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Proximal Renal Tubular Acidosis

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The proximal renal tubular acidosis (RTA) type Ⅱ is a defect in the reabsorption of bicarbonate in the proximal tubule. Read the article to know more.

Published At December 1, 2022
Reviewed AtAugust 18, 2023

What Is Proximal Tubular Acidosis Type 2 (Type Ⅱ RTA)?

Type Ⅱ RTA is a condition that occurs when the proximal tubule of the kidneys does not properly reabsorb bicarbonates. The term renal tubular acidosis (RTA) is used to describe a group of defects in the reabsorption of bicarbonate (HCO3), the excretion of hydrogen ions (H+), or aldosterone deficiency. There are four types of RTA’s. They are-

  1. Type Ⅰ RTA- Also known as distal RTA, it is caused due to impairment in hydrogen ion secretion in the distal tubule of the nephron resulting in a high urine pH (more than 5.5) and systemic acidosis.

  2. Type Ⅱ RTA- Also known as proximal RTA, it is the impairment in the reabsorption of bicarbonate in the proximal tubule resulting in urine with a pH more than seven.

  3. Type Ⅲ RTA- This is a combination of type Ⅰ and type Ⅱ RTA and is caused as a result of inherited carbonic anhydrase Ⅱ deficiency.

  4. Type Ⅳ RTA- This is the most common type of RTA and is caused due to aldosterone deficiency or unresponsiveness of the distal tubule to aldosterone.

What Are Proximal and Distal Tubules?

The human body has a pair of kidneys to remove waste and extra fluid, situated just below the rib cage, one on each side of the spine. Each kidney is made up of a million filtering units called nephrons.

The nephron has two parts:

A) Glomerulus- It is responsible for filtering the blood and is shaped like a cup.

B) Tubule- It is a long and convoluted structure that emerges from the glomerulus and is responsible for returning needed substances into the blood; the convulsions divide the tubule into three parts-

  1. Proximal Convoluted Tubule (PCT)- Primarily concerned with the reabsorption of already filtered bicarbonate (HCO3).

  2. Loop of Henle- It absorbs water and salts from the filtrate, making it highly concentrated.

  3. Distal Convoluted Tubule (DCT)- It is responsible for the excretion of acids and ammonia.

The reabsorption and excretion together combine to form a process called renal acidification. It is the malfunctioning of PCT that causes Type Ⅱ RTA.

What Are the Causes of Type Ⅱ RTA?

It is often seen during infancy and usually resolves by itself; some of the frequent causes are-

  1. Cystinosis- It is a condition where the body is unable to break down a non-essential amino acid called cysteine, which is responsible for making protein and collagen. It is a genetic condition with many systemic manifestations, and Type Ⅱ RTA is one of them.

  2. Drugs- Chemotherapeutic drugs like ifosfamide and antibiotics like Tetracycline and Acetazolamide have been known to interfere with bicarbonate (HCO3) absorption.

  3. Fanconi Syndrome- It is a disorder of the PCT that interferes with the absorption of electrolytes, and is always associated with other conditions like cystinosis, galactosemia- a hereditary disorder that affects the body’s ability to convert galactose to glucose, and tyrosinemia- a genetic disorder that interferes with the metabolism of an amino acid called tyrosine.

  4. Inherited Fructose Intolerance- It is a genetic condition that causes an inherited inability to digest fructose.

  5. Multiple Myeloma- It is a cancer of the plasma cells, which are a type of white blood cells capable of secreting antibodies. Myeloma is capable of damaging kidneys at different levels- namely glomerular, tubular (predominantly PCT), and interstitial.

  6. Primary Hyperparathyroidism- It is a condition where the body produces too much parathyroid hormone, which causes high calcium levels (hypercalcemia) in the blood, which in turn causes RTA. The most commonly seen RTA due to hyperparathyroidism is type Ⅰ, although type Ⅱ is not uncommon.

  7. Sjogren Syndrome- It is an autoimmune disorder with lymphocytic infiltration (build-up of white blood cells) of the exocrine glands (tear and salivary glands), RTA is one of its extraglandular manifestations.

  8. Wilson Disease- It is a genetic condition that causes copper accumulation in the liver, brain, and other vital organs. Patients with Wilson's disease have the autosomal recessive form of RTA.

  9. Vitamin D Deficiency- Vitamin D is responsible for the intestinal absorption of calcium, magnesium, and phosphate. Its deficiency will lower serum calcium and phosphorus levels and elevate alkaline phosphatase and parathyroid hormone levels. As a result, there will be a reduction in the bicarbonate recovery in the PCT (through the acid-base mechanisms), leading to type Ⅱ RTA.

  10. Gene Mutations- Mutations are alterations in the DNA (deoxyribonucleic acid) sequence of an organism. It has been observed that mutations in the SLC4A4 gene are responsible for type Ⅱ RTA.

What Are the Symptoms of Type Ⅱ RTA?

Type Ⅱ RTA is caused due to an impairment in the bicarbonate (HCO3) absorption in the PCT, and it is characterized by decreased renal bicarbonate threshold (which is 26 mmol/L in older children and adults). These patients have a lower urine pH (around 5.5) and excrete adequate amounts of ammonia. When the condition is not rectified, some or all of the below-mentioned symptoms will occur depending on the severity.

  1. Confused and decreased alertness.

  2. Decreased urine output.

  3. Dehydration.

  4. Weakness.

  5. Fatigue.

  6. Increased breathing rate.

  7. Muscle pain.

  8. Muscle cramps.

  9. Osteomalacia. It is the term used to describe softening of the bones.

  10. Skeletal deformities.

  11. Increased heart rate and irregular heartbeat.

How Is Type Ⅱ RTA Diagnosed?

A thorough patient history, along with clinical and laboratory investigations (urinalysis), will help in the diagnosis of type Ⅱ RTA. A urinalysis is a panel of medical tests that perform physical and chemical evaluations of urine.

Definitive diagnosis of type Ⅱ RTA can be achieved when the patient’s urine demonstrates the following features-

  1. A low pH at low bicarbonate (HCO3) concentration.

  2. Presence of normal PCO2 (partial pressure of carbon dioxide).

  3. High urine bicarbonate (HCO3) excretion at normal plasma bicarbonate (HCO3) concentration.

How Is Type Ⅱ RTA Treated?

In children with sporadic type Ⅱ RTA, the tubular defect improves over time; however, if associated conditions like Fanconi syndrome, inherited fructose intolerance, multiple myeloma, etc, are present, addressing the underlying etiology and additional treatments like alkali therapy will help in the management of type Ⅱ RTA.

Alkali therapy is the oral supplementation of a mixture of sodium (Na) and potassium (K) salts, preferably citrate, to treat different types of metabolic acidosis. The amount of alkali required for type Ⅱ RTA is large (up to 10 to 20 mmol/kg per 24 hours) due to heavy loss of bicarbonate.

It is important to split the daily dose into repeat portions day and night. The amount of alkali administered should compensate for the loss of urinary bicarbonate (HCO3) plus the amount of acid generated by the catabolism of proteins and skeletal growth.

Conclusion:

Type Ⅱ RTA is a relatively rare condition caused due to irregular function of PCT. It is often associated with conditions like multiple myeloma, Fanconi syndrome, vitamin D deficiency, etc., and can be diagnosed with urinalysis. The condition dissolves on its own in adults but, children will need alkaline therapy along with vitamin D and calcium supplementation to make a full recovery.

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Dr. Samer Sameer Juma Ali Altawil
Dr. Samer Sameer Juma Ali Altawil

Urology

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