Upper urinary tract urolithiasis

Introduction

Surgical intervention of the upper urinary tract is most commonly indicated when urolithiasis develops, resulting in partial or complete obstruction of urinary outflow. In cats, > 90% of upper urinary tract uroliths are composed of calcium oxalate (CaOx) although other urolith types, including struvite and dried solidified blood calculi, also occur 1 2. In dogs, CaOx and struvite uroliths are seen with a more equal frequency, with a reported incidence of struvite uroliths in the upper urinary tract between 20-60% 3. When an obstruction is not present, less invasive treatment using medical dissolution is recommended for certain urolith types (struvite and possibly cysteine and purine), but CaOx uroliths will not dissolve with medical management and surgical intervention is often necessary to relieve the obstruction and prevent further renal injury.

Renal and ureteral surgery in cats and dogs can be challenging for even the most experienced surgeon, mainly due to the sheer size of the ureter, particularly in cats. Meticulous surgical technique and appropriate magnification are essential in preventing both short and long-term complications. The choice of surgical treatment often depends on patient presentation, which can vary with regard to the number and location of ureteroliths and whether the disease is unilateral or bilateral, the presence or absence of concurrent nephrolithiasis, and any underlying kidney infection or dysfunction. Additionally, the duration of obstruction likely impacts the recovery of renal function; unfortunately in many cases, and particularly in patients with unilateral disease, this information is unknown. Depending on the type of urolith and location within the urinary tract, treatment may involve a combination of medical, interventional and/or surgical management.

Diagnosis

History and clinical exam

Historical information (including the onset and progression of clinical signs) and physical examination findings, as well as biochemical testing and imaging studies, help guide the clinician as to the best treatment approach for any given animal. A thorough evaluation is critical since many affected patients are older and may have concurrent disease. Animals with non-obstructive nephroliths are often asymptomatic. Cats with ureteroliths may be asymptomatic or display non-specific signs including lethargy, depression, weight loss, fever, anorexia, vomiting, polydipsia and polyuria. Oral uremic ulcers may be observed and hematuria may or may not be present. Dogs more often present with signs of dysuria (e.g., pollakiuria, stranguria, hematuria, polyuria and incontinence) and are frequently systemically ill, since pyelonephritis is commonly seen with obstruction. Abdominal palpation may reveal pain, splinting or renomegaly. A fundic examination should be performed to evaluate the retinas for detachment or hemorrhage which may be an indication of hypertension.

Biochemical testing

Most cats with ureteroliths are azotemic, even with unilateral obstruction, and studies confirm that many affected cats have pre-existing chronic kidney disease (CKD) 2 4 5. In addition cats often have a small, marginally functional, non-obstructed kidney in conjunction with an enlarged, hydronephrotic obstructed contralateral kidney (Big Kidney Little Kidney (BKLK) syndrome) 6. Anemia is often present and may suggest chronicity. An elevated white blood cell count may be seen in cases of ureteritis or to support a diagnosis of pyelonephritis secondary to obstructive urolithiasis.

Routine urinalysis, including culture and evaluation of the urine sediment, should be performed. Assessment of urine pH may help differentiate between CaOx and struvite uroliths and guide medical management during the postoperative period. In dogs, a history of urinary tract infection or evidence of bacteria, pyuria and/or hematuria on sediment evaluation should raise suspicion for struvite urolithiasis. Urease-producing bacteria, including Staphylococcus, Klebsiella and Proteus spp. are most commonly identified in dogs. Urinary tract infections have been identified in up to 32% of cats on presentation, with Escherichia coli identified as the most common isolate 4 7.

Imaging

Uroliths are often visualized on plain survey radiographs, although small or radiolucent ureteroliths, or those overlying vertebral bodies or colonic contents, are occasionally missed; the sensitivity of survey radiography for identifying feline ureteral uroliths is around 81% 7. The use of a compression paddle (e.g., a wooden spoon) during radiography to isolate the ureter from other abdominal viscera may help identification of a ureterolith.

