Ureteral obstructions are seen in both our canine and feline patients but are a more common problem in cats. Interventional radiology (IR) techniques can have many benefits as discussed in the previous article. This article aims to look at the different treatment approaches adopted for cats and dogs when using IR techniques.
Clinical signs and diagnosis
Ureteral obstructions can occur due to stones, neoplasia, strictures, dried solidified bloodstones or circumcaval ureters (McLauchlan, 2020). Clinical signs are often vague and non-specific in these cases, such as inappetence, weight loss, lethargy and abdominal pain, making it difficult for the owners to spot the problem or where it originates. Ureteral obstructions tend to be seen in middle-aged to geriatric patients, but it is possible to see them in young patients too.
Any patient seen with a suspected ureteral obstruction should have routine bloodwork performed. Most of these patients will show azotaemia on their biochemistry results. They may also have a concurrent hyperkalaemia and hyperphosphataemia due to the post-renal azotaemia. A metabolic acidosis is also common in these patients. If there is only a unilateral obstruction present, cases may be underdiagnosed due to a lack of increase in creatinine values. If there is a suspicion of an obstruction but the patient has creatinine levels within the reference range, then measurement of SDMA levels may prove useful for diagnosis. SDMA increases on average with 40 percent, and as little as 25 percent, loss of kidney function versus creatinine, which does not increase until up to 75 percent of kidney function is lost (Idexx, 2024).
Recent publications have reported on minimal pelvic and ureteral dilation, meaning more subtle obstructions could be easily missed by those less experienced in identifying these cases
Ultrasound of the urinary tract should also be performed in all azotaemic patients to look for dilation of the renal pelvis or proximal ureter. Abdominal radiographs can also be useful to identify stones as long as they are radiopaque, such as calcium oxalate. A diagnosis of a ureteral obstruction should be based on ultrasonographic findings of hydronephrosis and associated hydroureter. Recent publications have reported on minimal pelvic and ureteral dilation, meaning more subtle obstructions could be easily missed by those less experienced in identifying these cases.
Management of ureteral obstruction
In dogs, obstructions are best managed by placement of a ureteral stent which can be placed cystoscopically or surgically (in smaller patients). Ureteral stents have a pigtail on either end meaning they remain secured in the renal pelvis and allow drainage of urine. Stenting is the recommended treatment option in dogs with ureteral obstruction (Lulich et al., 2016). Stents are well tolerated in dogs and when placed cystoscopically can be performed as an outpatient procedure (Figure 1).
The mortality rate for ureteral stenting is around 15 percent, whereas for ureterotomy it is higher at 20 to 25 percent (McLauchlan, 2020). There is also a reduced risk of uroabdomen or post-operative stricture of the ureter when compared to ureterotomy. If required, it is also possible to remove these stents in a simple cystoscopic procedure, using grasping forceps. There is a risk of the patient exhibiting painful or more frequent urination due to the presence of the stent, but this is uncommon in dogs. Patients can also exhibit haematuria. Other risks include stent migration, encrusting of the stent and recurrent urinary tract infections. The recommended management of these stents includes radiographs, bloods and a urine culture every three months.
These stones are not able to be dissolved with medical treatment and diet modification, so they usually require removal or circumvention using traditional or minimally invasive surgical techniques
In cats, the most common cause of obstruction is calcium oxalate urolithiasis. Bartges (2016) commented that calcium oxalate accounts for 40 to 50 percent of urocystoliths in cats. That number is much higher for uroliths of the upper urinary tract. Lulich et al. (2016) reported that over 90 percent of nephroliths and ureteroliths in cats are composed primarily of calcium oxalate. These stones are not able to be dissolved with medical treatment and diet modification, so they usually require removal or circumvention using traditional or minimally invasive surgical techniques. Medical management to dislodge the obstruction can be attempted but this should be given a finite period of time before choosing a surgical method of correction.
Subcutaneous ureteral bypass placement
Ureteral stents are poorly tolerated in cats, causing dysuria due to irritation of the trigone (McLauchlan, 2020). Due to this, a subcutaneous ureteral bypass (SUB) device was designed by Norfolk Vet Products in 2009. The device consists of two catheters (one placed in the renal pelvis and the other in the urinary bladder), a Y or X adapter (depending on whether devices are being placed unilaterally or bilaterally) and a titanium shunting port which is secured to the body wall and sits subcutaneously (Figure 2). The port allows for future flushing and sampling.
The mortality rate for SUB device placement is considerably lower than ureterotomy (10 percent compared to 25 percent). There is no risk of post-operative ureteral stricture as the ureter isn’t opened and in some cases, once the device is placed, the stone causing the obstruction may shift due to a reduction in the back pressure following decompression, allowing patency of the ureter to be restored. The risk of uroabdomen is also decreased compared to ureterotomy.
A full laparotomy under general anaesthesia with fluoroscopic assistance is required for placement. Fluoroscopy is used with iodinated contrast medium (ICM) to confirm the patency of the device and no leakage. These patients are generally hospitalised for 72 hours post-operatively with daily bloods to monitor creatinine and electrolyte levels. Patients also have an oesophageal feeding tube placed at the time of surgery to allow for continued nutrition in the post-operative period as many of these patients will present as inappetant on admission.
In 2018, after evaluating 174 cases of SUB placement procedures, Berent et al. (2018) described the most common post-operative complications as mineralisation of the device, kinking of the device, device occlusion with blood clots and device leakage. Other potential complications are chronic urinary tract infections and dysuria (Berent and Weisse, 2009). Modifications of the SUB device (SUB 3.0 is the current version) have resulted in these complications being significantly reduced (Figure 3).
Regular flushing of the device with ultrasound guidance is recommended long-term and it is important to make the owner aware of this continued commitment, both financially and personally, prior to carrying out the procedure
Regular flushing of the device with ultrasound guidance is recommended long-term and it is important to make the owner aware of this continued commitment, both financially and personally, prior to carrying out the procedure (Figure 4). Initially the device is flushed one month following placement and then every three to four months thereafter. During this flush, a special Huber needle is inserted through the skin and into the port to allow saline to be injected. Ultrasound is used to confirm that the device is still fully patent. A urine sample also needs to be obtained for urinalysis and culture. Finally, tetrasodium ethylenediaminetetraacetic acid (tetra-EDTA) is injected into the port to help reduce mineralisation of the device and biofilm formation. During this recheck, bloods are also obtained to check biochemistry and electrolyte values.
