Renal tubular acidosis — formative SAQs

SAQ 1 (10)

A 7-month-old infant presents with failure to thrive, vomiting, polyuria, and muscle weakness. Bloods show sodium 138, potassium 2.6, chloride 118, and bicarbonate 12 mmol per litre. The anion gap is normal. The urine pH is 6.8, and a renal ultrasound shows bilateral nephrocalcinosis. [1][8]

1. Define renal tubular acidosis and explain why the anion gap is normal in this condition. (2) [9][2]
2. Identify the type of RTA, giving three supporting features from the stem, and explain the mechanism of the nephrocalcinosis. (4) [3][8]
3. Outline the immediate and definitive management, naming the drug, dose, route and rationale, and state two dangers of over-rapid bicarbonate correction. (4) [2][9]

Model answer

Definition and the normal anion gap. Renal tubular acidosis is a metabolic acidosis with a normal anion gap (hyperchloraemic) caused by a defect in renal acid-base handling rather than by kidney failure or unmeasured acid accumulation. The anion gap is calculated as sodium minus the sum of chloride and bicarbonate. In RTA the kidney loses bicarbonate, and chloride rises to preserve electroneutrality, so the gap stays normal (8 to 12, up to 16 mmol per litre). A high anion gap acidosis (DKA, lactic acidosis, renal failure, toxins) is never RTA. [9][2]

Type identification and nephrocalcinosis mechanism. This is type 1 (distal) RTA. Three supporting features: the urine pH is 6.8, persistently above 5.5 despite systemic acidosis, showing the alpha-intercalated cell cannot secrete hydrogen via the H-ATPase; the potassium is low at 2.6 mmol per litre from distal potassium wasting; and nephrocalcinosis is present. The nephrocalcinosis arises because chronic acidosis buffers calcium out of bone, producing hypercalciuria; intracellular acidosis increases proximal citrate reabsorption, so urinary citrate falls; and the alkaline urine favours calcium phosphate precipitation. Hypocitraturia plus hypercalciuria plus alkaline urine equals nephrocalcinosis. [3][8]

Management. Immediate management for severe symptomatic acidosis is cautious intravenous sodium bicarbonate to raise the bicarbonate only partially, to around 12 mmol per litre, with potassium replaced first because correction will shift potassium into cells. Definitive management is oral alkali (sodium or potassium citrate, or bicarbonate) at 1 to 4 mEq per kg per day, because in distal RTA the bicarbonate threshold is normal and little is wasted once the plasma level is restored; children need the higher end to restore growth. Identify the cause with genetics (ATP6V1B1, ATP6V0A4) and a hearing assessment. Two dangers of over-rapid correction: hypokalaemia from potassium shifting into cells, and hypocalcaemic tetany or seizures as ionised calcium falls (a third is hypernatraemia and fluid overload). [2][9]