Renal Tubular Acidosis (RTA)
Kumud P Mehta
Renal Tubular Acidosis (RTA) - Patient Education
My son aged 4 years has knock-knees. He has been treated for rickets to no avail. What should be done now?
If your 4-year-old son has knock-knees, for diagnosis of rickets, his blood should be checked for serum calcium, phosphorus and alkaline phosphatase and X-ray should be taken of knees and wrists to diagnose rickets. If both the tests suggest rickets and since the commonest cause of rickets in children in our country is Vit D deficiency, 600,000 units of oral or intramuscular Vit D should be given to the child and after 3-4 weeks, X rays and blood tests should be repeated to assess improvement. If there is no improvement in the x-rays and blood test, such a case is labeled as Vit D resistant rickets. Renal tubular acidosis (RTA) is one of the commonest causes of Vit D resistant rickets.

What is renal tubular acidosis?
RTA is a group of disorders in which there is metabolic acidosis due to defect in renal tubular acidification mechanism to maintain normal plasma bicarbonate and blood pH. In normal healthy children, daily 2-3 meq/kg of H+ ion (acid) is generated from protein, carbohydrate and fat metabolism. Kidneys play an important role in eliminating the acids by
- reabsorption of sodium bicarbonate (alkali) by proximal renal tubules.
- regeneration of bicarbonate (HCO3) and production of ammonia (NH3) by distal tubules which secrete H+ ion and maintain a gradient of H+ ion between cell and lumen.

Renal acidification mechanism keeps the blood pH within a narrow range of 7.35 - 7.45 which is vital for normal functioning of cellular and tissue metabolism. This mechanism is slow but sustained and is mainly by elimination of nonvolatile acids in urine unlike lungs which washout CO2 very fast to maintain PCO2, PO2, and blood pH via respiratory mechanism.

What are the various types of RTA?
There are 4 types of RTA.
- Type I or classical RTA or distal RTA: It is due to inability of distal tubules to maintain H+ion gradient between lumen and cell.
- Type II or Proximal tubular acidosis: It is due to excessive loss of sodium bicarbonate (alkali) in urine with low renal threshold for bicarbonate.
- Type III or distal RTA with excessive bicarbonate loss in urine, which occurs in infancy.
- Type IV or Hyperkalemic RTA due to obstructive uropathy or chronic tubulointerstitial disease. RTA can be primary/ idiopathic or secondary.

When does one suspect RTA and how is it diagnosed?
It is suspected whenever an infant or a child fails to put on weight or loses weight due to no apparent cause, becomes dehydrated, has excessive urine output (polyuria), excessive thirst, weakness, poor appetite, vomiting, constipation and muscle weakness which may be severe enough to cause paralysis of respiratory muscles due to low serum potassium levels (hypokalemia). Breathlessness with air hunger type of breathing due to acidosis may be seen in severe cases. Rickets and bony deformities occur late in the disease. In clinically suspected cases, arterial blood gas estimation will reveal low serum HCO3/TCO2 level with low blood pH and normal anion gap. Urinary pH may be inappropriately high (>5.5) for the level of acidosis in distal RTA.

What is the treatment for RTA?
Treatment of RTA is oral alkali therapy to correct acidosis and keep serum bicarbonate levels within the normal range continuously. If serum potassium is reduced, oral potassium supplements are required. Rarely, in severe cases intravenous potassium infusion is used to correct hypokalemia before starting alkali therapy.

Will the treatment cure the skeletal deformities? If not, why is treatment required?
Treatment of RTA with alkali therapy does not correct the skeletal deformities but if started below the age of one -one and half years can prevent skeletal deformities.

What are the complications of RTA?
Complications of RTA can be life threatening like hypokalemia (low blood potassium) or uncontrolled acidosis with dehydration and shock. Hypercalciuria (increased loss of calcium in the urine) with nephrocalcinosis in distal RTA can lead to chronic tubulointerstitial damage and CRF(Chronic renal failure).

How are the skeletal deformities that have already occurred treated?
Skeletal deformities due to RTA occur because the calcium from the bones is mobilized to buffer excess of H+ ion and bones become demineralised, deformed, bowed and can sustain fractures. These deformities can be corrected by surgery after sustained correction of acidosis.

Is it necessary to keep the acid level in the blood absolutely under control?
It is important to keep the acid level in the blood absolutely under control i.e. Serum NaHCO3 levels between 20-22 meq/L in infants and between 22-26 meq/L in children using Shohl's solution (sodium citrate + citric acid + water) or oral soda bicarbonate.

My son refuses to take Shohl's solution. Can I give him soda-mint tablet instead?
Since Shohl's solution contains Citric acid, it is sour and many children may refuse to take it. One can use oral Tablet soda-mint (325 mg= 3.4 meq of alkali) in equivalent doses.

How should a patient with RTA be monitored on follow up? How often should he be followed up?
Initially after starting oral soda- bicarb, the dose should be adjusted to correct acidosis by examining blood for HCO3 and blood pH and serum potassium if required every week till the lab tests reach normal values. Then onwards once in 2-3 months is enough. Monitoring of weight & height are important to assess clinical improvement. There is an impressive improvement in appetite, activity etc within few days of treatment.

Can RTA be cured? What are its causes?
RTA in children in majority of cases is congenital (from birth. Hence it needs life long treatment with oral alkali therapy. A small number of cases are acquired due to drugs like outdated tetracyclines, heavy metals, amphotericin B or diseases like medullary sponge kidney etc and withdrawal of the causative agent can result in cure.

Can omitting acidic foods in diet help the patient?
No dietetic manipulation helps the patient.

How do you prevent it from occurring in the next sibling?
Inherited form of RTA is known to occur in brothers/sisters. Prenatal diagnosis is not possible. Early detection in siblings and early institution of alkali therapy are the only measures to prevent complications of RTA.

What is the prognosis of a child with RTA?
Prognosis of a child with RTA is good if diagnosis made below 1-2 years and continuous oral alkali therapy with good monitoring is followed. Use of calcium and Vit D are responsible for nephrocalcinosis and accelerated tubulointerstitial damage. Short stature and bony deformities are the sequelae with which many children with RTA are left with eventually.

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