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Hydronephrosis, which refers to refers to distension and dilation of the renal pelvis and calyces, is a frequently reported renal disease in dogs associated with renal or post-renal urinary obstruction.
Hydronephrosis can be caused by any process which interferes with urine flow from the renal pelvis to the urethra. Hydroureter (distension of ureter with urine) is a common concurrent disease. Distension of the renal pelvis is often the result of urine accumulation and retropulsion into the pelvis from post-renal obstruction, but other causes can be involved. As the renal pelvis distends, intra-renal hydrostatic pressure rises (often >50 mmHg), leading to renal tubular cell death due to cell apoptosis, rather than necrosis. Cellular apoptosis is triggered by inflammatory responses within the kidney.
Increased hydrostatic pressure leads invariably to pronounced changes firstly in the renal collecting ducts, distal tubules and tubular interstitium and finally the cortical glomerulus. Histologically, an increase in apoptotic rate in medullary tubular cells is seen within the first 24 hours of urinary obstruction. Dilation is accompanied by flattening of the tubular epithelial cells. As obstruction persists, dilation extends to the proximal tubules and tubular atrophy is seen.. Contributing to these changes is a combination of pressure atrophy, pyelotubular reflux and ischemia. These effects have been shown, at least in vitro, to be inhibited by use of sodium nitroprusside or L-arginine.
Compensatory mechanisms within the kidney attempt to accommodate the elevated intrapelvic pressure, including dilation of the renal pelvis, afferent vasoconstriction and dilation of pelvic lymphatics with increased shunting of urine into the perirenal lymphatics. Intervention of ureteral obstruction allows a gradual resolution to normal over a 4- 6 weeks period. Sustained hydrostatic pressure due to congenital ureteral defects or neoplasia commonly lead to irreversible renal damage due to sustained hydrostatic hypertension.
Developmental renal defects are relatively common and may lead to unilateral or bilateral hydronephrosis, such as ectopic ureter (unilateral), ureteral duplication, ureteral atresia or absent ureter (unicornis; unilateral).
There are a number of underlying etiologies, including:
- - Amyloidosis
- - Circumcaval ureter associated with portosystemic shunt
- - Ureteral atresia
- - Ureteral duplication
- - Ureteral ectopia
- - Ureteral stenosis
- - Hereditary nephropathy - Cocker Spaniel
- - Polycystic kidney disease
- - Renal agenesis - Pekingese
- - Renal dysplasia
- - X-linked hereditary nephropathy
- Obstructive causes
- - Uroliths, ureteroliths, nephroliths
- - Iatrogenic ureteral ligation
- - Ureteral trauma during ovariohysterectomy
- - Renal biopsy
- - Chronic prostatitis
- - Chronic cystitis
- - Leptospira spp
- - Pyelonephritis and secondary pyonephrosis
- - Ureteral fibroepithelial polyps
- - Lymphoma - urinary bladder, retropelvic or prostatic
- - Transitional cell carcinoma - urinary bladder
- - Rhabdomyosarcoma - urinary bladder
- - Ureteral giant cell tumor
- - Ureteral sarcoma
In cases of ureteral or urethral blockage, hydronephrosis should be considered a medical emergency as renal failure can ensue rapidly due to increased intrapelvic hydrostatic pressure.
Clinically affected dogs shown varying degrees of acute or chronic renal disease, depending on cause, but consistent biochemical changes include hyponatremia, hypokalemia, elevated BUN and creatinine and metabolic acidosis.
A tentative diagnosis can be ascertained from ultrasonography, which shows hypoechoic renal medulla and pelvis. Renomegaly may be evident in dogs with acute ureteral obstruction, and renal shrinkage with chronic ureteral blockage.
A definitive diagnosis requires renal biopsy or exploratory laparotomy wedge biopsy.
Treatment is aimed at remedying the initiating cause and treatment of secondary renal disease. Ureteral defects such as ureteral atresia may be corrected surgically with autologous grafts or stents. Urinary bladder obstructions which cannot be relieved by urinary catheterization may require temporary marsupialization.
Response to therapy is dependent upon initiating cause.
- Uni of Penn
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