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Disorders of calcium metabolism frequently develop in neonates in the intensive care setting. Hypocalcemia is the most common. Hypocalcemia is defined as serum calcium below 8 mg/dL (ionized calcium < 1.1 mmol/L) in a full-term infant or serum calcium less than 7 mg/dL (ionized calcium < 1 mmol/L) in a preterm infant. The etiology and treatment of hypocalcemia is often determined by the age of onset.
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Premature infants are at risk for poor mineral stores because the fetus accrues 80% of its calcium stores in the third trimester.1 Similar accrual occurs for magnesium and phosphorus stores as well. Fetal parathyroid hormone is secreted in response to maternal calcium levels. After birth, there is a rapid drop in the serum calcium because of the loss of the transplacental infusion of calcium. The sudden removal of this calcium source requires the neonate to rely on endogenous parathyroid hormone production, ingested calcium, renal tubular reabsorption of calcium, and bone calcium stores. There is a physiologic nadir in the serum calcium level that occurs at 24–48 hours of life. In response to the falling calcium levels, there is a rise in the parathyroid hormone level, resulting in an increased serum calcium level. Abnormalities in this physiological response can lead to hypocalcemia in the neonate.
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Key aspects of the maternal history include a history of gestational diabetes, maternal vitamin D deficiency, and maternal hypercalcemia from hyperparathyroidism or large intake of calcium carbonate in antacids.
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Key aspects of the birth history include premature gestational age, low birth weight, intrauterine growth restriction/small for gestational age, large for gestational age, and perinatal stress.
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Key aspects of the patient’s history include diuretic use, daily calcium and phosphorus intake, vitamin intake, fat malabsorption, liver dysfunction, renal disease, and use of parenteral nutrition.
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Signs of hypocalcemia include the following:
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irritability,
jitteriness or tremulousness,
facial spasms,
tetany,
stridor, and
focal or generalized seizures.
Nonspecific signs of hypocalcemia include
apnea,
tachycardia,
cyanosis,
emesis, and
poor feeding.
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Note that the Chvostek sign (tapping over the facial nerve at the angle of the jaw, resulting in a twitch of the nose or lips) or Trousseau sign (inflating the blood pressure cuff above systolic pressure for 2 minutes, leading to carpal muscle spasm) are not commonly seen in neonates.
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Confirmatory Laboratory Studies
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Specific laboratory tests obtained during a period of hypocalcemia will provide important diagnostic information regarding the underlying etiology. The following laboratory tests should be obtained when the infant is hypocalcemic (Table 72-1):
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