Hypomagnesemia in childhood is relatively frequently noted in the neon
atal period due to maternal causes, such as decreased intake due to vo
miting, overuse of laxatives, and neonatal causes such as intrauterine
growth retardation, birth asphyxia and exchange transfusion. A very r
are cause of neonatal magnesium deficiency is called primary hypomagne
semia caused by impaired intestinal absorption of magnesium. Reference
values of serum magnesium in cord blood are slightly lowered. Erythro
cyte magnesium content is also lowered in cord blood and during the fi
rst month after birth. Mononuclear magnesium content shows no differen
ces with age. Renal magnesium loss is diagnosed by the presence of hyp
omagnesemia with an inappropriately high 24-hour urinary magnesium exc
retion. In isolated familial hypomagnesemia an autosomal dominant as w
ell as an autosomal recessive mode of inheritance was found. The renal
magnesium treshold is lowered in both forms but the tubular maximum i
s only lowered in the dominant form. In familial hypomagnesemia-hypoka
lemia (Gitelman syndrome) the renal magnesium treshold is lowered but
the tubular maximum is in the normal range. In this syndrome, with pro
bably an autosomal recessive mode of inheritance, the renal defect mig
ht be located in the distal nephron after the medullary part of the as
cending limb of the loop of Henle. The magnesium content of mononuclea
r cells and erythrocytes is in the normal and lower normal range, resp
ectively. In the familial hypomagnesemia-hypercalciuria syndrome, hypo
magnesemia is always combined with hyperuricemia and nephrocalcinosis.
Myopia and horizontal nystagmus are often present.