167
lacia include metabolic acidosis, hypercalciuria, hypophosphatemia, and low
1,25(OH) 2 D. Metabolic acidosis leads to impaired bone mineralization and
increased bone resorption. Activation of RANK ligand by systemic acidosis
results in increased bone resorption, which leads to release of calcium, phosphate,
and carbonate in order to counteract metabolic acidosis; however, it results in
hypercalciuria. In addition, acidosis leads to decreased 1α-hydroxylase activity
and consequently, reduced calcium and phosphate absorption from the intestine,
and secondary hyperparathyroidism. Defective tubular reabsorption of phosphate
in proximal RTA also contributes to hypophosphatemia. These alterations in min-
eral homeostasis result in decreased calcium–phosphate solubility product and
impaired mineralization, thereby leading to rickets/osteomalacia (Fig. 5.15).
- What is the indication of ammonium chloride loading test in a patient with
RTA?
Ammonium chloride loading test is indicated in children with a strong clinical
suspicion of distal RTA but with a normal blood pH. This is usually seen in
patients who have incomplete or milder forms of distal RTA. The recommended
dose of ammonium chloride is 0.1 g/Kg body weight administered over a period
of 1h to avoid gastric irritation, and blood and urine samples are taken every
hour for 4–6h. After administration of ammonium chloride, it is converted to
urea in the presence of bicarbonate in the liver, thereby reducing the levels of
a b
R
2 11 10
c
Fig. 5.15 (a) Windswept deformity in a child with rickets due to distal RTA. (b) X-ray of wrist
shows classical features of rickets with metaphyseal fraying, cupping, and splaying. (c) X-ray of
abdomen showing bilateral nephrocalcinosis
5 Rickets–Osteomalacia