What are the diagnostic tests for hypokalemic (low potassium levels) periodic paralysis?

Diagnostic Tests for Hypokalemic Periodic Paralysis

The primary diagnostic test for hypokalemic periodic paralysis is serum potassium measurement during an acute attack, which typically reveals levels below 3.5 mEq/L, with severe cases showing levels below 2.5 mEq/L. 1, 2

Initial Laboratory Evaluation

• Serum potassium level: Essential during an acute attack to document hypokalemia (<3.5 mEq/L) 2
• Comprehensive metabolic panel: To assess:
  • Renal function (BUN, creatinine)
  • Electrolytes (sodium, calcium, magnesium, phosphate)
  • Glucose levels 1
• Thyroid function tests: To rule out thyrotoxic hypokalemic periodic paralysis 1, 3
• Electrocardiogram (ECG): To detect cardiac abnormalities associated with hypokalemia (U waves, ST depression, T wave flattening) 1

Provocative Testing

In patients with suspected hypokalemic periodic paralysis but normal potassium levels between attacks:

• Glucose challenge test: Administration of glucose and insulin can provoke an attack by stimulating potassium shift into cells 4
  • CAUTION: Must be performed under close supervision with cardiac monitoring and immediate access to potassium replacement
  • Contraindicated in patients with severe hypokalemia or cardiac conditions

Genetic Testing

• DNA sequencing for mutations in:
  • CACNA1S gene (calcium channel): Most common cause (60.5% of cases) 5
  • SCN4A gene (sodium channel): Less common but important to identify 5
  • Genetic testing helps differentiate familial from acquired forms and guides treatment selection

Electrophysiological Studies

• Electromyography (EMG): May show reduced compound muscle action potential amplitudes during attacks
• Exercise test: Short exercise followed by rest may demonstrate characteristic decline in compound muscle action potential

Differential Diagnosis Testing

Additional tests to rule out other causes of hypokalemia and weakness:

• Urinary potassium excretion: To differentiate renal from non-renal potassium losses
• Arterial blood gas: To assess acid-base status 4
• Creatine kinase: To check for muscle damage

Monitoring During Treatment

• Serial potassium measurements: Every 1-2 hours during acute treatment and within 1-2 days after starting oral replacement therapy 2
• Continuous cardiac monitoring: During intravenous potassium replacement, especially in severe hypokalemia (<2.5 mEq/L) 2, 3

Important Considerations

• Avoid glucose-containing solutions when administering IV potassium, as this can worsen hypokalemia and weakness 6
• Monitor for rebound hyperkalemia during treatment, particularly in thyrotoxic periodic paralysis 3
• Consider testing family members of affected individuals due to the hereditary nature of familial forms

Pitfalls to Avoid

• Failing to measure potassium during an acute attack (may be normal between attacks)
• Misdiagnosing as conversion disorder or malingering
• Using glucose-containing IV fluids for potassium replacement 6
• Overlooking thyroid function testing, as hyperthyroidism is a common trigger 4, 3
• Delaying treatment while awaiting genetic confirmation in clear clinical cases

By following this diagnostic approach, clinicians can accurately diagnose hypokalemic periodic paralysis and initiate appropriate treatment to prevent potentially fatal complications affecting cardiac and respiratory function.