MUSCLE WEAKNESS CAUSES

Created: Monday, November 12, 2007

Examination

• Symmetrical proximal muscle weakness
• Muscle tenderness very rare with myopathy
• Fever with inflammatory causes
• There is usually no wasting but there may be hypertrophy of muscle (atrophy is a late sign)
• Reflexes and sensation usually normal
• Hypotonia is common in some myopathies (for example congenital myopathies)
• There may be helpful additional signs such as the skin changes of dermatomyositis
• Urine should be examined- myoglobinuria in acute alcoholic myopathy can cause renal tubular necrosis.

This list includes other conditions causing weakness:

• Guillain-Barré Syndrome
• Eaton-Lambert Myasthenic Syndrome
• Myasthenia Gravis
• Cerebral Palsy
• Spinal Muscular Atrophy
• Congenital hypomyelinating neuropathies.

• It may be difficult to distinguish myopathy from peripheral neuropathy. The distinguishing clinical features of peripheral neuropathy are:
  • Weakness affects distal muscles- although there are exceptions:
    • Myopathy where distal muscle groups are affected (myotonic dystrophy, myopathy of Welander)
    • Peripheral neuropathIies which affect proximal muscles (diabetic amyotrophy,motor neurone disease).
  • Reduced muscle- stretch reflexes
  • Fasciculations
  • Somatosensory abnormalities.
• Some complex cases may have both neurogenic and myopathic disorders which can lead to diagnostic confusion:
  • Diabetes mellitus can cause both neuropathy and inflammatory myopathy
  • Cancer can cause dermatomyositis and chemotherapy peripheral neuropathy in the same patient
  • Radiculopathy (from degenerative disc disease) can occur in patients with myopathy.

Blood and urine tests

These together with ECG examination are most useful in acute situations.

• Creatinine kinase (with isoenzymes)
• Electrolytes including calcium and magnesium
• Serum myoglobin
• Urea and serum creatinine
• Urinalysis and urine microscopy- myoglobinuria inferred by positive urinalysis with few red cells at microscopy
• Full blood count
• ESR
• Thyroid function tests
• Antinuclear antibodies.