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.
Diagnosis
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.
Types of RTA
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.
Type 1: Classic Distal RTA
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.
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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.
Type 2: Proximal RTA
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.
Type 4: Hyperkalemic RTA
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 no