Ultrasonography should be performed in all cases of suspected ureteral obstruction and can have excellent sensitivity and specificity 8. This also provides information as to the degree of hydronephrosis and/or hydroureter, and allows assessment of the renal parenchyma and the retroperitoneal space for any evidence of peri-renal inflammation or effusion. Note that renal pelvic dilation can be seen with other conditions including CKD, pyelonephritis, diuresis and ectopic ureters, and if the obstruction is subacute, pelvic or ureteral dilation may be minimal. In both dogs and cats, a renal pelvic height > 13 mm is consistent with obstruction and in cats, a ureteral diameter > 6 mm is consistent with obstruction 9. In some cases, ureteral dilation may not extend to the level of the obstruction (Figure 1). Ultrasound is also a useful monitoring tool to identify worsening dilation and the need for surgical intervention.

Antegrade pyelography may be indicated in some cases and can facilitate identification of both radiopaque and non-radiopaque causes of obstructions such as dried solidified blood calculi, ureteritis and strictures. The technique can be performed under ultrasonographic and/or fluoroscopic guidance; urine should be collected from the renal pelvis for analysis and culture at the same time. Computed tomography and magnetic resonance imaging are rarely used for cases of upper urinary tract obstruction.

Nephrolithiasis

If nephroliths are identified as incidental findings, patient monitoring without intervention is generally recommended. If urolith composition can be predicted based on signalment, urinalysis and radiographic appearance, medical dissolution should be attempted for amenable urolith types and preventative measures instituted whenever possible. One study in cats diagnosed with concurrent mild or moderate CKD found that surgical removal was not necessarily indicated, since the presence of nephrolithiasis was not associated with disease progression or increased mortality rate 10. Removal is considered when complications such as progressive renal injury, intractable pyelonephritis, obstruction of urine outflow, chronic pain or hematuria develop.

In humans, minimally invasive techniques are employed for treating nephrolithiasis, including extracorporeal shockwave lithotripsy (ESWL) and percutaneous nephrolithotomy. Although these techniques have been performed successfully in dogs, availability is often limited to specialist centers, and ESWL is not an effective treatment for nephrolithiasis in cats. If advanced equipment is not available for minimally invasive treatment, surgical removal is recommended for problematic nephroliths, and there are two options available.

Nephrotomy

The kidney is partially dissected from its retroperitoneal location and the renal artery and vein isolated and temporarily occluded (Figure 2a). A longitudinal incision is made through the convex surface of the kidney on midline (Figure 2b) and the parenchyma incised. Uroliths are removed from the renal pelvis and collecting ducts, and the area flushed with sterile saline. If a small urolith is present, a stab incision may be adequate to place retrieval forceps into the pelvis. Closure is accomplished by approximating the two “halves” of the kidney with the capsule sewn together with monofilament absorbable material in a simple continuous pattern. Vascular occlusion is removed and the two halves of the kidney are held firmly opposed for another five minutes (Figure 2c).

Pyelolithotomy

Pyelolithotomy is rarely performed in dogs and cats but may be warranted for smaller uroliths if dilation of the renal pelvis is present. The kidney is dissected free of its peritoneal attachments and folded medially (occlusion of the renal vasculature is not required) and an incision made over the proximal ureter and pelvis. The stones are removed, and ureteral patency is evaluated by passing a suture distally into the urinary bladder before the proximal ureter and pelvis are closed with a standard suture pattern.

Ureterolithiasis

The human ureter is described as having 3 anatomic sites of narrowing where uroliths typically become lodged, including the ureteropelvic junction (UPJ) where the ureter crosses the iliac vessels, and the ureterovesicular junction (UVJ) where the ureter courses through the bladder wall to the ureteric orifice 11. Males more commonly have uroliths lodged at the UVJ compared to females. In a study evaluating the radiographic distribution of feline ureteral uroliths, the proximal ureter was the most common site for obstruction, and the L4 vertebral body was the most frequently marked urolith location (which may correlate to the UPJ) (Figure 3). As in humans, uroliths located at the UVJ were more common in males and larger uroliths had a more proximal location 12.

