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A 9-year-old boy presented with short stature and progressive genu valgum (figure 1). He had frontal bossing, rachitic rosary and widening of wrist. X-Rays of the wrist (figure 2) and knee joint (figure 3) showed cupping, fraying and widening of metaphyses. Laboratory investigations showed serum calcium 9.4 mg/dl, phosphorus 2.2 mg/dl, chloride 105 mmol/l, potassium 3.0 mmol/l, alkaline phosphatase 1004 IU/l, and albumin 4.1 g/dl. 24-Hour urinary calcium excretion was 302 mg. Intact parathyroid hormone (iPTH) level was 50 pg/ml. Blood pH was 7.32 and urine pH 8.0. Intravenous pyelogram was normal. A longitudinal ultrasound scan of the kidneys is shown in figure 4. A test was done to confirm the diagnosis.
- What does the abdominal ultrasound show?
- What is the most likely diagnosis?
- Name the test for confirmation of diagnosis.
- How should this patient be managed?
The ultrasound scan of the kidneys shows hyperechoic renal pyramids with normal echotexture of the cortex. Renal size on both sides appear normal with normal central sinus echos. Pelvicalyceal system of both kidneys are normal. These features are suggestive of medullary nephrocalcinosis.
The diagnosis is distal renal tubular acidosis (RTA) with rickets. The patient had normal iPTH and hyperchloraemic acidosis with nephrocalcinosis in the presence of clinical and biochemical changes of rickets.
The confirmatory test is the NH4Cl challenge test: 0.1 g (1.9 mmol) NH4Cl/kg body weight is administered orally and blood and urine pH are followed for up to 6 h. In distal RTA the urine pH remains higher than 5.5.
The disease can be treated with sodium bicarbonate or Shohl's solution (Na+ and K+ citrate solution). The dose should be 0.5–2.0 mmol/kg body weight in four to five divided doses daily. The dose should be raised until acidosis and hypercalciuria are controlled. The patient should be followed by measurements of serum chloride, pCO2 in blood, and urinary calcium excretion, approximately twice yearly. Potassium supplementation is normally not required. The requirement of alkali usually rises during intercurrent illnesses. Corrective osteotomy may be done only when the rachitic changes have healed.
RTA is a disorder of renal tubules in which the renal excretion of acid is reduced out of proportion to reduction of glomerular filtration rate.1 Metabolic acidosis results but, in contrast to renal failure, the anions that accompany surplus hydrogen ions in the blood such as sulphate and phosphate, are excreted normally and are unavailable to balance the fall in serum bicarbonate. Therefore the kidneys reabsorb chloride in unusually large amounts and serum chloride rises to preserve electroneutrality in the extracellular fluid. The result is hyperchloraemic acidosis, and unmeasured anion gap is normal.1 2 There are at least four types of RTA.
Distal RTA is characterised by hypokalaemic hyperchloraemic metabolic acidosis and is due to selective deficiency in H+ ion secretion in the distal nephrons. Despite acidosis, urinary pH is high and it cannot be acidified below 5.5.3 Urinary excretion of NH4 + is decreased and the urinary anion gap is positive. Chronic acidosis lowers tubular reabsorption of calcium, causing renal hypercalciuria.4 The hypercalciuria, alkaline urine and low level of urinary citrate cause calcium phosphate stones and nephrocalcinosis.5 Growth in children is stunted because of rickets. The bone disease is due to the acidosis-induced loss of bone mineral and inadequate production of 1,25(OH)2 D3.1 Since kidneys do not conserve potassium or concentrate the urine normally, polyuria and hypokalaemia occurs. The diagnosis is suggested by rickets or osteomalacia, hyperchloraemic acidosis, alkaline urine and calcium phosphate stones or nephrocalcinosis. NH4Cl challenge (see above) confirms the diagnosis. Although systemic acidosis worsens, urine pH does not fall below 5.5.
Distal RTA is treated with sodium bicarbonate and/or Shohl's solution (Na+ and K+ citrate), as stated above. The total dose of alkali should be raised until acidosis and hypercalciuria are both eliminated and patients should be followed by measurement of serum chloride and pCO2 in blood and urine calcium excretion approximately twice yearly. K+ supplementation is required only when there is hypokalaemia-mediated muscle weakness and respiratory depression.1
distal RTA is a rare condition
in cases of rickets, if there is nephrocalcinosis in association with hyperchloraemic acidosis and alkaline urine, one should always suspect distal RTA
the diagnosis is proved by a positive NH4Cl challenge test
sodium bicarbonate or Shohl's solution is the cornerstone of treatment
Rickets with distal renal tubular acidosis.
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