Ureteral Obstruction
Ureterolithiasis
Also known as: Ureterolithiasis, Ureteral calculi, Hydroureter
Ureterolithiasis
Also known as: Ureterolithiasis, Ureteral calculi, Hydroureter
In short
Ureteral obstruction is a critical, life-threatening blockage of the tubes connecting the kidneys to the bladder, most commonly caused by calcium oxalate stones in cats, requiring immediate veterinary intervention.

TL;DR. Ureteral obstruction is a critical, life-threatening blockage of the tubes connecting the kidneys to the bladder, most commonly caused by calcium oxalate stones in cats, requiring immediate veterinary intervention.

A ureteral obstruction occurs when a stone blocks the narrow tube connecting the kidney to the bladder.
To understand a ureteral obstruction, it helps to look at how your pet's urinary system is built. The urinary tract is divided into two main sections: the upper urinary tract and the lower urinary tract. The upper tract consists of the kidneys, which filter waste products and toxins from the bloodstream to produce urine, and the ureters. The ureters are two extremely thin, delicate muscular tubes that transport this urine from each kidney down to the bladder. The lower tract consists of the bladder, which stores the urine, and the urethra, the tube through which urine exits the body.
A ureteral obstruction occurs when one or both of the ureters become partially or completely blocked. When a blockage happens, urine cannot flow freely into the bladder. Instead, it backs up into the kidney. This backup creates intense pressure, causing the ureter to stretch and dilate (a condition known as hydroureter) and the kidney itself to swell with trapped fluid (known as hydronephrosis).
This pressure is highly destructive. Within days, the delicate functional tissue of the kidney is compressed and damaged, which can lead to permanent kidney failure. If both ureters are blocked, or if a pet only has one functioning kidney and its ureter becomes obstructed, the body loses its ability to filter waste products entirely. This leads to a rapid, life-threatening buildup of toxins in the bloodstream, a medical emergency known as post-renal azotemia. While this condition can affect both dogs and cats, it is an especially common and devastating problem in feline medicine.
The overwhelming majority of ureteral obstructions in pets, particularly in cats, are caused by mineral stones that get stuck in the ureter. This condition is scientifically known as ureterolithiasis. These stones typically form first in the kidneys (where they are called nephroliths) and then migrate down into the narrow ureter, where they easily lodge.
In cats, these stones are almost exclusively composed of a mineral compound called calcium oxalate. A leading veterinary surgical textbook notes:
"A more recent case series of cats treated for ureterolithiasis found that approximately 98% of ureteroliths contain calcium oxalate."
Unlike some other types of urinary stones, calcium oxalate stones cannot be dissolved with specialized diets or medications. Once they form and migrate into the ureter, they must either pass on their own (which is rare due to the microscopic diameter of a cat's ureter) or be managed through medical or surgical intervention.
While ureteral stones are a major cause of kidney injury, they represent a small but growing portion of overall urinary stone cases. A leading veterinary internal medicine reference explains:
"Although ureteroliths account for only 2% of the total number of feline uroliths submitted to our laboratory each year, there has been a significant increase in the number of CaOx-containing ureteral calculi submitted over time."
This rising incidence may be linked to increased awareness among veterinarians, better diagnostic imaging, or a genuine increase in the prevalence of calcium oxalate stone formation in the pet population.
In addition to stone formation, other less common causes of ureteral obstruction include blood clots, inflammatory debris, strictures (narrowing of the ureter due to scarring), or tumors compressing the ureter from the outside.
Recognizing a ureteral obstruction can be incredibly challenging for pet owners. Cats, in particular, are evolutionary experts at hiding pain and illness. The clinical signs are often highly non-specific and can easily be mistaken for general aging, mild stomach upset, or arthritis.

