Hydronephrosis in Cats: Why Urine Backs Up Into the Kidney and What Treatment May Involve
Hydronephrosis means the kidney’s collecting system is stretching with trapped urine because normal outflow has been blocked somewhere downstream. In cats, that blockage often involves the ureter, the narrow tube carrying urine from the kidney to the bladder, and the pressure can gradually or suddenly damage kidney tissue. The frightening part is that some cats act only vaguely unwell until kidney values rise or the obstruction becomes complete (MSD Veterinary Manual, 2025; Merindol et al., 2023).
At-a-glance guide
| Question | Quick takeaway |
|---|---|
| What is hydronephrosis? | Dilation of the renal pelvis caused by partial or complete urine outflow obstruction. |
| What causes it in cats? | Most often ureteral stones, but also strictures, infection-related debris, masses, or surgical injury. |
| What signs may owners see? | Vomiting, lethargy, poor appetite, weight loss, pain, or changes in urination; sometimes signs are subtle. |
| How is it diagnosed? | Bloodwork plus abdominal ultrasound, and sometimes radiographs or contrast studies. |
| Can it be treated medically? | Some smaller distal stones may pass, but many cats need decompression or surgery. |
| What is a SUB? | A subcutaneous ureteral bypass that drains urine around an obstructed ureter in selected cases. |
What does hydronephrosis actually mean in a cat’s kidney?
Hydronephrosis is not a separate organ suddenly appearing; it is a structural consequence of urine meeting resistance. MSD defines it as dilation of the renal pelvis caused by partial or complete obstruction of urine outflow from one or both kidneys. As pressure builds upstream, the collecting space expands and the delicate kidney tissue responsible for filtration can be compressed or damaged over time, which is why persistent obstruction threatens kidney function even before a cat looks critically ill (MSD Veterinary Manual, 2025).
The speed of injury depends on whether the blockage is partial or complete, one-sided or bilateral, and whether infection is involved. A cat with one obstructed ureter may still make urine from the other kidney and therefore look only mildly sick at first, while a cat with severe bilateral obstruction can present in acute kidney crisis. Hydronephrosis is therefore both an imaging finding and a signal that the veterinary team must locate the obstruction quickly enough to preserve as much kidney tissue as possible (MSD Veterinary Manual, 2025; Merindol et al., 2023).
What usually causes hydronephrosis in cats?
In modern feline medicine, ureteral obstruction from stones is the cause owners hear about most often, and with good reason. In a 2023 review of benign ureteral obstruction in cats, Merindol and colleagues reported that uroliths accounted for 73% of affected kidneys, while strictures and pyonephrosis each accounted for 13.5%. That breakdown is useful because it reminds owners that stones are common, but not the only explanation; scar-like narrowing and infection-related obstruction are also part of the real-world case load (Merindol et al., 2023).
MSD expands the list further to include blood clots, tumors, iatrogenic ligation, and obstruction at other points in the urinary tract. In plain language, anything that plugs, squeezes, narrows, or accidentally ties off the urine pathway can create back pressure in the kidney. That is why the history matters: a cat with prior stone disease, abdominal surgery, chronic kidney disease, or recurrent urinary issues enters the exam room with a different obstruction risk profile than a young cat with one isolated vomiting episode (MSD Veterinary Manual, 2025).
What symptoms might an owner notice before diagnosis?
Owners often expect dramatic urinary signs, but the first clues may instead be appetite loss, vomiting, weight loss, lethargy, or a suddenly quieter cat. MSD lists lethargy and vomiting among common signs of obstructive uropathy, while Cornell’s urinary-disease overview emphasizes that lower urinary tract disease can produce pain, frequent urination, blood in the urine, or distress in the litter box. Hydronephrosis can overlap with those signs when the obstruction involves the ureter but the outward behavior remains nonspecific (MSD Veterinary Manual, 2025; Cornell Feline Health Center, 2025).
Pain can be especially hard to read because cats rarely point to the kidney region the way a person might hold a sore flank. Some cats become withdrawn, resist being picked up, or hide more. Others continue to urinate enough that owners assume the urinary tract is functioning normally, not realizing one kidney may be obstructed while the other continues working. That mismatch between internal severity and external subtlety is why unexplained vomiting and lethargy in a cat with a urinary history should not be written off as “just an upset stomach” (MSD Veterinary Manual, 2025; FelineVMA, 2024).
How do veterinarians confirm hydronephrosis and find the blockage?
Diagnosis usually starts with bloodwork and urinalysis but rarely ends there. MSD notes that identification of hydronephrotic kidneys on advanced abdominal imaging, together with laboratory abnormalities consistent with postrenal azotemia, supports the diagnosis of obstructive uropathy. Ultrasound is especially valuable because it can show pelvic dilation, ureteral enlargement, and sometimes the obstructing stone or debris directly, while radiographs and other studies add information about stone visibility and anatomy (MSD Veterinary Manual, 2025).
Cornell’s urinary guidance makes a broader point that applies here too: urinary signs often need urinalysis, blood work, x-rays, ultrasound, and sometimes culture because many conditions look similar early on. A cat with hydronephrosis may also need blood pressure assessment, repeat creatinine monitoring, and referral imaging if the obstruction is technically difficult to characterize. In some cases, the diagnosis becomes a serial picture rather than a single snapshot, especially when clinicians are deciding whether medical management is succeeding or the kidney is continuing to dilate (Cornell Feline Health Center, 2025; Merindol et al., 2023).
