Introduction to Renal tubular acidosis

A syndrome of persistent dehydration, hyperchloremia, hypokalemia, metabolic acidosis, and nephrocalcinosis, renal tubular acidosis (RTA) results from the kidneys’ inability to conserve bicarbonate. This disorder occurs as distal RTA (type I, or classic RTA) or proximal RTA (type II). The prognosis is usually good but depends on the severity of renal damage that precedes treatment.

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
Type I 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 patient 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 and systemic conditions, such as starvation, malnutrition, hepatic cirrhosis, and several genetically transmitted disorders.

Proximal RTA
Type II 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, metabolicacidosis results.

Proximal RTA occurs in two forms:

1. With primary proximal RTA, the reabsorptive defect is idiopathic and is the only disorder present.
2. With secondary proximal RTA, the reabsorptive defect may be one of several defects and results from proximal tubular cell damage from a disease such as Fanconi’s syndrome.

Signs and symptoms
In infants, RTA produces anorexia, vomiting, occasional fever, polyuria, dehydration, growth retardation, apathy, weakness, tissue wasting, constipation, nephrocalcinosis, and rickets.
In children and adults, RTA may lead to urinary tract infection, rickets, and growth problems. Possible complications of RTA include nephrocalcinosis and pyelonephritis.

Diagnosis
Demonstration of impaired urine acidification with systemic metabolic acidosis confirms distal RTA. Demonstration of bicarbonate wasting from impaired reabsorption confirms proximal RTA.
Other relevant laboratory results show the following:

• decreased serum bicarbonate, pH, potassium, and phosphorus levels
• increased serum chloride and alkaline phosphatase levels
• alkaline pH, with low titratable acids and ammonium content in urine; and increased urinary bicarbonate and potassium levels, with low specific gravity.
• In later stages, X-rays may show nephrocalcinosis.

Treatment
Supportive treatment of patients with RTA requires replacement of those substances being abnormally excreted, especially bicarbonate. It may include alkaline medications, such as potassium citrate and sodium bicarbonate, to control acidosis, and oral potassium to treat dangerously low potassium levels. If pyelonephritis occurs, treatment may include an antibiotic as well.
Treatment of renal calculi secondary to nephrocalcinosis varies and may include supportive therapy until the calculi pass or until surgery for severe obstruction is performed.