Hypernatremia

Definition

Hypernatremia is defined as a serum sodium concentration above 145 mmol/L. It indicates a deficit of total body water relative to body sodium: water intake is lower than water losses. An impaired thirst mechanism or lack of access to water is almost always involved, either as the main cause or as a contributing factor.

Classification

• Hypovolaemic hypernatremia: loss of sodium and water, with water loss predominating.
• Euvolaemic hypernatremia: an almost pure water deficit with near-normal sodium content.
• Hypervolaemic hypernatremia: increased sodium with normal or increased total body water, usually due to intake of sodium-rich solutions.
• Acute or chronic: in longer-standing hypernatremia, brain cells generate osmotically active substances ("idiogenic osmoles"); cerebral symptoms are therefore milder than with acute development.

Occurrence & epidemiology

Hypernatremia is common in older people, especially after surgery and during tube feeding or parenteral nutrition. Contributing factors are dependence on others for drinking, a diminished sense of thirst, reduced renal concentrating capacity (due to saluretics, impaired vasopressin release or age-related nephron loss) and reduced angiotensin II production.

Aetiopathogenesis

• Gastrointestinal losses: diarrhea and vomiting.
• Skin losses: burns and excessive sweating; because sweat is hypotonic, hypernatremia can develop before marked hypovolaemia occurs.
• Renal losses: saluretics acting on the loop of Henle, osmotic diuresis due to glucose, urea or mannitol, and renal diseases with impaired concentrating ability. The most common cause of osmotic diuresis is hyperglycemia in diabetes mellitus.
• Pure water deficit: tachypnea, fever, arginine vasopressin deficiency (formerly central diabetes insipidus) and lack of a renal response to vasopressin (formerly nephrogenic diabetes insipidus).
• Impaired thirst: primary hypodipsia due to midline malformations or damage to the thirst center and in chronically ill older people; also lack of access to water.
• Sodium excess: intake of hypertonic saline or sodium-containing solutions, parenteral nutrition and mineralocorticoid excess (e.g. hormone-secreting adrenal tumors, congenital adrenal hyperplasia due to 11β-hydroxylase deficiency).

Clinical features

• Cardinal symptom thirst: absence of thirst in alert patients suggests an impaired thirst mechanism. People with communication difficulties may express thirst only through agitation.
• Central nervous system signs: shrinkage of brain cells leads to confusion, neuromuscular excitability, hyperreflexia, seizures and coma. Subcortical and subarachnoid hemorrhage and venous thromboses have been described in children with severe hypernatremia.
• Associated findings: when body sodium content is altered, signs of volume depletion or overload are also present. In renal concentrating defects large volumes of hypotonic urine are excreted; in extrarenal losses urine sodium is low.

Diagnosis

• Serum sodium confirms the diagnosis; physical examination determines whether volume depletion or overload is also present.
• History: fluid intake and access to water, sense of thirst, gastrointestinal and skin losses, fever, diabetes mellitus, drugs and tube feeding or parenteral nutrition.
• Basic laboratory tests: glucose, urea, creatinine and other electrolytes.
• Urine volume and urine osmolality: when the cause is unclear or hypernatremia recurs despite access to water; particularly informative after water deprivation.
• Water deprivation test: used in polyuria to distinguish arginine vasopressin deficiency from lack of a renal response to vasopressin.

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Further reading (open access)

1. MSD Manual Professional: Hypernatremia
2. StatPearls: Hypernatremia

Cross-references