CONDITION
Hydronephrosis
Hydronephrosis describes swelling of the kidney that occurs when urine cannot drain away normally. The kidney's collecting system—the part that funnels urine toward the ureter and bladder—becomes stretched and enlarged under the pressure of backed-up fluid. This can affect one kidney or both, and may develop suddenly or over weeks to months. Many owners first encounter this term after imaging arranged for other reasons, or when investigating changes such as reduced appetite, weight loss, or discomfort that can accompany kidney problems. In some cases there are no outward signs at all, and the finding is incidental. The degree of swelling does not always match what an animal appears to feel, and the impact on kidney function depends on how complete the blockage is, how long it has been present, and whether the other kidney is working normally. This page explores the patterns that can prompt investigation, what may be causing the obstruction or pressure, how hydronephrosis is identified and monitored, and the range of approaches that exist depending on the underlying cause and the degree of kidney involvement.
Why this matters now
Hydronephrosis can appear at any age, though the underlying causes often follow predictable patterns. Congenital narrowing or malposition of the ureters tends to declare itself in younger animals, whilst stones, tumours, and age-related changes in the lower urinary tract are more commonly seen in middle-aged and older dogs and cats. Certain breeds carry higher risk for stone formation or anatomical variants that predispose to obstruction. In intact female dogs, pressure from masses in the reproductive tract can emerge during or after middle age.
The timeline varies widely depending on whether the obstruction develops suddenly or accumulates gradually. A stone that lodges in the ureter may cause swelling over hours to days, whilst a slowly growing tumour pressing on the drainage pathway can allow the kidney to stretch over weeks or months with little outward change. Partial obstructions may remain stable for long periods, and some degree of kidney function can persist even with significant swelling if the process is slow enough for compensatory changes to occur. Sudden complete obstruction, particularly if both kidneys are affected, tends to produce more rapid deterioration.
Signals & patterns
Early signals
Quieter than usual
Some animals become less interactive or sleep more, a subtle shift that can be easy to attribute to other causes. This may reflect low-grade discomfort or the metabolic effects of reduced kidney efficiency, though many animals with early hydronephrosis show no behavioural change at all.
Reduced appetite
Interest in food may wane gradually, or certain meals may be refused whilst others are accepted. The pattern is often inconsistent, and other explanations are common, but declining kidney function can alter the internal environment in ways that affect appetite.
Changes in drinking or urination
Increased thirst and larger volumes of urine can appear if kidney concentrating ability begins to decline, though this is not universal in early cases. Conversely, if obstruction is affecting urine outflow, an owner may notice straining or smaller amounts produced.
Intermittent discomfort
Occasional restlessness, shifting position more often, or brief episodes of standing and resettling can occur, particularly if the kidney capsule is stretching. Some animals show no sign of pain even with considerable swelling.
Weight loss over weeks
Gradual loss of condition, often noticed when lifting the animal or during routine grooming, can accompany chronic kidney changes. The loss tends to be steady rather than sudden, and muscle mass may diminish alongside fat.
Later signals
Persistent vomiting
As waste products accumulate in the bloodstream, nausea and vomiting can become more frequent and harder to settle. The vomit may be clear fluid, food, or bile, and episodes may occur at any time of day.
Marked lethargy
The animal may spend most of the day resting, show little interest in usual activities, and move with visible reluctance. This reflects the broader metabolic disturbance that can accompany significant loss of kidney function.
Visible abdominal swelling
In some cases the enlarged kidney becomes palpable or causes asymmetry in the abdomen, though this is more often identified during examination than by the owner. Fluid accumulation or other complications may contribute to a distended appearance.
Collapse or confusion
Severe imbalance in electrolytes, acid-base status, or accumulation of toxins can lead to weakness, disorientation, or sudden inability to stand. This tends to occur when both kidneys are affected or when the remaining function is critically low.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a review of what has been observed at home—changes in appetite, water intake, urination pattern, or general behaviour—alongside any known history of urinary stones, trauma, or masses elsewhere. Physical examination may reveal an enlarged kidney that can be felt through the abdominal wall, though this is often subtle or absent, particularly in cats or if only mild swelling is present. Imaging usually follows when clinical signs suggest kidney or urinary tract involvement, or when blood tests show changes consistent with impaired kidney function.
Blood biochemistry
Urinalysis
Abdominal ultrasound
Radiography
Advanced imaging
Options & trade-offs
Management is shaped by the underlying cause, the degree of obstruction, whether one or both kidneys are affected, and how much function remains. In many cases a combination of approaches is used, adjusted over time as the situation clarifies or changes. What is workable for one household or animal may be less practical for another, and the same condition can be managed in several different ways depending on individual circumstances.
Surgical relief of obstruction
When a stone, stricture, or anatomical variant is causing a blockage, surgery may be used to remove the obstruction or reconstruct the drainage pathway. Procedures range from removing a stone lodged in the ureter to reimplanting a ureter into the bladder or creating a permanent bypass. Success depends on how much kidney tissue remains viable and whether the obstruction can be fully resolved.
Trade-offs: Anaesthesia and surgery carry inherent risks, particularly in animals with compromised kidney function or other concurrent illness. Recovery may be prolonged, and some degree of permanent kidney damage is common if the obstruction has been present for more than a few days. Not all obstructions are accessible or correctable surgically.
Ureteral stenting or subcutaneous ureteral bypass
A stent is a small tube placed inside the ureter to hold it open, whilst a subcutaneous bypass involves implanting a device that reroutes urine from the kidney pelvis to the bladder via a tube under the skin. Both aim to restore drainage without removing the obstruction itself. These techniques are typically used when the blockage cannot be removed or when the ureter is too damaged or narrowed to function normally after surgery.
