Hey this is Dr K from imedicalschool and today we are going to talk about rental tubular
acidosis.
As the name sounds renal tubular acidosis is when the serum pH becomes more acidic due
to changes in the normal function of the kidneys.
Briefly let's talk about how acid and bases are regulated within the body.
The two main organs that are involved in maintaining adequate acid base balance are the lungs and
the kidneys.
If the serum pH is in the acidic range the lungs are able to expel acid in the form of
carbon dioxide.
On the other hand if the serum pH is alkaline the respiratory rate slows down and retains
CO2 to compensate.
The kidneys act in a similar manner but take longer to compensate for changes in the acid
base equilibrium.
In the future we will discuss renal physiology, but overall the kidney excretes ammonium in
the urine to release acid and in rare circumstances can excrete bicarbonate.
There are three type of renal tubular acidosis.
They are Type1, which affects the distal convoluted tubule, Type 2, which affects the proximal
convoluted tubule, and Type 4 which is due to hypoaldosteronism or a low aldosterone
state.
A type 3 RTA does exist but it is extremely rare and is caused by an enzyme deficiency.
I would not focus on type 3 RTA but really pay attention to types 1,2, and 4.
In terms of presentation the renal tubular acidoses present in a similar fashion.
If you obtain an ABG and basic lab work you may notice a metabolic acidosis with a normal
anion gap.
A normal anion gap with a metabolic acidosis is generally due to renal tubular acidosis,
gastrointestinal losses, or medications.
To differentiate we calculate the urine anion gap.
To do this sum the urine sodium and urine potassium then subtract the urine chloride
from the total.
If the urine anion gap is > 0 then it is due to renal tubular acidosis but if it is negative
then it is due to gastrointestinal losses.
This is an easy way to identify if metabolic acidosis is due to a renal tubular acidosis.
Now lets briefly talk about the anatomy of the nephron.
So the nephron is the basic unit that composes the kidneys.
There are millions of nephrons that comprise the kidney all of different sizes.
Blood flows through the afferent arteriole into the glomerulus.
Around the glomerulus is the Bowman's capsule.
The capsule allows for absorption of ions, water, and waste products into the tubules.
Whatever is not absorbed passes down into the efferent arteriole.
The filtrate that is absorbed passes into the the proximal convoluted tubule where bicarbonate
is absorbed then into the loop of Hnele which helps concentrate the urine and finally into
the distal convoluted tubule.
In the distal convoluted tubule is where most acid secretion takes place in the form of
ammonium and phosphate.
Now lets look at each type of
renal tubular acidosis.
Let's begin by talking about RTA type 1.
As we discussed before RTA type 1 is a process that affects the distal convoluted tubule.
As we remember the distal convoluted tubule is involved in acid secretion so if the distal
convoluted tubules unable to excrete acid we actually produce an alkaline urine.
Ttype I renal tubular acidosis presents with a hyperchloremic metabolic acidosis.
Causes of an RTA type I include autoimmune disorders such as lupus or rheumatoid arthritis,
amphotericin B, lithium, sarcoidosis, other medications, and Wilson's disease., Laboratory
findings include hypokalemia or low potassium.
An important point in the patient's history is if the patient has a history of kidney
stones or nephrolithiasis as kidney stones are commonly seen in RTA type I.
The serum bicarbonate in RTA type I is less than 10 which is a key distinctive factor
of RTA type I. In addition the urine pH is greater than six.
The treatment of RT type I is to give the patient bicarbonate.
The reason why is because the kidneys cannot excrete acid so acid builds up within the
serum.
Sodium bicarbonate helps balance the acid-base physiology.
Now let's proceed to discuss about renal tubular acidosis type II.
RTA type II presents with a hyperchloremic metabolic acidosis just like RTA type I.
The dysfunction in RTA type 2 is an impaired ability to reabsorb bicarbonate specifically
in the proximal tubule.
RTA type II can be due to Fanconi syndrome, a monoclonal gammopathy, such as multiple
myeloma or amyloidosis.
Edition acidIn addition acetazolamide can cause an RTA type II as well as glycogen storage
diseases.
In an RT type II you may see glucose, bicarbonate, and phosphate within the urine.
All patients with RTA type II should be screened for a monoclonal gammopathy such as multiple
myeloma or amyloidosis.
The key findings of an RTA type II include a urine pH greater than seven and a serum
bicarb about 14 to 20.
Now the reason why the urine pH is so high is because the bicarb in the urine cannot
be reabsorbed.
The bicarb actually gets lost with in the urine thus making the urine alkaline.
The treatment of an RTA type II includes diuretics as well as bicarbonate.
Let's finally talk about renal tubular acidosis type IV.
In RTA type 4 we have a hypo-renin and hypoaldosteronism state that is due to either aldosterone insufficiency
or resistance to the effects of aldosterone.
That insufficiency or resistance can be due to primal adrenal insufficiency , antibiotics,
ace inhibitors orspironolactone.
The key diagnostic finding is measuring the renin and aldosterone.
There are other features that you can look at to help diagnose and RTA type 4.
Generally the serum bicarb will be greater than 17 and the urine pH would be less than
5.5 but the key finding is hyperkalemia or elevated potassium.
The treatment of an RTA type four is a low potassium diet and a loop diuretic.
The key points to really take away are that there are three types of renal tubular acidoses
that are clinically significant.
They are RTA type I RTA type II RTA type IV.
RTA type I is distinctive in that the serum bicarb can be very low with a bicarb less
than 10.
In addition these patients are prone to develop kidney stones.
In RTA type 2 the urine pH is very high.
The urine pH is usually greater than seven which is a distinctive feature of RTA type
II.
Finally a distinctive feature of RTA type 4 is hyperkalemia or elevated potassium.
This is a brief review of renal tubular acidosis.
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This is Dr K and I will see you next time.