Hypocalcaemia and hypoparathyroidism — formative SAQs

SAQ 1 (10)

A 10-day-old infant born at term to a mother with poorly controlled type 1 diabetes presents with a generalised tonic-clonic seizure. The seizure self-terminates after two minutes. Blood glucose is normal. Ionised calcium is 0.7 mmol/L, total calcium 1.6 mmol/L (albumin 35 g/L), phosphate 3.2 mmol/L, and PTH 1.5 pmol/L (low). Magnesium is 0.45 mmol/L. The infant has a soft cardiac murmur and a cleft palate. ^[8][9]

1. Define hypocalcaemia, classify this case by PTH and timing, and state the most likely underlying diagnosis and the pathophysiological basis. (3) ^[8][9]
2. Give the emergency management including the drug, dose, route, and the critical safety rule for administration. (4) ^[1][3]
3. Outline the confirmatory investigation, the role of magnesium, and the long-term management plan. (3) ^[3][7]

Model answer

Definition and classification. Hypocalcaemia is a total calcium below 2.1 mmol/L or ionised below 1.1 mmol/L (this infant's ionised of 0.7 confirms it; the total of 1.6 is also low even before albumin correction). This is late neonatal hypocalcaemia (day 10) with a low PTH (1.5 pmol/L) and high phosphate (3.2 mmol/L) — the biochemical signature of hypoparathyroidism. The combination of late neonatal hypocalcaemia, low PTH, high phosphate, a cardiac murmur, and cleft palate points to 22q11.2 deletion syndrome (DiGeorge). In DiGeorge, the third and fourth pharyngeal pouches fail to develop, so the parathyroid glands and thymus are hypoplastic or absent, producing hypoparathyroidism and immune deficiency alongside the conotruncal cardiac defects and palatal abnormalities. ^[8][9]

Emergency management. The infant is seizing from symptomatic hypocalcaemia. Give intravenous 10% calcium gluconate at 0.5 mL per kilogram (maximum 20 mL) over 5 to 10 minutes with continuous cardiac monitoring. The critical safety rule is to give it slowly: rapid intravenous calcium causes bradycardia, arrhythmia, and can arrest the heart in systole. Calcium gluconate is used (not calcium chloride) for peripheral access because it is less vesicant if extravasation occurs. After the bolus, transition to a continuous infusion or oral calcium and calcitriol once the infant is stable. ^[1][3]

Investigation, magnesium, and long-term plan. Confirm the diagnosis with 22q11.2 deletion testing (FISH or chromosomal microarray), a cardiac echo (to define the conotruncal anatomy), and an immune panel (T-cell subsets, given thymic hypoplasia). The magnesium is low (0.45 mmol/L) and must be replaced, because severe hypomagnesaemia both suppresses PTH secretion and causes PTH resistance — the hypocalcaemia will not correct fully until magnesium is restored. Long-term management is oral elemental calcium (30 to 75 mg/kg/day) and calcitriol (20 to 60 ng/kg/day), because native vitamin D cannot be activated without PTH. Target a calcium just below normal to avoid hypercalciuria and nephrocalcinosis. A coordinated endocrine, cardiac, immunology, and developmental team is required, with a MedicAlert identifier and a sick-day plan. ^[3][7]