Overview
Definition
Polymyositis (PM) is an autoimmune inflammatory myopathy that presents as symmetric proximal muscle weakness. Antisynthetase syndrome is a subtype that shows the presence of antisynthetase antibodies and certain extramuscular manifestations.
Etiology
The exact etiology is unknown.
Possible genetic predisposition:
- HLA-A1
- HLA-B8
- HLA-DR3
Environmental factors:
- Infectious:
- HIV
- Human T cell lymphotropic virus type-1 (HTLV-1)
- Hepatitis B and C
- Influenza
- Coxsackie B virus
- Drug induced:
- ACE inhibitors
- D-penicillamine
- Hydralazine
- Procainamide
- Phenytoin
Associated medical conditions:
- Malignancy
- Connective-tissue disorders
Pathophysiology
- Unclear triggering event → activation of CD8 T cells and macrophages
- Inflammatory cells infiltrate healthy muscle fibers.
- Cytotoxic response to unspecified muscle antigens → direct cellular damage to the endomysial layer of skeletal muscles → necrosis of muscle fibers
Clinical Presentation
Symptoms
- Muscle weakness:
- Symmetric
- Develops gradually over weeks to months
- Particularly evident when:
- Climbing stairs
- Brushing hair
- Standing up from a seated position
- Reaching for overhead objects
- Holding up the head
- Associated myalgias occur in a few patients.
- Arthralgias or arthritis:
- Symmetric
- Joints involved:
- Knees
- Wrists
- Hands
- Usually associated with antisynthetase syndrome
- Constitutional symptoms:
- Anorexia
- Weight loss
- Fatigue
- Low-grade fever
Physical examination
- Reduced muscle strength:
- Affects proximal muscle groups:
- Deltoid
- Hip flexors and extensors
- Neck extensors → dropped head
- Late findings:
- Muscle trophy
- Loss of deep-tendon reflexes (due to severely weakened or atrophied muscles)
- Affects proximal muscle groups:
- Sensory examination is normal.
- Raynaud’s phenomenon may be present.
- There is usually no cutaneous involvement. Exception: “mechanic’s hands”:
- Hyperkeratotic eruptions over the finger pads and lateral aspects of fingers
- Associated with antisynthetase syndrome
“Dropped head” in polymyositis: caused by weakness in the extensor muscles of the neck
Image by Lecturio.
“Mechanic’s hands” associated with antisynthetase antibodies in polymyositis: hyperkeratosis resulting in fissures, roughness, and scaling on the pad of the thumb and the lateral aspect of the index finger
Image: “Patient 3” by Department of Respiratory Medicine, Kyorin University School of Medicine, 6-20-2 Shinkawa, Mitaka City, Tokyo 181-8611, Japan. License: CC BY 2.0
Systemic manifestations
- Respiratory:
- Interstitial lung disease (5%–30% of cases)
- Dry cough
- Dyspnea
- Bilateral crackles
- Seen with anti-synthetase syndrome
- Diaphragm and chest-wall muscle weakness
- Respiratory insufficiency
- Interstitial lung disease (5%–30% of cases)
- GI: pharyngeal and esophageal involvement (approximately 30% of patients)
- Dysphagia
- Dysphonia
- Gastroesophageal reflux
- Cardiovascular (rare):
- Abnormalities in atrioventricular conduction
- Myocarditis
- Dilated cardiomyopathy
Diagnosis
Diagnostic workup
- Initial laboratory tests:
- CBC
- Leukocytosis
- Thrombocytosis
- Inflammatory markers
- ↑ Erythrocyte sedimentation rate (ESR)
- ↑ CRP
- Muscle enzymes
- ↑↑ CK
- ↑ Aldolase
- ↑ AST and ALT
- ↑ LDH
- ANA positivity
- Nonspecific and not diagnostic
- Seen in ⅓ of patients
- CBC
- Followed up based on myositis-specific antibody testing:
- Anti-signal recognition protein (anti-SRP) antibodies
- Specific for inflammatory myopathies
- Associated with severe myositis
- Anti-Jo-1 antibodies
- Anti-histidyl tRNA synthetase antibody
- Seen in anti-synthetase syndrome
- Occurs in 20%–30% of patients
- Anti-signal recognition protein (anti-SRP) antibodies
- Additional studies if the diagnosis is unclear due to nonspecific or atypical findings:
- Electromyography (EMG):
- Consists of nerve-conduction studies, including repetitive nerve stimulation and needle examination of muscles
- Altered in 90% of cases
- Common findings: ↑ insertional activity, spontaneous fibrillation, short-duration motor unit potential, and complex repetitive discharges
- Can support the diagnosis, but is not diagnostic
- MRI of skeletal muscles:
- Indicates muscle inflammation, edema, fibrosis, and calcification
- Nonspecific, and cannot distinguish between rhabdomyolysis, muscular dystrophy, or metabolic myopathies
- Muscle biopsy:
- Most accurate test to confirm the diagnosis
- Findings: endomysial invasion of lymphocytes, areas of necrosis with some regeneration, fibrosis
- Electromyography (EMG):
Histological findings in polymyositis:
Muscle biopsy from a patient with polymyositis showing endomysial inflammatory cells (arrows) and variations in fiber size
Electromyography findings in polymyositis: comparison chart showing the differences in EMG activity for different pathologies
Image by Lecturio.Evaluation of systemic manifestations
- Interstitial lung disease:
- Chest radiography
- Pulmonary function tests
- Dysphagia:
- Barium swallow
- Esophageal manometry
- Conduction abnormalities: ECG
- Myocarditis or cardiomyopathy: echocardiography
Screening for malignancy
- Patients diagnosed with PM should be evaluated for possible underlying malignancy.
