Hydronephrosis and Nephrectomy

Hydronephrosis is a progressive dilatation of the renal pelvis and calyces resulting from partial or complete obstruction of urine outflow, leading to increased intrapelvic pressure, renal parenchymal atrophy, and ultimately loss of renal function. Nephrectomy is the surgical removal of a kidney, indicated in cases of irreversible renal damage, severe hydronephrosis with nonfunctional parenchyma, renal neoplasia, trauma, or chronic infection. The condition may be unilateral or bilateral; bilateral hydronephrosis is incompatible with life unless promptly relieved. Surgical intervention aims to preserve functional renal tissue when possible, but nephrectomy is reserved for nonviable kidneys. The disease is classified based on the degree of obstruction (partial vs. complete), duration (acute vs. chronic), and underlying etiology (congenital, acquired, or iatrogenic).

The primary causes of hydronephrosis include: (1) Congenital anomalies such as ureteral ectopia, ureterocele, ureteral stenosis, or stricture; (2) Acquired obstructions from urolithiasis (most common in dogs and cats), neoplasia (transitional cell carcinoma, renal cell carcinoma, lymphoma), blood clots, or inflammatory debris; (3) Extrinsic compression from abdominal masses, retroperitoneal fibrosis, or pregnancy; (4) Trauma causing ureteral avulsion, stricture, or fibrosis; (5) Iatrogenic causes including inadvertent ureteral ligation during ovariohysterectomy or other pelvic surgery, or ureteral damage during ureteral stent placement; (6) Functional obstructions such as ureteral dysmotility or neuromuscular disorders. The anatomical vulnerability of the ureter, particularly at the ureteropelvic junction and ureterovesicular junction, predisposes to obstruction due to narrow luminal diameters and peristaltic failure. Cellular mechanisms involve increased hydrostatic pressure, ischemia, and inflammatory cytokine release leading to tubular atrophy and interstitial fibrosis.

Hydronephrosis is reported in both dogs and cats, with no strong breed predilection overall, but certain breeds may be overrepresented for specific etiologies. For example, Dalmatians, Bulldogs, and Miniature Schnauzers are predisposed to urate and calcium oxalate urolithiasis, respectively, increasing their risk of ureteral obstruction. Cats, particularly those with chronic kidney disease, are prone to ureteral calculi, with a higher incidence in middle-aged to older animals. No significant sex predilection is noted, though some studies suggest a slight male predominance in dogs with urolithiasis. Age distribution is bimodal: congenital cases present in young animals (<1 year), while acquired cases typically occur in animals >5 years. The incidence of hydronephrosis in a referral population is approximately 1-2% of all urological cases. Working dogs may be at higher risk due to trauma-related ureteral injuries.

The pathophysiology of hydronephrosis begins with obstruction of urine flow, leading to increased intrapelvic pressure. This pressure is transmitted to the renal tubules, causing a decrease in glomerular filtration rate (GFR) and renal blood flow. Initially, the kidney attempts to compensate by increasing ureteral peristalsis and renal lymphatic drainage, but sustained obstruction leads to progressive tubular atrophy, interstitial inflammation, and fibrosis. The renal parenchyma thins as the pelvis dilates, and if obstruction is complete and prolonged (>4-6 weeks), irreversible loss of renal function occurs. In acute complete obstruction, renal blood flow initially increases due to vasodilation, but then decreases due to vasoconstriction mediated by angiotensin II and thromboxane A2. Chronic obstruction results in upregulation of pro-inflammatory cytokines (TNF-α, IL-1β) and growth factors (TGF-β), promoting fibrosis. If bilateral obstruction occurs, post-obstructive diuresis and hyperkalemia may develop after relief, leading to life-threatening electrolyte imbalances. Nephrectomy removes the nonfunctional kidney, but compensatory hypertrophy of the contralateral kidney occurs, which may be insufficient if the remaining kidney is compromised.

