Managing ureteral and renal calculi in cats (Proceedings)

Analysis of urolith composition in cats at veterinary urolith centers in North America has demonstrated a progressive decline in the percentage of struvite uroliths since 1970, while at the same time recording a progressive increase in percentage of calcium oxalate uroliths recovered from the urinary tract of cats.  During this time period a progressive rise in the diagnosis of upper tract stones has also been observed.  The majority of upper urinary tract calculi (98%) are calcium oxalate.  The remainder is composed of dried solidified blood clots, and other mineral types.  While the development of calcium oxalate urolithiasis appears to be multifactorial, attempts by pet food companies to reduce the incidence of struvite urolithiasis by dietary manipulation has likely inadvertently contributed to the rise in calcium oxalate uroliths in cats.

Ureteral calculi are challenging to manage in the cat in part because calcium oxalate uroliths cannot be dissolved medically.  The normal feline ureteral diameter is only 0.4 mm (1.2 Fr) and patency can easily be compromised by mild wall swelling, such as what might occur after handling of tissues during surgery.  It is easy to see how a lumen of such small size can create great surgical difficulty, and both significant operative and postoperative risks.

Signalment

Calcium oxalate uroliths are more commonly seen in middle-aged to older cats.  Upper urinary tract stones can be seen in cats of any breed.  Breeds predisposed to calcium oxalate urolithiasis are Persian, Himalayan, and Burmese breeds.

History and physical exam

Clinical signs in upper tract uroliths will often depend on whether stones are blocking urine flow and whether secondary pyelonephritis is present. 

Lethargy, vomiting, and anorexia are typically reported in cats with ureteral obstruction.  Concurrent renal failure is common in patients with obstructive ureteroliths and presenting signs in these cats are often due to uremia.  However, signs may be absent in patients where the contralateral kidney still has normal kidney function, or in cases where stones are not significantly impacting urine flow as they pass, or in cases where tiny non-obstructive mineral fragments are embedded in the ureteral wall.  In the presence of ureteral obstruction, the affected kidneys are often large and may be painful on palpation.  Pronounced pain from ureteral colic is less commonly recognized in animals than in humans.

Most nephroliths are asymptomatic.  However, in cases where pyelonephritis or obstructive uropathy has developed, signs related to these processes may be present (e.g., anorexia, lethargy, abdominal pain, hematuria, etc.).

Nephroliths and ureteroliths are often first discovered when cats are evaluated for azotemia (new or worsening), are worked up for signs of lower urinary tract disease (hematuria, dysuria, pollakiuria, obstruction), or in patients who undergo imaging for other illnesses.  Nephroliths are most commonly found incidentally and infrequently associated with clinical signs. 

Diagnosis

A minimum database consisting of CBC, chemistry, and urinalysis should be performed in all patients in whom upper urinary tract stones are discovered. 

Ureteral calculi may be unilateral or bilateral, and affected cats may present either with acute or chronic renal failure.  Many are azotemic (83% in one study), and acute decompensation of preexisting chronic renal disease is a common clinical scenario in cats presenting with a ureteral obstruction.  Approximately 35% of cats with calcium oxalate stones are also hypercalcemic.  Urinalysis may demonstrate hematuria and/or pyuria.  Urine cultures should be obtained, as renal impairment and the presence of mineral within the urinary tract can both predispose to infection.

Imaging

Calcium oxalate uroliths are radiopaque and are therefore often easily identifiable on radiographs.  Ureteral calculi can be diagnosed on radiographs in 81% of cats, using ultrasound in 77% of cats, and in 90% of cases when both modalities are used.  Ultrasonography is also helpful in determining the degree of hydronephrosis, ureteral dilation, and renal parenchymal effacement in the affected kidney.  Renal pelvis dilation is not specific for obstructive disease and may be absent in early stages of the obstructive process.  After an obstruction, dilation of the urine collection system begins at the renal pelvis and then progresses from the proximal ureter distally, gradually moving towards the site of ureteral obstruction.  The exact level of the ureteral obstruction may therefore not always be initially discernable in patients where a stone is not readily visible.  Differentiating between nephrolithiasis and renal mineralization can be challenging in some cases.

Because obstructive disease due to stones is potentially treatable, all cats presenting with new azotemia or a sudden deterioration of preexisting chronic renal disease should have imaging performed (radiographs, +/- ultrasonography) to investigate for the possibility of ureteral calculi.