Cats experiencing ureteral pain often show subtle signs like hiding and a hunched posture.
It is vital to understand the diagnostic significance of blood in the urine. If your pet is urinating blood but is not showing typical signs of a bladder infection (like straining or frequent small urinations), the problem may lie higher up in the urinary tract. A leading veterinary internal medicine reference states:
"Patients may also present with hematuria, without concurrent lower urinary tract signs, such as stranguria, pollakiuria, and dysuria. When hematuria alone is noted in a cat without concurrent lower urinary tract signs, evaluation for renal and/or ureteroliths is warranted."
If your pet stops producing urine entirely, is vomiting repeatedly, or is extremely weak, this is a red-flag emergency that requires immediate veterinary care.
Diagnosing a ureteral obstruction requires a systematic approach, combining physical examination, blood work, and advanced imaging. During the physical exam, your vet will carefully palpate your pet's abdomen. They may feel one abnormally large kidney and one abnormally small kidney. This clinical presentation is known as "big kidney, little kidney" syndrome. It often occurs when one kidney was silently destroyed by a past, unnoticed blockage and has shrunken over time, while the other kidney has swollen to compensate or is currently facing an acute blockage.
A leading veterinary internal medicine reference describes this visual finding on imaging:
"Lateral radiograph of a cat that illustrates the so-called big kidney, little kidney syndrome. The smaller kidney is superimposed over the larger kidney in this projection."
To confirm a diagnosis and locate the blockage, your vet will utilize several key diagnostic tests:
"During conservative management, it is crucial to critically evaluate patient stability and fluid status. Patients should be monitored by serial measurements of serum creatinine and blood urea nitrogen concentrations, because these are often the best clinicopathologic indicators currently available that the obstruction has improved or progressed."

Ultrasound imaging allows veterinarians to visualize the swelling and fluid backup in an obstructed kidney.
Treating a ureteral obstruction is complex and depends heavily on how stable the patient is, whether one or both kidneys are blocked, and the severity of the kidney damage.
If your pet is stable, hydrated, and does not have life-threateningly high kidney values, your vet may attempt conservative medical management to help the stone pass naturally.
If first-line therapies are not enough, your vet may add medications designed to relax the smooth muscles of the ureter, making it easier for the stone to slide through.
If medical therapy fails to move the stone within 24 to 72 hours, or if the pet's kidney values continue to climb, emergency intervention is required. Traditional surgery to cut open the ureter and remove the stone (ureterotomy) is highly challenging and carries a high risk of complications due to the microscopic size of the tubes.
Instead, modern veterinary medicine often utilizes advanced interventional techniques. These include placing a Subcutaneous Ureteral Bypass (SUB) system—an artificial tube that bypasses the blocked ureter entirely to carry urine directly from the kidney to the bladder—or placing a ureteral stent, which is a tiny, flexible tube inserted inside the ureter to hold it open.
The prognosis for pets with ureteral obstruction is guarded to fair, and the condition requires a significant commitment to long-term care.
Statistics show that the 12-month survival rate for cats undergoing medical treatment for ureteral stones is approximately 66%. However, about 32% of patients fail to respond to conservative medical therapy and either die or are humanely euthanized within the first month due to progressive kidney failure.
For pets that undergo successful surgery or interventional procedures (like SUB placement), the immediate survival rate is higher, but the long-term management remains challenging. There is a high recurrence rate of approximately 40% after surgery, as these pets are prone to forming new calcium oxalate stones. Regular, lifelong veterinary checkups, blood work, and ultrasound monitoring are essential to catch new blockages before they cause irreversible damage.
Because calcium oxalate stones cannot be dissolved with medication or diet, prevention focuses entirely on stopping new stones from forming.
Ureteral obstruction is a Grade 5 veterinary emergency. If you suspect your pet is suffering from a blockage, do not wait to see if they improve. Contact your veterinarian or an emergency animal hospital immediately if you notice any of the following red flags:
While any dog or cat can develop ureteral obstructions, certain breeds have a known genetic predisposition to forming calcium oxalate stones. Himalayan and Persian cats are highly overrepresented in clinical cases. If you own one of these breeds, it is highly recommended to discuss preventative urinary health and routine screening with your veterinarian early in your pet's life.
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
Ureteral obstruction is a critical, life-threatening blockage of the tubes connecting the kidneys to the bladder, most commonly caused by calcium oxalate stones in cats, requiring immediate veterinary intervention.
Anorexia、Hematuria、Lethargy、Vomiting、Weight loss、Lumbar pain、Polydipsia、Polyuria
combination of survey radiography and ultrasonography、Abdominal ultrasonography、Computed tomography、antegrade pyelography、serial measurements of serum creatinine and blood urea nitrogen、survey abdominal radiography
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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