Chapter 91: Calcium Disorders (Hypocalcemia, Hypercalcemia)

INTRODUCTION

Abnormalities of calcium (Ca²⁺) and magnesium (Mg²⁺) metabolism are not infrequent occurrences among infants admitted for neonatal intensive care. Moreover, the disturbances of Ca²⁺ may be mirrored by Mg²⁺, or conversely, as in hypocalcemia and hypomagnesemia. Infants of diabetic mothers (IDMs) and infants with fetal growth restriction (FGR) may present with low serum levels of either Ca²⁺ or Mg²⁺ or both. Serum values for Ca²⁺ and Mg²⁺ above or below accepted normal values are of concern in any infant and warrant further clinical studies. Magnesium disorders are discussed in Chapter 105.

I. HYPOCALCEMIA

1. Definition. Hypocalcemia is determined by either total serum calcium (tCa) or ionized calcium (iCa) values. Clinical chemistry values for serum levels vary by units (ie, mEq/L, mmol/L, or mg/dL), by gestational age, and by day of age following the immediate newborn period. Reference textbooks reflect considerable variance of serum values for Ca²⁺ and Mg²⁺. Interpretation of serum values for any given patient is dependent on recognition of one’s institution laboratory values and range of acceptable values.

   A generally accepted value for hypocalcemia is <2.0 mmol/L (<8.0 mg/dL) for a term infant or <1.75 mmol/L (<7.0 mg/dL) for a preterm infant. A typical range of normal values for a term newborn can be 2.25 to 2.65 mmol/L (9.0–10.6 mg/dL) throughout the first week of life. Preterm infant tCa levels closely parallel those for term infants. Of greater significance is the ionized fraction of Ca²⁺. It is the active physiologic component and is dependent on the interaction of tCa²⁺, acid-base status, and serum albumin. Typical iCa²⁺ values for term infants over the first 72 hours of life are 1.22 to 1.24 mmol/L (4.88–4.96 mg/dL). Preterm infant mean values are similar for 24 to 72 hours: 1.21 to 1.28 mmol/L (4.84–5.12 mg/dL). Thereafter, preterm infants have slightly increased iCa²⁺ levels, whereas term infants experience a slight decline. iCa levels of <1.2 mmol/dL (4 mg/dL) are considered hypocalcemic.

2. Incidence. Hypocalcemia is likely the most common disorder of either Ca²⁺ or Mg²⁺ in newborn infants, and it affects both preterm and term infants. It occurs in up to 30% of infants with birthweight <1500 g. Late-onset hypocalcemia is more common in developing countries where cow’s milk or formulas with phosphate concentrations are used.

3. Pathophysiology. iCa²⁺ is the biologically important form of calcium. The tCa²⁺ levels have been repeatedly shown to not be predictive of iCa²⁺ levels. Therefore, tCa²⁺ levels are unreliable as criteria for true hypocalcemia. In premature infants, it has been shown that tCa²⁺ levels as low as ≤6 mg/dL correspond to iCa²⁺ levels >3 mg/dL.

4. Risk factors

   1. Early-onset neonatal hypocalcemia. During the third trimester of pregnancy, the human fetus receives at least 120 to 150 mg/kg/d of elemental Ca²⁺ via the umbilical cord. ...