December 13, 2024

A 9-month-old male presents with failure to thrive

By Med-Challenger · 2 minute read

A 9-month-old male presents with failure to thrive. Over his life, he has fallen below the growth curve on both length and weight. A dietary history reveals good intake (130-150cc/kg/day plus some solids), but delayed development. He is not sitting on his own and has trouble with truncal control, but he does have head control. Aside from appearing small and delayed, his exam is normal.

At previous visits, the following information has been obtained: CBC is normal without evidence of anemia. A CXR is normal, including a normal cardiac silhouette. A sweat chloride is normal and the child’s newborn screen is entirely normal.

Today, the basic metabolic panel shows a sodium of 140, a potassium of 2.3, a chloride of 114, a bicarbonate of 12, BUN of 16 and creatinine of 0.4. A venous blood gas confirms acidosis with a pH of 7.21. Urinanalysis shows a pH of 7.0, and 1-4 white cells, but is otherwise normal. You ask for urine electrolytes which yield a urine sodium of 48, potassium of 18, and chloride of 30.

What is your diagnosis?

distal renal tubular acidosis
congenital adrenal hyperplasia
Munchausen syndrome by proxy
posterior urethral valves
proximal renal tubular acidosis

The correct answer is:

distal renal tubular acidosis

Looking at his labs, you can see that this patient has a metabolic acidosis with a normal anion gap (14). In the United States, the anion gap is calculated with the equation AG = [Na+] – [Cl-] – [HCO3-], which in this case is 140 – 114 – 12 = 14. Although the serum is acidotic, the urine is alkalotic. Acidification of the urine occurs in two steps. First, in the proximal tubule, bicarbonate is exchanged for hydrogen ion -- bicarbonate is pulled back into the tubule, while hydrogen is secreted into the lumen. 85% of bicarbonate is filtered here. The other 15% of bicarbonate is reabsorbed in the distal tubule, where, in a normal kidney, all the bicarbonate is reabsorbed. Also, the distal tube acidifies the urine by the excretion of both hydrogen ion and ammonia.

In distal renal tubular acidosis (RTA), hydrogen ion is not excreted as it should be. Sodium bicarbonate is lost in the urine, causing the hyperchloremia and hypokalemia seen in our patient (disabled exchange mechanisms).

One clue that this patient has distal RTA rather than proximal is that the urine chloride is less than the sum of the urine sodium and urine potassium.

Proximal renal tubular acidosis tends to have a more acute and severe onset, whereas its distal counterpart can present much like this patient, with a delay in diagnosis.

Reference:

Marx: Rosen's Emergency Medicine, 7th ed.: Mosby; 2009. CHAPTER 172 - Genitourinary and Renal Tract Disorders

This question appears in Med-Challenger Pediatric Medicine Exam Review with CME

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