RENAL TRANSPLANTATION COMPLICATIONS

Postoperative complications
Ultrasound has an essential role in assessing the transplant and makes a significant contribution towards graft survival through the early recognition of postoperative complications. Complications are varied and include acute rejection, ureteric obstruction, vascular occlusions, perirenal fluid collections, renal dysfunction (of various aetiologies) and infection. Drug toxicity from the immunosuppressive therapy can also compromise graft function. Finally, in the long term, the original disease, for which transplantation was performed, may recur.

Complications can be divided into three main categories: immediate postoperative complications, primary and secondary renal dysfunction.
● Immediate
non-perfusion, normally the result of an occluded or twisted renal artery; correction is surgical
haematoma
● Primary dysfunction
non-perfusion (arterial occlusion), total or lobar
acute tubular necrosis
renal vein thrombosis
obstruction
acute or accelerated acute rejection
● Secondary dysfunction
acute rejection
ciclosporin nephrotoxicity
acute tubular necrosis
obstruction
RAS
postbiopsy fistula
infection
chronic rejection.

Renal transplant dilatation
A mild degree of PCS dilatation is normal postoperatively, due to oedema at the site of the vesicoureteric anastomosis. This phenomenon is usually transient, and serial scans in conjunction with biochemistry (urea, creatinine) is normally all that is required. More severe dilatation may be indicative of obstruction, especially if the individual calyces are also dilated. A trend of increasing dilatation is a poor prognostic indicator. A ratio between the area of the PCS and the renal outline in two planes, the dilatation index, has been found to predict obstruction and differentiate obstructive from non-obstructive dilatation. The degree of dilatation of the PCS correlates well with the severity of obstruction. Obstruction of the transplant kidney may be due to an ischaemic related stricture at the vesicoureteric anastomosis, or may be the result of a blood clot or infected debris in the ureter. Haematoma or debris within the PCS may appear echogenic but requires to be differentiated from fungal balls. Percutaneous nephrostomy is the method of choice to relieve obstruction.

Rejection
This can be acute or chronic. Acute rejection may be responsible for delayed graft function whereas chronic rejection is a gradual deterioration in renal function that may begin any time after 3 months of transplantation. Ongoing episodes of acute rejection should raise the possibility of non-compliance with therapy. Acute rejection cannot be differentiated on ultrasound from other causes of delayed function, particularly acute tubular necrosis, and therefore biopsy is invariably necessary. Pathologically, rejection can be either cellular (98%) or vascular (now accounting for only 2% of cases). Improved immunosuppressive therapy has greatly reduced the problems of rejection. Ultrasound appearances of rejection These are varied and non-specific. In the majority of cases the kidney appears normal; however, greyscale findings include enlargement due to oedema (this change is subtle in the early stages and not a reliable ultrasonic indicator), increased corticomedullary differentiation with prominent pyramids, infundibular thickening (thickening of the PCS walls) and decreased fat in the renal sinus. These findings are subjective, non-specific and limited in the diagnosis of rejection. In chronic rejection there may be an overall increase in the echogenicity of the kidney with reduced corticomedullary differentiation. Eventually the kidney will shrink. The Doppler resistance indices are increased in rejection but, again, this finding is non-specific. In general, the higher the RI or PI, the more likely is the diagnosis of acute rejection. The cause of renal dysfunction is established by biopsy.

Fluid collections associated with transplantation
Up to 50% of renal transplants will demonstrate perirenal fluid. The size of the collection should be monitored with ultrasound, as significant growth may require intervention. While it is not possible to classify the collection on the ultrasound appearances alone, the clinical picture, including the time interval following transplantation, can often be helpful.
● Lymphocoele The commonest perirenal fluid collection, lymphocoeles usually occur several weeks or months after the transplant. They may resolve spontaneously but occasionally require percutaneous drainage if large. They may compress the kidney, causing an increase in vascular resistance on spectral Doppler. The collection is anechoic but may contain loculations or septa. If treated, then surgical laparoscopic marsupialization is the treatment of choice.
● Haematoma An immediate postoperative phenomenon which usually resolves spontaneously. If the haematoma is due to an anastomotic leak at the main artery or vein, it can compress the renal vein, causing thrombosis in rare cases. On ultrasound, the haematoma can appear hyperechoic and illdefined in the early stages. As it resolves and liquefies, the margins become more defined and the centre becomes anechoic. Hyperechoic blood clots and strands of fibrin may be seen within the haematoma.
● Urinoma This occurs as a result of an anastomotic leak in the ureter. Urinomas are uncommon, but may progress to urinary ascites. They occur early following the surgical procedure, unlike lymphocoeles.
● Abscess If any of the above fluid collections becomes infected, this leads to an abscess. Hyperechoic debris can be seen in the collection and this may be treated with percutaneous drainage.

