Late-onset hypercalcemia in Williams-Beuren syndrome: importance of early and frequent screening and intervention.

Williams-Beuren syndrome (WBS) affects multiple systems and has a known association with infantile hypercalcemia that is typically mild and transient. We report a 12-month-old female previously diagnosed with WBS by a chromosomal microarray, who was admitted for failure to thrive. Upon evaluation, serum calcium of 19.0 mg/dL (4.75 mmol/L) (normal 9-11 mg/dL, SI: 2.25-2.75 mmol/L) and serum ionized calcium of 2.33 mmol/L (normal 1.22-1.37 mmol/L) were revealed. Her hypercalcemia correlated with symptoms of irritability, poor feeding, mild hypotonia, and constipation, which were increasingly present for 6 months prior to admission. This calcium level is one of the highest reported in association with WBS. Additionally, while hypercalcemia associated with WBS typically resolves by the first year, this case represents a later presentation as compared to other reports. The patient initially responded to conservative treatment with intravenous fluids administration, loop diuretic therapy, and dietary calcium restriction. However, she subsequently had rebound hypercalcemia 5 weeks after treatment and received one dose of intravenous bisphosphonate with subsequent resolution of her hypercalcemia. Our report highlights the importance of screening, early management, and recognition of late presentation hypercalcemia in the setting of WBS.