Education And Debate

Distinguishing between salt poisoning and hypernatraemic dehydration in children

BMJ 2003; 326 doi: http://dx.doi.org/10.1136/bmj.326.7381.157 (Published 18 January 2003) Cite this as: BMJ 2003;326:157
  1. Malcolm G Coulthard ([email protected]), consultant paediatric nephrologista,
  2. George B Haycock, professor of paediatricsb
  1. a Department of Paediatric Nephrology, Royal Victoria Infirmary, Newcastle upon Tyne NE1 4LP
  2. b Guy's, King's, and St Thomas's School of Medicine, London SE1 9RT
  1. Correspondence to: M G Coulthard
  • Accepted 4 October 2002

Hypernatraemia caused by salt poisoning or dehydration must be distinguished correctly, as the two situations need different legal and medical approaches. Two nephrologists discuss the physiology of hypernatraemia and explain how to differentiate between cases caused by salt poisoning and dehydration

The criteria most widely used to diagnose salt poisoning in children were formulated by Meadow.1 These criteria focus on hypernatraemia with high urinary concentrations of sodium and chloride, but this combination may also be found in children with dehydration caused by diarrhoea. The medical and legal management of the two conditions fundamentally are different, so reliable ways to distinguish them are needed.

We consider the physiology of salt overload and hypernatraemic dehydration.2 We explain how to differentiate the two situations on the basis of history; examination; and biochemical analysis of serial, paired, blood, and “spot” urine samples. We illustrate the method with two cases that have been tested in court.

Summary points

Medical causes of hypernatraemia other than salt poisoning and dehydration are persistent and easy to exclude

Urinary sodium concentrations may be high in cases of salt poisoning and dehydration, so they cannot distinguish between the two conditions

Fractional excretions of sodium and water can differentiate salt poisoning from dehydration

Serial pairs of spot plasma and urine samples should be taken during recovery

“Spot balances” for salt and water estimated from fractional excretion values give a clear picture of the physiology in individual cases

Illustrative cases

Salt poisoning

In case 1, a 7 year old boy had severe colitis, for which he underwent a colostomy and received intravenous and nasogastric nutrition, for five years. He presented twice with confusion, hypernatraemia, and weight gain but without fever, diarrhoea, or vomiting. Paired plasma and urine samples collected during his second presentation led to a diagnosis of salt poisoning (table). His mother then confessed to …

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