Introduction

The main feature of muscle disorders is weakness without reflex loss. This helps us to differentiate these disorders from:
  • UMN lesion – weakness with increased reflexes
  • LMN lesion – weakness with decreased reflexes

Investigating Muscle Disorders

Distribution pattern of:

  • weakness
  • Wasting
  • Hypertrophy
  • Consistency

Serum muscle enzymesserum creatinine phosphokinase is the most prominent of these. Levels of this will be raised in dystrophies, and inflammatory conditions, but will be normal in myasthenia gravis, LEMS, myotonias, and other innervations problems.
Genetic testing of family members can be useful for some inherited conditions
Electromyography – in weakness, the amplitude is likely to be reduced. There are also characteristic signs for other conditions:

  • Myopathy – short duration, polyphasic action potentials. Sometimes there may also be spontaneous fibrillation
  • Denervation –fibrillation
  • Myotonic discharge –high frequency ‘whine’
  • Myasthenia gravis – gradually reducing amplitude – known as decrement
  • LEMS –gradually increasing amplitude – known as increment

Muscle biopsy – again, this can reveal specific abnormalities for the different disorders:

  • Denervation – atrophy/hypertrophy of particular fibre groups
  • Dystrophy and myositisdiffuse abnormal cells – nuclei become central, there may be necrosis, and there is invasion of the tissue by inflammatory cells

MRI – may show changes in some cases of myositis

Examples of Muscle Disorders

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