If patients are stable on presentation, medical management (including the administration of intravenous fluid therapy alone or with the diuretic mannitol) may be attempted for 24-48 hours to determine if spontaneous passage of uroliths into the bladder will occur. Smooth muscle relaxants (e.g., prazosin), the tricyclic antidepressant amitryptiline and other alpha antagonists (e.g., tamsulosin) have also shown anecdotal efficacy with ureteral relaxation and stone passage 13, but note that aggressive medical therapy can also result in complications, including fluid overload, electrolyte abnormalities, migration of a nephrolith into the ureter or migration of a ureterolith that was causing only a partial obstruction to a location where complete obstruction occurs. In the author’s experience, successful passage of stones with medical management is uncommon and often only identified in patients presenting initially with a distal ureteral obstruction 6 7. Because the incidence of urinary tract infections in dogs is > 50% with ureteral obstructions, broad-spectrum antimicrobial therapy is also indicated 14.

When surgical intervention is necessary, the preferred option (i.e., a traditional surgical procedure, stenting or subcutaneous ureteral bypass (SUB)) is made based on clinical presentation, imaging studies and surgical findings. In certain cases a combination of techniques may be warranted, but the availability and cost of equipment can be a major factor; substantial (e.g., 8-10X) magnification via an operating microscope is usually necessary, although in larger dogs surgical loupes (2.5-4.5X magnification) may be adequate. Placement of ureteral stents also requires a considerable array of specialist equipment including a fluoroscopic C-arm. For cystoscopic stent placement, which is not possible in male cats, a rigid endoscope or flexible ureteroscope may also be necessary. Human ureteral stents can be used for most dogs, but special commercial stents are required for cats.

Ureterotomy and ureteral reimplantation

For urolith removal the author prefers traditional surgical techniques, including ureterotomy and ureteral reimplantation, over the use of long-term implants in order to avoid complications often associated with these devices 15.

A ureterotomy is suitable for patients presenting with one or two stones in the proximal ureter. Once the location of the stone(s) is/are identified (Figure 4a) the affected segment of ureter is isolated using silastic material proximally and distally (Figure 4b); this decreases urine flow into the surgical field and prevents spontaneous retrograde movement of ureteroliths back into the kidney. If the ureter is dilated proximal to the obstruction, a longitudinal incision can be made in this location and the stone gently manipulated out of the ureterotomy site (Figure 5); occasionally, more than one urolith can be removed via the same incision. More commonly, the urolith is embedded within the wall of the ureter and the incision needs to be made directly over the urolith. Care is taken when manipulating the ureter so as not to disrupt the blood supply or inadvertently traumatize the ureter. Following urolith removal, the proximal silastic can be loosened temporarily to verify urine flow from the kidney, and a suture can be passed distally from the ureterotomy site to confirm patency of the distal ureter. Closure of the ureterotomy site is routine, but absorbable material is preferred so that the suture does not act as a nidus for new urolith formation.

Stones lodged in the mid to distal ureter may be removed by ureterotomy, or the ureter can be transected adjacent to the most proximal urolith, the distal portion of the ureter removed, and the ureter reimplanted into the bladder (ureteroneocystostomy) using either an intravesicular or an extravesicular technique (Figure 6); the reader is referred to specialist texts for full details. Although uncommon, ureteroneocystostomy can also be performed when only the proximal third of the ureter is available for anastomosis, using one or more specialist techniques to avoid tension on the tissues postoperatively (Figure 7) 15. These include;

• Renal descensus, where the kidney is mobilized from its retroperitoneal attachments and shifted caudally so that the renal capsule can be sutured to the adjacent body wall.
• Cystopexy, where the bladder is fixed cranially to the body wall or the tendon of the psoas muscle (psoas hitch).
• Nephrocystopexy, where sutures are placed between the caudal pole of the kidney and the apex of the bladder.