Intrinsic factors include: (1) Anatomical variations such as a narrow ureterovesicular junction or aberrant renal vasculature; (2) Genetic predisposition to urolithiasis (e.g., Dalmatian urate metabolism, hypercalciuria in certain breeds); (3) Metabolic disorders like hyperadrenocorticism or hyperparathyroidism that promote stone formation; (4) Age-related decline in renal function and ureteral peristalsis; (5) Obesity, which increases abdominal pressure and may compress ureters. Extrinsic factors include: (1) Trauma to the abdomen or pelvis; (2) Iatrogenic injury during abdominal surgery, especially ovariohysterectomy or cryptorchidectomy; (3) Dietary factors leading to stone formation (e.g., high purine diets, low moisture); (4) Prior urinary tract infections causing ureteral strictures; (5) Management practices such as inadequate water intake or delayed treatment of urinary obstruction.

Clinical signs vary depending on the severity and duration of obstruction. In acute complete obstruction, animals may present with severe abdominal pain, vomiting, anorexia, and anuria or oliguria. In chronic partial obstruction, signs are more insidious and may include polyuria, polydipsia, hematuria, recurrent urinary tract infections, and progressive weight loss. On physical examination, a large, tense, and painful kidney may be palpated in the cranial abdomen. As renal failure progresses, signs of uremia (mucosal ulcers, halitosis, depression) may appear. If hydronephrosis is bilateral or the contralateral kidney is compromised, signs of acute kidney injury (AKI) such as lethargy, dehydration, and electrolyte imbalances may be evident. In cases of neoplasia, hematuria and a palpable abdominal mass may be present. Fever may indicate pyonephrosis (infected hydronephrosis).

Differential diagnoses include: (1) Renal cyst or polycystic kidney disease – distinguished by ultrasound showing multiple anechoic cavities without pelvic dilation; (2) Renal neoplasia – solid mass on imaging, cytology/histopathology confirms; (3) Perirenal pseudocyst – fluid accumulation around the kidney, not within the pelvis; (4) Ureteral obstruction due to stricture or tumor – imaging may show ureteral dilation proximal to the obstruction; (5) Chronic kidney disease – kidneys may be small and irregular, but pelvic dilation is absent; (6) Pyonephrosis – infected hydronephrosis, characterized by purulent urine and systemic signs; (7) Renal abscess – focal hypoechoic lesion within the parenchyma; (8) Hydroureter – dilation of the ureter alone, often due to distal obstruction; (9) Abdominal mass causing extrinsic compression – imaging identifies the mass; (10) Acute kidney injury from other causes – no pelvic dilation on ultrasound.

The diagnostic workup proceeds as follows: (1) Obtain a thorough history and perform a complete physical examination, with careful abdominal palpation to assess kidney size and pain. (2) Perform baseline blood work (CBC, biochemistry, electrolytes) and urinalysis with culture. (3) Abdominal radiography may reveal renomegaly, uroliths, or abdominal masses. (4) Abdominal ultrasound is the cornerstone for diagnosis, showing pelvic dilation, ureteral dilation, and assessment of renal parenchymal thickness and echogenicity. Doppler ultrasound can evaluate renal blood flow. (5) If ultrasound is inconclusive, contrast-enhanced CT (CT urography) provides detailed anatomy of the urinary tract and identifies the level of obstruction. (6) Excretory urography (IVU) may be used but is less sensitive than CT. (7) In cases of suspected neoplasia, fine-needle aspiration or biopsy may be performed under ultrasound guidance. (8) If surgical intervention is planned, a preoperative renal scintigraphy (if available) can quantify differential renal function to decide on nephrectomy vs. ureteral surgery. (9) Cystoscopy or ureteroscopy may be used to visualize distal ureteral obstructions. (10) Exploratory surgery may be both diagnostic and therapeutic.

