Acute ureteral obstruction (Proceedings)

Upper tract uroliths have been relatively rare in cats until the last ten years. Many are asymptomatic and are discovered when lower urinary tract stones are creating clinical signs, or when abdominal radiographs or ultrasound are performed for other reasons. However, nephroliths and ureteroliths are being found more frequently in cats presented for acute or chronic renal failure, and present unique challenges to the feline practitioner. Most dramatically, ureteroliths, virtually unreported before 1990, became the most common cause of acute azotemia in cats in this decade. Ureteral obstruction is now a common challenge for practitioners, internists and surgeons in veterinary medicine.

Most (> 70%) upper urinary tract uroliths in cats are now calcium oxalate in composition. Small, multiple stones are common, usually found in both kidneys and/or ureters. Small ureteroliths appear to migrate over time in cats; it is unknown how many cats have passed and voided smaller uroliths prior to detection of upper tract uroliths. Approximately 15% of cats with upper tract uroliths had concurrent lower tract uroliths in one retrospective analysis. Purebred cats, including Siamese, Persians, Burmese and Himalayans, appear to be at increased risk for formation of nephroliths. Upper tract urolithiasis usually affects middle-aged to older cats, although some young (<6 years of age) cats are being presented now with severely compromised urinary tracts, presumably due to previous or long-standing stone disease.

Nephroliths and ureteroliths appear to be associated with chronic kidney disease in cats. While screening cats with chronic kidney disease for a large prospective study, Ross et al found upper tract uroliths in almost 50% of potential subjects. Nephroliths and ureteroliths can lead to upper tract obstruction and deterioration of renal function, and can act as a nidus for persistent bacterial urinary tract infections; nonobstructive nephroliths may be harmful to long term function as well. Obstructing material may include blood or inflammatory debris, rather than a discrete urolith. Obstructive hemoliths (dried solidified blood, or DSB) were recently described in a series of cats.

Table 1. Causes of ureteral obstruction

Pathophysiology

Uroliths and other material are moved down the ureter by passive flow and ureteral peristalsis. In dogs, the ureteral lumen is fairly distensible and can accommodate 2 – 3 mm uroliths easily. The canine ureteral also distends significantly with fluid diuresis. In cats, however the normal ureteral lumen is tiny (0.4 mm) and fairly unforgiving. Although some cats can pass 1 -2 mm obstructions, many are obstructed with the tiniest of stones.

With obstruction, progression varies depending on age, species, degree of obstruction, and whether obstruction is unilateral or bilateral. In general, complete outflow obstruction raises ureteral pressure, which is then transmitted to the proximal tubules and affects renal filtration forces and blood flow. Blood flow and glomerular capillary hydrostatic pressure actually increase initially, features that usually increase the driving force for glomerular filtration, however these increases are soon offset and exceeded by the high opposing pressure in the tubules. Because of this, the net hydrostatic pressure gradient across the glomerular capillaries decreases, resulting in a decline in filtration (decreased GFR). Later (after 5-24 hours), intratubular pressures decline, but glomerular capillary pressures decline faster, so GFR stays significantly decreased (> 50%).

Renal function can deteriorate by 70 – 80% within 2 weeks. Partial obstruction can be expected to have less effect on renal blood flow and GFR. Reversibility depends predominantly on duration of obstruction, but also is affected by severity of obstruction and function of the contralateral kidney. In dogs, complete reversibility may be obtained if the obstruction is removed within 4 – 7 days; reversibility declines to approximately 30% by 4 weeks and 0% by 6 – 8 weeks of complete obstruction. With long term obstruction, histological changes include interstitial fibrosis and tubular basement membrane thickening, signs of irreversible renal damage. In the obstructed ureter, distension is accompanied by smooth muscular hypertrophy, and eventual fibrosis. With this in mind, treatment decisions often need to be made quickly to protect renal function.

Clinical findings in cats

In the affected animal, clinical signs depend on whether an unobstructed, functional contralateral kidney is present. In these cases, clinical signs may be inapparent. Dogs or cats with chronic, nonobstructive or partially obstructive ureteroliths may be evaluated because of mild azotemia. Movement or lodging of a urolith, or rapid distension of a ureter, can cause significant pain. Distension of the renal pelvis, along with ensuing renal parenchymal edema causes stretching of the highly innervated renal capsule, the most likely source of abdominal pain. In cats, pain is usually manifest usually as abdominal or back pain, crouching or hiding behavior, along with anorexia or vomiting. Some cats may exhibit atypical aggression and become quite fractious. Dogs remarkably lack the excruciating pain associated with upper tract urolithiasis in people or cats; however, observant owners have noticed some change in behavior (albeit transient).

Other clinical signs include signs of renal failure or pyelonephritis, vague abdominal or back discomfort and persistent hematuria. More acute illness is observed when previously non-obstructive uroliths migrate and completely obstruct urine flow. Acute azotemia is expected with bilateral obstruction, or acute unilateral obstruction of a sole functional kidney (usually when the other kidney has already failed). Most often these patients are lethargic, anorectic, and may appear painful or exhibit vomiting. Severely uremic cats can appear moribund, hypothermic and hyperkalemic, similar to a cat with advanced urethral obstruction. For example, ureteral obstruction has become the most frequent etiology in cats presenting for hemodialysis. These cats usually have a severe azotemia (median SCr > 17 mg/dl), metabolic acidosis, and often are hyperkalemic (median serum potassium 6.8 mmol/L).

