Pathophysiology

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Pathophysiology Diagnosis of Renal Artery
Stenosis

• By Saleem Thompson, MSIII

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Renal Artery Stenosis

• Most common cause of surgically correctable
  hypertension.
• May result from atherosclerosis, fibromuscular
  dysplasia, or post traumatic subintimal
  dissections.

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Renovascular Hypertension

• Accounts for only 5 of the total incidence of
  hypertension.
• Accounts for a disproportionate fraction of
  hypertension in children, young adults, and
  new-onset hypertension in the elderly.

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Renovascular Hypertension (cont.)

• Often refractory to medical management
• Pt. requires multiple antihypertensive
  medications to partially control their HTN.
• Other causes of surgical correctable hypertension
  include pheochromocytoma, aldosterone-secreting
  tumors, descending thoracic aortic coarctation.

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Atherosclerotic Renal Artery Lesions

• Usually occur in the proximal to middle portion
  of the renal artery often represent an
  extension of an aortic plaque into the renal
  artery ostium.

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Fibromuscular Dysplasia

• A hyperplastic, fibrosing process of the intima,
  media, or adventitia.
• It is three times more common in women than in
  men.
• Usually occurs in the 2nd to 4th decade.
• Bilateral involvement is seen in as many as 50
  of cases.

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Pathophysiology of RAS

• Critical stenosis of the renal artery causes
  decreased blood pressure flow to the kidney as
  well as decreased GFR.
• This change stimulates the renal JGA to produce
  renin (which catalyzes the conversion of
  angiotensinogen to angiotensin I.
• ACE converts angiotensin I to angiotensin II.

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Action of Angiotensin II

• Acts as a potent vasoconstrictor.
• Stimulates the production of aldosterone, causing
  sodium retention increased plasma volume.
• This combination of vasoconstriction and sodium
  retention causes a profound hypertensive state.

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On Physical Examination

• Most untreated patients have profound diastolic
  hypertension exceeding 120 mm Hg.
• An epigastric or flank bruit may be noted of
  auscultation of the abdomen and suggests
  turbulent flow within the renal artery.

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Diagnosis of RAS

• Renal scan, renal function studies, intravenous
  urography are used as screening studies to
  separate patients with renal vascular
  hypertension from the general hypertensive
  population.
• However the high incidence of false-negative
  results limits reliance on these studies.

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Urogram may indicate

• A decrease in renal size secondary to
  hypoperfusion.
• Ureteral notching as a result of compression by
  collateral blood vessels.
• A delayed nephrogram with hyperconcentration of
  contrast.
• An exclusion of other primary renal parenchymal
  pathology.

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Renal Functional Studies(Captopril Challenge
Test)

• Captopril, an ACE inhibitor, prevents the
  conversion of Angio-I to Angio-II.
• When ACE is blocked, renin increases.
• This leads to a decrease in the glomerular
  filtration rate because of the blockage of the
  effect of Angio-II on the efferent arteriole of
  the renal glomeruli.
• Unfortunately, in patients with bilateral renal
  artery stenosis, the ratio of renin renal vein
  sampling may be 1.

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Renal duplex Ultrasonography

• Can assess the flow velocity profile in both
  renal arteries as well as the juxtarenal aorta.
• A significant difference in renal artery
  velocities suggests the presence of a
  hemodynamically significant renal artery
  stenosis.

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Renal Duplex Ultrasonography (cont.)

• This scan can also determine the renal
  parenchymal size to determine whether one of the
  kidneys is atrophying as a result of ischemia.
• The sensitivity specificity for detecting
  significant renal artery stenosis are greater
  than 90.

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Definitive Diagnosis of RAS

• Confirmed by angiography.
• Renal arteriogram not only detects RAS, but also
  assists in determining kidney size by evaluating
  the postinjection nephrogram.

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Case Author(s) Thomas H. Vreeland, M.D., Tom R.
Miller, M.D., Ph.D., Jerold W. Wallis, M.D.,
Henry D. Royal, M.D. , 7/21/94 .
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Treatment

• Antihypertensive medications are usually
  ineffective in controlling renovascular HTN.
• ACE inhibitors are avoided in cases of bilateral
  renal artery stenosis because these drugs
  decrease the GFR may cause renal failure.
• In patients w/severe HTN hemodynamically
  significant RAS, some form of revascularization
  should be considered.

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Treatment (cont.)

• Fibromuscular dysplasia responds exceptionally
  well to percutaneous transluminal balloon
  angioplasty in all age groups. (95 of patients
  show significant improvement)
• Atherosclerotic lesions of the proximal renal
  artery are less responsive to angioplasty. (These
  lesions are more likely to return to their
  original degree of stenosis.)
• Metallic stent after angioplasty provides better
  results.

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When is surgical treatment indicated?

• When is surgical treatment indicated?
• - Recurrent stenosis after angioplasty.
• - Lesions are not correctable with angioplasty.
• Surgical interventions include
• - Endarterectomy of atherosclerotic lesions.
• - Bypass the renal artery from the aorta,
  hepatic arteries, or splenic arteries.

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Clinical Case

• This is a 60 year-old physician otherwise in good
  health who experienced sudden onset of
  hypertension.

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Findings

• For the first study the patient was given 2.5 mg
  enalapril intravenously over a five minute period
  followed by administration of Tc-99m MAG3. There
  is marked asymmetry in function, with normal
  uptake and excretion by the right kidney, but
  significant retention of radiopharmaceutical by
  the left kidney with little or no excretion.
• The second study, performed without
  administration of enalapril, shows dramatically
  improved left renal function. Uptake and
  excretion are only minimally delayed on the left.

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Discussion

• Renal scintigraphy with angiotensin converting
  enzyme inhibitors is a sensitive and specific way
  to screen patients with suspected renovascular
  hypertension. Either the rapid-onset intravenous
  ACE inhibitor enalapril, or the slower-acting
  oral captopril agent may be used. Enalapril is
  probably preferable because of its faster and
  more reproducible effect. If the initial study
  performed with the ACE inhibitor is normal, some
  practitioners will not do the baseline study. If
  the initial study is abnormal, as in the present
  case, it is essential that the baseline
  examination be performed to evaluate the
  possibility of unilateral, non-renovascular renal
  disease.

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Follow-up

• After the renal scinitgraphy an arteriogram was
  performed showing a high-grade left renal artery
  lesion just distal to the ostium with only
  minimal disease on the right. The left renal
  artery lesion was treated by balloon angioplasty
  with only a minimal residual stenosis. The blood
  pressure promptly returned to a level close to
  normal.