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Case Study: Ureteral Calculi in Cats

By Linda E. Luther, DVM, DACVIM (Internal Medicine)
(02/02/06)

Indications

Anorexia
Lethargy
Weight Loss
Vomiting
Abdominal Pain

Case Study

• A ten-year-old neutered male Domestic Longhair cat presented with an acute history of lethargy and vocalization. Physical examination revealed that he was depressed and dehydrated, and he had a painful abdomen when it was palpated.

Lethargy and abdominal pain are nonspecific findings that can be consistent with gastrointestinal disease, pancreatitis, abdominal masses, pyelonephritis and ureteral calculi.

• A complete blood count and blood chemistry profile revealed a neutrophilia, mild azotemia and hypokalemia. A urinalysis revealed a specific gravity of 1.025, pH of 6.5, hematuria and proteinuria. A urine culture was negative. Survey abdominal radiographs revealed a small, mineralized density in the left proximal ureteral region.

These findings suggested that a ureteral calculus was present. Renal failure was diagnosed based on the azotemia and inappropriately concentrated urine. The hypokalemia was likely due to renal failure. The neutrophilia could have been due to stress or an undetected pyelonephritis. A complete abdominal ultrasound was performed to evaluate the kidneys and ureters, and to assess for evidence of ureteral obstruction.

• The abdominal ultrasound revealed moderate to severe left renal pelvic dilation (Figures 1 & 2). The left proximal ureter was tortuous and dilated (0.31 - 0.48 cm) to the level where a calculus (0.30 cm) was seen approximately 2 cm distal from the renal pelvis (Figure 3). The right kidney was larger than the left, and had decreased corticomedullary distinction. There was no evidence of pelvic dilation in that kidney (Figures 4 & 5).

A diagnosis of left ureteral obstruction secondary to a ureteral calculus was made. Concurrent pyelonephritis was considered. Medical therapy with fluids, potassium and antibiotics was given, and surgery was recommended if the calculus did not move. His clinical condition improved 100% with the medical therapy. The owner declined additional follow-up, and 7 months later she reported that he was still clinically well. It is assumed that either the calculus moved distally, or that the condition stopped being painful and the right kidney was providing enough renal function that he was asymptomatic. The assumption is that he still has renal failure.

Diagnosis

Both survey abdominal radiographs and abdominal ultrasound will help to diagnose ureteral calculi in cats, and the methods are complementary, with the sensitivity of detecting a case being highest when both are done (90%). The increasingly common practice of omitting abdominal radiography and performing only abdominal ultrasound may delay a diagnosis of ureteral calculi.

Abdominal Radiography

A lateral survey abdominal radiograph is very useful to examine the retroperitoneal space where the ureters are located without having superimposition of the gastrointestinal tract. The ventrodorsal view is needed to distinguish which ureter is affected. Ureteral calculi are typically composed of calcium oxalate, and are therefore radiopaque. Concurrent renal calculi will frequently be seen, with concurrent cystic or urethral calculi being less common. Renal calculi cannot be distinguished from renal mineralization radiographically.

Contrast radiography (excretory urography or antegrade pyelography) is considered to be the most reliable method for definitive diagnosis of the presence, extent and exact location of ureteral obstruction. For cases when a definitive diagnosis and clinical status is not certain after survey abdominal radiographs and ultrasound has been done, these contrast procedures may be indicated. CT scanning can also be useful to evaluate the kidneys and ureters, and can localize the ureteral obstruction, but this imaging method is frequently less available.

Abdominal Ultrasonography

Abdominal ultrasonography is valuable to evaluate renal parenchymal detail, the presence and degree of renal pelvic dilation, ureteral dilation, renal calculi, and many ureteral calculi. Overlying intestine with gas-shadowing can obscure visualization of some ureteral calculi, especially if the ureter proximal to the calculus is not dilated.

The left kidney is located in the left cranial abdomen, medial and at times dorsal to the spleen and caudal to the stomach. The right kidney is in the right cranial abdomen, somewhat more cranial than the left kidney, just caudal to the liver. In small, thin cats, the two kidneys often are near the midline, and it is not uncommon to be able to visualize both kidneys in one image. An ultrasound exam of the normal kidney reveals three regions. A hyperechoic central region represents the renal sinus and peripelvic fat, a hypoechoic region surrounding the renal sinus represents the medulla, and the outer zone of intermediate echogenicity is the cortex. As the kidney is scanned medially and laterally, diverticulae, interlobar vessels and arcuate arteries can often be seen within the medulla. In the midtransverse view, the hyperechoic renal sinus forms a ‘C’ shape in the left kidney, and a reverse ‘C’ shape in the right kidney. Bright, hyperechoic, linear pelvic diverticulae can be seen within the sinus. The hypoechoic tissue just lateral to the sinus is called the renal crest or papilla.

