A Case of Proximal Renal Tubular Acidosis without Fanconi Syndrome

Introduction: Proximal renal tubular acidosis (pRTA) is characterized by impaired resorption of bicarbonate in the proximal tubule. This occurs most commonly with Fanconi syndrome, as isolated pRTA is rare. The most common inherited form is an autosomal recessive mutation in the Na/HCO3 transporter; infants with a sporadic form tend to spontaneously improve over time. We present a case of isolated pRTA confounded by a history of diarrhea in a neonate. Case Description: A 4-week-old term baby girl presented with poor weight gain (+160 g from birth) and fussiness, found to have metabolic acidosis. Initial labs showed hypokalemia, hyperchloremia and anion gap metabolic acidosis (AGMA) with a gap of 15. After fluid resuscitation, subsequent labs showed a non-AGMA. Urine pH was 5.0 with no protein or glucose; urine chloride level was <15mmol/L. Mom reported persistent diarrhea, which was favored to be the cause of her acidosis. She began supplementation with potassium citrate during admission with improvement in labs. Two weeks later, she returned with recurrence of diarrhea, non-AGMA, and hypernatremia in the setting of adenovirus infection and overconcentrated formula due to improper preparation. Her acidosis improved with fluids and increase of potassium citrate. After two more weeks she was readmitted from Nephrology Clinic with persistent non-AGMA. The urine pH was 6.0 with a gap of -24.9. Mom reported persistent diarrhea, but inpatient stools were observed to be normal. A clinical diagnosis of isolated pRTA was then made; the family refused genetic testing. Magnesium citrate was added to regimen, with improvement in hypokalemia and bicarbonate levels prior to discharge. She continues to grow well in follow up. Discussion: This case highlights pRTA without Fanconi syndrome as the cause of poor growth and non-AGMA in a neonate. It also demonstrates the difficulty in recognizing an isolated pRTA if more common etiologies are reported, especially when patients have confounding historical factors or lab results. An accurate history is paramount in making the diagnosis when genetic testing is unavailable to help distinguish between stool or renal bicarbonate losses, and a pRTA diagnosis should not be discounted in the absence of Fanconi syndrome.