Hyponatraemia and hypernatraemia

Sodium sits in a narrow band. The body defends a serum sodium of 135 to 145 mmol/L with a system that is essentially a water-control machine: arginine vasopressin (also called antidiuretic hormone, ADH) from the posterior pituitarius, the thirst it generates, and the kidney's ability to dilute or concentrate the urine. When that system fails, the serum sodium drifts, and because sodium is an effective osmole that does not cross cell membranes easily, the water follows it — into the brain cells in hyponatraemia, out of them in hypernatraemia. The brain, locked inside a rigid skull, cannot tolerate either swing, and that is why every sodium disorder is at heart a brain-cell-volume problem. [1] [5]

This page treats the two disorders as a mirror image. Hyponatraemia is the commoner and the one most often made in hospital, so it gets the 3 percent saline resuscitation framework and the volume-status classification. Hypernatraemia is rarer on the ward but commoner in infants and at the extremes of illness, so it gets the free-water deficit and the slow-correction framework. Both share the same organising logic: classify by water balance, let the neurological examination set the tempo, and never correct faster than the brain can follow. [5] [2]

Overview & Definition

Hyponatraemia is a serum sodium below 135 mmol/L. It is graded by severity: mild (130 to 135), moderate (125 to 130), and severe (below 125). The number alone is a poor guide to danger — what matters is whether the brain is symptomatic. A child with a seizure or coma at any sodium level needs emergency treatment; a child who is alert and well may need only investigation and slow correction. [3] [1]

Hypernatraemia is a serum sodium above 145 mmol/L, with moderate disease from 150 to 160 and severe above 160. Like hyponatraemia, severity is neurological rather than numerical, but the danger here is cerebral dehydration and shrinkage, with the added risk of bleeding from torn bridging veins. Both thresholds are serum concentrations, and both demand a repeat sample to exclude artefact before you act on an asymptomatic, mildly abnormal result. [2] [5]

The serum sodium is a concentration, not a content. It tells you the ratio of body sodium (and its anions) to body water at that instant, not how much of either the body holds. A child may have hyponatraemia with excess total-body sodium — as in heart failure, where the problem is even greater excess water — or hypernatraemia with a normal or low total-body sodium, as in diabetes insipidus, where the problem is pure water loss. Treating the number without understanding the water balance is the root of most sodium-management errors. [1] [2]

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1. [1]Adrogué HJ; Madias NE Hyponatremia. N Engl J Med, 2000.PMID 10824078
2. [2]Adrogué HJ; Madias NE Hypernatremia. N Engl J Med, 2000.PMID 10816188
3. [3]Spasovski G; Vanholder R; Allolio B; et al Clinical practice guideline on diagnosis and treatment of hyponatraemia. Nephrol Dial Transplant, 2014.PMID 24569496
4. [4]Verbalis JG; Goldsmith SR; Greenberg A; et al Hyponatremia treatment guidelines 2007: expert panel recommendations. Am J Med, 2007.PMID 17981159
5. [5]Sterns RH Disorders of plasma sodium--causes, consequences, and correction. N Engl J Med, 2015.PMID 25551526
6. [6]Rondon-Berrios H; Sterns RH Hypertonic Saline for Hyponatremia: Meeting Goals and Avoiding Harm. Am J Kidney Dis, 2022.PMID 34508830
7. [7]Moritz ML; Ayus JC Hyponatraemia: Isotonic fluids prevent hospital-acquired hyponatraemia. Nat Rev Nephrol, 2015.PMID 25599620
8. [8]McNab S Isotonic vs Hypotonic Intravenous Fluids for Hospitalized Children. JAMA, 2015.PMID 26284724
9. [9]Padua AP; Macaraya JR; Dans LF; et al Isotonic versus hypotonic saline solution for maintenance intravenous fluid therapy in children: a systematic review. Pediatr Nephrol, 2015.PMID 25576065
10. [10]Didsbury M; See EJ; Cheng DR; et al Correcting Hypernatremia in Children. Clin J Am Soc Nephrol, 2023.PMID 36888887
11. [11]Ayus JC; Moritz ML Misconceptions and Barriers to the Use of Hypertonic Saline to Treat Hyponatremic Encephalopathy. Front Med (Lausanne), 2019.PMID 30931308