IDIOPATHIC INFANTILE HYPERCALCEMIA:

Idiopathic infantile hypercalcemia is a rare inborn form of severe hypersensitivity to vitamin D which tends to abate by 1 year of age. We report clinical studies of an infant with this syndrome which bear on the pathogenetic mechanism. Our patient was born at term to a diabetic mother and was transiently hypocalcemic in the first week of life. From 1-3 months, he was irritable and failed to thrive on breast milk (Ca = 31.5 mg/dl) and supplementary vitamin D2 (400 U/day). At 3 months, hypercalcemia (17.7 mg/dl) was recognized in association with depressed serum levels of PTH (undetectable), normal TPR (90%), 1,25 dehydroxy-D3 (11 mg/ml) and 25-hydroxy-D3 (< 17 ug/ml) and hypercalciuria (U Ca/creat = 1.3). Parents were unaffected and there was no evidence of malignancy, subcutaneous fat necrosis or stigmata of Williams syndrome. Radiologic bone density was increased and GFR was reduced secondary to nephrocalcinosis demonstrable by ultrasound. Hypercalcemia resolved with prednisone therapy (1 mg/kg/d) and GFR returned to normal, but growth was poor. On an oral calcium binding agent (cellulose phosphate, 0.5 g × 5/d) serum calcium has been well controlled, U Ca/creat has decreased to 0.4 and growth has accelerated. We suggest that hypercalcemia in this syndrome results from the post-natal induction of unregulated intestinal vitamin D receptors allowing massive hyperabsorption of dietary calcium. Studies of vitamin D receptors in the patient's fibroblasts are in progress.