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Hypernatraemia is more often caused by water deficiency than by sodium excess. Water deficiency may be due to inadequate intake, abnormal losses or a combination of the two. Causes of hypernatraemia are listed in table 1.
HYPERNATRAEMIA CAUSED BY DEFICIENT WATER INTAKE
A mentally competent, healthy adult or child will not become hypernatraemic from water deficiency alone provided there is access to water. Even mild hypertonicity is a powerful stimulus to thirst. Hypernatraemia caused by inadequate intake is therefore confined to infants, the old and frail, and the mentally impaired except for the unusual circumstance that an individual has no access to water over a sustained period.
Several recent reports have described infants who became hypernatraemic in the neonatal period because of inadequate breast feeding by inexperienced mothers.1–3 This is not an argument against breast feeding, but makes the point that mothers need adequate support to establish lactation, especially for their first born infants, and that even though human milk has a low sodium concentration a minimum water intake is necessary to prevent dehydration and hypernatraemia.
HYPERNATRAEMIA CAUSED BY INCREASED RENAL WATER LOSS
Diseases that impair the ability of the kidney to concentrate urine lead to hypernatraemia if replacement of urinary water is inadequate. Provided the thirst mechanism is intact, hypernatraemia does not occur in the competent subject with access to water; polyuria and polydipsia are the cardinal symptoms in such cases. Young infants with severe concentrating defects are at high risk of hypernatraemia however, because the distressed crying caused by thirst is frequently misinterpreted. Central (cranial or hypothalamic) and nephrogenic diabetes insipidus (DI) are the best known causes of failure of urinary concentration, and typically present in infancy with recurrent episodes of dehydration with hypernatraemia.
Central DI is due to failure of synthesis or secretion of antidiuretic hormone (ADH). It is uncommon in infancy and usually secondary …