Distal renal tubular acidosis

Synonyms

2

Overview

A rare disorder where increased bone density is caused by carbonic anhydrase II deficiency.

Symptoms

• Mental retardation
• Growth failure
• Lung disease
• Increased risk of fractures
• Metabolic acidosis

Causes

Metabolic acidosis usually results from renal excretion of bicarbonate. However, metabolic acidosis associated with RTA results from a defect in the kidneys’normal tubular acidification of urine. Distal RTA results from an inability of the distal tubule to secrete hydrogen ions against established gradients across the tubular membrane. This results in decreased excretion of titratable acids and ammonium, increased loss of potassium and bicarbonate in the urine, and systemic acidosis. Prolonged acidosis causes mobilization of calcium from bone and, eventually, hypercalciuria, predisposing the kidney to the formation of renal calculi. Distal RTA may be classified as primary or secondary. ❑ Primary distal RTA may occur sporadically or through a hereditary defect and is most prevalent in females, older children, adolescents, and young adults. ❑ Secondary distal RTA has been linked to many renal or systemic conditions, such as starvation, malnutrition, hepatic cirrhosis, and several genetically transmitted disorders. Proximal RTA 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. Ultimately, metabolic acidosis results. Proximal RTA occurs in two forms: ❑ In primary proximal RTA, the reabsorptive defect is idiopathic and is the only disorder present. ❑ In secondary proximal RTA, the reabsorptive defect may be one of several defects and is due to proximal tubular cell damage from a disease such as Fanconi’s syndrome.

Diagnosis

Demonstration of impaired acidification of urine with systemic metabolic acidosis confirms distal RTA. Demonstration of bicarbonate wasting due to impaired reabsorption confirms proximal RTA. Other relevant laboratory results show: ❑ decreased serum bicarbonate, pH, potassium, and phosphorus ❑ increased serum chloride and alkaline phosphatase ❑ alkaline pH, with low titratable acids and ammonium content in urine; increased urinary bicarbonate and potassium; low specific gravity. In later stages, X-rays may show nephrocalcinosis.

Treatment

Supportive treatment for patients with RTA requires replacement of those substances being abnormally excreted, especially bicarbonate, and may include sodium bicarbonate tablets or solution to control acidosis. Potassium may be given by mouth for dangerously low potassium levels. Vitamins D and calcium supplements are usually avoided because the tendency toward nephrocalcinosis persists even after bicarbonate therapy. If pyelonephritis occurs, treatment may include antibiotics 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.