Renal Tubular Acidosis (RTA): Symptoms, Causes and Treatment

The process of breaking down of food in the body produces acids. These acids circulate in the blood and it is the function of the kidneys to remove the excess acid from the blood and excrete it through urine. This filtration task is mainly done by the tubules of the nephron, which is the functional unit of the kidneys. There are around 800,000 to one million nephrons in a human kidney. In people affected with Renal Tubular Acidosis (RTA), this capability of the kidneys get impaired due to various reasons. This condition results in a rise in the acid content of the blood. The high levels of acid in blood is called metabolic acidosis, but RTA specifically refers to the condition in which the kidneys fail to remove the acids through urine.

Type 1 or Distal RTA

Type 1 or Distal RTA is characterized by the inability to excrete acid through the urine due to the inability of the cells of the distal (distant) nephron to secrete acid and excrete it through urine. This makes the blood acidic and eventually, leads to high levels of acid in the blood, low levels of potassium in the blood (hypokalemia), calcium deposits in the kidneys causing kidney stones and low level of minerals in the bones, leading to rickets in children and osteomalacia in adults. Distal renal tubular acidosis can also cause dehydration, weakness of muscles, paralysis and kidney failure. Since the disease is caused by the impairment of distal nephron, it is named distal RTA.

The symptoms may vary from person to person with some of the affected people not experiencing any sign of the disease. The symptoms include severe rickets in children, loin pain and hematuria (red blood cells in urine) from kidney stones. Sometimes, distal RTA may lead to renal failure and death. Distal RTA can be caused by autoimmune disease like Sjögren's syndrome, rheumatoid arthritis, systemic lupus erythematosus and hypergammaglobulinemia. In such cases, renal tubular acidosis can lead to severe hypokalemia. It can be hereditary too. If linked to heredity, the disease may be associated with hypokalemia and sensorineural deafness. The other causes include liver cirrhosis, sickle cell anemia, chronic urinary tract obstruction and renal transplantation. The treatment for distal RTA includes oral sodium bicarbonate or sodium citrate (alkaline agents) for normalizing acid levels in the blood. This helps to fight against bone demineralisation too. Hypokalemia, urinary stones and calcium deposits are treated with potassium nitrate.

Type 2 or Proximal RTA

This type is caused by the malfunctioning of the proximal (meaning 'near') tubular cells. These cells fail to reabsorb filtered bicarbonate from the urine. Bicarbonate neutralizes acid in the blood. As the kidneys fail to retain bicarbonate, the levels of acid in the blood increases. The bicarbonate levels of the urine also rises. This form is not as severe as the distal RTA, because in this case the distal intercalated cells function normally. Proximal RTA is mainly caused due to the malfunctioning of the proximal tubular cells, which is termed as Fanconi's syndrome. This condition is characterized by demineralization of bones causing rickets or osteomalacia. The other causes of proximal RTA include hereditary disorders like Lowe syndrome, Wilson's disease, cystinosis, glycogen storage disease (type I) and galactosemia. This disease can be a result of acquired disorders, like, multiple myeloma, toxins like lead and cadmium and amyloidosis. Proximal renal tubular acidosis is mainly treated with oral bicarbonate. This can increase the loss of bicarbonate through urine, but the amount of bicarbonate supplements should be large enough to reverse high acid content of the blood and bone demineralization.

Type 4 or Hyperkalemic RTA

Type 4 RTA is caused by the low levels of the hormone aldosterone (hypoaldosteronism), which directs the kidneys to regulate the levels of sodium, potassium and chloride in the blood, by excretion of excess amounts through urine. It may also be caused when the kidneys do not respond to this hormone. Hyperkalemic RTA is characterized by high levels of potassium in the blood or hyperkalemia. One of the causes of this type of renal tubular acidosis is deficiency of aldosterone due to malfunctioning of the adrenal glands, congenital adrenal hyperplasia or aldosterone synthase deficiency. Another cause is hyporeninemic hypoaldosteronism occurring due to diseases, like, HIV, renal dysfunctioning or some drugs like NSAIDS and ACE inhibitors In some people, aldosterone resistance is the reason for Type 4 RTA. This may be caused by some drugs or by pseudohypoaldosteronism. Symptoms are very rare in this type of RTA. If the level of potassium in the blood is very high, irregular heartbeats and muscle paralysis may occur. The treatment for this form of RTA includes mineralocorticoid for aldosterone deficiency and fludrocortisone for hyporeninemic hypoaldosteronism.

Type 3 RTA, which is now thought to be a combined form of Type 1 (Distal) and Type 2 (Proximal) was earlier recognized as a distinct form of this disease. But now this classification is not used much. This was commonly found in infants and kids and was known as 'Juvenile RTA'. This type of renal tubular acidosis is very rare.

If RTA is treated properly, kidney failure can be averted. Ultimate goal of any treatment plan for RTA is the neutralization of acid in the blood. If the acid is not neutralized, it can lead to complications, like, kidney stones, bone diseases, growth retardation, chronic kidney disease and even kidney failure. Persons with this disorder have to be on therapy which may have to be maintained for a lifetime.