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:

• Primary distal renal tubular acidosis is most prevalent in women, older children, adolescents, and young adults. The disease may result from a hereditary defect. Occasionally, it may occur for unknown reasons.
• Secondary distal renal tubular acidosis has been linked to many renal or systemic conditions, such as starvation, malnutrition, and hepatic cirrhosis, and to several genetically transmitted disorders.

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:

• In primary proximal renal tubular acidosis, the reabsorptive defect is idiopathic and is the only disorder present.
• In secondary proximal renal tubular acidosis, the reabsorptive defect may be one of several and is due to proximal tubular cell damage from a disease such as Fanconi's syndrome.

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.

Causes

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

• fatigue
• weakness
• confusion or decreased alertness
• increased rate of breathing
• muscle pain

Diagnostic tests

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:

• decreased serum bicarbonate, potassium, and phosphorus levels
• increased serum chloride and alkaline phosphatase levels
• alkaline pH, with low titratable acid and ammonium content in urine, increased urine bicarbonate and potassium levels, and low specific gravity.

In later stages, X-rays may show nephrocalcinosis.

Treatment

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.

Prevention

Most of the disorders that cause proximal renal tubular acidosis are not preventable.