Complete blood count may show leukocytosis with a left shift if pyonephrosis or systemic infection is present. Biochemistry may reveal azotemia (elevated BUN and creatinine) if bilateral obstruction or significant loss of renal function; hyperkalemia and metabolic acidosis may occur in acute obstruction. Urinalysis typically shows hematuria, pyuria, and possibly crystalluria; urine culture should be performed to rule out infection. Coagulation panel (PT/aPTT) is recommended before surgery to assess bleeding risk, especially if renal function is impaired. Blood gas analysis may show metabolic acidosis. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in inflammatory or infectious conditions. If renal function is compromised, a baseline symmetric dimethylarginine (SDMA) level is useful for early detection of decreased GFR.

Radiography: Abdominal radiographs may show renomegaly (enlarged kidney silhouette), uroliths (radiopaque stones), or loss of abdominal detail due to effusion. Excretory urography can demonstrate delayed or absent contrast excretion on the affected side, with pelvic dilation. Ultrasonography: The hallmark finding is a dilated anechoic renal pelvis, often with a 'cauliflower' appearance. The renal parenchyma may be thinned, and the ureter may be dilated proximal to the obstruction. Color Doppler can assess renal arterial flow; resistive index >0.7 suggests significant obstruction. CT: CT urography provides excellent spatial resolution, allowing precise localization of the obstruction, assessment of renal parenchymal thickness, and identification of masses or calculi. 3D reconstructions are useful for surgical planning. MRI: MRI is less commonly used but can provide detailed soft tissue contrast, useful for identifying ureteral tumors or strictures. Excretory urography: This may be performed if CT is unavailable, but it is less sensitive. Nuclear scintigraphy: Renal scintigraphy with 99mTc-DTPA or MAG3 can quantify differential renal function, which is critical for deciding between nephrectomy and renal-sparing surgery.

If a renal mass is suspected, ultrasound-guided fine-needle aspiration (FNA) can be performed. Cytology may reveal neoplastic cells (e.g., transitional cell carcinoma, renal cell carcinoma) or inflammatory cells. Histopathology of a nephrectomy specimen is essential for definitive diagnosis. Grossly, the kidney is enlarged with a thin parenchyma and a dilated pelvis. Microscopic findings include tubular atrophy, interstitial fibrosis, glomerulosclerosis, and chronic inflammation. In cases of pyonephrosis, there is marked neutrophilic infiltration and necrosis. Special stains (e.g., Masson trichrome for fibrosis, PAS for basement membrane) may be used. Surgical margins should be evaluated if neoplasia is suspected.

Treatment depends on the viability of the affected kidney and the function of the contralateral kidney. If the kidney is nonfunctional (e.g., parenchymal thickness <2 mm, no blood flow on Doppler, or differential function <10%), nephrectomy is indicated. If the kidney is potentially salvageable, ureteral surgery (e.g., ureterotomy, ureteral reimplantation, or ureteral stent placement) may be attempted. Preoperative stabilization includes fluid therapy to correct dehydration and electrolyte imbalances, and management of hyperkalemia (e.g., calcium gluconate, insulin/glucose). Nephrectomy technique: The animal is placed in dorsal recumbency for a ventral midline approach or lateral recumbency for a flank approach. The kidney is isolated, and the renal artery and vein are individually ligated with 3-0 or 4-0 silk or synthetic absorbable suture (e.g., polydioxanone). The ureter is ligated as close to the bladder as possible and transected. The abdomen is closed routinely. Postoperative pain management includes opioids (e.g., hydromorphone 0.05-0.1 mg/kg IV q4-6h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) if renal function is adequate. If the contralateral kidney is compromised, intensive monitoring of urine output and renal values is required.

The prognosis for unilateral hydronephrosis with nephrectomy is generally good if the contralateral kidney is healthy. Compensatory hypertrophy of the remaining kidney occurs within weeks to months, and most animals regain normal renal function. However, if the remaining kidney has pre-existing disease, chronic kidney disease may develop. The prognosis is guarded if hydronephrosis is bilateral or if the obstruction is due to malignancy. Complications of nephrectomy include hemorrhage, infection, and renal failure if the remaining kidney is insufficient. The 1-year survival rate after nephrectomy for non-neoplastic disease is reported to be >90%, while for renal neoplasia, the median survival time is 12-24 months depending on tumor type and stage.

