Corticosterone methyl oxidase type II deficiency: a cause of failure to thrive and recurrent dehydration in early infancy.

Corticosterone methyl oxidase type II (CMO II) deficiency is an uncommon cause of salt-wasting in infancy. We describe a boy who presented with recurrent dehydration and severe failure to thrive in the first 3 months of life, associated with mild hyponatraemia (serum Na+ 127-132 mEq/l) and hyperkalaemia (serum K+ 5.3-5.9 mEq/l). The diagnosis was suggested by an elevated plasma renin activity (PRA): serum aldosterone ratio, and subsequently confirmed by an elevated serum 18-hydroxycorticosterone: aldosterone ratio. Treatment with 9 alpha-fluorohydroxycortisone normalized growth parameters and PRA levels. CMO II deficiency should be considered in infants with recurrent dehydration and failure to thrive, even when serum sodium and potassium levels are not strikingly abnormal.