SHM's Clinical Quick Talks-Hyponatremia

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This document provides a concise yet comprehensive overview of hyponatremia, focusing on its pathophysiology, diagnostic approach, and clinical considerations. Hyponatremia is defined as a serum sodium level below 135 mEq/L, corrected for hyperglycemia. Key regulators involved include the renin-angiotensin-aldosterone system (RAAS) and antidiuretic hormone (ADH), whose activities are influenced by serum osmolality, effective circulating volume, and hypothalamic stimuli.<br /><br />The diagnostic approach follows a stepwise assessment:<br /><br />1. Confirm true hyponatremia by measuring serum osmolality. Hypo-osmolar hyponatremia (<275 mOsm/kg) indicates true hyponatremia, requiring further evaluation. Pseudohyponatremia (normal osmolality with lab interference) and hypertonic hyponatremia (due to hyperglycemia or mannitol) are differentiated at this step.<br /><br />2. Assess ADH activity via urine osmolality. Suppressed ADH (urine osmolality ≤100 mOsm/kg) suggests excess free water intake states (e.g., primary polydipsia, beer potomania). Elevated ADH (urine osmolality >100 mOsm/kg) indicates water retention despite hyponatremia.<br /><br />3. Measure urine sodium to evaluate RAAS activation and volume status. Urine sodium >30 mEq/L with euvolemia suggests SIADH or mimics (hypothyroidism, secondary adrenal insufficiency). Urine sodium <30 mEq/L usually indicates hypovolemia or hypervolemia due to extrarenal losses or conditions like heart failure, cirrhosis, and nephrotic syndrome.<br /><br />4. Assess extracellular fluid (ECF) volume status (hypovolemia, euvolemia, or hypervolemia) to guide diagnosis and management.<br /><br />Clinical pearls emphasize that hyponatremia is primarily due to increased ADH activity except in water intoxication states where ADH is suppressed. SIADH diagnosis is one of exclusion and shares urine profiles with hypothyroidism and secondary adrenal insufficiency. Recognizing these nuances aids effective diagnosis in inpatient and consult settings.<br /><br />This guide references leading clinical guidelines and recent reviews, making it a valuable resource for understanding and managing hyponatremia.

Asset Subtitle

Edwin Rosas

Keywords

Hyponatremia

Serum sodium

Renin-angiotensin-aldosterone system

Antidiuretic hormone

Serum osmolality

Urine osmolality

Urine sodium

SIADH

Volume status

Water intoxication