Excretory urography (Intravenous urography, intravenous pyelography) is a diagnostic test used to visually clarify renal function in the canine patient, allowing contrast media-enhanced visualization of the kidneys, ureters, urinary bladder and urethra.
Radiographic imaging using intravascular iodinated contrast agents was the mainstay of many advanced diagnostic procedures prior to the 1990s, prior to the advent of diagnostic ultrasonography and CT scans, which resulted in a marked decrease of abdominal and cardiac contrast procedures in many referral veterinary centers. However, it is still a viable procedure in many non-referral veterinary practices.
Although lacking the accuracy of visual inspection afforded with laparotomy, it is a relatively non-invasive, low-risk procedure for detection of renal parenchymal filling defects, renal pelvic dilatation or pelvic defects, pyelonephritis, renal agenesis, renal dysplasia, ectopic kidney, hydroureter, hydronephrosis, ureterolithiasis, ureteral stenosis, ureteral atresia, ectopic ureters and urinary tract ruptures.
This diagnostic test involves induction of general anesthesia and serial radiographic images taken following the intravenous injection of radio-opaque contrast dyes such as diatrizoate meglumine, iothalamte, iohexol or iodixanol. These dyes are categorized according to their physical and chemical properties.
The most commonly used second generation iodinated compounds (iopamidol and iohexol) are low osmolar, non-ionic monomers which have improved vascular tolerability and thus have fewer side effects.
Third generation iodinated contrast media (iodixanol and iotrolan) are iso-osmolar, non-ionic dimers and are reported to be the safest contrast agents to date. However, their high cost limits their use in acute renal injury patients. Anaphylaxis has been reported with use of gadobenate dimeglumine used as an MRI contrast medium, and antihistamines should be employed prior to use of this drug.
Excretory urography is normal indicated in dogs with a history of incontinence, acute renal injury and non-age-related chronic renal disease and complicated urolithiasis cases. Plain radiographs should be performed in most cases prior to this test, to assess normal kidney size (usually bean-shaped and 2.5 - 3.5 times large than the length of the second lumbar vertebra. The right kidney is usually one-half length cranial to the left kidney and have a soft-tissue or watery density throughout and are more dense than perirenal fat.
As a prior workup on cases, contrast cystography should be conducted to exclude urinary bladder position, size or shape and evidence of filling defects. Ultrasonography will also help exclude mass effects, urinary calculi and extrinsic nonskeletal abnormalities such as amyloidosis or renal tumors.
Prior to excretory urography, the canine patient should be fasted for 24 hours and administered a cleansing enema the evening before as well as 1 - 3 hours prior to procedure.
General anesthesia is then induced and an intravenous bolus injection given at 500 mg of iodine/kg bodyweight.
Imaging is best performed using either normal radiography or CT scans, showing greater accuracy than digital fluoroscopic excretory urography but inferior to transurethral cystoscopy when detecting ectopic ureters at the ureterovesicular junction.
Following injection of the iodinated dye, a ventrodorsal radiographs of the abdomen is obtained immediately, then at 1, 5, 10, 20, 40 and 60 minutes after injection, including left to right lateral radiographs at 10 minutes after injection.
The addition of furosemide (at 4 mg/kg), ascorbic acid and low dose dopamine have been shown to improve visualization of the renal pelvis and protect renal tubules from prolonged exposure to concentrated contrast medium..
Side-effects associated with iodinated contrast dyes are not uncommon, including transient vomiting, anaphylactoid reactions, hypotension, tachycardia, muscle tremors, contrast-induced renal failure, urticaria and bronchospasm.
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