In many cats, renal size is asymmetrical, with one kidney small (end-stage or nonfunctional) and the remaining kidney large due to compensatory hypertrophy or hydronephrosis. Some have hypothesized that the small kidney was previously obstructed and reached end-stage status without detection; however evidence of a previous obstructive lesion or contralateral urolithiasis is not always observed.

Diagnostic approach

Once upper tract obstruction is suspected, the diagnostic workup is aimed at determining the likely nature of the obstruction, composition of the urolith (if present), the structural and functional effects on the urinary tract, and the most appropriate therapeutic plan. A CBC, serum biochemical panel, urinalysis and urine culture is indicated. Abdominal radiographs, sonographic imaging, excretory urography or computed tomography may be required to determine the total number, position and obstructive potential of uroliths or other material.

Nephroliths can be difficult to distinguish from renal parenchymal mineralization, which does not require specific treatment. Radio-opaque intestinal contents, gallstones and calcification of other abdominal structures also can be mistaken for uroliths. Fecal material may interfere with detection of ureteroliths. Despite these limitations, survey radiography is extremely useful for the diagnosis of radio-opaque uroliths, to assess renal size and shape, urinary bladder size and shape, and to screen for fluid accumulation in the abdomen or retroperitoneum, suggesting urinary tract rupture. The retroperitoneal space must be carefully reviewed for evidence of small ureteroliths. Serial abdominal ultrasonography adds to the information gained by radiography; the combination of radiographs and ultrasound detected ureteroliths in 90% of affected cats in one large retrospective study. Renal pelvic dilation and ureteral dilation can be picked up by a skilled sonographer even when quite subtle. Dilation may not develop for several days after acute obstruction, however. It may be impossible to visualize more distal obstructions, or multiple obstructive sites. Multiple sonographic evaluations may be necessary to determine whether a nephrolith or ureterolith is progressively obstructive.

Specialized imaging techniques, including computed tomography and antegrade pyelography, may be particularly helpful in characterizing ureteroliths and other ureteral pathology. Quantitative assessment of individual kidney function is helpful prior to planning surgery or lithotripsy treatment, because structural appearance does not always correlate with function. Additionally, scintigraphic methods may be used to estimate or confirm degree of obstruction.

Management of ureteroliths

     Monitoring and medical strategies

Many ureteroliths will pass into the lower urinary tract over time and can be voided or removed from the urinary bladder. In nonazotemic or mildly azotemic, asymptomatic cats without evidence of obstruction, a "wait and see" approach is reasonable. Complete migration of mobile ureteroliths may take weeks to months. Risks include episodes of acute obstruction or slow progressive deterioration of the kidney(s) and ureter(s) affected. Although creatinine measurements may remain stable for months to years in cats with ureteroliths, creatinine is a fairly crude indicator of renal function. More subtle damage to the urinary tract may go undetected, particularly in cats with two functional kidneys. Regular, serial radiographic and sonographic examinations must be done to determine the rate of change in the urinary tract. It is still unclear whether earlier intervention or cautious monitoring of partially obstructive stones is the best strategy for long term success. In one group of cats reported retrospectively, cats with surgically managed ureteroliths had a greater short-term morbidity and mortality, but better long term outcome, than cats managed medically. Because of the significant morbidity associated with ureteral surgery in cats, availability of a skilled surgeon plays a key role in treatment decisions. Dramatic advances in interventional techniques, including placement of nephrostomy tubes and ureteral stents for short or long term bypass of ureteral obstructions, is extremely helpful in the emergency or peri-operative management of these cats but is challenging in some cases and limited in availability.

Fluid diuresis (up to two or three times maintenance requirements depending on degree of obstruction and renal function) may be employed to facilitate migration of small ureteroliths, and is recommended as a first line attempt in cats with obstructive ureteroliths. Use of smooth muscle relaxants (alpha antagonists, calcium channel blockers), diuretics (particularly mannitol), steroids or non-steroidal anti-inflammatory agents, or glucagon infusion to facilitate ureteral movement is theoretically useful, but the effects of these agents are unproven in cats. Amitriptyline shows promise for facilitating urethral, and possibly ureteral stone passage in cats. Tamsulosin, a uroselective alpha antagonist, along with NSAIDs is the preferred approach to enhance passage of ureteroliths in humans. In dogs and cats, it appears reasonable to monitor partially obstructive stones (or inflammatory material) for 2 – 7 days, provided azotemia and pelvic/ureteral dilation is stable or decreasing. However, some ureteroliths may be embedded in diseased, edematous or inflamed ureters and will not move. Intervention is not recommended for ureteroliths in nonfunctional kidneys.

