High Blood Pressure (cont.)
What are the causes of secondary high blood pressure?
As mentioned
previously, 5% of people with hypertension have what is called secondary
hypertension. This means that the hypertension in these individuals is secondary
to (caused by) a specific disorder of a particular organ or blood vessel, such
as the kidney, adrenal gland, or aortic artery.
Renal (kidney) hypertension
Diseases of the kidneys can cause secondary hypertension. This type of secondary
hypertension is called renal hypertension because it is caused by a problem in
the kidneys. One important cause of renal hypertension is narrowing (stenosis)
of the artery that supplies blood to the kidneys (renal artery). In younger
individuals, usually women, the narrowing is caused by a thickening of the
muscular wall of the arteries going
to the kidney (fibromuscular hyperplasia).
In older individuals, the narrowing generally is due to hard, fat-containing
(atherosclerotic) plaques that are blocking the renal artery.
How does narrowing of the renal artery cause
hypertension? First, the narrowed renal artery impairs the circulation of blood to the affected kidney. This
deprivation of blood then stimulates the kidney to produce the hormones, renin
and angiotensin. These hormones, along with aldosterone from the adrenal gland, cause a constriction and increased stiffness (resistance) in the peripheral arteries throughout the body, which results in high blood pressure.
Renal hypertension is usually first suspected when high
blood pressure is found in a young individual or a new onset of high blood
pressure is discovered in an older person. Screening for renal artery narrowing
then may include renal isotope (radioactive) imaging, ultrasonographic (sound
wave) imaging, or magnetic resonance imaging (MRI) of the renal arteries. The
purpose of these tests is to determine whether there is a restricted
blood flow to the kidney and whether angioplasty (removal of the restriction in
the renal arteries) is likely to be beneficial. However, if the ultrasonic
assessment indicates a high resistive index within the kidney (high resistance
to blood flow), angioplasty may not improve the blood pressure because chronic
damage in the kidney from long-standing hypertension already exists. If any of
these tests are abnormal or the doctor's suspicion of renal artery narrowing is high enough,
renal angiography (an x-ray study in which dye is injected into the renal
artery) is done. Angiography is the ultimate test to actually visualize the
narrowed renal artery.
A narrowing of the renal artery may be treated by balloon angioplasty. In this
procedure, the physician threads a long narrow tube (catheter) into the renal
artery. Once the catheter is there, the renal artery is widened by inflating a
balloon at the end of the catheter and placing a permanent stent (a device that stretches the
narrowing) in the artery at the site of the narrowing. This procedure usually
results in an improved blood flow to the kidneys and lower blood pressure.
Moreover, the procedure also preserves the function of the kidney that was
partially deprived of its normal blood supply. Only rarely is surgery needed these days to open up the narrowing of the renal artery.
Any of the other types of chronic kidney disease that reduces the function of
the kidneys can also cause hypertension due to hormonal disturbances and/or
retention of salt.
It is important to remember that not only can kidney disease cause hypertension,
but hypertension can also cause kidney disease. Therefore, all patients with
high blood pressure should be evaluated for the presence of kidney disease so
they can be treated appropriately.
Next: Adrenal gland tumors »
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