Ureteral obstructions can be difficult to diagnose and treat in veterinary medicine. This article outlines the diagnostic process and discusses some recent advances in treatment options. Ureters are thick-walled fibromuscular ducts that sit in the retroperitoneal space and connect the renal pelvises with the urinary bladder (Lamb, 1998) (Figure 1). The ureter is lined with transitional cell epithelium surrounded by a connective tissue layer called the lamina propria. Together, these two layers make up the mucosa. The lumen is normally collapsed, opening only when urine passes through (Hardie, 2014). The internal diameter of the feline ureter is approximately 0.4 mm.
Ureterolithiasis is the most common cause of ureteral obstruction in both dogs and cats although ureteral strictures and neoplasia are also reported. More than 98% of feline and 50% of canine uretoliths are reported to be calcium oxalate. These types of stones do not dissolve medically and need to be passed spontaneously, removed surgically via a urethrotomy or medically managed to permit urine passage (Berent, 2011).
Studies in dogs have demonstrated that ureteral pressure increases immediately following obstruction and can take 24 hours after the obstruction is removed to fall. This increased pressure causes renal blood flow to diminish to 40% of its normal rate over the first 24 hours then to 20% of normal by 2 weeks. The excessive pressure is transmitted to the entire nephron and a decrease in glomerular filtration rate occurs (Berent, 2011). The other kidney will respond by increasing the glomerular filtration rate. The longer the ureter remains obstructed, the more damage occurs, which may be irreversible depending on the severity and duration of obstruction. If only one kidney is affected, clinical signs can be mild so cats may not appear unwell until the second ureter becomes obstructed. Cats will not present with raised urea and creatinine until 70% of the kidney nephrons are damaged. This is known as ‘big kidney little kidney’ syndrome, as the previously obstructed kidney is reduced in size (Hattersley, 2015). The smaller kidney results from the initial obstruction and chronic renal failure, while the larger kidney is due to the sudden acute obstruction causing renal pelvis dilation.
Clinical presentation
Clinical signs associated with ureteral obstruction in the cat are non specific. Feline patients typically present with vague signs associated with vomiting, lethargy, anorexia and weight loss. If the patient is azotaemic, then signs of uraemia, such as polyuria, polydipsia, vomiting, uraemia ulcerations and weakness, may be present. Pain on palpation of the affective kidney may be seen in acute obstructions and, on physical examination, it is common to palpate one enlarged kidney and one small kidney (Berent, 2011).
Diagnosis
Ultrasound is the main diagnostic tool used at the author's practice to confirm ureteral obstruction. Renal pelvis dilation and a dilated ureter tapering to a focal point can be seen on ultrasound examination (Figures 2 and 3). A renal pelvis dilation of more than 13 mm on ultrasonographic image is almost always associated with an obstruction, but a diameter of more than 8 mm should also raise suspicion (Hattersley, 2015).
Radiography can also be useful to document the size, number and location of any uroliths. As gas and faeces within the colon can obscure smaller uroliths, an enema may be useful.
Percutaneous antegrade pyelography is mentioned in some literature. This involves inserting a needle into the renal pelvis with ultrasound guidance, withdrawing urine and injecting a contrast material; radiographs can then be taken. This technique incurs the risks of bleeding or urinary leakage, so noninvasive techniques such as ultrasonography and plain radiography are preferable.
Treatment options
Historically, the options for ureteral obstructions were medical management or surgical correction via ureterotomy, ureteral resection and anastomosis or ureteronephrectomy. As the internal diameter of a ureter is only 0.4 mm, surgical intervention can prove problematic and ureteronephrectomy is not an option for bilaterally affected patients.
The most common surgical complications include a uroabdomen from leakage at the surgical site, persistent or recurrent ureteral obstruction (from nephrolith migration, failure to remove all the obstructive stones or surgical oedema) and postsurgical stricture formation.
Medical management, while noninvasive, is unlikely to be successful. Horowitz et al (2013) report that only 8–17% of cases resolve with medical management. It can be considered in patients that are not anuric and that are normokalaemic for 24–48 hours, and involves intravenous fluid therapy and prazosin. Progress should be monitored based on the reduction in the degree of azotaemia and also an improvement or resolution of the affected renal pelvis and ureteral dilation via ultrasound monitoring (Hattersley, 2015).
Newer devices such as the double pigtail ureteral stent and the subcutaneous ureteral bypass device have offered treatment options to patients with ureteral obstructions where medical management has failed while avoiding complications associated with traditional surgical techniques. A ureteral stent is placed to decompress the renal pelvis and restore urine flow to the bladder.
