Differential Diagnosis in Neurology

12.15. Myopathy of Electrolyte Disorders

Hypokalemia 

• General Features:
  • Differential diagnosis:
    • Alcoholism
    • Severe prolonged vomiting
    • Post-surgery
    • Renal tubular acidosis
    • Bartter's syndrome
    • Laxative abuse
    • Amphotericin B
    • Hemodialysis
    • Ureterosigmoid diversion
    • Licorice (glyceric acid)
  • Weakness is manifest at a level of 2.0–3.0 meq/dl levels below 2.0 m Eq/dl may cause vacuolar myopathy and myoglobinuria
  • Reduced serum potassium:
    • Hyperpolarizes the muscle membrane which makes it inexcitable
    • Muscle necrosis putatively caused by relative ischemia
• Clinical Presentation:
  • Insidious proximal weakness that develops over hours to days
  • Cramps
  • Rare for diaphragmatic or bulbar muscles to be involved
  • There are no cognitive or sensory changes (muscles may feel tight)
  • Reflexes are depressed or absent; absent myotatic reflex (muscle is inexcitable)
  • Cardiac arrhythmia is a dangerous complication
  • Hypokalemia often coexists with other electrolyte abnormalities
  • Proximal muscles may take weeks to months to recover; in mild cases recovery occurs in 1–2 hours
• Laboratory evaluation:
  • Cardiac arrhythmia; prominent U waves
  • CK is usually normal
• EMG:
  • Myopathic
  • Occasional signs of muscle irritability
• Pathology (muscle biopsy):
  • Multiple vacuoles in some muscles (central in hereditary periodic paralysis; subsarcolemmal in acid maltase deficiency)
  • Differential diagnosis of vacuoles:
    • Severe SLE myopathy
    • Paraneoplastic myopathy
    • Hyperkalemic periodic paralysis
    • Alcoholic myopathy
    • Congential myotonia (rare)
    • Osteomalacic myopathy
    • Rapidly progressive dystrophies
    • Acid maltase deficiency

Hyperkalemia 

• General features:
  • In a general hospital setting hyperkalemic occurs with:
    • Potassium sparing diuretics
    • Addison's disease
    • Transfusion of old blood
    • Severe acidotic states
    • Muscle necrosis (trauma or ischemia)
    • Oral ingestion of potassium salts
  • Causes abnormal depolarization of the motor nerve
• Clinical presentation:
  • Rapid onset of paresis to plegia
  • No sensory or cognitive loss
  • May mimic GBS
  • Absent deep tendon reflexes
  • Sparing of cranial nerves; diaphragm and intercostal muscles may be involved

Hypernatremia

• General Features:
  • Most often hypernatremia does not affect muscle function
  • Sodium concentrations of 169–216 meq/dl may be symptomatic
  • Mechanism:
    • Possible depletion of intramuscular energy stores demonstrated by MR spectroscopy
  • Clinical Presentation:
    • Paralytic episodes may occur with high levels of sodium
    • Proximal weakness
    • Rare rhabdomyolysis
    • Myalgia
  • EMG:
    • Myopathic MUAP
    • Normal motor and sensory NCVs
  • Laboratory Evaluation:
    • Elevated CK
  • Pathology (muscle biopsy):
    • Normal muscle biopsy

Phosphorous 

• General features:
  • General setting in which phosphorous depletion occurs:
    • IV hyperalimentation
    • Diabetic ketoacidosis (<1 mg/dl)
    • Acute alcoholism (<1 mg/dl)
    • Burn patients
    • Antacids that bind phosphate
    • Chronic diarrhea
• Clinical Presentation:
  • Chronic proximal myopathy
  • Guillain–Barré syndrome
  • Parenteral alimentation deficits (severe < 1 mg/dl):
    • Paraesthesias around the mouth and extremities
    • Weakness and areflexia
    • Disorientation and seizures
    • Oculomotor paresis and ptosis