Renal artery stenosis
Updates to Article Attributes
Renal artery stenosis (RAS) refers to a narrowing of a renal artery. When the process occurs slowly, it leads to secondary hypertension. Acute renal artery stenosis does not lead to hypersecretion of renin.
Pathology
When the stenosis occurs slowly, collateral vessels form and supply the kidney. The kidney wrongly senses the reduced flow as low blood pressure (via juxtaglomerular apparatus) and releases large amount of renin, that converts angiotensinogen to angiotensin I. Angiotensin I is then converted to angiotensin II with the help of angiotensin converting enzyme (ACE) in lungs. Angiotensin II is responsible for vasoconstriction and release of aldosterone which causes sodium and water retention, thus resulting in secondary hypertension.
Renal artery stenosis may be caused by several pathological processes:
- atherosclerosis ~75% involves proximal renal artery
- fibromuscular dysplasia ~20% involves distal renal artery
- vasculitides (especially polyarteritis nodosa, Takayasu arteritis, radiation)
- neurofibromatosis type 1
- abdominal aortic coarctation
- aortic dissection
- segmental arterial mediolysis
Occurrence is not uncommon following a renal transplant.
Radiographic features
Ultrasound
Ultrasound although most freely available, cheap and often used first line, is relatively operator dependent and time consuming.
- increased renal arterial resistive index (RI): a cut-off value of 0.7 may be a good approximation in clinical practice 9
- RI difference between kidneys >5% 9
- increased peak systolic velocity (PSV): some advocate 180cm/s 4
- increased renal-interlobar ratio (RIR): some advocate values greater than 5 3
- increased renal aortic ratio (RAR) i.e. PSVrenal/PSVaorta: usually taken as >3.5 although some advocate greater than 3 4
- turbulent flow in post stenotic area
- pulsus parvus et tardus waveform (slow-rising) due to stenosis
CT angiography
The three-dimensional reconstruction of the renal vascular tree provides a reliable method of visualizing the whole vascular tree. Images are acquired with thin collimation and bolus tracking on the abdominal aorta. Sensitivity and specificity varying between 90 to 99% have been reported 7. Both the raw data and 3D reconstructions should be viewed. Additionally supranumery arteries may be identified.
MR angiography
Two different imaging methods can be used:
- time of flight (TOF): whereby the high velocity of the blood jet at the level of stenosis appears as a loss of signal (black)
- phase contrast technique: wherein gadolinium is used as a contrast agent
Three-dimensional reconstruction technique offers a sensitivity and specificity values around 90 to 100% 7. In some cases renal impairment doesn'tdoes not permit the use of contrast, in which case TOF imaging is beneficial.
Nuclear imaging
ACE inhibitor scintigraphy
- affected kidney with renovascular hypertension shows impaired function due to ACE inhibition; based on this principle scintigraphy has been very much useful for diagnosis of renal artery stenosis
- it is performed by IV administration of enalapril maleate after 15 minutes
- sequential images and scintigraphic curves are drawn for renal cortex and pelvis; renal uptake is measured for every 1-2 min interval after giving IV injection
- typical isotopes used are
Tc99 mTc-99m MAG3, Tc-99m DTPA or I131OIH-123 0-iodohippurate 6. - scintigram will be intereprted as either low, intermediate or high probability
-</ul><p>Three-dimensional reconstruction technique offers a sensitivity and specificity values around 90 to 100% <sup>7</sup>. In some cases renal impairment doesn't permit the use of contrast, in which case TOF imaging is beneficial.</p><h4><sup>Nuclear imaging</sup></h4><h5><sup>ACE inhibitor scintigraphy</sup></h5><ul>- +</ul><p>Three-dimensional reconstruction technique offers a sensitivity and specificity values around 90 to 100% <sup>7</sup>. In some cases renal impairment does not permit the use of contrast, in which case TOF imaging is beneficial.</p><h4><sup>Nuclear imaging</sup></h4><h5><sup>ACE inhibitor scintigraphy</sup></h5><ul>
-<li>typical isotopes used are Tc99 m MAG3 or I 131OIH</li>- +<li>typical isotopes used are <a title="Tc-99m MAG3" href="/articles/tc-99m-mag3">Tc-99m MAG3</a>, <a title="Tc-99m DTPA" href="/articles/tc-99m-dtpa">Tc-99m DTPA</a> or I-123 0-iodohippurate <sup>6</sup>.</li>
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