Sep 24, 2007 - causes of CRF in decreasing prevalence are diabetes mellitus, .... Nephrogenic systemic fibrosis/nephrogenic fibrosing dermopathy (NSF/NFD) ...
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RENAL FAILURE Brad P. Barnett and Satomi Kawamoto
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CLINICAL INFORMATION Radiologic evaluation in the setting of renal failure includes determining the number, size, position, and degree of perfusion of the kidneys, and whether urinary obstruction is present. Obstructive uropathy is a surgically correctable cause of acute renal failure (ARF). Imaging is also useful in detecting underlying vascular or embolic causes of renal failure. The size of the kidneys and the thickness of the parenchyma should be noted. ■ ■ ■
AQ1
Normal adult kidney size is 9–12 cm in length. Normal parenchymal thickness is 2–2.5 cm. Enlarged kidneys with thick parenchyma are usually indicative of an acute disorder, whereas small kidneys with thin parenchyma are often present in chronic renal disease. Causes of ARF are classically placed in one of three categories • Prerenal etiologies account for approximately 70% of all ARF cases and should be diagnosed by nonradiologic methods ( ↑ blood urea nitrogen [BUN] and creatinine with inherently normal renal function). Hypoperfusion secondary to volume depletion, sepsis, cardiac failure, liver failure, burns, bilateral renal artery stenosis, or pharmacologic agents (such as cyclooxygenase [COX] inhibitors or angiotensin-converting enzyme [ACE] inhibitors) are the most common causes of prerenal failure. • Renal etiologies result from damage to the kidney parenchyma itself. Acute tubular necrosis is the most common type of intrinsic renal disease and is found in cases of prolonged hypotension, secondary to gram-negative sepsis, trauma, hemorrhage, or direct toxins (e.g., mercuric chloride). Other causes include large vessel disease (thrombosis, emboli, dissection), small vessel disease (vasculitis, thrombotic thrombocytopenic purpura [TTP], and disseminated intravascular coagulation [DIC]), interstitial nephritis (urate, myeloma, drugs), and cortical necrosis. Glomerulonephritis is a relatively uncommon intrinsic renal disease, which may occasionally cause ARF but more commonly presents in the context of chronic renal failure (CRF). • Postrenal failure is secondary to obstruction of urine outflow from the kidneys. To cause renal failure, obstruction must be bilateral unless the contralateral kidney is absent or diseased. The most common sources of obstruction include stricture, prostatic hyperplasia, and bladder neck obstruction. Postrenal failure is a relatively uncommon cause of ARF, accounting for less than 5% of cases.
Causes of CRF are commonly classified according to underlying disease. The major causes of CRF in decreasing prevalence are diabetes mellitus, hypertension, and glomerulonephritis. Disease progression can be characterized by three stages: (1) diminished renal reserve, (2) renal insufficiency (azotemia), and (3) uremia.
IMAGING WITH RADIOGRAPHS Indications. Radiographs of the abdomen are not typically helpful for evaluation of renal failure. However, incidental findings suggestive of ARF can be seen on abdominal films acquired for other reasons. Protocol.
Supine position. Include kidneys and area of ureters and bladder.
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Possible Findings ■ ■ ■ ■ ■
Abnormal renal size, shape, and position. Radiopaque calculi in kidneys, ureters, or bladder. Abnormal gas collections in urosepsis. Calcifications of vascular structures, lymph nodes, cysts, or tumors. Abnormal bones in renal osteodystrophy.
IMAGING WITH ULTRASONOGRAPHY Indications. Because of lack of ionizing radiation and low cost, ultrasonography (US) should be the first imaging modality employed in every new case of renal failure. US is considered the method of choice for differentiating the postrenal and renal causes of ARF after prerenal failure has been excluded clinically. Protocol. Patients should be well hydrated and the bladder should be empty because a distended bladder can cause upper tract dilatation in a normal patient, giving a false impression of obstruction. Kidneys are imaged with at least a 3.5 MHz transducer (preferably, 5.0 MHz for children). Color Doppler is used to evaluate renal perfusion, renal vasculature, and ureteral bladder jets.
