What is the appropriate diagnosis term for hyponatremia?

Hyponatremia: Appropriate Diagnostic Terminology

Hyponatremia is defined as a serum sodium concentration less than 135 mEq/L (or mmol/L), and this is the universally accepted diagnostic term and threshold. 1, 2, 3

Diagnostic Classification Framework

Once hyponatremia is identified, the diagnosis should be further refined using a systematic classification approach:

Severity-Based Classification

• Mild hyponatremia: Serum sodium 130-134 mEq/L (or 126-135 mEq/L by some definitions) 1, 3
• Moderate hyponatremia: Serum sodium 120-129 mEq/L (or 125-129 mEq/L) 1, 3
• Severe hyponatremia: Serum sodium less than 120 mEq/L (or <125 mEq/L) 1, 3

Volume Status Classification

The diagnostic term should specify the patient's extracellular fluid volume status, as this fundamentally guides treatment 1, 4:

• Hypovolemic hyponatremia: Characterized by ECF contraction, orthostatic hypotension, dry mucous membranes, decreased skin turgor, and typically urine sodium <30 mmol/L 1, 4
• Euvolemic hyponatremia: No clinical signs of volume depletion or overload, normal skin turgor, moist mucous membranes, no edema 1, 4
• Hypervolemic hyponatremia: Presence of peripheral edema, ascites, jugular venous distention, or pulmonary congestion 1, 4

Temporal Classification

• Acute hyponatremia: Onset less than 48 hours 1, 5
• Chronic hyponatremia: Onset greater than 48 hours 1, 5

This temporal distinction is critical because acute hyponatremia causes more severe symptoms at the same sodium level and can be corrected more rapidly without risk of osmotic demyelination syndrome 6, 5.

Osmolality-Based Classification

After confirming true hyponatremia, measure serum osmolality to exclude pseudohyponatremia 1, 4:

• Hypotonic (hypoosmolar) hyponatremia: Serum osmolality <275 mOsm/kg—this represents true hyponatremia requiring treatment 1
• Isotonic hyponatremia (pseudohyponatremia): Normal serum osmolality (275-290 mOsm/kg)—caused by hyperlipidemia or hyperproteinemia 4
• Hypertonic hyponatremia: Elevated serum osmolality—caused by hyperglycemia or mannitol administration 4

Etiologic Diagnostic Terms

Once classified by volume status, the specific underlying cause should be identified in the diagnostic terminology 1:

For euvolemic hyponatremia:

• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)—characterized by urine sodium >20-40 mmol/L, urine osmolality >300 mOsm/kg, and serum uric acid <4 mg/dL 1, 7
• Cerebral salt wasting (CSW)—particularly in neurosurgical patients, distinguished from SIADH by true hypovolemia with CVP <6 cm H₂O despite elevated urine sodium 1

For hypervolemic hyponatremia:

• Heart failure-associated hyponatremia 1, 4
• Cirrhosis-associated hyponatremia 1, 4
• Nephrotic syndrome-associated hyponatremia 4

For hypovolemic hyponatremia:

• Diuretic-induced hyponatremia 1, 4
• Gastrointestinal loss-associated hyponatremia 1, 4
• Renal salt-wasting hyponatremia 1

Symptom-Based Diagnostic Modifiers

The diagnostic term should include symptom severity, as this determines urgency of treatment 1, 3:

• Asymptomatic hyponatremia: No neurological symptoms 1
• Mildly symptomatic hyponatremia: Nausea, vomiting, headache, weakness, mild cognitive deficits 6, 3
• Severely symptomatic hyponatremia: Confusion, delirium, seizures, coma, altered consciousness, respiratory distress 6, 3

Critical Diagnostic Threshold

Hyponatremia warrants full diagnostic workup when serum sodium falls below 131 mmol/L, though even mild hyponatremia (130-135 mmol/L) should not be ignored as it increases fall risk and mortality 60-fold. 1, 6, 2

Common Diagnostic Pitfall

Never use the term "asymptomatic hyponatremia" to dismiss mild chronic hyponatremia (130-135 mEq/L) as clinically insignificant—even this range is associated with cognitive impairment, gait disturbances, increased falls (23.8% vs 16.4%), fractures, and significantly increased hospital mortality (11.2% vs 0.19%). 6, 2