Diagnostic Imaging: Ultrasound of cats with chronic renal disease not always black and white
Chronic renal disease is one of the most common ultra-sonographic findings in older cats. If you're doing ultrasounds on cats, you're sure to see signs of chronic renal changes daily.
The normal kidney
Normal kidneys in cats are oval or bean-shaped (Image 1). They're shorter than canine kidneys, but also wider. The left kidney is located caudal to the spleen, and the right kidney is slightly more cranial and deeper when scanning in dorsal recumbency. The length of the kidney should be 3.8-4.4 cm, and the surface is smooth and regular. Compare the echogenicity of the medulla and cortex to the spleen and liver. The order of echogenicity, from hypoechoic to hyperechoic is medulla, cortex, liver, spleen and prostate. Try the mnemonic "My Cat Loves Sunny Places." The cortex should be about twice the thickness of the medulla, and you should be able to differentiate them easily.
Chronic renal disease
One of the things you'll hear in describing kidneys with chronic changes is "poor corticomedullary distinction." If you compare the first image (normal kidney) with the second image (chronic renal disease), you'll see that there is less difference in the shade of gray between the cortex and the medulla. The change in echogenicity implies that there is alteration of the composition of the renal tissue.
Compare the echogenicity of the cortex to the spleen and/or liver if possible, because both the cortex (hyperechoic in Image 3) and medulla (hyperechoic in Image 4) can be affected.
Kidneys affected with chronic renal disease usually are small, and may have altered surface contour. Renal infarcts look like wedge-shaped, hyperechoic areas when acute (base toward the periphery) and an inward defect when chronic (fibrosis and scarring). They can drastically alter the shape of the kidney.
The anechoic area in the distal third of the kidney is the dilated pelvis (asterisk). This is normally a potential space, with no visible fluid within it. Certain diuretic conditions, such as fluid therapy or polyuria/polydipsia, can dilate the pelvis 1-2 mm, but more than that is pathologic. Kidneys with pyelectasia (renal pelvic dilation without loss of renal tissue) may have previous or current obstruction or pyelonephritis.
Mineralization can occur in the pelvis or the parenchyma. In the parenchyma, it often is an irregularly shaped, hyperchoic focus with distal acoustic shadowing. Take care to differentiate this from the arcuate vessels, which are very echogenic but look like double parallel lines.
The kidney in Image 5 has some mineralization of the pelvis that is causing a faint acoustic shadow. This can be hard to tell from a pelvic stone that's at risk for obstructing a ureter.
The end-stage kidney
All of the changes described previously are chronic and progressive. Regardless of the underlying disease, the kidney tissue becomes replaced by fibrous tissue over time. The kidney gets smaller, the cortex often appears equal or greater thickness than the medulla, and eventually they become indistinguishable. A small kidney can be very hard to find, so use your anatomic and vascular landmarks (spleen, aorta, renal artery) if you need to.
Although ultrasound is able to detect changes in chronic renal disease, it is not highly sensitive. Cats can have significant renal dysfunction without ultrasonographic signs of renal disease. Conversely, many cats cope with their chronic renal disease though they have severe ultrasonographic changes. The history and blood work are important in assessing the significance of the findings. These changes are non-specific, so the underlying mechanism can't be determined unless a biopsy is performed.
Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.