Living Donor Evaluation
About 20% of transplanted kidneys are obtained from living donors, and most donors are related to the recipients. After appropriate HLA matching is performed, the donor undergoes radiologic evaluation to be sure that the kidney considered for donation does not have a morphologic or vascular abnormality that would contraindicate surgery. The remaining kidney must be sufficiently normal that the donor is not at risk for subsequent renal insufficiency.
Laparoscopic nephrectomy has become the preferred method of harvesting the donor kidney. Since the exposure is limited, it is important to provide an imaging evaluation to guide the procedure. Computed tomography (CT) is the preferred modality due to its high spatial resolution and sensitivity to vascular calcification and renal stones.
Both computed tomography angiography (CTA; Fig. 9.1
) and magnetic resonance angiography (MRA; Fig. 9.2
) have proven to be accurate in distinguishing whether each kidney has one or two
main renal arteries. CT is used more commonly than MR to evaluate prospective renal donors, and images are routinely reformatted to display the anatomy of the artery and to identify the point at which the first branch occurs (Fig. 9.3
). These CT-reformatted images are also used to depict renal venous anatomy (Figs. 9.4
). MRA has the disadvantage of low sensitivity for small renal stones and is more likely than CTA to miss tiny polar accessory arteries, further supporting CT as the primary imaging modality for evaluating prospective renal donors.
FIGURE 9.1. CTA. Single renal arteries are seen on this volume-rendered image. Portions of the renal veins are also visible.
FIGURE 9.2. MRA. Single renal arteries are seen on this T1-weighted three-dimensional spoiled gradient-echo image.
Several techniques are available to reduce the radiation dose of the donor CT examination. Unenhanced views of the kidneys are necessary to find small stones and are helpful in detecting vascular calcification. Arterial phase images are obtained to assess the vascular supply, and delayed images are useful to evaluate the collecting system. The scout view from a CT urogram depicts the renal collecting systems, bladder, and ureter (Fig. 9.7
). Axial images collimated to 1.5 mm as well as coronal, sagittal, and 3D-reconstructed images should be available for review at the workstation.
Evaluation of the kidneys should include an assessment of renal volume, as this parameter closely predicts renal function. Large kidneys are preferable; donors with large remaining kidneys achieve better renal function than those with small ones; large donated kidneys have fewer complications in recipients; and small transplanted kidneys may have insufficient function to serve a large recipient. Simple cysts and small nonobstructing stones may not absolutely disqualify a donor candidate, but tumors, postinflammatory fibrosis, hydronephrosis, and other conditions can jeopardize the health of the donor and/or recipient and usually prohibit transplantation.
Renal vascular assessment is crucial. Arterial conditions such as atherosclerosis or fibromuscular disease (Fig. 9.8
) are contraindications to donor nephrectomy. Accessory renal arteries are important to demonstrate (Fig. 9.9
). Duplicated renal arteries are not an absolute contraindication, but arterial luminal diameters should be measured because arteries smaller than 3 mm are difficult to anastomose to recipient vessels. Triplicated renal arteries usually preclude donor nephrectomy. Very small polar arteries can often be sacrificed, but since small lower pole arteries may give branches to the renal pelvis and proximal ureter, postoperative ureteral complications are more likely if these vessels are occluded. The distances between the origin from the aorta and first bifurcation of all major renal arteries should be measured; if these segments are short, surgery becomes more difficult.
FIGURE 9.3. CTA. A single right renal artery is reformatted to clearly display the first branching vessel.
FIGURE 9.4. CTA. A single right renal vein is clearly demonstrated.
FIGURE 9.5. CTA. A single left renal vein is clearly demonstrated.
FIGURE 9.6. CT angiogram reveals a single artery and vein supplying each kidney.
FIGURE 9.7. CT urography. A scout view demonstrates a single ureter bilaterally. The prostate is enlarged, there is a TURP defect, and the bladder is trabeculated.
FIGURE 9.8. Fibromuscular dysplasia is seen bilaterally.
FIGURE 9.9. Accessory renal artery. Volume-rendered image from CTA demonstrates two left renal arteries (arrows).
Renal venous anatomy must also be analyzed. Retroaortic or circumaortic left renal veins are found in approximately 5% of potential donors and are important to recognize, as are duplicated right renal veins, found in almost 15% of donor evaluations. The distance between the junction of the vein with the inferior vena cava and the first bifurcation should be measured. Finally, renal venous tributaries, such as adrenal, lumbar, and gonadal veins, should be described.
The left kidney is preferred for donation, as the left renal vein is longer and surgical resection is often easier (Fig. 9.5
). Single renal arteries are preferred, though small accessory arteries may sometimes be ignored. Potential donors with accessory arteries at the lower pole may have branches supplying the renal pelvis or ureter and are usually avoided.
Perirenal fat is removed from the donor kidney before transplantation. Thus, an assessment of the amount of fat in the perirenal space is useful. Simple renal cysts are seldom a problem as they are easily excised. Indeterminant lesions should be carefully evaluated before surgery. Ureteral duplication, found in 1% of the population, presents a technical challenge, but is not an absolute contraindication for transplantation.
Chu et al. found renal or extrarenal abnormalities in more than 40% of potential renal donors, though most were incidental findings that did not prevent organ donation. Renal abnormalities, which are absolute contraindications to transplantation including solitary kidney, horseshoe kidney, or polycystic kidney disease, were found in <1% of potential donors.
Due to the success of heterotopic renal transplantation, orthotopic transplantation is rarely performed. With heterotopic transplantation, the donor kidney is placed in an extraperitoneal location in the iliac fossa. CT is helpful in confirming that there is space within the fossa for the transplanted kidney. The renal artery and vein are anastomosed to the external iliac vessels. There are many techniques for reconstructing the urinary tract, but an antirefluxing ureteroneocystostomy is commonly created.
Virtually all recipients have their kidneys imaged in the course of treatment for renal failure, often a combination of ultrasound, CT, MRI, and radionuclide studies. Sometimes, specific imaging of the recipient’s native kidneys will be necessary during the pretransplant workup. Evaluation for acquired renal cystic disease and the neoplasms that such kidneys develop is best performed by CT. Patients with autosomal dominant polycystic kidney disease may also benefit from a CT examination prior to transplantation if extreme enlargement or persistent bleeding makes them candidates for nephrectomy. Patients with severe vesicoureteral reflux may require surgical therapy, so voiding cystourethrography may be necessary in patients with recurrent urinary tract infections. Voiding cystourethrography may also be indicated in patients who have been anuric so long that their ability to void normally after transplantation is questioned. Bladders in such patients may have only a small capacity and, when studied by cystography, may demonstrate benign extravasation. This finding does not indicate gross perforation of the bladder and is not a contraindication to transplantation.
Evaluation of the vessels to which the transplanted kidney’s artery and vein will be anastomosed is important. An unenhanced CT permits assessment of the degree of recipient vascular calcification; the severity of calcification is directly proportional to the degree of difficulty in performing the anastomosis. Some centers perform CTA of the common and external iliac arteries as well to evaluate them for stenosis or occlusion.
Patients with end-stage renal disease often have undergone one or more biopsies. These may lead to an arteriovenous fistula or pseudoaneurysm. While most of these are asymptomatic, some may lead to hematuria or hypertension and should be treated prior to transplantation.