With renal artery stenosis, ‘stenosis’ means narrowing, which refers to a progressive
narrowing of the renal artery, which carries blood to the kidney.
This means that the blood downstream of the narrowed spot that goes to the kidney is at
lower pressure, which gets sensed by the kidney.
Since an important role of the kidney is to sense and help the maintain a normal blood
pressure, the kidney then tries to raise blood pressure throughout the body.
Inside the kidney, there are millions of nephrons, each of which help to filter the blood and
then fine-tune the composition of blood by carefully reabsorbing and secreting electrolytes
as fluid passes through various parts of the nephron.
Blood approaches the nephron via the afferent arteriole.
You can remember it as ‘A’ for approach, and then forms a tangle of capillaries called
the glomerulus, before exiting via the efferent arteriole - “e” for exit.
That efferent arteriole goes on to split into another set of capillaries - the vasa recta
- which surround the nephron, and then blood leaves via the venule.
So there are two capillary beds per nephron, usually we think of it going arteriole - capillary
- venule, but in the nephron it goes arteriole - capillary - arteriole - capillary - and
finally venule.
So nephrons have the general shape of the letter “U”, with the beginning and end
portions getting pretty close to each other.
The reason that this matters, is that over here, lining the inside of the afferent arteriole
are endothelial cells.
Wrapped around them are juxtaglomerular cells which are super special smooth muscle cells
that contract down like normal smooth muscle cells, but also have the ability to release
a hormone called renin in response to low blood pressure.
Over here, close to the distal convoluted tubule, there is another special group of
cells that line the tubule called macula densa cells which are sodium-chloride-sensing cells
that detect the sodium concentration in the tubule.
These two clusters of cells work together because if the blood pressure falls, less
blood is filtered and less sodium gets into the tubule.
This is then detected by the macula densa cells and they send out a local prostaglandin
signal that reaches the juxtaglomerular cells and causes them to release renin.
In addition the juxtaglomerular cells are able to directly sense low pressure in the
afferent arteriole, and also respond to sympathetic nerve fibers to release renin.
Now, renin helps to constrict blood vessels and increases sodium reabsorption in the nephrons,
which ultimately causes blood pressure to rise.
Normally, as the blood pressure rises, the juxtaglomerular cells are no longer triggered
to release renin, so balance is restored.
One cause of renal artery stenosis is atherosclerotic plaque buildup, which is where a mix of fat,
calcium, and immune cells form a crusty rim on the inside of the artery.
Another cause, though, is renal fibromuscular dysplasia, where fibro- refers to fibrous
collagen connective tissue, -muscular refers to smooth muscle in the artery wall, and -dysplasia
means abnormal development.
So, renal fibromuscular dysplasia is when something goes wrong with connective tissue
and smooth muscles in the walls of renal artery, resulting in a series of bulges and narrow
spots that leaves the artery looking like a “string of beads” instead of a uniform
cylinder.
Fibromuscular dysplasia classically develops in young women, but the exact underlying cause
is still unclear.
In both atherosclerosis and renal fibromuscular dysplasia, the renal artery narrows and that
causes the blood pressure to stay low in the kidney as well as all the kidney’s nephrons.
As a result, the juxtaglomerular cells release renin, which might raise the blood pressure
in the body and vessels before the stenosis, but the pressure in the kidney and the afferent
arterioles stays low, so they just keep releasing renin.
So renal artery stenosis creates a vicious cycle of high blood pressure, or hypertension,
which puts incredible stress on the blood vessels and can contribute to having a stroke
or heart attack.
Also, since the affected kidney receives less blood than usual, it eventually starts to
atrophy or shrink in size, as the kidney cells die off and get replaced by fibrotic scar
tissue.
If the renal artery stenosis involves only one of the kidneys, then the other kidney
can often pick up the slack.
But if both kidneys are affected, then it can lead to acute kidney injury or acute renal
failure.
Symptoms are often a result of the hypertension, which if it becomes severe, it can cause symptoms
like headaches and blurry vision.
The diagnosis is therefore based on seeing if there’s an underlying cause for hypertension.
Physical examination might reveal a bruit - which is a whooshing sound heard with a
stethoscope over the renal arteries in the abdomen caused by the turbulence of blood
flowing through the stenotic part of the renal artery.
Basic screening labs like serum creatinine and a urinalysis can help assess kidney function
as well, and imaging studies like ultrasound, CT, and MRI can visualize the kidney.
For example, in this CT scan of the abdomen, the atrophied kidney is a lot smaller than
the healthy one on the other side.
Ultimately, since the problem is in the renal artery, a renal arteriography can be done
to visualize the renal artery with dye, to see if there’s evidence of a blockage.
Generally atherosclerosis forms in the beginning portion of the renal artery where it branches
from the aorta, whereas fibromuscular dysplasia develops in the middle to far end of the renal
artery, nearest to the kidney.
Treatment of renal artery stenosis is aimed largely at trying to manage hypertension,
through using medications, as well as adopting healthy eating and exercising habits.
In addition, a surgical procedure called balloon angioplasty can sometimes be used - and this
is where a balloon is used to open up the narrowed portions of the renal artery and,
in rare cases, a tiny stent is left behind to keep the artery propped open.
An alternative option is to simply bypass the stenosis with another artery that is grafted
into place.
Ultimately, if one kidney becomes atrophied and nonfunctional, it also might be necessary
to remove it.
All right, as a quick recap.
Renal artery stenosis is where the artery that carries blood to the kidney becomes more
narrow, which reduces the amount of blood that the kidney receives as well as the blood
pressure.
The kidney senses the low blood pressure and responds by releasing the hormone renin which
increases blood pressure - ultimately causing hypertension.
Over time the lack of blood can also cause the affected kidney
to atrophy.