Hydroelectrolytic syndromes in neuroanesthesia and neurocritical care

Table 2 Hypernatremia syndromes and laboratory workup

Etiology	Volume status	Urine osmolality	Urine Na+	Key lab/clinical features
Extrarenal water losses	Hypovolemic	High (> 450 mOsm/kg)	Low (< 30 mEq/L)	↑ BUN/Cr ratio, hypotension/tachycardia, clinical signs of volume depletion
(Fever, hyperventilation, GI losses, drains, wounds)
Osmotic diuresis	Hypovolemic	High (> 300 mOsm/kg)	High (> 30 mEq/L)	Polyuria, osmotic diuresis; mannitol-induced free-water loss
(Mannitol, diuretics)
Central diabetes insipidus	Euvolemic	Low (< 200 mOsm/kg)	Low (< 30 mEq/L)	Polyuria (> 200 mL/hour), polydipsia, ↑ serum Na+, responds to desmopressin
Nephrogenic diabetes insipidus	Euvolemic	Low (< 200 mOsm/kg)	Variable (< 30 mEq/L)	Polyuria, ADH-resistance; no response to desmopressin
Hypertonic saline infusion	Hypervolemic	High (> 450 mOsm/kg)	High (> 30 mEq/L)	Positive fluid balance, volume overload, exogenous Na+ load (3%-23.4% NaCl)
Exogenous sodium load	Hypervolemic	High (> 450 mOsm/kg)	High (> 30 mEq/L)	Iatrogenic Na+ gain (bicarbonate drips, enteral salt), often with ↑ chloride
(NaHCO3, salt tablets)

Serum osmolality is invariably high (> 295 mOsm/kg) in true hypernatremia. Differentiation hinges on volume status (clinical exam, hemodynamics) and urinary indices (urine osmolality, urine Na⁺) to guide targeted therapy. ADH: Antidiuretic hormone; BUN: Blood urea nitrogen; Cr: Creatinine; GI: Gastrointestinal.