Renal Tubular Acidosis
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Your body's cells use chemical reactions to carry out tasks such
as turning food into energy and repairing tissue. These chemical
reactions generate acids. But too much acid in the blood (acidosis)
can disturb many bodily functions. Healthy kidneys help maintain
acid-base balance by excreting acids into the urine and returning
bicarbonate (an alkaline, or base, substance) to the blood. This
"reclaimed" bicarbonate neutralizes much of the acid that is created
when food is broken down in the body.
Renal tubular acidosis (RTA) is a disease that occurs when the
kidneys fail to excrete acids into the urine, which causes a
person's blood to remain too acidic. Without proper treatment,
chronic acidity of the blood leads to growth retardation, kidney
stones, bone disease, and progressive renal failure.
One researcher, pediatric neurologist Donald Lewis, has theorized
that Charles Dickens may have been describing a child with RTA when
he created the character of Tiny Tim in his famous story, "A
Christmas Carol." Tiny Tim's small stature, malformed limbs, and
periods of weakness are all possible consequences of the chemical
imbalance caused by RTA. Among the evidence cited to support this
theory is the fact that Tiny Tim's condition, while fatal in one
scenario, is reversible when Scrooge pays for medical treatments,
which in those times would likely have included sodium bicarbonate
and sodium citrate, which are alkaline agents that would neutralize
the acid in Tiny Tim's blood. Whether the literary diagnosis of Tiny
Tim is correct or not, the good news is that medical treatment can
indeed reverse the effects of RTA.
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To diagnose RTA, your doctor will check the acid-base balance in
samples of your blood and urine. If the blood is more acidic than it
should be and the urine less acidic than it should be, RTA may be
the reason, but additional information is needed first to rule out
other causes. If RTA is suspected, additional information about the
sodium, potassium, and chloride levels in the urine and the
potassium level in the blood will help identify which of the three
types of RTA you have. In all cases, the first goal of therapy is to
neutralize acid in the blood, but different treatments may be needed
to address the different underlying causes of acidosis.
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At one time, doctors divided RTA into four types.
- Type 1 is also called classic distal RTA. "Distal," which
means distant, refers to the point in the urine-forming tube where
the defect occurs. It is relatively distant from the point where
fluid from the blood enters the tiny tube (or tubule) that
collects fluid and wastes to form urine.
- Type 2 is called proximal RTA. The word "proximal," which
means near, indicates that the defect is closer to the point where
fluid and wastes from the blood enter the tubule.
- Type 3 is rarely used as a classification today because it is
now thought to be a combination of type 1 and type 2.
- Type 4 RTA is caused by another defect in the distal tubule,
but it is different from classic distal RTA and proximal RTA
because it results in high levels of potassium in the blood
instead of low levels. Either low potassium (hypokalemia) or high
potassium (hyperkalemia) can be a problem because potassium is
important in regulating heart rate.
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This disorder may be inherited as a primary disorder or may be
one symptom of a disease that affects many parts of the body.
Researchers have now discovered the abnormal gene responsible for
the inherited form. More often, however, classic distal RTA is a
complication of diseases that affect many organ systems (systemic
diseases), like the autoimmune disorders Sjögren's syndrome and
lupus.
Other diseases and conditions associated with distal RTA include
hyperparathyroidism, a hereditary form of deafness, analgesic
nephropathy, rejection of a transplanted kidney, renal medullary
cystic disease, obstructive uropathy, and chronic urinary tract
infections.
A major consequence of classic distal RTA is low blood-potassium.
The level drops if the kidneys excrete potassium into urine instead
of returning it to the blood supply. Since potassium helps regulate
nerve and muscle health and heart rate, low levels can cause extreme
weakness, cardiac arrhythmias, paralysis, and even death.
Untreated distal RTA causes growth retardation in children and
progressive renal and bone disease in adults. Restoring normal
growth and preventing kidney stones, another common problem in this
disorder, are the major goals of therapy. If acidosis is corrected
with sodium bicarbonate or sodium citrate, then low blood-potassium,
salt depletion, and calcium leakage into urine will be corrected.
Alkali therapy also helps decrease the development of kidney stones.
Potassium supplements are rarely needed except in infants, since
alkali therapy prevents the kidney from excreting potassium into the
urine.
This form of RTA occurs most frequently in children as part of a
disorder called Fanconi's syndrome. The symptoms of Fanconi's
syndrome include high levels of glucose, amino acids, citrate, and
phosphate in the urine, as well as vitamin D deficiency and low
blood-potassium.
Proximal RTA can also result from inherited disorders that
disrupt the body's normal breakdown and use of nutrients. Examples
include the rare disease cystinosis (in which cystine crystals are
deposited in bones and other tissues), hereditary fructose
intolerance, and Wilson's disease.
Proximal RTA also occurs in patients treated with ifosfamide, a
drug used in chemotherapy. A few older drugs--such as acetazolamide
or outdated tetracycline--can also cause proximal RTA. In adults,
proximal RTA may complicate diseases like multiple myeloma, or it
may occur in people who experience chronic rejection of a
transplanted kidney.
When possible, identifying and correcting the underlying causes
are important steps in treating the acquired forms of proximal RTA.
The diagnosis is based on the chemical analysis of blood and urine
samples. Children with this disorder would likely receive large
doses of oral alkali, such as sodium bicarbonate or potassium
citrate, to treat acidosis and prevent bone disorders, kidney
stones, and growth failure. Correcting acidosis and low potassium
levels restores normal growth patterns, allowing bone to mature
while preventing further renal disease. Vitamin D supplements may
also be needed to help prevent bone problems.
This form of RTA is most often referred to as type 4. It occurs
when blood levels of the hormone aldosterone are low or when the
kidneys do not respond to it. Aldosterone directs the kidneys to
regulate the levels of sodium, potassium, and chloride in the blood.
Type 4 RTA is distinguished by a high blood-potassium level.
Hyperkalemic distal RTA may result from sickle cell disease,
urinary tract obstruction, lupus, amyloidosis, or
transplantation.
Aldosterone's action may be impeded by drugs, including
- diuretics used to treat congestive heart failure such as
spironolactone or eplerenone
- blood pressure drugs called angiotensin-converting enzyme
(ACE) inhibitors and angiotensin receptor blockers (ARBs)
- the antibiotic trimethoprim
- an agent called heparin that keeps blood from clotting
- the antibiotic pentamidine, which is used used to treat
pneumonia
- a class of painkillers called nonsteroidal anti-inflammatory
drugs (NSAIDs)
- some immunosuppressive drugs used to prevent transplant
rejection
For people who produce aldosterone but cannot use it, researchers
have now identified the genetic basis for their body's resistance to
the hormone. To treat type 4 RTA successfully, patients may require
alkaline agents to correct acidosis as well as medication to lower
the potassium in their blood.
If treated early, most people with RTA will not develop permanent
kidney failure. Therefore, the goal is early recognition and
adequate therapy, which will need to be maintained and monitored
throughout the patient's lifetime.
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The National Institute of Diabetes and Digestive and Kidney
Diseases (NIDDK) conducts and supports research into many kinds of
kidney disease, including renal tubular acidosis. NIDDK-supported
researchers are exploring the genetic and molecular mechanisms that
control acid-base regulation in the kidney. These studies will point
the way to more effective treatments for RTA.
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American Association of Kidney
Patients
American Kidney Fund
National Kidney Foundation
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