing Syndrome
- Polymyalgia Rheumatica
- Rheumatoid Arthritis
- Systemic Lupus Erythematosus (SLE)
Routine Blood Tests
Muscle Enzyme Levels
Serum creatine kinase levels are elevated. Serum creatine kinase levels may be used to monitor myositis activity.
Other muscle enzymes that may be elevated are
- Lactic dehydrogenase
- Aspartate aminotransferase
- Alanine aminotransferase
The following antibody findings may exist in polymyositis
- Antinuclear antibody
- Positive in one-third of patients with polymyositis
- Myositis-specific antibodies
- Jo-1 antibodies
- Signal-recognition particle antibodies
Pulmonary function tests
For evaluation of interstitial lung disease may be appropriate.
Muscle-imaging techniques such as magnetic resonance imaging (MRI) and ultrasonography may be useful to document and localize the extent of muscle involvement. MRI scans show signal intensity abnormalities of muscle due to inflammation, edema, or scarring.
MRI scans may also be used to guide muscle biopsy and to monitor disease activity.
Chest radiography and high-resolution CT scanning of the chest are helpful for the evaluation of interstitial lung disease. CT scanning of the chest, abdomen, and pelvis is considered for the screening of associated malignancy. Chest radiography may also reveal evidence of associated malignancy.
Also consider mammography, pelvic ultrasonography, and upper and lower GI endoscopy in screening for associated malignancy.
Electromyographic findings are abnormal in almost all patients (90%) with polymyositis. Various abnormalities consistent with polymyositis may be found, depending on the stage of the disease
Muscle biopsy (eg, deltoid or quadriceps femoris) is crucial in helping to diagnose polymyositis and in excluding other rare muscle diseases
Muscle biopsy shows muscle fibers in varying stages of inflammation, necrosis, and regeneration.
Inclusion body myositis is histologically similar to polymyositis, with the additional presence of intracytoplasmic inclusion bodies observed on electron microscopy.
Treatment of Polymyositis
Prednisone 1 mg/kg/day is usually continued for 4-8 weeks, until the creatine kinase level returns to reference ranges. Then the steroid is tapered.
Read more about Side Effects of Corticosteroid Therapy
Immunosuppressive agents are indicated in patients who do not improve with steroids within a reasonable period (ie, 4 wk) or in whom adverse effects from corticosteroids develop.
Methotrexate is the most common agent used. Azathioprine, cyclophosphamide, chlorambucil, and cyclosporine have also been used.
Intravenous immunoglobulin is used for the short-term treatment of steroid-resistant cases of polymyositis.
Tacrolimus [the calcineurin inhibitor] is safe, and well-tolerated in patients with polymyositis that is refractory to other treatments.
A high protein diet may benefit these patients. Patients should take a supervised exercise program early in the disease course.
Dysphagia may be severe enough to require enteral feeding through a gastrostomy tube or parenteral nutrition.
In most patients, polymyositis responds well to treatment, although residual weakness occurs in approximately 30% of patients. Osteoporosis, a common complication of long-term corticosteroid therapy, may cause significant morbidity.
Poor prognostic factors are
- Advanced age
- Female sex
- Interstitial lung disease
- Presence of anti-Jo-1 and anti-SRP antibodies
- Associated malignancy
- Delayed or inadequate treatment
- Dysphagia, dysphonia
- Cardiac and pulmonary involvement
Complications of Polymyositis
Complications of polymyositis may include the following:
- Interstitial lung disease
- Aspiration pneumonia
- Cardiac complications
- Heart block
- Congestive heart failure
- Myocardial infarction
- Breast cancer or lung cancer
- Steroid myopathy and other complications of steroid treatment
- Zampieri S, Valente M, Adami N, et al. Polymyositis, dermatomyositis and malignancy: a further intriguing link. Autoimmun Rev. 2010 Apr. 9(6):449-53.
- Carroll MB, Newkirk MR, Sumner NS. Necrotizing Autoimmune Myopathy: A Unique Subset of Idiopathic Inflammatory Myopathy. J Clin Rheumatol. 2016 Oct. 22 (7):376-80.
- Schnabel A, Hellmich B, Gross WL. Interstitial lung disease in polymyositis and dermatomyositis. Curr Rheumatol Rep. 2005 Apr. 7(2):99-105.
- Cherin P, Pelletier S, Teixeira A, et al. Results and long-term followup of intravenous immunoglobulin infusions in chronic, refractory polymyositis: an open study with thirty-five adult patients. Arthritis Rheum. 2002 Feb. 46(2):467-74.
- Aggarwal R, Oddis CV. Therapeutic advances in myositis. Curr Opin Rheumatol. 2012 Nov. 24(6):635-41.