Renal tubular acidosis types: distal, proximal and hyperkalaemic explained

The kidneys keep the blood's acid-base balance by excreting acids into urine and reclaiming bicarbonate. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) explains that in RTA the kidneys fail to excrete acids into the urine, which causes the blood to remain too acidic. A clinical review of distal RTA describes RTA as a group of disorders in which excretion of hydrogen ions or reabsorption of filtered bicarbonate is impaired, leading to chronic metabolic acidosis with a normal anion gap.

It differs from the acidosis of kidney failure, because in RTA the defect is in the tubules, the tiny tubes that process the filtered fluid, rather than in the filters themselves.

The review explains that distal RTA (type 1) is the inability of the distal nephron to secrete hydrogen ions maximally; type 2 reflects impaired bicarbonate reabsorption in the proximal tubule; type 3 corresponds to mixed forms; and type 4 is caused by aldosterone deficiency or resistance. NIDDK adds that type 3 is rarely used as a classification because it is now thought to be a combination of types 1 and 2.

NIDDK says untreated classical distal RTA causes growth retardation in children and progressive kidney and bone disease in adults, with kidney stones, and that a major consequence is a low blood potassium level, which can cause extreme weakness, irregular heartbeat, paralysis and even death. The review adds that growth retardation develops during infancy and nearly all patients have hypokalaemia, nephrocalcinosis (calcium deposits in the kidneys) and hypercalciuria.

In inherited forms, hearing loss can occur: the review says variants in the ATP6V1B1 or ATP6V0A4 genes can be associated with sensorineural hearing loss, which may be progressive and is not reversed by alkali therapy, so children need monitoring. Proximal RTA in children often forms part of Fanconi syndrome, with loss of glucose, amino acids and phosphate in the urine and vitamin D deficiency, according to NIDDK.

NIDDK says doctors check acid-base balance in blood and urine samples: if the blood is more acidic than it should be and the urine less acidic than it should be, RTA may be the reason, but additional information is needed to rule out other causes. Sodium, potassium and chloride levels in urine, and the potassium level in blood, help identify the type.

The review notes that molecular genetic testing has greatly helped the diagnosis of inherited distal RTA, which is mostly diagnosed in infancy, childhood or young adulthood, whereas acquired secondary forms, a consequence of other disorders or medicines, can happen at any age and are more common in adults. In acquired cases the aim is to find and treat the underlying cause or stop an offending medicine.

The first goal in all types, according to NIDDK, is to neutralise acid in the blood, though different treatments may be needed for different causes. For distal RTA, alkali therapy with sodium bicarbonate or sodium citrate corrects the acidosis, which in turn corrects low potassium, salt depletion and calcium leakage into urine, decreases kidney stones and stabilises kidney function. Infants may need potassium supplements.

The review adds that alkali should be prescribed promptly to prevent osteopenia and allow normal growth in children, that maintaining normal bicarbonate raises urinary citrate and lowers stone formation, and that nearly all children grow normally with appropriate therapy. For proximal RTA, children may receive large doses of an oral alkali, and vitamin D supplements may be needed. For type 4, NIDDK says treatment may require alkaline agents and medicine to lower potassium, and that drugs that contribute may need to change.

Treatment is long-term and needs monitoring of blood bicarbonate, potassium and kidney function. NIDDK says that if treated early most people with any type of RTA will not develop permanent kidney failure, so early recognition and adequate therapy, maintained and monitored throughout life, is the goal.

The review cautions that distal RTA is not as benign as previously assumed. It cites a cohort of more than 340 patients in which the majority of adults had chronic kidney disease, about a quarter of them at stage 3 to 4. That makes regular follow-up, adherence to alkali medicines and attention to kidney stones and bone health important, and it highlights the need for planned transition from paediatric to adult nephrology care.

Practical points include staying well hydrated, taking alkali doses regularly as prescribed, and telling doctors about the diagnosis before any new medicine is started, since some drugs worsen acid-base or potassium balance.

Children are usually managed by paediatric nephrology and adults by a nephrologist, often within a rare disease service; genetics input is useful for inherited forms. Private nephrology can arrange tests quickly, but long-term monitoring and prescribing are best shared with the NHS team.

We could not find an NHS patient page for RTA, so this guide relies on a US government fact sheet and a clinical review; your nephrologist can advise on UK practice.