Q: My 13-year-old cat Patches was recently diagnosed with kidney failure. The veterinarian said that urinary stones have formed in her bladder and ureter on one side, and that these may be contributing to her kidney problems because the ones in the ureter are blocking the kidneys’ ability to flow.

He also said that removing the stones would be dangerous because there are so many. What would you recommend?

A: As you are probably aware the kidneys filter the blood of certain waste products and eliminate them in the urine, which flows from the kidney to the bladder through a tube known as the ureter. The urine is then voided through the urethra.

Stones can form or lodge anywhere along that tract and can cause a blockage to the flow of urine. If this occurs in the urethra and the cat or dog is completely obstructed, severe damage to the bladder will occur, often within 24 hours. The bladder may rupture or urine may cease to be produced. Either way, the untreated patient will rapidly become toxic and die.

In your case, multiple stones are in the ureter, which makes both the diagnosis and determination of treatment a bit more difficult. Anyone who even knows someone who has passed a kidney stone is aware that it is an incredibly painful event. In dogs and cats, if it is painful, they don’t tell us.

In fact, a ureter can be completely obstructed and result in complete loss of function in the associated kidney and the owner never notices a problem. Only when there is a complete obstruction and infection at the same time do they usually appear sick enough that we know there is a problem, and this can be rapidly fatal.

Otherwise, we often find these cases when more subtle signs of kidney failure arise, or through bloodwork or incidentally on X-rays.

When a kidney is completely obstructed, it will become permanently damaged due to pressure. Removing an obstruction after one month will only result in about 30-40 percent of normal function regained. After 6 weeks, less than 3 percent will remain after relieving obstruction.

So, recovery is inversely related to the degree and duration of the obstruction. Therefore, early recognition and treatment are critical to the preserving or regaining kidney function.

In many cases, the stones are not completely blocking the flow of urine but create enough of an obstruction that the back pressure slowly destroys the kidney and compromises its function. This can put a cat with declining kidneys into kidney failure.

If there is a single stone, it can often be removed with a relatively simple surgery called a ureterotomy. In these cases, a small incision in the ureter is made and the stone is removed. This is easy when the ureter is enlarged and there is a single stone.

When the ureter is not enlarged and/or when there are multiple stones, this surgery becomes more difficult and riskier. So, in a case like this, which sounds like Patches, we would first assess the kidney and see if there is any apparent function. If the kidney is functional, the obstruction should be removed or bypassed to try and salvage whatever kidney function remains.

In our hospital, we offer a new alternative to the risks faced with opening the ureter. Our surgeons prefer to pass a stent, which is a small plastic tube, from the kidney to the bladder. This will bypass the stone and re-establish flow without cutting into the ureter.

Over time, the stent causes the ureter to dilate and the stone will pass harmlessly into the bladder. This procedure also has the advantage of allowing future stones to pass as well.

In dogs, this procedure can be performed in a minimally invasive manner, utilizing fluoroscopy (moving X-rays) or ultrasound to guide us, while avoiding an open surgery. We use a newly developed coated polyurethane stent. This is an innovative product made from a polymer called ThermoStar.

This product will begin as a firm structure but will soften at body temperature for long-term comfort. This type of procedure has given a completely new direction to an old problem in maintaining the kidney function in stone-forming cats and dogs.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.