When to worry
Same-day or emergency care: repeated vomiting, urinary distress, severe lethargy, collapse, or a known stone cat that suddenly stops eating.
Bottom line
Hydronephrosis in cats means urine is backing up into the kidney because outflow is blocked, most often by ureteral stones but sometimes by strictures, infection, or masses. Diagnosis depends on imaging and lab work, medical management helps only selected cases, and many cats need a procedure such as a stent or SUB to protect kidney function. Prompt evaluation matters because the kidney can be losing ground long before the cat looks dramatically sick (MSD Veterinary Manual, 2025; Merindol et al., 2023).
Can hydronephrosis in cats be treated without surgery?
Sometimes, but not reliably enough to assume waiting is always safe. In the 2023 study of benign ureteral obstruction, medical management succeeded in 30% of obstructed kidneys overall, with smaller distal uroliths being more likely to pass and the median time to a successful outcome reported as 16 days. That means medical treatment can work in selected cases, especially with tiny distal stones, but a large share of kidneys either failed medical management or improved only partially (Merindol et al., 2023).
This is why many treatment plans begin with stabilization and rapid reassessment rather than an immediate promise that fluids and time will solve the problem. MSD emphasizes correcting life-threatening fluid and electrolyte abnormalities first, then relieving the obstruction to urine flow. Medical options may include intravenous fluids, pain control, anti-nausea therapy, antibiotics when infection is present, and close imaging follow-up, but owners should understand that ‘trying medicine first’ is still an active, monitored strategy, not casual observation at home (MSD Veterinary Manual, 2025).
When do cats need procedures like stents or a SUB?
A procedure becomes more likely when the obstruction is complete, persistent, associated with worsening kidney values, or anatomically unlikely to resolve on its own. Cornell’s account of a cat treated for ureteral obstruction highlights how modern surgery has shifted outcomes, particularly through use of a subcutaneous ureteral bypass, or SUB. In that system, a tube drains the kidney and diverts urine to the bladder around the blocked ureter, bypassing the obstruction rather than trying to make an irreversibly damaged ureter behave normally again (Cornell University College of Veterinary Medicine, 2023).
SUB devices and ureteral stents are specialist tools, but for the right patient they can be kidney-saving. The same Cornell report notes that complication rates for surgery improved substantially after the development of the SUB system, making these cases less hopeless than they once seemed. Owners should still expect discussions about referral, anesthesia risk, cost, long-term flushing and monitoring of the device, and the possibility that a cat with one damaged kidney may also have underlying chronic kidney disease that affects overall prognosis (Cornell University College of Veterinary Medicine, 2023; MSD Veterinary Manual, 2025).
What hydronephrosis-related symptoms make this urgent?
Urgent care is needed if your cat is repeatedly vomiting, becomes profoundly lethargic, cries or strains in the litter box without passing urine, or shows collapse or severe weakness. Cornell calls urethral obstruction an absolute emergency, and while hydronephrosis often involves the ureter rather than the urethra, the same principle applies: blocked urine flow can destabilize the whole cat, not just the kidney (Cornell Feline Health Center, 2025; MSD Veterinary Manual, 2025).
Cats with only one obstructed kidney may not stop urinating altogether, which can create false reassurance. If a cat with kidney disease or prior stones suddenly loses appetite, vomits daily, or seems painful, same-day evaluation is warranted because imaging may reveal a silent obstruction before irreversible damage accumulates. Kidneys do not complain loudly; they fail quietly until the numbers and the cat both worsen (Merindol et al., 2023; MSD Veterinary Manual, 2025).
Bottom line
Hydronephrosis in cats means urine is backing up into the kidney because outflow is blocked, most often by ureteral stones but sometimes by strictures, infection, or masses. Diagnosis depends on imaging and lab work, medical management helps only selected cases, and many cats need a procedure such as a stent or SUB to protect kidney function. Prompt evaluation matters because the kidney can be losing ground long before the cat looks dramatically sick (MSD Veterinary Manual, 2025; Merindol et al., 2023).
References
- Cornell Feline Health Center. (2025, n.d.). Feline lower urinary tract disease. Cornell University College of Veterinary Medicine. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/feline-lower-urinary-tract-disease
- Cornell University College of Veterinary Medicine. (2023, March 29). Cornell tackles cat’s kidney failure. https://www.vet.cornell.edu/about-us/news/20230329/cornell-tackles-cats-kidney-failure
- Feline Veterinary Medical Association. (2024, January). How do I know if my cat is in pain? https://catvets.com/wp-content/uploads/2024/01/PainManagement-WebView.pdf
- Merindol, I., Vachon, C., Juette, T., & Dunn, M. (2023). Benign ureteral obstruction in cats: Outcome with medical management. Journal of Veterinary Internal Medicine, 37(3), 1047–1058. https://pubmed.ncbi.nlm.nih.gov/37073892/
- MSD Veterinary Manual. (2025, March). Obstructive uropathy in dogs and cats. https://www.msdvetmanual.com/urinary-system/noninfectious-diseases-of-the-urinary-system-in-small-animals/obstructive-uropathy-in-dogs-and-cats