Excretory urography (EU) or Ultrasound-guided antegrade pyelography (AP) may be helpful in some patients when the cause of the obstruction is not readily apparent on radiographs or ultrasound and may be more sensitive for assessing the location and extent of obstruction as well as number of uroliths.  EU and AP both require general anesthesia.  Decreased renal function in the obstructed kidney can lead to inadequate opacification of that ureter and may limit interpretation of EU.  Systemic contrast administration also carries the risk of contrast medium-induced nephropathy.  It should be avoided in dehydrated patients.  Antegrade pyelography (AP) can be useful in localizing the site of ureteral obstruction and uses both ultrasonographic and fluoroscopic assistance. Because contrast is injected directly into the renal pelvis and not intravenously, there is minimal risk of contrast medium-induced nephropathy.  However if contrast leaks from the renal pelvis, it may obscure interpretation. AP was nondiagnostic in 1 out of 3 cats in 1 study where its use was documented. Laceration of the pelvis, the dilated proximal ureter (resulting in uroabdomen) and injury to the vascular pedicle are possible.

While not used commonly, computed tomography can identify the number and location of stones and is better at differentiating nephrolithiasis from renal pelvic mineralization.  Nuclear scintigraphy can be useful in assessing remaining renal function of the unaffected kidney in azotemic patients, when planning surgical intervention.

Treatment

Initial treatment in cats with ureteral obstructions consists of medical management to stabilize the patient and to promote passage of the stone.  Medical management should be instituted as quickly as possible to avoid further progression and to reduce the risk for potentially irreparable damage to renal function.  Goals are to improve hydration and acid-base imbalances.  Fluid diuresis and diuretic therapy (e.g., mannitol in cats without cardiac compromise: 0.25-0.5 g/kg over 20-30 minutes followed by a CRI at 1 mg/kg/min for 24 hours) are typically recommended to encourage stone movement towards the bladder.  Additional attempts to improve ureteral relaxation and peristalsis involve the use of amitriptyline (1 mg/kg PO q24h), analgesics, and smooth muscle relaxants (e.g., prazosin, phenoxybenzamine; benefits unproven).  The use of glucagon to promote stone passage did not show any benefit in the single study which reported its use.  Appropriate therapies to manage the patient's concurrent renal failure should also be provided.  Severely affected cats that do not show rapid improvement with standard medical therapy or those who are oliguric or anuric may best be stabilized with hemodialysis, peritoneal dialysis, or by placing a temporary nephrostomy tube.  Nephrostomy tubes can be challenging to maintain.  Serial imaging (radiography/ultrasonography) should document progressive movement of the ureterolith(s) towards the urinary bladder and a lack of progressive hydronephrosis and hydroureter.  Evaluation of renal values should indicate gradual improvement or stabilization of values.

Lack of progress in renal function and ureterolith movement towards the bladder indicate the need to intervene more aggressively in order to avoid a progressive decline in function of the obstructed kidney.  As time passes, the reversibility of the decline in renal function diminishes.  Options for relief of the obstruction include surgical removal, stenting, implantation of a subcutaneous bypass device, or extracorporeal shock-wave lithotripsy.

Surgical removal of ureteroliths is achieved by ureterotomy, ureteroneocystostomy, or pyelolithotomy and carries considerable risks. The exact procedure chosen depends on the location of the ureterolith.  Surgery requires an operating microscope, microsurgical techniques and should only be attempted by surgeons with expertise in this type of surgery.  Feline ureters pose special surgical challenges because of the extremely small size of their lumen.  Even a minimal amount of postoperative ureteral inflammation or edema can compromise the lumen.  Urine leakage and stricture formation can present significant postoperative problems.  A nephrectomy may be required if the ureter cannot be salvaged and is deemed beyond repair.

Recent preliminary data indicates that ureteral stenting may offer an improved outcome in cats with obstructive ureteroliths that have failed medical therapy.  This novel intervention requires specialized equipment, specialized training to master the technique, and a facility that is able to provide 24-hour critical care monitoring of these often extremely compromised patients.  A stent is placed to bypass the obstruction.  Its presence also allows for gradual ureteral dilation, which may facilitate subsequent spontaneous passage of the ureterolith.  Ureteral stents can also be useful adjuncts in surgical cases to prevent post-procedural obstruction.  The stents are placed either retrograde (from ureteral opening in bladder to the renal pelvis) or antegrade (via percutaneous entry into the renal pelvis, with the double pig-tail catheter extending all the way to the bladder).