Nonspecific changes that can be seen with chronic renal disease include increased echogenicity of the renal cortex and decreased renal corticomedullary distinction. The kidneys may also blend into the surrounding tissues. Frequently, normal feline kidneys are hyperechoic due to increased fat deposition in the kidney tubules. An echogenic rim can be seen between the cortex and medulla in diseased as well as in normal kidneys. Renal mineralization can be distinguished from renal calculi by assessing for an acoustic shadow which will occur with calculi, but not mineralization. To optimize visualization of acoustic shadowing, the highest frequency should be chosen, the focal point appropriately set, and the gain and power output settings minimized. Both radiopaque and nonradiopaque calculi will shadow. Renal pelvic dilation, or pylectasia, is seen with fluid diuresis (only mild dilation results from this), ectopic ureters, pyelonephritis and ureteral obstruction. When there is pelvic dilation, an anechoic space is seen between the echogenic linear pelvic diverticulae. Very mild pelvic dilation is most easily detected in the mid transverse view. Observation of renal pelvic dilation should prompt thorough evaluation of the ureters for dilation and calculi or masses, as well as thorough evaluation of the bladder wall in the dorsal trigonal region for masses causing ureteral obstruction.

The normal ureter is not seen ultrasonographically. If there is ureteral dilation, it can often be imaged ultrasonographically, but interference from an overlying gas-filled intestine can interfere with adequate imaging. The ureter will extend medially and caudally from the hilus of the kidney. The renal pelvis will usually be dilated whenever the ureter is, and differential diagnoses for ureteral dilation are similar to those for renal pelvic dilation, although diuresis has not been shown to result in ureteral dilation. Dilation of the ureter and renal pelvis may not be apparent for 4-7 days after ureteral obstruction has occurred.

Treatment & Prognosis

Renal Calculi: Surgical removal of renal calculi is rarely indicated as the risk of renal injury is usually higher than the benefit of removing the calculi.

Ureteral Calculi: If ureteral calculi are nonobstructing, they can be monitored for size changes and movement with serial radiographs. Calculi may continue to move very slowly, over months to years. Medical management consisting of intravenous fluid therapy, diuretics, pain management therapy and drug therapy that causes smooth muscle relaxation, such as amitriptyline, can be considered, although these drugs have not been proven to be effective. Emergency nephrostomy tube placement (surgically or with ultrasound-guidance) and hemodialysis may be indicated in some cases. As most feline ureteral calculi are calcium oxalate, medical dissolution with special diets is not a treatment option.

Surgery is indicated when there is obstruction that is resulting in decreased renal function and destruction of nephrons, when the renal lesions are suspected to be reversible, or when there is unresolved infection. Most cats have preexisting renal dysfunction, thus after therapy for ureteral calculi they will likely still have chronic renal failure. Post-operative complications have been reported to be high (31%), with urine leakage into the abdomen and persistent ureteral obstruction being most common. A 20% perioperative mortality rate has been reported. Taking into account the high perioperative mortality rate, survival of ureteral calculi cats has been observed to be similar with or without surgery (short-term survival [6 months] 72%, and long-term survival [24 months] 66%).

Summary

Ureteral calculi and resulting ureteral obstruction are being observed with increased frequency in cats, and because of the vague clinical signs that are present, may remain undetected in many cases. Abdominal radiographs and ultrasound are indicated in cats with nonspecific clinical signs, as well as in all cats with acute or chronic renal failure. In chronic renal failure cats, ureteral obstruction could lead to further loss of already limited nephrons and result in acute decompensation.

Figure 1. A mid sagittal view of the left kidney demonstrating moderate to severe pelvic dilation. The proximal ureter was also dilated. The cortex blended into the surrounding tissue.

Figure 2. A mid transverse view of the kidney seen in Figure 1.

Figure 3. A mid sagittal view of the left proximal ureter demonstrating dilation extending distally to the region of the ureteral calculus. Note the acoustic shadow that extended from the calculus. Only a small segment of the ureter could be seen in each plane as it was tortuous.

Figure 4. A mid sagittal view of the right kidney from the same cat. Note that the renal pelvic dilation was not present in this kidney. The corticomedullary distinction was decreased.

Figure 5. A mid transverse view of the kidney in Figure 4. The hyperechoic renal sinus forms a reverse ‘C’ shape. The hyperechoic linear pelvic diverticulae can be seen within the sinus.