Vascular complications
Vascular occlusion
Colour and spectral Doppler are essential for the diagnosis of postoperative vascular complications. Non-perfusion may be total or lobar. Focal areas of hypoperfusion may be due to oedema in focal infection, arteriovenous fistula or severing of an accessory artery during harvesting of the transplant or at the time of implantation. Total vascular occlusion is rare, but occurs early. Patients may be asymptomatic and non-perfusion of the transplant may be inadvertently seen on either a routine scan or isotope study. Graft nephrectomy is the most likely outcome. Conversely, the appearance of good renal perfusion throughout the kidney on colour or power Doppler does not vascular rejection or acute tubular necrosis can be present under such circumstances. Vascular complications can include arterial stenosis or thrombosis, venous stenosis or thrombosis, pseudoaneurysms and arteriovenous fistulae.

Renal artery stenosis
This generally occurs at the site of the anastomosis close to the iliac artery but can also occur along the length of the artery or even affect the intrarenal branches. The patient may present with severe, difficult-to-control hypertension, graft dysfunction, or both. Alternatively the patients renal function may deteriorate following angiotensin-converting enzyme inhibitor therapy and this is also an indication of a possible underlying RAS. Careful Doppler examination is now the accepted first-line investigation in the diagnosis of RAS. In most cases it is possible to trace the artery back to its anastomosis with the iliac artery, using colour Doppler. If the site of the stenosis is identified, spectral Doppler will demonstrate an increase in peak systolic velocity at the lesion, followed by poststenotic turbulence. This can be difficult to pinpoint with MRA, especially if bowel is overlying the vessel. A delayed systolic rise (the parvus tardus waveform) can be identified in the intrarenal spectral Doppler waveforms, as for the native kidney (see above). The diagnosis however is primarily made on the peak systolic velocity within the renal artery. A value of < 2.5 m/s is normal while > 2.5 m/s constitutes RAS. If the stenosis is severe, it may be difficult to identify colour flow in the kidney and the waveform may be reduced in velocity with a tiny, damped trace in the main vessel. A stenosis affecting an interlobar artery may result in focal, segmental changes in the kidney. In general, contrast angiography is only used to grade and treat stenoses after a positive ultrasound scan, or when a high index of clinical suspicion persists, despite a negative ultrasound.

Renal vein thrombosis
The occlusion may be partial or complete and the venous Doppler spectrum may therefore be absent. If venous thrombosis is partial, the arterial spectral waveform becomes very pulsatile, with reverse end diastolic flow; in the clinical setting of an oliguric patient with a tender graft in the early postoperative period, this is almost pathognomonic for RVT. During the early stages, when thrombosis is incomplete, venous flow may be seen in the kidney, but the artery is of reduced velocity. The ultrasound findings of renal vein thrombosis may be indistinguishable from severe rejection; however venous flow is generally unaffected in the latter. Thrombosis is rare, occurring typically in the immediate postoperative period. It may be associated with a faulty venous anastomosis, secondary to compression of the vein, for example by a large, perivenous collection, or the patient may have an increased thrombotic tendency for a number of reasons.

Pseudoaneurysms and arteriovenous fistulae
These may sometimes form as a result of vascular damage during biopsy procedures. They are usually not significant and tend to resolve spontaneously. An arteriovenous fistula shows an irregular knot of vessels on colour or power Doppler with a pulsatile venous waveform and high peak and end diastolic velocity in the feeding artery. A large draining vein may also be seen. A pseudoaneurysm may appear cystic on the grey-scale image, but will demonstrate filling on colour Doppler with a pulsatile flow velocity waveform. Careful biopsy technique helps to avoid such lesions.

Infection
This is characterized by swelling of the uroepithelium, especially with fungal infections. Fungal balls may be visible as relatively hyperechoic structures within the PCS.

Acute tubular necrosis
This may demonstrate prominent medullary pyramids on ultrasound, with low end diastolic flow. Reverse end diastolic flow is uncommon but recognized. A biopsy is required for confirmation.

Ciclosporin nephrotoxicity
The toxic nature of the immonosuppressive regime requires the dose to be very carefully adjusted. There may be increased Doppler resistance, as for acute tubular necrosis, but normally indices remain unaffected. Histology is required to confirm the diagnosis, or a clinical improvement following reduction or withdrawal of the immunosuppressive agent.

Renal transplant dysfunction and Doppler correlation
Doppler correlation with the different types of renal graft dysfunction is not possible. However, by taking the clinical picture into account it is possible to differentiate these situations.