What is Polymyositis?
Polymyositis (PM) is a rare, chronic, idiopathic inflammatory myopathy characterised by immune-mediated inflammation of skeletal muscle, resulting in predominantly proximal muscle weakness. Unlike Dermatomyositis, Polymyositis lacks the characteristic skin manifestations. It is a diagnosis of exclusion — made after other causes of proximal muscle weakness (hypothyroidism, inclusion body myositis, muscular dystrophies, drug-induced myopathy) have been systematically excluded.
Modern reclassification of inflammatory myopathies has led many experts to question the entity of "pure" Polymyositis, recognising that many previously diagnosed PM cases actually represent Anti-Synthetase Syndrome, IMNM (Immune-Mediated Necrotising Myopathy), or Overlap Myositis — subtypes with distinct antibody profiles, different prognoses, and specific treatment implications. This makes comprehensive myositis-specific antibody (MSA) testing absolutely essential.
Difficulty rising from a chair without using arms — a clinical hallmark of proximal hip and thigh muscle weakness in Polymyositis.
Symptoms: The Progressive Weakness Syndrome
The onset of PM is typically subacute — developing over weeks to months rather than suddenly. The clinical features include:
- Symmetric proximal muscle weakness: Difficulty rising from a chair or low seat, climbing stairs, lifting objects above the head, combing hair, carrying shopping bags.
- Dysphagia: Difficulty swallowing, particularly liquids, from pharyngeal muscle weakness — a serious symptom indicating risk of aspiration pneumonia.
- Dysphonia: Hoarseness or a nasal voice quality from laryngeal muscle involvement.
- Respiratory muscle weakness: Breathlessness, reduced exercise tolerance, and in severe cases, respiratory failure.
- Myalgia: Muscle aching and tenderness, though often not as prominent as the weakness.
- Constitutional features: Fatigue, fever, and weight loss.
- Interstitial Lung Disease: Present in 30-50% of Anti-Synthetase Syndrome (a subtype of PM/DM), causing progressive breathlessness and dry cough.
Muscle MRI showing STIR signal enhancement (oedema) in inflamed muscles — a non-invasive tool for diagnosis and monitoring of inflammatory myopathies.
💊 Statin-Induced Myopathy vs. Immune-Mediated Necrotising Myopathy (IMNM)
Statin medications commonly cause mild myalgia, but can rarely trigger Immune-Mediated Necrotising Myopathy (IMNM) — characterised by very high CK levels, severe proximal weakness, and anti-HMGCR or anti-SRP antibodies. IMNM requires immunosuppression, not just statin withdrawal. If you are on a statin and develop progressive muscle weakness, please seek specialist evaluation.
Diagnostic Investigations
A systematic workup is required to confirm the diagnosis and identify the specific inflammatory myopathy subtype:
- Creatine Kinase (CK): Usually markedly elevated (10-50x the upper limit of normal in acute PM). Also LDH, AST, and aldolase.
- MSA/MAA Panel: Essential. Identifies specific autoantibodies (e.g., anti-Jo-1 for Anti-Synthetase Syndrome, anti-SRP for IMNM, anti-MDA5 for CADM-ILD).
- Electromyography (EMG): Shows a myopathic pattern — short-duration, small-amplitude, polyphasic motor unit potentials with spontaneous activity.
- Muscle MRI: STIR sequence identifies muscle oedema and fascial inflammation, guiding the optimal biopsy site.
- Muscle Biopsy: The gold standard for definitive diagnosis — identifies perimysial, endomysial, or perivascular inflammatory infiltrate patterns.
Dr. Prateek's Approach to Polymyositis
The key to excellent outcomes in PM is precise MSA-guided diagnosis, aggressive early immunosuppression, and vigilant monitoring for pulmonary and swallowing complications. Dr. Prateek Deo's training in complex myositis management at PGIMER ensures the highest standard of care.
MSA-Guided Diagnosis
Comprehensive antibody panel testing to identify the specific myopathy subtype — enabling prognostication and subtype-specific treatment decisions from day one.
Physiotherapy Integration
Structured physiotherapy and graded exercise rehabilitation, supervised by specialists, is a core component of PM management alongside immunosuppression.
Swallow Assessment
All PM patients with dysphagia are referred for a videofluoroscopic swallow study and SALT assessment to prevent aspiration pneumonia — a major cause of morbidity.
ILD Surveillance
Baseline HRCT chest and pulmonary function tests for all PM patients, with repeat testing in those with anti-Jo-1 or anti-MDA5 antibodies where ILD risk is highest.