Proximal renal tubular acidosis

Proximal Renal Tubular Acidosis

Proximal renal tubular acidosis (pRTA) is a type of renal tubular acidosis (RTA) characterized by a defect in the reabsorption of bicarbonate (HCO₃⁻) in the proximal tubule of the nephron. This condition leads to a decrease in blood bicarbonate levels, resulting in metabolic acidosis. pRTA is also known as Type 2 RTA.

Pathophysiology

In pRTA, the proximal tubule of the nephron fails to reabsorb bicarbonate efficiently. Normally, the proximal tubule reabsorbs about 80-90% of the filtered bicarbonate. In pRTA, this reabsorption is impaired, leading to bicarbonate wasting in the urine and a subsequent decrease in blood bicarbonate levels. This results in a non-anion gap metabolic acidosis.

The defect in bicarbonate reabsorption can be due to various causes, including genetic mutations affecting transporters, acquired conditions, or as part of a generalized proximal tubular dysfunction known as Fanconi syndrome.

Causes

Proximal RTA can be caused by:

• Genetic mutations: Mutations in genes encoding for bicarbonate transporters, such as SLC4A4, can lead to inherited forms of pRTA.
• Acquired conditions: Conditions such as multiple myeloma, certain medications (e.g., carbonic anhydrase inhibitors), and heavy metal poisoning can cause pRTA.
• Fanconi syndrome: pRTA is often a component of Fanconi syndrome, which involves generalized dysfunction of the proximal tubule, leading to the loss of multiple substances in the urine, including glucose, amino acids, phosphate, and bicarbonate.

Clinical Features

Patients with pRTA may present with:

• Metabolic acidosis: Due to bicarbonate loss, patients develop a non-anion gap metabolic acidosis.
• Hypokalemia: Potassium wasting can occur, leading to low blood potassium levels.
• Growth retardation: In children, chronic acidosis can lead to growth retardation.
• Bone demineralization: Chronic acidosis can lead to bone demineralization and osteomalacia.

Diagnosis

The diagnosis of pRTA involves:

• Blood tests: Showing metabolic acidosis with normal anion gap and low bicarbonate levels.
• Urine tests: Demonstrating bicarbonate wasting in the urine, especially after bicarbonate loading.
• Genetic testing: In cases of suspected hereditary pRTA, genetic testing may identify mutations in relevant genes.

Treatment

The primary treatment for pRTA involves:

• Alkali therapy: Oral bicarbonate or citrate supplements are used to correct acidosis.
• Potassium supplementation: If hypokalemia is present, potassium supplements may be necessary.
• Treating underlying causes: Addressing any underlying conditions contributing to pRTA, such as discontinuing offending drugs or treating associated diseases.

Prognosis

The prognosis of pRTA depends on the underlying cause and the effectiveness of treatment. With appropriate management, many patients can maintain normal growth and development, although some may experience complications related to chronic acidosis.

Also see

• Renal tubular acidosis
• Fanconi syndrome
• Metabolic acidosis
• Nephron