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
Options & trade-offs
Last reviewed: Invalid Date ·