Trade-offs: Stents and bypass devices can become blocked, infected, or displaced over time, and may require replacement or revision. Long-term monitoring and occasional intervention are often needed. The procedures are performed at specialist centres and involve costs that are higher than some other surgical options.
Medical dissolution of stones
Certain types of urinary stone can be dissolved by altering the urine's pH and concentration through dietary change and, occasionally, medication. This approach is well established for sterile struvite stones in cats, where dietary dissolution alone is often effective within weeks. In dogs, struvite stones are usually associated with a urinary tract infection, so dissolution requires diet combined with antibiotic therapy, and it is not effective for calcium oxalate or other non-struvite stone types. Dissolution takes weeks to months, and imaging is repeated to track progress.
Trade-offs: Dissolution is only effective for specific stone compositions, and there is no guarantee that a stone will dissolve completely or that new stones will not form once the diet is stopped. The approach does not address anatomical narrowing or masses, and is unsuitable when obstruction is severe or causing rapid decline in kidney function.
Supportive care and monitoring
When the obstruction is partial, slowly progressive, or confined to a kidney that contributes little to overall function, monitoring without immediate intervention may be appropriate. This involves regular blood tests, urinalysis, and imaging to track changes, alongside dietary adjustments and management of complications such as infection or high blood pressure. The aim is to preserve remaining function and maintain comfort whilst avoiding procedures that carry significant risk or are unlikely to alter the outcome meaningfully.
Trade-offs: This approach accepts that kidney function may continue to decline, and does not reverse existing damage. It suits animals with limited life expectancy, those for whom anaesthesia is high-risk, or situations where the other kidney is compensating well. It requires ongoing commitment to monitoring and may involve difficult decisions later if function deteriorates or discomfort emerges.
Nephrectomy
Removal of a severely damaged or non-functional kidney may be considered when the other kidney is working well and the affected kidney is contributing little beyond being a source of infection, discomfort, or progressive illness. This is typically reserved for cases where the hydronephrosis is long-standing, unilateral, and irreversible, or when a tumour is present.
Trade-offs: The animal is left dependent on a single kidney, so any future illness or injury affecting the remaining kidney has greater consequences. Anaesthesia and surgery carry risks, and the procedure is irreversible. It is unsuitable when both kidneys are affected or when the remaining kidney is not functioning adequately.
Common misconceptions
"A kidney that appears very swollen on imaging has lost all function and cannot recover."
The degree of dilation visible on ultrasound or other imaging does not always correlate closely with remaining function or potential for recovery. Kidneys that have been obstructed for days to a few weeks can regain a useful degree of filtration once drainage is restored, though the extent of recovery depends on how complete the obstruction was and how much tissue damage occurred. Prolonged or complete obstruction tends to leave more lasting deficits, but even visibly enlarged kidneys may retain some working capacity.
"If blood test results are normal, there cannot be a problem with the kidneys."
Bloodwork reflects the combined output of both kidneys, so values such as creatinine and urea may remain within normal limits even when one kidney is severely affected, provided the other is compensating. Changes may only become apparent once a significant proportion of total kidney function is lost, or if both kidneys are involved. Normal blood results do not rule out hydronephrosis or other structural kidney disease.
"Once a stone or blockage is removed, the kidney will return to normal and no further monitoring is needed."
Relief of obstruction can allow remaining kidney tissue to recover some function, but damage sustained during the period of blockage often leaves permanent scarring and reduced capacity. Animals that have formed stones or developed strictures may be predisposed to recurrence, and ongoing monitoring of kidney function, urine composition, and imaging is often part of long-term care. The kidney's structure may remain visibly altered even after successful treatment.
Related conditions
Ureteral Obstruction in Cats
Ureteral obstruction is often the direct cause of hydronephrosis in cats, as blockage of the ureter prevents urine from draining away from the kidney. When imaging reveals hydronephrosis in a cat, investigation typically focuses on identifying whether a stone or other obstruction is present in the ureter itself.
Ureteroliths
Ureteroliths—stones within the ureters—are one of the most common causes of hydronephrosis, as they can block the flow of urine from the kidney toward the bladder. The degree of hydronephrosis often reflects how complete the obstruction is and how long it has been present.
Polycystic Kidney Disease in Cats
Polycystic kidney disease can lead to hydronephrosis if enlarging cysts compress the collecting system within the kidney or obstruct the ureter. In some cases, both conditions may be identified during the same imaging study, particularly when investigating progressive kidney dysfunction.
Acute Kidney Injury
Hydronephrosis can lead to acute kidney injury if the obstruction is sudden and severe enough to compromise kidney function rapidly. Conversely, acute kidney injury from other causes may prompt imaging that incidentally reveals hydronephrosis as a contributing or unrelated finding.
Pyelonephritis
Pyelonephritis may develop as a consequence of hydronephrosis, as stagnant urine in the dilated collecting system can provide an environment for bacterial infection to take hold. In some cases, infection and obstruction can coexist, complicating both diagnosis and management.
Hydronephrosis sits within a wider group of conditions affecting the kidneys and urinary tract, and the underlying cause often has its own trajectory and considerations. Chronic kidney disease, urinary stone formation, and tumours each have their own pages within the Metabolic Health pillar, and may offer useful context for patterns observed over time. For animals receiving ongoing management, conversations at routine appointments can be a useful opportunity to discuss monitoring intervals, dietary adjustments, or when imaging might next be helpful.