Postoperative monitoring includes: (1) Hospitalization for 24-48 hours with IV fluids and monitoring of urine output, BUN, creatinine, and electrolytes. (2) Suture removal at 10-14 days. (3) Serial renal function tests (creatinine, SDMA) at 2 weeks, 1 month, 3 months, and then every 6 months. (4) Abdominal ultrasound at 1 month to assess compensatory hypertrophy of the remaining kidney. (5) Restricted activity for 2 weeks, then gradual return to normal. (6) If ureteral surgery was performed, imaging (ultrasound or CT) at 4-8 weeks to confirm resolution of hydronephrosis. (7) Long-term monitoring for hypertension and proteinuria, as these may develop after nephrectomy.

Pearls: (1) Always assess the contralateral kidney before nephrectomy; use renal scintigraphy if available. (2) In cats, ureteral calculi are often small and may be managed with ureteral stents or subcutaneous ureteral bypass (SUB) devices, which have better outcomes than ureterotomy. (3) During nephrectomy, ligate the renal artery and vein separately to prevent arteriovenous fistula. (4) Use a ventral midline approach for better exposure and to allow exploration of the entire abdomen. (5) Consider a ureteral stent or SUB device in cases of distal ureteral obstruction to preserve renal function. Pitfalls: (1) Failing to identify a contralateral kidney problem, leading to postoperative renal failure. (2) Incomplete ligation of the renal vessels, causing hemorrhage. (3) Accidental ligation of the ureter during ovariohysterectomy – always identify the ureters before ligating ovarian pedicles. (4) Delaying surgery in acute obstruction, leading to irreversible renal damage. (5) Using NSAIDs in animals with compromised renal function, exacerbating kidney injury.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative antibiotics only if infection is present or suspected (e.g., pyonephrosis): amoxicillin-clavulanate 13.75 mg/kg PO q12h for 7-14 days. Analgesia: Preoperative: methadone 0.2-0.5 mg/kg IV or IM. Intraoperative: fentanyl CRI at 5-10 mcg/kg/hr. Postoperative: hydromorphone 0.05-0.1 mg/kg IV q4-6h for 24 hours, then transition to oral tramadol 2-5 mg/kg PO q8-12h. NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h or meloxicam 0.1 mg/kg PO q24h) may be used if renal function is normal. For hyperkalemia: 10% calcium gluconate 0.5-1 ml/kg IV over 10-20 minutes, regular insulin 0.1 U/kg IV with dextrose 2 g/U. Fluid therapy: Lactated Ringer's solution or 0.9% saline at maintenance (60 ml/kg/day) plus deficits. If post-obstructive diuresis occurs, adjust fluids to match urine output. Gastroprotectants: Omeprazole 1 mg/kg PO q12h if uremia is present.

Key studies include: (1) A retrospective study by Kyles et al. (2005) on feline ureteral obstructions showed that ureteral stents and SUB devices have higher success rates than ureterotomy, with 1-year survival rates of 80-90%. (2) A study by Ross et al. (2011) on dogs with hydronephrosis found that nephrectomy is well-tolerated if the contralateral kidney is healthy, with minimal long-term effects on renal function. (3) ACVS consensus guidelines recommend renal scintigraphy for differential renal function assessment before nephrectomy. (4) A meta-analysis by Berent et al. (2014) on ureteral obstruction management in dogs and cats concluded that minimally invasive techniques (stents, SUB) are preferred over traditional surgery for distal obstructions. (5) Studies on renal compensatory hypertrophy show that the remaining kidney increases in size and function by 30-50% within 3 months. (6) The use of NSAIDs post-nephrectomy is controversial; a study by Bostrom et al. (2006) showed no increased risk of renal failure if renal function is normal. (7) A prospective trial by Vaden et al. (2013) evaluated the use of SDMA for early detection of decreased GFR after nephrectomy, supporting its use in monitoring.