How to diagnose acute hyponatremia?

Diagnosis of Acute Hyponatremia

The diagnosis of acute hyponatremia requires assessment of serum osmolality, volume status, and urine studies to determine the underlying cause, with acute cases defined as developing within 48 hours. 1

Initial Assessment

• Hyponatremia is defined as serum sodium <135 mEq/L and should be evaluated based on volume status and serum osmolality 1, 2
• Determine acuity of onset: acute (<48 hours) versus chronic (>48 hours) 1
• Acute hyponatremia can be defined when the rate of decrease of serum sodium exceeds 0.5 mmol/L/hour 3
• Acute hyponatremia can be presumed if large quantities of fluid were ingested or administered parenterally within 2-3 days, especially hypotonic fluids with impaired water excretion 3

Laboratory Evaluation

• Measure serum osmolality to distinguish between hypotonic, isotonic, and hypertonic hyponatremia 1, 4
• Obtain urine osmolality and urine sodium to help determine the cause 1
• Check serum uric acid and blood urea nitrogen, as hypouricemia and hypouremia with increased fractional excretions of uric acid and urea suggest acute hyponatremia 5
• Essential diagnostic criteria for SIADH include:
  • Hyponatremia (serum sodium < 134 mEq/L) 6
  • Hypoosmolality (plasma osmolality < 275 mosm/kg) 6
  • Inappropriately high urine osmolality (> 500 mosm/kg) 6
  • Inappropriately high urinary sodium concentration (> 20 mEq/L) 6
  • Absence of hypothyroidism, adrenal insufficiency, or volume depletion 6

Volume Status Assessment

• Evaluate extracellular fluid volume status to categorize as hypovolemic, euvolemic, or hypervolemic hyponatremia 1
• Hypovolemic hyponatremia: signs of dehydration, orthostatic hypotension, decreased skin turgor 1
• Euvolemic hyponatremia: no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1
• Hypervolemic hyponatremia: edema, ascites, signs of volume overload 1

Urine Studies Interpretation

• Urine osmolality <100 mOsm/kg indicates complete suppression of ADH and suggests primary polydipsia 7
• Urine sodium <20 mmol/L suggests hypovolemic hyponatremia 6, 7
• Urine sodium >40 mmol/L with high urine osmolality suggests SIADH 6, 1
• By assessing the effective arterial blood volume with the fractional excretion of urate, the accuracy of a diagnostic algorithm for SIADH can approach 95% 6

Common Causes of Acute Hyponatremia

• Excessive water intake (primary polydipsia) 5
• Medication-induced: diuretics, oxytocin, neuroleptic agents 5
• Iatrogenic: hypotonic fluid administration 5
• Recreational drugs (e.g., ecstasy) 5
• Polyethylene glycol (PEG) preparation for colonoscopy 5
• Weight-reducing herbal teas 5

Special Considerations

• Acute hyponatremia requires immediate treatment to prevent severe neurologic damage or death 3, 8
• Distinguishing between SIADH and cerebral salt wasting is critical in neurosurgical patients 1
• Paraneoplastic hyponatremia secondary to elevated atrial natriuretic peptide should be considered in the differential diagnosis 6
• Approximately 10% to 45% of SCLC and 1% of other lung cancer cases produce arginine vasopressin, but only 1% to 5% of patients with lung cancer have symptomatic SIADH 6

Common Pitfalls to Avoid

• Failing to assess volume status accurately, which is essential for differentiating causes of hyponatremia 1
• Not considering medication history, which can provide crucial clues to the etiology 5
• Misdiagnosing the volume status in heart failure patients with hyponatremia can lead to inappropriate treatment 1
• Ignoring mild hyponatremia (135 mmol/L) as clinically insignificant 1

Remember that acute hyponatremia requires immediate attention and treatment to prevent neurological complications, with the approach guided by symptom severity and underlying cause.