This article will focus on the subcutaneous ureteral bypass device (SUB) as this technique is commonly used in the author's practice (Figure 4).
Surgical intervention should generally be performed as soon as it is safe to do so to maximise the return of renal function. Anuria, metabolic acidosis and hyperkalaemia are all signs that rapid intervention is needed. A significant proportion of these patients will have concurrent conditions such as cardiac disease, so this should be considered when choosing an anaesthetic protocol.
The procedure is performed through a ventral midline laparotomy. The perirenal fat is dissected off the caudal pole of the kidney to expose a portion of renal capsule. Using fluoroscopy, the nephrostomy tube is placed into the renal pelvis and then secured (Figure 5).
A pyelogram is performed to confirm that there is no leakage and that the catheter is placed appropriately.
The urinary bladder catheter is then placed and secured, then leak tested (Figure 6).
Both the nephrostomy and cystostomy tubes are passed through the body wall and attached to the flushing port then secured with non-dissolvable sutures under the skin (Figure 7).
Using a Huber needle, the system is then flushed with saline and the catheters examined carefully at the renal capsule entry point, the apex of the bladder and at both sides of the port for any leakage. In addition, all catheters should be examined for leaks. Fluoroscopy can be used to ensure that there are no kinks and that the catheters are situated within the renal pelvis and the urinary bladder (Figure 8).
Postoperative care
Following surgery, cats are at risk of fluid overload due to postobstructive diuresis and should be monitored carefully. This involves regular weighing and monitoring of urine output. The fluid rate needs to ensure proper hydration and cardiovascular stability, and to improve azotaemia without overloading the patient. Berent (2011) reports that cats are at particularly high risk of developing postobstructive diuresis (sometimes exceeding 100 ml/hour of urine production) and are therefore at risk of fluid overload.
In the author's experience, the majority of patients that develop congestive heart failure after their procedure usually have a normal echocardiogram before anaesthesia. For this reason, all cats are treated as though they are at risk of fluid overload and are monitored accordingly. Regular blood samples are taken to monitor urea, creatinine and electrolyte levels and to ensure these are improving. A jugular catheter is routinely placed at the time of surgery so samples can be taken easily and without stressing the patient.
The literature recommends monitoring central venous pressure to prevent fluid overload although this is not done routinely at the author's practice (Berent, 2011). An oesophagostomy tube is routinely placed at the time of surgery as many of these patients present with anorexia and the tube can also be used to give enteral water to maintain hydration.
The SUB system should be flushed every 3–6 months to ensure no encrustation and full patency. This is done using ultrasound guidance. The fur is clipped over the port and the skin is aseptically prepared. A 22 gauge Huber needle with a three-way tap and extension set is used to take a urinary sample for urinalysis and culture and to flush the system with sterile saline.
Long-term, persistent azotaemia is a widespread problem and monitoring for renal disease progression is needed. Following successful intervention, Berent (2011) reports that 40–50% of cats will experience this.
Management of chronic kidney disease is often necessary, which may involve diet, phosphate binders, antacids and ACE-inhibitors if proteinuria is present. Prognosis for renal recovery depends on the length of obstruction, the cause and degree of obstruction and the postoperative care given. Berent (2011) reports that recovery can take weeks to months and has noted dramatic improvements in creatinine concentrations 4–6 months following ureteral decompression.
Berent et al (2014) also reported that creatinine levels at 3 months after ureteral obstruction surgery were predictive for longer-term survival.
Conclusion
Despite the ureter being a difficult area to access for diagnostic and therapeutic purposes, recognition of ureteral obstruction is increasing due to advances in diagnostic techniques and as veterinary surgeons become more adept at identifying clinical signs.
Medical management may not be effective so surgical intervention is the only hope for some of these patients. Newer interventional techniques such as SUB placement carry lower perioperative complications than traditional uretotomy techniques and address the issue of recurrence of ureteral obstruction. These techniques require extensive training and specialised equipment so are unlikely to be used in general practice.
However, although techniques such as SUB device placement can greatly increase the short-term survival of these cases, most cats will continue to have elevated creatinine levels after surgery as they have chronic kidney changes and renal insufficiency. Therefore, the long-term prognosis is variable. As these techniques are relatively new, there is also a limited amount of data regarding the long-term prognosis of these patients. Having a subcutaneous ureteral bypass device will also mean the patient will require life-long, regular veterinary intervention to ensure patency of the SUB device. This means that patient temperament and owner commitment should be considered when deciding if this technique is the most suitable treatment option for an individual patient.