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Freebase(0.00 / 0 votes)Rate this definition: Hydronephrosis â literally water inside the kidney â refers to distension and dilation of the renal pelvis and calyces, usually caused by obstruction of the free flow of urine from the kidney. Untreated, it leads to progressive atrophy of the kidney.
Technically, hydronephrosis specifically describes dilation and swelling of the kidney, while the term hydroureter is used to describe swelling of the ureter. Hydronephrosis may be unilateral involving just one kidney or bilateral involving both.ydronephrosis describes the situation where the urine collecting system of the kidney is dilated. This may be a normal variant or it may be due to an underlying illness or medical condition. Normally, the kidney filters waste products from blood and disposes of it in the urine.