Table 2: Medical strategies and pharmacologic agents used to facilitate ureterolith passage

     Surgical management

Persistently obstructive ureteroliths must be removed by surgical or lithotripsy methods if renal function is to be protected or preserved. A modified nephrotomy or pyelolithotomy is employed for nephroliths or proximal ureteroliths. Resection and reimplantation of the ureters is effective for surgical management of distal ureteroliths. Ureterotomy also has been successful for the removal of obstructive ureteroliths in some cats; this approach requires a highly experienced surgical team due to the small size of the feline ureter. With a trained surgeon, microscopic surgical techniques can be employed to remove obstructive ureteroliths with good long term outcomes. Renal transplantation has also been used as a salvage procedure; cats may form uroliths in their renal allograft, however. Nephroureterectomy is reserved for severely hydronephrotic, infected, nonfunctional kidneys.

     Lithotripsy treatments

Shock wave lithotripsy for renal pelvic stones or ureteroliths can by accomplished by intracorporeal methods, where shock waves are generated at the tip of an instrument placed directly on the stone, or extracorporeal methods, where shock waves are generated by an electrohydraulic or electromagnetic source outside the body and transmitted to the stone via a water interface (ESWL). ESWL treatments have been successfully adapted to the treatment of canine uroliths for some time and are quite reliable for the treatment of ureteroliths in dogs. Laser lithotripsy methods are also likely to prove useful in medium to large dogs. In those cases, a ureteroscope can be used to reach the ureterolith and apply laser energy. In cats, percutaneous nephrostomy approaches may allow laser access to proximal ureteroliths.

Feline ureteroliths are a bit more challenging to treat with ESWL than are dogs. Sufficient fragmentation for passage of 13 of 14 canine ureteroliths and 6 of 8 feline ureteroliths has been achieved using dry ESWL in our hospital; significant morbidity was associated with the procedure in some cats. Feline fragments must be extremely small to pass through the narrow ureteral lumen. In addition, feline upper tract uroliths appear more difficult to fragment with ESWL than dog uroliths. In addition, transient or permanent worsening of renal function occurred in several cats after ESWL At this time, it appears that lithotripsy may be a reasonable treatment choice for single, partially obstructive ureteroliths in a medically stable cat. Because cats tend to form multiple, recurrent small upper tract uroliths, lithotripsy is unlikely to achieve a stone-free state; recurrent obstruction is possible.

     Complications

All traumatic manipulations of the upper urinary tract have potential risks. Urine leakage and uroabdomen are common, transient complications of renal pelvic or ureteral surgery. Nephrolithotomy and lithotripsy can lead to decreases in renal function over time. With lithotripsy, progressive ureteral obstruction prior to urolith passage, persistent urolith fragments, and ureteral rupture are potential complications. For both methods, intensive peri-operative care is imperative and costs can be high. At a few centers, peri-operative hemodialysis can be utilized to manage azotemia while treatments are planned and carried out, which improves outcome but increases cost.

Recurrence of urolithiasis also is common in dogs and cats. Over the longer term, small animals with upper tract uroliths should have periodic follow-up examinations to assess progression or recurrence of uroliths, renal function, urine pH and composition and to monitor for urinary tract infection. Administration of medications and strategies for chronic kidney disease may be useful in slowly progression of renal interstitial changes and fibrosis. Some patients will remain static for years, whereas others will demonstrate progressive renal failure.

Prognosis

In cats affected with acute ureteral obstruction, it is estimated that 30 – 40% of ureteroliths will spontaneously pass with conservative management. For cats requiring surgery, survival rates of 60 – 76% can be achieved with peri-operative dialytic support. Short term morbidity and mortality are significant, especially in surgically managed cats. Uroabdomen, persistent obstruction and other complications are likely in 25 to 30% of patients. In a retrospective assessment of 153 cats with ureteroliths that survived the first month after initial treatment, 12 month survival rates were 91% in cats treated surgically and 66% in cats treated medically. In dogs, outcome of ureteral lithotripsy or surgery is generally good. In one group of 14 dogs followed after surgical removal of ureteroliths, long term (> 90 day) survival was gained in 13, with a median survival of 904 days. Recurrence of urolithiasis is possible in both dogs and cats.

Key points

• Ureteral obstruction is a common cause of acute azotemia as well as chronic progressive renal disease in dogs and cats.

• Recognition of upper urinary tract obstruction requires imaging studies such as radiographs and ultrasonographic examination.

• Many ureteroliths will spontaneously pass into the lower urinary tract with time and/or diuresis.

• Persistently obstructive ureteroliths require surgical or lithotripsy intervention.

• Prognosis for recovery of urinary function depends on the degree and duration of obstruction

• Prognosis for ureteroliths in dogs is generally good

• Prognosis for ureteroliths in cats is good if the urolith passes, or if surgical or lithotripsy removal is successful prior to significant loss of renal function.

• Short term morbidity and mortality in cats with severe uremia is high unless dialytic support is available (hyperkalemia, acidosis, death).

• Short term morbidity in cats following ureteral surgery is high (urine leakage, renal deterioration, dehiscence, stricture).

References

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