Extracorporeal shock-wave lithotripsy uses shock waves generated outside of the body to break up urinary stones.  Feline calcium oxalate uroliths are more difficult to fragment than canine uroliths and feline kidneys are more sensitive to shock-wave-induced renal injury.  Even if stones are reduced to a consistency of fine sand, the small ureteral diameter may still predispose to obstruction from this minute debris in cats.  Availability of ESWL is limited to centers with the equipment and with the expertise in its use. 

The majority of nephroliths, unlike ureteroliths, are asymptomatic and most do not cause obstructive disease unless they move into the ureter. Most nephroliths therefore do not require removal.  However, ongoing monitoring is still indicated long-term.  Surgical removal of nephroliths in cats poses significant risks to renal function and is reserved for patients with an outflow obstruction, persistent UTIs, progressive stone growth accompanied by declining renal function, or signs of marked pain and/or severe hematuria that fail to resolve with medical management.  Stone removal is accomplished either via nephrotomy or pyelolithotomy.  Reduction in renal function and urine leakage are potential complications.

Prevention of upper tract uroliths

Calcium oxalate urolith removal does not alter the factors that promoted its development, and recurrence is therefore unfortunately common.  Calcium oxalate stones cannot be medically dissolved.  The goal of preventative medical treatment protocols is to minimize remaining urolith growth and recurrence.  Preventative strategies should follow currently available guidelines for reducing the risk of calcium oxalate urolithiasis recurrence and for routine monitoring.  Dietary management can reduce urinary saturation with calcium and oxalate.  Enhanced water intake further influences saturation level via dilution of urine. If these measures are insufficient in controlling growth and recurrence of calcium oxalate uroliths, potassium citrate can be considered with the goal of alkalinizing the urine.  Dose should be titrated to a urine pH of 7.0 to 7.5.  If hypercalcemia is present, identify and correct the underlying cause.

References

Cannon AB, Ruby AL, Westropp JL, et al. Evaluation of trends in urolith composition in cats: 5,230 cases (1985–2004). J Am Vet Med Assoc 2007;231:570–576.

Lekcharoensuk C, Osborne CA, Lulich JP, et al. Trends in the frequency of calcium oxalate uroliths in the upper urinary tract of cats. J Am Anim Hosp Assoc 2005;41:39–46.

Westropp JL, Ruby AL, Bailiff NL, et al. Dried solidified blood calculi in the urinary tract of cats. J Vet Intern Med 2006;20:828–834.

Hardie EM, Kyles AE. Management of ureteral obstruction. Vet Clin North Am Small Anim Pract 2004; 34:989–1010.

Kyles AE, Hardie EM, Wooden BG, et al. Clinical, clinicopathologic, radiographic, and Ultrasonographic abnormalities in cats with ureteral calculi: 163 cases (1984–2002). J Am Vet Med Assoc 2005; 226:932–936.

Fisher JR. Acute ureteral obstruction. In: August JR, editor. Consultations in Feline Internal Medicine.5 St. Louis:Elsevier Saunders, 2005;379–387.

Lulich JP, Osborne CA. Upper tract uroliths: questions, answers, questions, in Consultations in Feline Internal Medicine, ed. August JR, Elsevier Saunders, St. Louis, 2005, 399–406

Forman MA, Francey T, Cowgill LD. Use of glucagon in the management of acute ureteral obstruction in 25 cats.J Vet Intern Med 2004: 18, 417 (A).

Lane IF, Labato MA, Adams LG. Lithotripsy. In: August JR, editor. Consultations in Feline Internal Medicine.5 St.Louis: Elsevier Saunders, 2005;407–414.

Adams LG, Williams JC, Jr., McAteer JA, et al. In vitro evaluation of canine and feline urolith fragility by shock wave lithotripsy. Am J Vet Res 2005; 66:1651–1654.

Lane IF. Lithotripsy: an update on urologic applications in small animals. Vet Clin North Am Small Anim Pract 2004; 34:1011–1025.