Rickets and metabolic bone disease — formative SAQs

SAQ 1 (10 marks) — The hypocalcaemic seizing infant

Stem: A six-month-old, exclusively breastfed infant of a veiled migrant mother presents in late winter with a brief generalised seizure. The child is afebrile and looks well between events. The ionised calcium is 0.85 mmol per litre, the phosphate is 1.2 mmol per litre, the alkaline phosphatase is 920 units per litre, and the parathyroid hormone is raised. Outline your immediate and definitive management, and explain the principle that governs the order of the drugs. ^{[3] [1]}

Model answer

Resuscitation — intravenous calcium first (3 marks). This is symptomatic hypocalcaemia from nutritional vitamin D deficiency rickets, an emergency. Give intravenous calcium gluconate slowly through a secure large-bore line with continuous cardiac monitoring, because rapid infusion can cause bradycardia and arrest. Terminate the seizure with calcium rather than standard anticonvulsants, which are ineffective for hypocalcaemic seizures. Check and correct magnesium and avoid alkalosis, which lowers ionised calcium. ^{[3] [1]}

The principle — calcium before vitamin D (2 marks). Give oral calcium before or with the cholecalciferol, never vitamin D alone. High-dose vitamin D accelerates bone mineralisation and can deepen the hypocalcaemia through hungry bone, the rapid uptake of calcium and phosphate by the healing skeleton; giving a stoss vitamin D dose without concurrent calcium is the classic avoidable error that can trigger further seizures and, rarely, cardiac compromise. ^{[1] [4]}

Definitive treatment (3 marks). Once the calcium is stable, start cholecalciferol 2000 IU daily for three months, or a stoss dose of 100 000 to 300 000 IU, together with oral calcium at 30 to 50 mg per kilogram per day elemental. The choice of daily versus stoss depends on adherence; the stoss regimen suits a family for whom a daily dose is uncertain. Send a 25-hydroxyvitamin D to confirm the deficiency and a wrist radiograph to stage the rickets. ^{[1] [4]}

Monitoring and public health (2 marks). Check the alkaline phosphatase and calcium at four to six weeks — the alkaline phosphatase should be falling — and repeat the radiograph at three months to document healing, which is expected by three to six months. Screen and supplement the mother and any siblings, because nutritional rickets in one infant signals household risk. Counsel the family that exclusive breastfeeding was the right choice and that the supplement is safe, and arrange primary-care follow-up to sustain 400 IU daily through adolescence. ^{[2] [5]}