Renal Transplantation

Benjamin Tubb

PREOPERATIVE EVALUATION OF RENAL DONORS

Imaging of renovascular and ureteral anatomy in prospective renal donors is valuable for surgical planning, especially as living donor nephrectomies are increasingly performed laparoscopically. Traditional catheter angiography has largely been replaced by computed tomographic (CT) angiography and magnetic resonance (MR) angiography for preoperative donor evaluation. Radiologists must assess for variant anatomy such as accessory renal arteries, prehilar renal artery branching, and renal venous variants such as retroaortic or circumaortic left renal vein and large connections to lumbar veins which might necessitate open surgical approach rather than laparoscopic. Cross-sectional imaging allows identification of duplicated collecting systems and parenchymal abnormalities (masses, calculi, abnormally located kidney, or cysts suggesting possible polycystic kidney disease) that might alter or preclude graft harvesting. CT and MR can both be used for evaluation of prospective donors, although MR is less sensitive for small calculi.

SURGICAL TECHNIQUE

Transplant kidneys are usually placed in extraperitoneal location within the iliac fossa, with end-to-side vascular anastomosis to the external iliac vessels. The common method of urinary drainage is ureteroneocystostomy, with transplant ureter implanted into the wall of the bladder. Variations include intraperitoneal graft placement, anastomosis to internal iliac artery, and connection of transplant collecting system to the recipient ureter or interposed bowel segment. Operative notes should be reviewed to clarify transplant anatomy.

COMMON POSTOPERATIVE FINDINGS

Doppler ultrasound examinations of renal allografts are routinely performed in the immediate postoperative period to evaluate blood flow in the transplant organs and to exclude early complications. Mild hydronephrosis is often seen, possibly due to denervation of the collecting system during transplantation and edema at the ureteral anastomosis. Small perinephric fluid collections are often present after surgery, usually representing postoperative seromas or small hematomas. Doppler examination of intrarenal vessels should demonstrate antegrade diastolic flow with low resistance waveform, with resistive index (RI) less than 0.70. However, cadaveric allografts can have transient edema related to ischemia (some degree of ischemia is inevitable during transport), which can cause high resistance flow in intrarenal arteries during the immediate postoperative period.

RENAL TRANSPLANT COMPLICATIONS

Acute tubular necrosis (ATN) occurs postoperatively in 30-50% of cadaveric transplants due to ischemia during transport and transplantation. Most will return to normal function in 2 weeks. ATN is rare in living donor transplants. Recovery usually occurs, often heralded by slow rise in urine output followed by large diuresis. ATN causes elevated creatinine and
oliguria. These are very nonspecific findings, and imaging studies are often performed to exclude other causes of decreased transplant function.

Perinephric collections can cause compression of transplant vasculature or ureter and may present with fever, tenderness, worsening renal function, oliguria, or hypertension. Causes include the following:

Seroma. Virtually a normal postoperative finding, often appearing as crescentic collection of simple fluid along border of the transplanted kidney. Only problematic if compressing adjacent structures or enlarging.
Hematoma. May indicate bleeding from vascular anastomoses. Usually occurs in first few days postoperatively. Small hematomata usually resolve spontaneously. May also occur after biopsy of transplanted kidney.
Abscess. Increased risk due to immunosuppression. Usually seen a few days after surgery. Any form of perinephric collection can become infected and lead to abscess formation, necessitating drainage.
Urinoma. Due to leakage at ureteral anastomosis, collecting system rupture from obstruction, or rarely calyceal leakage from segmental infarction in transplant organ (e.g., accessory renal artery was not identified or anastomosed). Usually occurs within days to weeks after surgery. Often located between transplant and bladder; can occur in unusual locations such as scrotum/labia or thigh. May rapidly increase in size. Aspirated fluid demonstrates higher creatinine than other collections. Treated with nephrostomy tube placement, ureteral stenting, and/or surgical repair.
Lymphocele. Secondary to disruption of native lymphatic channels in the operative bed or leakage from lymphatics of the allograft. Lymphoceles usually present 4-12 weeks after surgery but can occur years later. They usually occur medial to transplant around iliac vessels and may appear septated. These are the most common collection to cause transplant hydronephrosis; can even cause leg swelling from pressure on iliac vein. They commonly recur after drainage and may require sclerotherapy or marsupialization.

Urinary obstruction occurs in 2% of transplantations, usually within the first 6 months postoperatively. Obstruction may not be evident until nephroureteral stent is removed. The most common cause is stenosis of distal ureter at site of implantation in bladder, related to ischemia or rejection. Other causes include obstructing material such as calculi, blood clots, fungus balls, or papillary necrosis. Extrinsic compression of ureter by perinephric collections can also cause hydronephrosis. Importantly, the transplanted kidney is denervated, so patients will not experience colicky pain due to obstruction. The only indication of obstruction may be elevated serum creatinine and/or oliguria. Diagnosis of urinary obstruction can be complicated in transplanted kidneys. Incompetence of the ureteral anastomosis to the bladder may cause mild to moderate collecting system dilatation when the patient’s bladder is full. It is important to reevaluate after bladder is empty. Denervation of the transplant collecting system decreases its tone, so mild dilatation may be a stable postoperative finding. Moreover, prior episodes of obstruction may yield persistently dilated collecting system, despite resolution of the obstruction. Alternatively, fibrosis related to rejection may prevent hydronephrosis in some cases of obstruction. For these reasons, assays such as Whitaker test (invasive test with direct measurement of pressure gradient from renal pelvis to bladder during fluid instillation in renal pelvis) or functional assays such as nuclear medicine examinations are used to diagnose true urinary obstruction with consequent delayed renal function.

Vascular complications occur in approximately 2% of transplant recipients. These include renal artery stenosis or thrombosis, renal infarction, renal vein thrombosis, and postbiopsy complications such as arteriovenous (AV) fistula or pseudoaneurysm formation.

Renal artery stenosis usually manifests in the first year and occurs most commonly at anastomosis (related to surgical technique or fibrosis), but can occur distally in transplant renal artery (due to rejection occurring in vessel wall, extrinsic compression by perinephric collections, or mechanical kinking or twisting of vessels related to transplant position) or proximal to the anastomosis (native vessel atherosclerotic disease). Renal artery stenosis may present with severe/persistent hypertension, refractory to medical therapy, and decreased renal function.
Renal artery thrombosis may be due to hyperacute rejection (usually evident at surgery), occlusion or kinking at anastomosis, or renal artery dissection. Acute transplant infarction can present with anuria and swelling and tenderness over the kidney. Despite denervation of the transplant, any cause of inflammation in the transplanted kidney can irritate adjacent structures in the pelvis and cause pain.
Vasculitis usually affects smaller renal vessels and leads to small segmental infarcts.
Renal vein thrombosis is a rare postoperative complication and a surgical emergency requiring immediate intervention (venous collaterals are absent in transplant organ). This usually occurs in the first week postoperatively and classically presents with abruptly decreased renal function and swelling and tenderness over the graft.
AV fistula or pseudoaneurysm formation affecting small vessels in the renal parenchyma are occasional complications of renal biopsy; these are usually managed conservatively when they are small. Gross hematuria occurs in 5-7% of patients following biopsy and usually resolves spontaneously.

Infections occur in 80% of transplant recipients during the first year after surgery. In the first several weeks, these infections are similar to those of other surgical procedures: pneumonia, wound infection, urinary tract infection, and infected perinephric fluid collections leading to abscess formation. Pyelonephritis can mimic acute rejection. Opportunistic infections related to immunosuppression characteristically occur 1-6 months postoperatively.

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