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Polymyositis

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Overview

Polymyositis (poly – multiple, myos- muscle, -itis – inflammation) is a connective tissue disease characterised by autoimmune inflammation of muscles. It may be idiopathic or associated with connective tissue disease or malignancy.

Epidemiology

  • Tends to present between 30 and 60 years of age
  • Twice as common in women as in men

Risk Factors

  • Female sex
  • Black ethnicity
  • UV radiation
  • Malignancy

Presentation

The main feature of polymyositis is proximal muscle weakness:

  • Onset is over weeks or months and progression is steady
  • Diffuse weakness in the proximal muscles
  • Distal muscles are usually spared
    • They are usually affected late in the disease
  • Pharyngeal weakness may cause dysphagia or dysphonia
  • There are no skin manifestations
  • 1/3 of patients have pain
  • Extraocular muscles are unaffected
  • Patients may have difficulty holding their head up

Signs on Examination

  • Muscles may be tender on palpation and may feel nodular or grainy
  • Muscular atrophy may be present with preserved tendon reflexes and normal sensation

Investigations

  • Serum creatine kinase (CK):
    • Elevated
  • Serum aldolase:
    • Elevated
    • Lower sensitivity than creatine kinase (CK)
  • Muscle biopsy – may show:
    • Perivascular or inter-fascicular inflammation
    • Endothelial hyperplasia in the intramuscular blood vessels
    • Perifascicular atrophy
  • EMG – not essential for diagnosis if CK and muscle biopsy findings are positive:
    • May show reduced amplitude and fibrillation
  • Anti-Jo-1 antibodies:
    • Positive in polymyositis

Management

Patients should be screened for malignancy.

For acute flares or severe disease:

  • IV methylprednisolone
  • IV immunoglobulin (IVIG)
  • Immunosuppression e.g. methotrexate or azathioprine

Ongoing management:

  • 1st line: oral prednisolone
  • 2nd line: methotrexate or azathioprine
  • 3rd line: other immunosuppressants e.g. ciclosporin

Monitoring

  • Patients should have regular reviews to monitor muscle strength and biomarkers of disease activity
  • Patient should be assessed for the development of cardiac or pulmonary involvement
  • Patients should be monitored for malignancy
    • Screening should be done at presentation and if no malignancy is found, annually for 3 years following diagnosis
  • Patients should be monitored for drug toxicity and adverse effects

Patient Advice

  • Patients should use high-factor sunscreen to protect themselves from UV radiation
  • Patients should participate in physiotherapy
  • Patients should be counselled on adverse effects and monitoring of the drugs involved in their treatment

Complications

  • Respiratory tract infections
    • Due to respiratory muscle weakness
  • Interstitial lung disease
  • Cardiac disease
  • Dysphagia
  • Malignancy
  • Cutaneous calcinosis

Prognosis

  • Associated malignancy indicates a poorer prognosis
  • Around 20-40% of patients will achieve remission
  • Mortality is 2-3 times higher than the general population with cancer, lung, and cardiac complications and infections being the most common causes of death

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