It is the most common vascular complication. It may appear at any time after transplantation. Most of them develop at the anastomotic site. End to end arterial anastomosis (with the internal iliac artery) is more prone to induce stenosis that end to side anastomosis (with the external iliac artery or with the aorta). A stenosis is considered hemodynamically significant when the arterial luminal narrowing is above 50%. USD may detect stenosis in the absence of clinical symptoms (bruits over the graft, hypertension, and increased creatinine level). Based upon the adult literature, direct signs include: a reduction in size of the main renal artery, a peak velocity in the main renal artery far above 2 m/s, marked turbulences with spectral broadening and vibration of the adjacent tissues but overall a renal/iliac arteries diameter ratio above 1.8 [11]. Indirect signs are also present especially for sever stenosis; they include small systolic amplitude and rounding of systolic peak (parvus), a prolonged systolic acceleration above 0.08 s (tardus) and a diminished resistive index (<0.56) obtained from interlobar or segmental arteries. 3D–Gadolinium MR angiography is preferred to the ionizing CE-CT in order to differentiate if necessary, stenosis from arterial kinking. Angioplasty is the treatment of choice for significant stenosis (Fig. 23.2).

This very rare complication occurs at the anastomotic site; if unrecognized (by US), pseudo-aneurysms carry a high risk of rupture with an associated high mortality rate.

This early complication may be suspected clinically in the presence of soft tissue swelling around the graft and/or ipsilateral lower extremity edema. Etiologies include anastomotic dysfunction and compression by fluid collections. Hypovolemia, hypercoagulability, and acute rejection are predisposing factors. On US, the main vein may look normal (fresh clots can be isoechoic or the main vein is not yet thrombosed) or contains echoic clots. On Doppler, there is no venous flow and the arterial flow is highly resistive or even presents with reversed diastolic flow. However, these arterial findings are non-specific and may be found in case of acute rejection, severe hydronephrosis, extrinsic graft compression [11]. Surgical thrombectomy is the only chance to save the graft during the first postoperative week while thrombolytic therapy can be attempted later.

Venous stenosis may occur before thrombosis. Diminished diameter of the vein, turbulences, and increased speed within the vein on USD are the main findings.

The site of ureteral anastomosis is the most frequent site of stenosis usually related to local surgical difficulties and poor ureteral blood supply. Ischemia may also responsible for stenosis at any level of the ureter and will usually be demonstrated at distance from the transplantation. US reveals progressive increasing dilatation. This abnormal evolution should not be confused with a properly functioning transplant where the renal pelvis is slightly dilated, because of an increase in urine outflow and also due to the loss of ureteral tonicity after denervation. First treatment includes balloon dilatation of the stenosis and JJ stenting (Fig. 23.3). When this measure is unsuccessful surgical repair is required. The differential diagnosis of urinary tract obstruction due to stenosis includes obstruction secondary to blood clots, stones, and fungus balls, all frequent complications. To be noted, the typical symptoms of renal colic are absent due to denervation of the upper urinary tract.