Renal Tubular Acidosis
Renal tubular acidosis is a syndrome that causes persistent dehydration, hyperchloremia, hypokalemia, metabolic acidosis, and nephrocalcinosis. Renal tubular acidosis results from the kidneys' inability to conserve bicarbonate. This disorder is classified as distal (type I, or classic renal tubular acidosis) or proximal (type II).
Distal renal tubular acidosis occurs in two forms:
Proximal renal tubular acidosis results from defective reabsorption of bicarbonate in the proximal tubule. This causes bicarbonate to flood the distal tubule, which normally secretes hydrogen ions, and leads to impaired formation of titratable acids and ammonium for excretion. Metabolic acidosis ultimately results.
Proximal renal tubular acidosis also occurs in two forms:
Renal tubular acidosis affects people of all ages, including infants. The prognosis is usually good but depends on the severity of renal damage that precedes treatment.
The kidneys normally remove substances such as acid from the blood and excrete them through the urine. There are three types of renal tubular acidosis, called Types 1, 2, and 4 (there is no Type 3). Type 1 may be hereditary or can be triggered by an autoimmune disease (such as lupus), certain drugs, chronic kidney obstruction, or kidney transplant. Type 2 is usually caused by a hereditary disease, such as hereditary fructose intolerance, Lowe’s syndrome, Wilson’s disease, or Fanconi’s syndrome; heavy metal poisoning; kidney transplant; vitamin D deficiency; and certain drugs. Type 4 is the only type that is not hereditary, but is caused by autoimmune disease, diabetes mellitis, urinary tract obstruction, or sickle cell disease.
Signs and Symptoms
Demonstration of impaired urine acidification with systemic metabolic acidosis confirms distal renal tubular acidosis. Demonstration of bicarbonate wasting due to impaired reabsorption confirms proximal renal tubular acidosis.
An ammonium chloride loading test helps determine the type of renal tubular acidosis. If the urine pH stays above 5.5 after oral administration of ammonium chloride, despite systemic acidosis, distal renal tubular acidosis is present; if the urine pH falls below 5.5, proximal renal tubular acidosis is present.
Other relevant laboratory results show:
In later stages, X-rays may show nephrocalcinosis.
Supportive treatment requires replacement of substances being excreted, abnormally, especially bicarbonate. Treatment may include sodium bicarbonate tablets or Shohl's solution to control acidosis, oral, potassium for dangerously low potassium levels, and vitamin D for bone disease. If pyelonephritis occurs, antibiotics may be prescribed as well.
Treatment for renal calculi secondary to nephrocalcinosis varies and may include supportive therapy until the calculi pass or until surgery for severe obstruction is performed.
Most of the disorders that cause proximal renal tubular acidosis are not preventable.
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