Possible Findings (Fig. 35-1) ■ ■
Abnormal number or position. Absent kidney: renal agenesis, previous nephrectomy. Ectopic kidney: pelvic, intrathoracic (rare), crossed fused. Abnormal renal size. Normal adult kidney size is 9–12 cm in length. Normal or large-sized kidneys generally indicate ARF and small kidneys generally indicate CRF. (Exception: adult or infantile polycystic kidney disease and bilateral hydronephrosis.)
Figure 35-1. Sagittal ultrasonographic (US) image of the right kidney demonstrates dilatation of the renal pelvis (arrow) and renal calyces (arrowheads) in a patient who presented with renal failure. Subsequent evaluation demonstrated cervical cancer that caused ureteral obstruction.
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Dilatation of upper urinary tract (calyces, renal pelvis, ureters)/hydronephrosis, indicating obstructive uropathy. The false positive rate is 8–26%. False positives (i.e., nonobstructive dilatation of the pelvocalyceal system) include the following • Reflux nephropathy. • Congenital megacalyces. • Congenital megaloureters and prune belly syndrome. • Parapelvic cysts, calyceal cysts, and polycystic kidney disease may also lead to confusion. False negatives can occur in four circumstances, as follows: • Acute or early obstruction, when dilatation of the collecting system may be minimal to absent. In these cases, intravenous pyelogram (IVP) is more sensitive. • Staghorn calculi, which may obscure visualization of pelvocalyceal dilatation due to dense echoes and acoustic shadowing. • ARF superimposed on chronic obstruction, where urine output is so low that pelvocalyceal or ureteral dilatation cannot occur. • Encasement of the pelvocalyceal system or ureter by a retroperitoneal mass or fibrosis so that these structures are not distensible even in the presence of obstruction. Increased cortical echogenicity (≥ adjacent liver) is seen in renal parenchymal disease. However, the false positive rate is 72% and the false negative is 63%. Decreased cortical echogenicity can be seen in lymphoma, acute pyelonephritis, and acute or subacute renal vein thrombosis. Increased medullary echogenicity can be seen in gouty nephropathy, medullary nephrocalcinosis, renal tubular acidosis, and medullary sponge kidney. Abnormal Doppler waveform in the main renal artery or vein • In renal artery stenosis: (1) peak velocity greater than 100 cm/second for greater than 50% stenosis (sensitivity 0–42%, specificity 79–92%); (2) ratio of peak renal artery velocity to peak aortic velocity greater than 3.5 for greater than 60% stenosis (sensitivity 0–91%, specificity 37–97%). • In renal vein thrombosis: absence of a venous waveform in the renal vein (sensitivity 100%, specificity 75%). Renal calculi, which appear as echogenic foci with acoustic shadowing. Incidental mass.
IMAGING WITH COMPUTED TOMOGRAPHY Indications. Computed tomography (CT) should be employed if US is inconclusive. CT is particularly useful in localizing small or ectopic kidneys, in detecting obstructions and their causes, in evaluating retroperitoneal pathology, and detecting nephrocalcinosis and urolithiasis. Protocol. Noncontrast CT is required to evaluate nephrocalcinosis or urolithiasis. Intravenous (IV) contrast is not required in most cases and may be contraindicated in severe renal failure (especially diabetic nephropathy and multiple myeloma–associated nephropathy). For patients with borderline renal function in which contrast is necessary, N-acetylcysteine (NAC) may be given orally on the day before and on the day of contrast administration unless acute etiologies are suspected and the administration of NAC would delay therapeutic intervention.
Possible Findings (Fig. 35-2) ■ ■ ■
Kidney size, position, and configuration, and degree of cortical atrophy can be evaluated. Obstruction can be confirmed or excluded in many cases and the level of ureteral obstruction can be determined with or without IV contrast. Small or ‘‘nonopaque’’ calculi can be detected. Indinavir (Crixivan) may appear lucent on CT scans. Very rare pure matrix stones of mucoprotein and fibrin may be undetectable on CT.
