Diagnosis of Ischemic Nephropathy
The tests used to diagnose RAS and ischemic nephropathy are performed by radiologists (experts in imaging procedures, such as x-ray). These tests determine whether there is significant renal artery stenosis (narrowing) in association with ischemic nephropathy. To definitively diagnose RAS, the radiologist performs a renal arteriogram, an imaging test that highlights the renal arteries. This is an invasive procedure and is associated with complications; therefore, many alternative, less invasive tests have been developed to help identify RAS. The tests and their advantages and disadvantages follow.
Renal angiogram (arteriogram) is the "gold standard" of RAS diagnosis, but it can have adverse effects. A contrast dye is injected to obtain images of the renal arteries. Patients with normal or nearly normal kidney function are at minimal risk for kidney damage caused by the injected dye; however, the risk increases in patients whose renal function is already impaired. The greater the impairment of renal function, the greater the risk of damage. Contrast dye can cause acute tubular necrosis (ATN, the death of tubular cells), which usually is reversible (on occasion, dialysis is needed while awaiting renal recovery). Patients with advanced, chronic renal failure (CRF) may progress to end-stage renal disease (ESRD) as a consequence of ATN.
Another possible complication is a cholesterol emboli "shower." This is a rare, potentially devastating complication in which small pieces of cholesterol are dislodged from the walls of the arteries during the test. The emboli (clots or "lugs") get stuck in the smaller blood vessels of the kidneys and other organs causing more ischemia (deficient blood flow) and organ damage.
Ultrasound of the kidney, with Doppler study of the renal arteries creates images of internal organs by means of ultrasound echos. The echos identify tissue-density changes and compare them with the blood flow in underlying vessels. Ultrasound of the kidney provides a useful indication of renal artery stenosis (RAS) if the test is positive. If the test is negative, however, RAS cannot be ruled out. Patients who are obese or are unable to hold their breath are not good candidates for Doppler studies.
If the kidneys are asymmetrical (one kidney being significantly smaller than the other), the physician may suspect that the smaller one has atrophied (shrunk) due to prolonged hypoperfusion (poor blood flow). Kidney atrophy is a hallmark of significant RAS. A shrunken kidney usually is not worth salvaging because irreversible damage has occurred.
Renal scan with ACE challenge is a safe, nuclear medicine test that helps the physician diagnose unilateral (one-sided) RAS in patients with renal vascular hypertension (high blood pressure in the kidney's blood vessels). The test relies on the difference between the kidneys' blood flow, which is accentuated by the use of an ACE (angiotensin, converting enzyme) inhibitor—an antihypertensive (anti-high blood pressure) medication. Renal scanning is not useful in patients with chronic renal failure (CRF), who have impaired function in both kidneys.
Magnetic resonance angiography (MRA) is becoming the test of choice for diagnosing RAS and ischemic nephropathy. This noninvasive test uses magnetic resonance imaging (MRI). Magnetic dye is injected into a peripheral vein, and then the MRI takes pictures of the renal arteries. This dye is not toxic to the kidneys.
Because MRA is relatively new, not all institutions have technicians experienced in administering this test. Patients who are unable to hold their breath have limited results, and patients with pacemakers cannot undergo this test because the MRI scanner is a large magnet.
Significant renal artery stenosis (RAS)—with or without ischemic nephropathy—presents a treatment challenge. Patients must be evaluated individually to determine which option will provide the best possible result. Treatments include the following:
- Angioplasty alone
- Angioplasty (surgical reconstruction of blood vessels) with placement of a stent (device used to support tubular connections)
- Medical therapy
- Surgical revascularization (restoration of blood supply) to bypass the RAS
Aggressive interventions to open the narrowed artery (either surgically or with angioplasty and/or stenting) carry some degree of risk.
RAS tends to progress in most cases. Over time, the kidney suffers reduced circulation, continues to atrophy (shrink), and loses more function. An untreated artery ultimately becomes completely occluded (blocked) in many patients.