Acute renal failure is defined as a potentially reversible condition characterized by an abrupt deterioration in renal function resulting in impaired excretion and subsequent accumulation of uremic waste products. Acute renal failure is a relatively common emergency presentation in the cat and is often associated with numerous metabolic, electrolyte, fluid balance and acid-base abnormalities. There are many causes of acute renal failure in the cat, including ischemic insult, toxin ingestion, infectious causes, and neoplasia. More recently, ureteral obstruction secondary to calculi has been recognized as a common cause of acute renal failure in cats. The following will discuss the pathophysiology, history, clinical signs, clinicopathologic changes, diagnostic testing, treatment options, and prognosis in cats with acute renal failure secondary to ureterolithiasis.
The first descriptive study on a series of cats with ureteral calculi was published in 1998, and the incidence of ureteral calculi increased steadily over the 18 years of the study. The reason for this increase is likely multifactorial. Factors that may play a role include increased awareness, increased incidence of calcium oxalate ureteroliths, and increased use of advanced diagnostic imaging modalities in cats with acute and/or chronic renal failure. The nonspecific clinical signs exhibited in these cats, combined with the fact that this is a relatively new condition can make early diagnosis challenging. However, chronic ureteral obstruction can lead to permanent and irreversible damage, so early diagnosis and intervention are an integral part of the treatment and management of these cats. The severity of renal compromise in cats affected with ureteral calculi is variable, and in some cats it is actually an incidental finding during diagnostic testing for other reasons. The focus of this talk will be cats with acute renal failure secondary to ureteral obstruction, and therefore many of the physical examination and laboratory findings are applicable to acute renal failure from any inciting cause. It is also important to remember, however, that not all cats with ureteral calculi will have clinical signs as severe as those discussed in this talk.
In cats, the majority of ureteral calculi are composed of calcium oxalate. Unfortunately, the pathogenesis of calcium oxalate stone formation in these cats is not fully understood. Although some of the affected cats are hypercalcemic, this is not a consistent finding. In the cats that are hypercalcemia, no studies have been done to determine whether the hypercalcemia was a causative factor in the formation of calcium oxalate stones, or if the hypercalcemia was secondary to chronic renal failure (secondary renal hyperparathyroidism). In addition, all the previous studies report the total calcium levels, and no studies have been done evaluating ionized calcium levels in cats with calcium oxalate ureteroliths. Calcium oxalate urolithiasis in cats has also been associated with diet. Diets low in sodium or potassium, as well as diets formulated to acidify the urine have been implicated in calcium oxalate stone formation. Further studies evaluating diet in cats with ureteral calcium oxalate calculi are needed. As the incidence of this disease increases, additional investigations into the pathogenesis of this disease may provide new insight into both treatment and prevention.
Clinical and Clinicopathologic Abnormalities
Unfortunately, as with many other disease processes in cats, there are no pathognomonic findings in cats with acute renal failure secondary to ureteral calculi. Affected cats are middle aged with no sex or breed predilection. Clinical signs are generally vague and nonspecific. Common historical findings include anorexia, lethargy, vomiting, weight loss, polyuria and polydipsia. Less commonly reported clinical signs include lower urinary tract signs (stranguria, hematuria, pollakiuria), inappropriate urination, abdominal pain, hypersalivation and obtundation. Presenting clinical signs associated with acute renal failure include halitosis, oral ulceration, lingual necrosis, increased respiratory rate and effort, tremors, ataxia or seizures. The affected kidneys are generally normal to large in size, and they are often painful on palpation. Many of the cats that present with acute renal failure have decreased size of the contralateral kidney, giving rise to the term big kidney little kidney (BKLK).
Although bloodwork will not provide a definitive diagnosis in these cats, it is important to assess both the severity of renal compromise as well as investigate dysfunction in other organ systems. For definitive diagnosis of ureteral obstruction, advanced imaging modalities are required.
Despite the fact that calcium oxalate is present in the majority of ureteroliths, the sensitivity of survey radiographs remains quite low. In the veterinary literature, the sensitivity of survey radiographs alone to identify ureteral obstruction varies from 60–81%. Other radiographic signs may include renomegaly or unequal kidney size, but these changes are nonspecific.