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Figure 35-2. Contrast-induced acute tubular necrosis. Noncontrast computed tomography (CT) scan obtained in a patient with acute renal failure after cardiac catheterization shows persistent bilateral renal nephrograms (arrows) and vicarious contrast excretion into the gallbladder (arrowhead). No contrast is seen elsewhere. ■ ■ ■ ■
Delayed nephrogram if IV contrast was given. Renal vein thrombosis sensitivity is approximately 100% with dynamic CT. Renal arterial stenosis or dissection requires IV contrast and may require multiplanar or 3D reconstruction. Differential perfusion of the kidneys is an indirect finding. Wedge-shaped perfusion defects compatible with embolic disease.
IMAGING WITH NUCLEAR MEDICINE Indications. Radionucleotide scanning provides a good means of assessing global function and renal reserve, and is helpful in differentiating reversible causes of renal failure such as acute tubular necrosis from other conditions with a poorer prognosis like cortical necrosis. Unfortunately, the accuracy of nuclear medicine in determining global renal function decreases as renal function deteriorates due to predominance of background activity. AQ3
Protocol. 99m Technetium (99m Tc)-diethylene triamine pentaacetic acid (DTPA) (glomerular filtration) and 99m Tc-mercaptoacetyltriglycine (MAG3) (tubular secretion) are the two most commonly employed agents. Typical study includes the following: ■ ■ ■ ■
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Bolus injection. Flow imaging with 1 second/frame for 60 seconds. Delayed imaging with 1 minute/frame for 30 minutes. If persistent collecting system activity is present at 30 minutes, diuretic scan should be performed by administering furosemide (Lasix) 0.5 mg/kg IV. Perform additional imaging with 1 minute/frame for 30 minutes. ACE inhibitor scan requires a baseline study for comparison, which is typically performed with low dose of 99m Tc-DTPA. Following 4–6 hours to allow for tracer washout, another renal scan is performed 1 hour after oral administration of captopril, 50 mg. Before the test, withhold diuretics for 48 hours and ACE inhibitors for at least 1 week.
Possible Findings Normal ■
Renal activity should be seen within 6 seconds after aortic peak in normal kidneys.
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Peak cortical activity should occur at 3–5 minutes. Collecting system activity should appear at 5 minutes.
Obstruction ■
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More than 20 minutes renal parenchymal clearance. Tracer accumulation in dilated nonobstructed collecting system may create a false positive requiring diuretic administration. Lack of response to Lasix within 15 minutes with persistent parenchymal and or collecting system activity. Lasix may not work in patients with severe renal failure resulting in a false-positive study. Nonvisualization of ureter in severe obstruction.
Renovascular Hypertension ■
Unilateral renal artery stenosis results in the following: • Decreased flow. • Prolonged cortical transit time. • Prolonged washout. • Bilateral renal artery stenosis is a common cause of false negative study.
IMAGING WITH MAGNETIC RESONANCE Indication. Magnetic resonance imaging (MRI) is a useful alternative to CT in patients in whom the use of iodinated contrast material is contraindicated due to the perceived negligible nephrotoxicity of gadolinium(Gd)- based contrast media. The indications and findings are similar to those of CT, although magnetic resonance angiography (MRA) to assess for renal artery stenosis is the most common indication. Further, diffusion-weighted (DW) MRI may provide information as to degree of kidney dysfunction, assist in the differentiation of various renal abnormalities, and be applicable for follow-up of patients with glomerulonephritis or tubulointerstitial disease. Protocol. A renal MRI study should include a T1- and T2-weighted image in at least one plane. Fast gradient echo images allow for acquisition of images in time-efficient manner. Gd can be delivered as a bolus with dynamic image acquisition. This approach permits evaluation of the enhancement properties of a focal renal lesion or estimation of the degree of functional impairment associated with hydronephrosis or parenchymal abnormalities.
