Polymyositis

Background

Idiopathic inflammatory myopathy causing symmetric proximal muscle weakness, elevated CK,and characteristic EMG findings

Clinical Features

• Symmetrical proximal muscle weakness with insidious onset
• Generally painless (though 30% have myalgia)
• Difficulty with kneeling, climbing stairs, combing hair, or rising from a seated position
• Weak neck extensors causing difficulty of holding head up
• Associated arthralgias

Differential Diagnosis

• Hypokalemia
• Hypophosphatemia
• ALS
• Thyroid disorder (hypothyroidism or hyperthyroidism)
• Myastenia Gravis
• Myopathies
• Inclusion body myositis
• Drug Induced myopathies (EtOH, antimalarials, colchicine, antifungals)

Weakness

• Neuromuscular weakness
  • Upper motor neuron:
    • CVA
    • Hemorrhagic stroke
    • Multiple sclerosis
    • Amyotrophic Lateral Sclerosis (ALS) (upper and lower motor neuron)
  • Lower motor neuron:
    • Spinal and bulbar muscular atrophy (Kennedy's syndrome)
  • Spinal cord disease:
    • Infection (Epidural abscess)
    • Infarction/ischemia
    • Trauma (Spinal Cord Syndromes)
    • Inflammation (Transverse Myelitis)
      • Acute flaccid myelitis
    • Degenerative (Spinal muscular atrophy)
    • Tumor
  • Peripheral nerve disease:
    • Guillain-Barre syndrome
    • Toxins (Ciguatera)
    • Tick paralysis
    • Diabetic peripheral neuropathy
  • Neuromuscular junction disease:
    • Myasthenia gravis crisis
    • Botulism
    • Organophosphate toxicity
    • Lambert-Eaton myasthenic syndrome
  • Muscle disease:
    • Rhabdomyolysis
    • Dermatomyositis
    • Polymyositis
    • Alcoholic myopathy
• Non-neuromuscular weakness
  • Can't miss diagnoses:
    • ACS
    • Arrhythmia/Syncope
    • Severe infection/Sepsis
    • Hypoglycemia
    • Periodic paralysis (electrolyte disturbance, K, Mg, Ca)
      • Hypokalemic periodic paralysis
      • Thyrotoxic periodic paralysis
    • Respiratory failure
  • Emergent Diagnoses:
    • Symptomatic Anemia
    • Severe dehydration
    • Hypothyroidism
    • Polypharmacy
    • Malignancy
    • Aortic disease - occlusion, stenosis, dissection
  • Other causes of weakness and paralysis
    • Acute intermittent porphyria (ascending weakness)

Diagnosis

• CBC, ESR, CRP, CK, UA, RF, ANA, Anti-Jo-1

Management

• Prednisone 1mg/kg/day for 4-8 weeks until CK returns to reference range
• Followed by prednisone taper
• Methotrexate as second line for poor response to corticosteroids
• Other agents with less evidence: IVIG, TNF Inhibitors

Disposition

• Rheumatology or neurology consultation either in ED or as outpatient depending on severity of symptoms

References

Tintinalli