Abdominal ultrasound: Findings on abdominal ultrasound may include renomegaly, hydronephrosis and hydroureter. It is important to note, however, that ureteral dilation may not continue all the way to the level of the calculi. The sensitivity of abdominal ultrasound alone in the aforementioned studies is 77–100%. Although the sensitivity of abdominal ultrasound in 1 veterinary publication was 100%, the specificity was only 33%. In the study which reported a sensitivity of 77% for ultrasound alone, the combination of survey radiographs and abdominal ultrasound increased the sensitivity to 90%.
Additional imaging modalities may be necessary in some cats - both for diagnosis as well as surgical planning. Antegrade pyelography provides contrast enhancement of the ureter, while avoiding systemic contrast administration. This decreases the chances of contrast-induced nephropathy. Under general anesthesia and ultrasound guidance, a needle is passed into the renal pelvis and contrast injected. One potential complications of this procedure is leakage of contrast material from the renal pelvis, resulting in a non-diagnostic study. Other reported complications include laceration of the renal pelvis and hemorrhage into the renal pelvis. In a report of 11 cats in which antegrade pyelography was utilized for suspected ureteral obstruction, the sensitivity was 100%. In addition, the correct anatomic location of the obstruction was identified in 100% of the cats with diagnostic studies. Leakage of contrast material prevented diagnostic interpretation in 5 out of 18 studies. Computed tomography provides another method by which to evaluate the urogenital tract. Although CT does require general anesthesia, the information often provides information for both diagnosis as well as surgical planning. In a study of 163 cats with ureteral obstruction, CT was performed in 7 cats. The CT confirmed dilation of the renal pelvis and ureter in all 7 cats, and identified additional ureteral calculi not visible by other means in three cats.
The first priority in the treatment of cats with acute renal failure secondary to ureteral calculi is stabilization of any life threatening electrolyte abnormalities. Urine output should be monitored closely and this can be done with an indwelling urinary catheter, weighing diapers, or measuring urine output from a litterbox using nonabsorbent litter. Urine output of < 1.0 ml/kg/hour is inadequate, and < 0.5 ml/kg/hour is considered oliguria. Fluid therapy should be assessed regularly, particularly in the oliguric patient as these cats are very susceptible to fluid overload. If urine output remains low and the patient is showing evidence of overhydration, fluid therapy should be adjusted (match ins to outs) and diuretic therapy (Lasix, mannitol) may be necessary to resolve the fluid burden. Mannitol should not be administered to patients that are anuric and overhydrated, as this can exacerbate the hypervolemia. In cats that have life threatening metabolic abnormalities unresponsive to medical management, dialysis (hemo or peritoneal) may be necessary for stabilization prior to surgical intervention.
If the ureteral calculus does not pass despite medical management, surgical intervention is warranted. The main factor in renal recovery in these cats is the duration of ureteral obstruction. In experimental studies, complete recovery can occur if the obstruction is relieved within 4 days, but only 46% of GFR returns after 14 days of obstruction and after 40 days of obstruction almost no GFR returned.
Surgical or interventional procedures for removing ureteral calculi include ureterotomy, ureteroneocystotomy, extracorporeal shockwave lithotripsy, ureteral stents and subcutaneous ureteral bypass (SUB).
The outcome of cats with acute renal failure secondary to ureteral obstruction varies depending on the severity of the renal failure, the degree of underlying renal dysfunction and any concurrent diseases. The reported postoperative complication and mortality rate for cats treated surgically are 31% and 18% respectively. The overall 12 month survival rate reported for cats treated medically is 66%, and the 12 month survival rate for cats treated surgically is 91%. Many cats have persistent azotemia even after the obstruction is relieved.
Feline ureteral obstruction is a common cause of acute renal failure in the cat. The clinical signs and clinicopathologic abnormalities associated with this condition are often vague and nonspecific. Both medical and surgical management are associated with significant morbidity, and many cats will continue to have impaired renal function after definitive treatment. Aggressive diagnosis and treatment are necessary in order to provide the best outcome.
References are available upon request.