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26th January 2024

Differential Diagnoses

  • Muscular dystrophies may present similarly to inflammatory myopathy. However, features such as calf hypertrophy, winging of the scapular would point more to a diagnosis of muscular dystrophy, which can be confirmed by muscle biopsy. 
  • Myasthenia gravis may also have similar features, such as muscle weakness and dysphagia. However, symptoms in MG tend to be intermittent and physical examination findings would show muscle fatigability, whereas those of myositis would have a more progressive or insidious onset. Laboratory testing for autoantibodies and electrophysiology (EMG) studies may help distinguish between both. 
  • Drug-induced myopathy can mimic inflammatory myopathies, and as such, a thorough history needs to be taken to determine whether the patient is on any drugs that may possibly trigger this, including glucocorticoids, statins, antimalarials, Penicillamine, anti-retroviral therapies, Colchicine, alcohol or cocaine. If the diagnosis remains unclear, then an EMG and/or muscle biopsy may aid in making a definitive diagnosis. 
  • Inherited metabolic myopathies (disorders of carbohydrate and lipid metabolism) – patients tend to develop chronic muscle weakness. However, this would be preceded by years of acute episodes of intermittent muscle pain and tenderness induced by exertion followed by myoglobinuria, which is not a finding in inflammatory myopathies. 
  • Motor Neuron disease (Amyotrophic Lateral Sclerosis) can present with muscle weakness but will also have features of hyperreflexia and muscle spasticity (long-tract signs), unlike myositis. Investigations such as EMG would not show myopathic changes, and muscle enzymes such as creatinine kinase will be normal or only mildly elevated. 

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