- Workup is based on age and sex of the patient.
- Initial workup:
- Mammography
- Pap smear
- Colonoscopy
- CT of thorax, abdomen, and pelvis
- Additional studies ordered based on risk factors, symptoms, or abnormal initial workup:
- Cancer antigen 125 (CA 125)
- Transvaginal ultrasound (for women at high risk of ovarian cancer)
- Upper endoscopy (for esophageal cancer)
- MRI
- PET
Management
A multidisciplinary approach including pharmacological and non-pharmacological therapies is used in the management of PM.
- Rheumatology consultation
- Medical therapy:
- Systemic glucocorticoids (1st line)
- 50% of patients with PM will not respond to steroids alone.
- Therapy is usually needed for 9–12 months.
- Slowly tapered off
- Levels of muscle enzymes are monitored to determine response to therapy.
- Immunosuppressive therapy:
- Methotrexate, azathioprine
- Used in conjunction with glucocorticoid therapy
- IV immunoglobulin (IVIG):
- Used in patients with severe life-threatening weakness (e.g., severe dysphagia, respiratory insufficiency)
- Provides more rapid onset of action than steroids
- For recurrent or refractory disease:
- Rituximab
- Mycophenolate mofetil
- Tacrolimus
- Systemic glucocorticoids (1st line)
- Physiotherapy to maintain and improve muscular function
- Management of dysphagia:
- Speech therapy
- Elevate the head of the bed.
- Diet modifications
- Enteral feeding for patients with severe dysphagia
- Prophylaxis (while on long-term corticosteroids):
- Osteoporosis:
- Calcium and vitamin D supplementation
- Bisphosphonates for high-risk patients and individuals with osteoporosis
- Pneumocystis jirovecii: trimethoprim-sulfamethoxazole
- Appropriate immunization
- Osteoporosis:
Complications and Prognosis
Complications
- Malignancy:
- Usually detected within the 1st year of diagnosis
- Incidence lower than in dermatomyositis
- Observed in men > 60 years of age with severe systemic involvement
- Most common: non-Hodgkin’s lymphoma, and bladder, lung, breast, ovarian, stomach, pancreatic, prostate, and colorectal cancer
- Respiratory complications:
- Interstitial lung disease → pulmonary hypertension and cor pulmonale
- Aspiration pneumonia due to dysphagia
- Cardiovascular involvement (rare):
- Conduction abnormalities → arrhythmia
- Myocarditis or dilated cardiomyopathy → heart failure
- ↑ Risk of myocardial infarction and thromboembolism
- Adverse effects of medical therapy:
- Corticosteroid therapy:
- Osteoporosis and compression fractures
- Insulin resistance → diabetes mellitus
- Hypertension
- Dyslipidemia
- Weight gain
- Growth delay in children
- Steroid-induced myopathy
- Immunosuppression therapy:
- Opportunistic infections
- Upper respiratory tract infections and pneumonia
- Skin infections
- Bacteremia and sepsis
- Corticosteroid therapy:
Prognosis
- PM is a chronic illness.
- Most patients are highly responsive to medical therapy.
- 30% of patients may have residual weakness.
- Mortality:
- 5-year survival rate > 80%
- Often related to:
- Malignancy
- Pulmonary complications
- Cardiac involvement
- Aspiration
Differential Diagnosis
- Dermatomyositis (DM): an autoimmune inflammatory myopathy resulting from immune-complex deposition in the muscle capillaries. Patients present with symptoms similar to those observed in PM, and also exhibit characteristic cutaneous manifestations (e.g., heliotrope rash, Gottron’s sign). Diagnosis is based on clinical presentation and antibody evaluation, and may be confirmed with biopsy. Management includes systemic glucocorticoids, immunosuppressants, and screening for malignancy.
- Inclusion body myositis (IBM): an inflammatory myopathy characterized by slowly progressing muscle weakness. However, the weakness is asymmetrical and involves distal muscles, especially those of the hands. Physical examination reveals significant muscle atrophy. Diagnosis is confirmed based on biopsy, which shows inclusion bodies within the muscle tissue. Inclusion body myositis is refractory to steroids and must be excluded in refractory cases of PM.
- Polymyalgia rheumatica: an inflammatory condition that affects adults > 55 years of age. Patients present with pain and stiffnesses of the proximal muscles. There is no muscle weakness or atrophy. Diagnosis is clinical, revealing findings of elevated inflammatory markers. Management is with corticosteroids. Patients should be evaluated for temporal arteritis.
- Drug-induced myopathy: Certain drugs (e.g., statins) can cause myopathy and myotoxicity, which can result in muscle weakness and myalgias. The CK levels are also elevated. Patient history and a review of medications can lead to the diagnosis. Management includes withdrawal of the offending medication. Treatment with steroids may be necessary.
- Hypothyroidism: a thyroid-hormone deficiency caused by thyroid, hypothalamic, or pituitary disorders. Proximal muscle weakness is a common presenting complaint. Patients usually exhibit multiple other systemic manifestations, including intolerance to cold conditions, neuropsychiatric changes, dry skin, constipation, and bradycardia. Thyroid function tests can aid in the diagnosis. Management is with thyroid hormone replacement.
- Cushing’s syndrome: an endocrine disorder characterized by chronic exposure to exogenous or endogenous corticosteroids. Patients with Cushing’s syndrome may display proximal muscle weakness due to atrophy of the gluteal and upper-leg muscles. The characteristic truncal obesity, moon face, and easy bruising may also be present. Diagnosis is based on clinical history and cortisol studies. Management depends on the underlying cause.
References
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