Ureteral stenting

In humans with urolithiasis, ureteral stents are often used in conjunction with ureteroscopy and ESWL, usually as a short-term measure to maintain drainage of the kidney until swelling resolves. The stents are often removed within a few days following the procedure, although longer-term placement is possible. If stents do need to be left long term, they are exchanged every few months to prevent complications. In contrast, both temporary and long-term use of ureteral stents are described for dogs and cats 4 16 17 18 19 20. Although cystoscopic placement is common in dogs, the vast majority of cats require laparotomy for ureteral stent placement because of the small size of the feline ureter.

Following ureterotomy, if concerns exist regarding healing of the ureteral incision, a temporary stent can be placed to divert urine during healing; this may also be useful when an obstructive stone has been associated with a pyonephrosis, and again a stent allows urinary diversion and continued drainage of purulent material post-surgery. In these cases, stents are removed approximately one month later. Note that if using a stent in conjunction with a ureterotomy, closure of the incision is aided by placing the stent first. Ureteral stenting has also been used for patients with multiple ureteroliths (unilaterally or bilaterally) with or without the presence of nephroliths (Figure 8a) (Figure 8b). In this situation, stent exchange may be done every few months, but permanent removal is rarely possible.

Ureteral stents can be placed via an anterograde or retrograde approach, although in dogs with ureterolithiasis, cystoscopic placement in a retrograde manner is usual 16 17, whilst anterograde surgical placement is preferred for cats. For both techniques, surgical dissection and digital manipulation of the ureter may be necessary to remove tortuosity and to straighten the ureter prior to guide wire passage. Ureterotomy may also be necessary to facilitate wire and stent passage. Both techniques require fluoroscopy and a high level of surgical expertise.

Potential complications following a traditional surgical intervention include urine leakage and obstruction, but are relatively uncommon. Complications following stenting are more common and include urine leakage and uroabdomen, persistence of the ureteral obstruction or re-obstruction, sterile cystitis, urinary tract infection, and stent migration. Potentially in patients that have concurrent nephrolithiasis and ureterolithiasis, nephroliths can pass into a ureter that was recently unobstructed 6. Various studies have evaluated success rates and long-term outcomes 16 18 19 20 21, with at least one study reporting a 21% incidence of perioperative mortality in cats, although the cause of death was frequently associated with progression of CKD and not related to surgical complications 22. Careful counseling of owners before undertaking such procedures is therefore essential.

Subcutaneous ureteral bypass

The initial indication for using a SUB device was for patients in which stent placement had been unsuccessful or was contraindicated, but in the author’s facility the device has been used most commonly for presumptive strictures in the proximal ureter. The device consists of 2 locking loop pigtail catheters (one placed within the renal pelvis and the other within the urinary bladder) and a shunting port 23. Placement of the device requires fluoroscopic assistance and the renal pelvis should be at least 5 mm in size to allow accurate placement of the renal portion of the system. If the pelvis is small, the catheter can be placed in the proximal ureter without locking the loop. Cyanoacrylate glue is used to help secure the system in place to the kidney and bladder and prevent urine leakage. The access port allows urine samples to be collected for bacterial culture. It is recommended that the port be flushed one month post-surgery and then every three months to help maintain patency. However, as above, potential complications are considerable and include fluid overload, dysuria, persistently high-creatinine levels, catheter malfunction (kinking, obstruction or mineralization), urine leakage, infection, inappetence, and the need for revision surgery 23 24.

New developments

Brief mention should be made of a novel technique recently developed at the author’s facility to treat a proximal ureteral obstruction in a cat. The method is based on a modification of a tubularized bladder flap that relies on the surrounding natural tissues for treatment, and in future may potentially avoid complications with long-term implant use 25.

Surgery of the upper urinary tract in cats and dogs can be challenging, even for the experienced clinician. Regardless of technique chosen, thorough diagnostic evaluation and follow-up for each patient, the availability of equipment, and appropriate surgical training, are all critical to prevent or limit complications with these techniques.