Paediatrics: Proximal renal tubular acidosis

Proximal renal tubular acidosis

This type of RTA results from reduced proximal tubular reabsorption of bicarbonate.

·  25% of urinary bicarbonate is lost.

·  Plasma bicarbonate level falls until it reaches a threshold when urinary bicarbonate wasting ceases (approximately 15–18mmol/L).

·  Urinary acidification to pH values <5.5 is not possible.

Proximal RTA may occur as an isolated disorder with no other abnor-malities of tubular function. This form may be transient and is occasion-ally inherited. Proximal RTA also occurs as a more generalized defect of proximal tubular transport characterized by:

·  RTA.

·  Excessive urinary loss of glucose, phosphate, amino acids, sodium, potassium, calcium, and uric acid. This generalized form is known as Fanconi syndrome, which may be p or s to several inherited and acquired disease states.

This is due to deficiency in hydrogen ion secretion by the distal renal tubules and collecting ducts. Urine pH cannot be reduced 5.8. Hyperchloraemia and hypokalaemia are characteristic, but less severe than that found in proximal RTA. Nephrocalcinosis may be present. Distal RTA may be isolated or secondary.

Clinical features of RTA Children with isolated forms of proximal and distal RTA usually present with failure to thrive in infancy. Those with the s forms of RTA may present in a similar way.

Diagnosis

Other causes of systemic acidosis (e.g. chronic diarrhoea, lactic acido-sis, diabetic ketoacidosis) should be excluded. Investigation to establish a diagnosis of RTA should include:

·  Blood: pH; bicarbonate (low); potassium (low); chloride (high).

·  Urine—early morning sample:

·  pH < 5.5 suggests proximal RTA;

·  pH ≥ 5.8 suggests distal RTA.

If proximal RTA is detected, blood and urinalysis to establish other tubular defects should be undertaken.

Treatment

The main aims are correction of acidosis and maintenance of normal bicar-bonate and potassium. This can be achieved by alkali (citrate or bicarbon-ate)/potassium-containing solutions.