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.