NOTE OF CAUTION Nephrogenic systemic fibrosis/nephrogenic fibrosing dermopathy (NSF/NFD) is a newly described complication of Gd in patients with renal failure. NSF/NFD is seen in patients who have noticeably advanced renal failure. The disease causes fibrosis of the skin and connective tissues throughout the body. Patients develop skin thickening that may prevent bending and extending joints, resulting in decreased mobility of joints. In addition, patients may experience fibrosis that has spread to other parts of the body such as the diaphragm, muscles in the thigh and lower abdomen, and the interior areas of lung vessels. The clinical course of NSF/NFD is progressive and may be fatal. Before Gd is administered to patients with renal failure, your department’s policy regarding NSF/NFD should be consulted.
ANGIOGRAPHY Indication. Renovascular disease is an important cause of CRF. Approximately 10% of dialysis patients have renovascular disease as their primary diagnosis. Correction of
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renovascular disease by angioplasty or other means can significantly improve renal function. Over recent years, contrast-enhanced MRA and CT angiography have rapidly developed into an acceptable alternative to traditional angiography. Conventional angiography is typically performed if other imaging is inconclusive, or if a stenosis has been detected in a viable kidney and therapeutic angioplasty/stenting is requested.
IMAGE-GUIDED RENAL BIOPSY Indication. To provide definitive diagnosis in renal parenchymal disease and in patients with unexplained ARF image guided renal biopsy is indicated. Small kidneys with parenchymal thickness of 1 cm or less are not usually biopsied due to nonspecific histologic findings and increased associated complication rate. Protocol. Real-time US guidance is most commonly employed, although CT may also be used. The patient is placed prone and instructed to hold breath on inspiration for each needle advance. Core biopsy specimens are usually taken from the lower pole of the kidney just deep to the capsule, thereby obtaining cortex and not medulla. Further Readings Davidson AJ, Hartman DS. Radiology of the kidney and urinary tract, 3rd ed. Philadelphia: WB Saunders, 1999. Grainger RG, Allison DJ. Diagnostic radiology, 4th ed. New York: Churchill Livingstone, 2001:1671–1679. Hagen-Ansert SL. Textbook of diagnostic ultrasonography, 5th ed. St. Louis: Mosby, 2001:274–289. Hideki O, Takase K, Rikimaru H. Quantitative vascular measurements in arterial occlusive disease. Radiographics 2005;25:1141–1158. Kawashima A, Sandler CM, Ernst RD, et al. CT evaluation of renovascular disease. Radiographics 2000;20(5):1321–1340. Kawashima A, Vrtiska TJ, LeRoy AJ, et al. CT Urography. Radiographics 2004;24(Suppl 1): S35–S54. Kim SH. Uroradiology illustrated. Philadelphia: WB Saunders, 2003. Liu R, Deepu N, Joachim I, et al. N-Acetlycysteine for the prevention of contrast-induced nephropathy. J Gen Intern Med 2005;20:193–200. Resnick MI, Rifkin MD. Ultrasonography of the urinary tract, 3rd ed. St. Louis: Mosby, Elsevier Science, 2004. Rumack CM, Wilson SR, Charboneau JW. Diagnostic ultrasound, 3rd ed. St. Louis: Mosby, Elesevier Science, 2005. Sandler CM, Newhouse JH, Amis ES, et al. Textbook of uroradiology, 3rd ed. Williams & Wilkins: Baltimore, 2001. Williamson MR, Smith A. Fundamentals of uroradiology. Philadelphia: WB Saunders, 1999.
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Queries in Part III
AQ1. We have expanded TTP as ‘‘thrombotic thrombocytopenic purpura’’ and DIC as ‘‘disseminated intravascular coagulation’’. Please confirm if this is fine. AQ2. We have defined ‘‘IVP’’ as ‘‘intravenous pyelogram.’’ Kindly confirm if the expansion is correct. AQ3. We have expanded DTPA as ‘‘diethylene triamine pentaacetic acid’’. Please confirm if this is fine.
