Bowed legs, a wrist swelling and a diet history in a 10-month-old
A 10-month-old boy is referred for bowed legs and a swelling at both wrists. He was born at term, was exclusively breastfed to 6 months, and since then has taken a home-made rice porridge with no fortified cereal and no formula. He is still breastfed on demand 8-10 times a day and eats little else. The family moved from a northern latitude two years ago; the mother wears a niqab, has had no vitamin supplementation, and keeps the blinds closed in the flat. Examination: weight 8.2 kg (10th centile), length 71 cm (3rd centile), frontal bossing, palpable widening of the wrists, and a rachitic rosary. Investigations: calcium 8.9 mg/dL (2.22 mmol/L), phosphate 3.1 mg/dL (1.0 mmol/L), alkaline phosphatase 890 U/L, 25-hydroxyvitamin D 9 ng/mL, PTH 96 pg/mL, creatinine normal. Wrist radiograph shows fraying and cupping of the metaphyses. The mother asks whether she should stop breastfeeding and whether she needs tests herself.
1. Confirm the diagnosis and explain how the biochemistry separates nutritional from hypophosphataemic rickets.
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The diagnosis is nutritional (vitamin D deficiency) rickets: low 25-hydroxyvitamin D (9 ng/mL), low-normal calcium, low phosphate, very high alkaline phosphatase, and an appropriately raised PTH with metaphyseal fraying and cupping.
Reasoning through the numbers:
- 25-hydroxyvitamin D 9 ng/mL is deficient (deficiency is usually defined as below 12-20 ng/mL depending on the guideline). This is the rate-limiting substrate for active vitamin D.
- PTH 96 pg/mL is raised, which shows the parathyroid is responding normally. Secondary hyperparathyroidism mobilises calcium from bone (hence the high alkaline phosphatase and the cupping), maintains the calcium near normal (8.9 mg/dL), and drives phosphate down by increasing renal phosphate excretion.
- Phosphate 3.1 mg/dL is therefore low as a consequence of the high PTH, not because of a primary renal phosphate leak.
Hypophosphataemic rickets would look different: 25-hydroxyvitamin D normal, PTH normal or only mildly raised, and phosphate low despite a normal vitamin D, with evidence of renal phosphate wasting (fractional phosphate excretion or tubular reabsorption of phosphate) and, in X-linked disease, a family history and often a normal or low alkaline phosphatase relative to the degree of deformity. If the vitamin D repletion produces no biochemical or radiological improvement within 4-8 weeks, revisit that diagnosis and check fibroblast growth factor 23.
Also consider dietary calcium deficiency as a contributor: prolonged exclusive breastfeeding without fortified foods supplies little calcium, and rickets can occur with a normal vitamin D intake if calcium intake is very low. Both are corrected by the same dietary plan.
2. Write the treatment plan: doses, route, duration and monitoring. Why would a single large 'stoss' dose be a poor choice here?
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Vitamin D: 2,000 IU/day of cholecalciferol orally for 12 weeks (an alternative regimen is 50,000 IU once weekly for 6 weeks, but the daily regimen is preferred in infants because it is easier to titrate). Then reduce to the standard 400 IU/day maintenance dose.
Calcium: 500 mg/day of elemental calcium in divided doses with food for the first 4-6 weeks, then reassess against dietary intake. Do not omit this: repleting vitamin D in a child on a very low calcium intake can precipitate hypocalcaemia as bone mineralises ('hungry bone'), and calcium supplementation improves healing.
Monitoring: serum calcium, phosphate, magnesium, alkaline phosphatase and PTH at 1, 2, 4 and 12 weeks; a repeat wrist radiograph at 3 months; and measure growth at every visit. Watch for hypercalcaemia and hypercalciuria (urine calcium-to-creatinine ratio) after the loading phase, and stop or reduce the dose if they occur. Check adherence explicitly, because failure to heal is far more often non-adherence than a wrong diagnosis.
Why not a single large stoss dose: in infants, stoss regimens (for example 150,000-600,000 IU given at once) are associated with hypercalcaemia and hypercalciuria, are less controllable if the child develops vomiting or poor feeding, and make it impossible to distinguish transient overshoot from genuine hypervitaminosis D. They are also more difficult to reverse once given. The aim is to refill the empty store slowly while keeping calcium in a safe range.
Do not stop breastfeeding - continue it, since the calcium and fat content are useful, but add the supplements, fortified cereal and a dietitian review; breast milk alone cannot supply the vitamin D requirement.
3. Address the mother's own health and the family's prevention plan. What tests and advice do they need?
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Maternal testing: the history is high risk (covered skin, northern latitude, closed blinds, no supplementation, two pregnancies). Check serum 25-hydroxyvitamin D, calcium, phosphate, alkaline phosphatase, PTH, and a random glucose if there is any weight or fatigue concern. If she is deficient, treat with a loading course of vitamin D followed by maintenance, according to local guidance, and check calcium and 25-hydroxyvitamin D after 3 months. Breastfed infants of deficient mothers are doubly at risk, and treating the infant without treating the mother guarantees recurrence in the next pregnancy.
The infant's siblings and future pregnancies: any sibling under 5 years should be screened clinically and, if there is any doubt, biochemically; all breastfed infants need 400 IU/day vitamin D from birth, and higher doses should be prescribed rather than guessed. The mother should be started on antenatal supplementation in any future pregnancy.
Practical family advice: continue breastfeeding and add fortified infant cereal, dairy or calcium-fortified alternatives, eggs and oily fish as age-appropriate; refer to a dietitian who can work with the foods actually available and affordable. Discuss safe sun exposure - short periods of sunlight to the face and hands, avoiding the midday heat - and explain that the closed blinds and covered skin together remove both routes of synthesis. Write the supplement dose and the duration on paper, give a repeat prescription so the 12-week course is not interrupted, and arrange follow-up at 2 and 6 weeks rather than 3 months, so that adherence and the biochemical response can be checked while there is still time to correct them.