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An uncommon cause of metabolic alkalosis that has been seen as a presenting feature of CF as well as a complication in those with known disease. It is accompanied by chronic salt depletion and sometimes failure to thrive without severe dehydration. It can also present acutely often as part of heat stroke so is commoner in hot weather when there has been inadequate salt and fluid replacement with dehydration. Principal findings are hypokalaemic hypochloraemic metabolic alkalosis, sometimes with hyponatraemia. This may be preceded by anorexia, nausea, vomiting, fever and weight loss, in the acute setting, this can be mistaken for infective gastroenteritis. Judging degree of dehydration in an acute presentation can be hard, the classic clinical signs of dehydration (sunken eyes, loss of skin turgor) are not always apparent and a comparison of acute presentation weight with last clinic weight is helpful.
Check venous sample in blood gas machine for bicarbonate, or venous blood for Cl, Na and K. Acutely oral rehydration solution (Dioralyte or equivalent) or sometimes IV fluids (normal saline +/- potassium chloride) is required. In the more chronic, indolent presentation treatment is with sodium +/- potassium chloride supplements, which may be required for many months or long term. After salt replacement, the metabolic abnormality resolves and weight gain follows rapidly.
Unexplained failure to thrive should always have urinary electrolytes checked, a spot urine Na+ <20 mmol/l indicates low total body sodium that needs correcting. A serum potassium at the lower end of the normal range may still be associated with body depletion.
It is quite usual for a newborn screened infant under 3 months to have low urine Na levels and normal range is less well defined, so it should not be used to guide sodium supplementation in this age group (see salt supplement recommendations in section 7.3).
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