Renal tubular acidosis

Renal tubular acidosis - proximal

Definition:

A condition where the body is in an acidic state caused by abnormal reabsorption of bicarbonate in the proximal tubule of the kidney.

Alternative Names:

Proximal renal tubular acidosis; Type II RTA; RTA - proximal; Renal tubular acidosis type II

Causes, incidence, and risk factors:

NORMAL ACID-BASE REGULATION
Acid-base balance is defined by the concentration of hydrogen ions (the pH). The normal pH of the body is very slightly alkaline. Acidic substances in the body include carbon dioxide and molecules containing hydrogen ions. These are buffered (counteracted) by alkaline substances, primarily bicarbonate, with some buffering by phosphate, proteins, hemoglobin, and other substances. Increased carbon dioxide or decreased bicarbonate levels can create an acidic state in the body, called acidosis. Decreased carbon dioxide or increased bicarbonate levels can create an excess alkaline state, called alkalosis. If the defect is caused by changes in carbon dioxide levels, it is a respiratory acidosis or respiratory alkalosis. If the defect is caused by changes in bicarbonate levels, it is a metabolic acidosis or metabolic alkalosis.

The respiratory system regulates the levels of carbon dioxide through changes in the breathing rate. Carbon dioxide is lost with faster breathing and increased when breathing slows. This provides fast but temporary regulation of body pH.

The major, long-term regulation of body pH occurs in the kidneys, which excrete acids, and excrete or create bicarbonate for use in the body.

PROXIMAL RENAL TUBULAR ACIDOSIS
Proximal renal tubular acidosis (Type II RTA) is a disorder caused by a partial defect in the secretion of hydrogen ions in the proximal renal tubule. This causes a reduction in the reabsorption of bicarbonate from the tubule back into the bloodstream. Renal tubular acidosis is one cause of metabolic acidosis.

Type II RTA is an uncommon but well-known form of renal tubular acidosis. It is less common than classical (Type I) RTA. It most commonly occurs during infancy, and spontaneous resolution may occur. The exact cause of the defect varies, including inherited and acquired conditions.

Renal tubular acidosis causes disorders of the body related to the loss of bicarbonate and the inability to excrete hydrogen. The body attempts to maintain an electro-chemical balance between positively charged and negatively charged molecules. If the excretion of a molecule is abnormal, the excretion of other molecules may become abnormal in an attempt to maintain a balance. Type II RTA causes the loss of bicarbonate and is also associated with the loss of glucose and amino acids in the urine. There is excessive loss of phosphate and calcium in the urine. There is a marked loss of potassium in the urine. The body may also lose fluid because water accompanies the excretion of these molecules.

The acidic condition of the body causes calcium to dissolve from the bones. Calcium accumulates in the bloodstream and excess serum calcium is excreted by the kidneys, causing a loss of total body calcium and resulting in osteomalacia or rickets, impaired growth of children, skeletal deformities, and muscle weakness. Unlike type I RTA, there is no increased incidence of kidney stones and nephrocalcinosis associated with type II RTA.

Symptoms:

Signs and tests:

The respiratory rate may be rapid as the body attempts to decrease carbon dioxide in response to decreased bicarbonate levels. Signs and symptoms of osteomalacia or rickets, kidney stones or nephrocalcinosis, or electrolyte disturbances may appear. Signs of dehydration may develop from loss of water into the urine that may accompany excretion of electrolytes.

The simultaneous measurements of the blood pH and the urine pH may show inadequate hydrogen ion excretion.
A urinalysis shows abnormalities including excessive levels of phosphate and calcium in the urine. The urine may be alkaline, although this may change as the body becomes more acidic and increasing amounts of bicarbonate are reabsorbed to counteract the acidic condition. A urinalysis may show abnormalities including glucose and amino acids lost as filtration from the kidney becomes abnormal.
Arterial blood gases and blood chemistries may indicate metabolic acidosis and electrolyte abnormalities.

This disease may also alter the results of the following tests:

Treatment:

The goal of treatment is to restore the normal pH (acid-base level) and a normal electrolyte balance to the body. This will indirectly correct bone disorders and reduce the risk of osteomalacia and osteopenia in adults. Some adults may need no treatment, but all children need alkali therapy to prevent bone disease (rickets in particular) and to permit normal growth. The underlying cause should be corrected if it can be identified.

Alkali therapy:
Alkaline medications such as sodium bicarbonate and potassium citrate are administered to correct the acidic condition of the body and to correct low potassium levels. Thiazide diuretics may indirectly increase reabsorption of bicarbonate.

Vitamin D and calcium supplements may be needed to aid in the reduction of skeletal deformities resulting from osteomalacia or rickets.

Expectations (prognosis):

Although the cause of proximal renal tubular acidosis may resolve spontaneously, the disorder must be treated to reduce its effects and complications, which can be permanent and/or life-threatening. Most cases resolve successfully with treatment.

Complications:

osteomalacia
rickets
electrolyte disturbances including (but not limited to) hypokalemia

Calling your health care provider:

Call your health care provider if symptoms indicating proximal renal tubular acidosis may be present.

Call your health care provider if new symptoms develop, including bone pain, pain in the back or flank or abdomen, skeletal deformities, increased heart rate or irregular heartbeat, muscle cramps, decreased urine output, bloody urine, or other symptoms.

Severe decrease in alertness or orientation, decreased consciousness, and seizures, are emergency symptoms that can develop.

Prevention:

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

Review Date: 12/7/2001
Reviewed By: Andrew Koren, M.D., Department of Nephrology, NYU-Mount Sinai Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org). URAC's accreditation program is the first of its kind, requiring compliance with 53 standards of quality and accountability, verified by independent audit. A.D.A.M. is among the first to achieve this important distinction for online health information and services. A.D.A.M. is also a founding member of Hi-Ethics (www.hiethics.com) and subscribes to the principles of the Health on the Net Foundation (www.hon.ch).

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. Copyright 2003 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.