Early complications: arterial and venous thrombosis
Vascular thromboses are relatively rare (about 1–5%), but they develop during the critical early postoperative period. Causes may include twisting or angulation of the surgical anastomosis, as well as severe rejection or renal tubular necrosis.
Diagnosis is based on ultrasound:
- Renal vein thrombosis: venous blood flow is completely absent, and a reverse (reversed) diastolic flow is recorded in the main and intrarenal arteries.
- Renal artery thrombosis: on color Doppler mapping, blood flow is not visualized either in the main trunk or within the renal parenchyma.
- Segmental infarction: appears as a wedge-shaped zone of decreased echogenicity without vascularization. It may occur, for example, during forced ligation of a renal artery branch during surgery.
The diagnosis is confirmed using CT, MRI, or selective angiography. Treatment requires urgent intervention: from mechanical and chemical thrombolysis and thrombectomy with plasty to nephrectomy if the organ is already non-viable.
Late complications: renal artery stenosis
Stenosis (Renal Artery Stenosis) usually develops at later stages in the area of the donor-recipient vascular anastomosis. The problem should be suspected if the patient develops severe arterial hypertension or deterioration of the transplanted kidney function.
Ultrasound diagnostic criteria:
- Blood flow velocity in the renal artery exceeds 200 cm/s.
- The velocity gradient between the stenotic and pre-stenotic segments is greater than 2:1.
- Downstream from the narrowing, turbulence, spectral broadening, and a decrease in signal amplitude are noted (forming the tardus parvus pattern — delayed upstroke and low amplitude of the systolic peak).
It is important to remember potential false-positive results due to vessel kinking or tortuosity; therefore, the study requires an experienced specialist. If the clinical picture and ultrasound findings do not match, CT or MR angiography is indicated. If the data converge, direct catheter angiography is performed with possible conversion to angioplasty and stenting.
Malformations and pseudoaneurysms
Arteriovenous malformations (AVMs) and pseudoaneurysms are usually iatrogenic in nature — they occur after previous percutaneous interventions such as biopsy or nephrostomy placement.
They are often asymptomatic and resolve spontaneously, but sometimes they lead to prominent hematuria or ischemia of the renal parenchyma.
On ultrasound, an intrarenal pseudoaneurysm resembles a cyst, but color Doppler shows its active communication with the vascular bed. Extrarenal variants may be located near the anastomosis or be a consequence of an infectious process. AVMs manifest as arterialization of the venous bed or a focal zone of disorganized blood flow.
Differential diagnosis with nephrogenic pathologies
The ultrasound picture in vascular catastrophes can overlap with signs of acute rejection or acute tubular necrosis (ATN).
- Acute rejection (peaking at 1–3 weeks) manifests as renal edema, pyramid swelling, and blunting of the corticomedullary differentiation. In severe situations, diastolic flow disappears and the resistive index (RI) increases.
- ATN is the most common cause of cadaveric graft dysfunction due to prolonged cold ischemia. Ultrasound symptoms can be identical to acute rejection (including a high RI, although it may also remain normal).
Percutaneous biopsy serves as the gold standard for accurately differentiating these conditions, as Doppler data lack sufficient specificity.
Briefly: answers to topical questions
What resistive index (RI) values are considered normal for a kidney transplant?
A value of 0.8 and below is considered normal for the Doppler resistive index (RI) of a renal allograft. An RI value > 0.8 in a renal allograft is considered abnormal. The normal Doppler waveform is characterized by low-resistance flow ("ski slope" pattern), in which diastolic velocity is approximately one-third of systolic velocity.
What are the options for vascular anastomoses in kidney transplantation?
In heterotopic kidney transplantation, the vessels of the donor organ are connected to the recipient's iliac vessels.
- Common iliac artery and vein (a. et v. iliaca communis) — the vessels of the donor kidney are sutured to them during retroperitoneal placement of the graft in the iliac fossa.
No other variants of vascular anastomoses are described in the sources.
What urological complications can occur after kidney transplantation?
Various urological complications associated with fluid accumulation or impaired urine outflow can occur after kidney transplantation.
- Urinoma (urinoma) — an accumulation of urine due to leakage at the ureteral anastomosis.
- Lymphocele (lymphocele) — an extensive accumulation of lymph capable of causing external compression of the urinary tract.
- Ureteral strictures (ureteric strictures) — narrowing resulting from ischemia of the distal ureter.
- Transplant hydronephrosis (transplant hydronephrosis) — dilation of the pelvicalyceal system.
Postoperative hematomas and seromas are also classified as urological complications.
What are the causes of acute tubular necrosis in a renal transplant?
The main cause of acute tubular necrosis in a renal transplant is prolonged cold ischemia time.
- Cold ischemia increases the risk of pathology development; therefore, necrosis is significantly more common in cadaveric transplants than when receiving an organ from a living donor.
- Hypotensive and hypoxic episodes can lead to acute tubular necrosis in neonates.
Which clinical symptoms indicate transplant renal artery stenosis?
Development of transplant renal artery stenosis is indicated by:
- Severe arterial hypertension.
- Renal dysfunction.
What treatments are used for transplant renal artery stenosis?
The following treatments are used for transplant renal artery stenosis:
- Angioplasty.
- Stenting of the arterial anastomosis area.
If the clinical picture and ultrasound data match, direct angiography is indicated with possible conversion to angioplasty and stenting.
To remember the blood flow profile distal to the arterial stenosis, use the tardus parvus pattern: tardus (late/slow) — delayed upstroke of the systolic peak, parvus (small/weak) — low amplitude.
Class and exam questions
What does renal transplant vein thrombosis look like on ultrasound?
Absence of venous flow is recorded, and a reverse (reversed) flow appears in the arteries during the diastole phase.
When does renal artery stenosis most commonly occur?
This is a late complication of transplantation, usually localized at the site of the surgical anastomosis and clinically manifesting as prominent hypertension.
What are the risks of a false-positive ultrasound diagnosis of stenosis?
Vessel tortuosity or kinking can mimic the ultrasound criteria of stenosis (high velocity, turbulence), so correlation with the clinical picture and, if necessary, MR/CT angiography is required.
Can ultrasound accurately differentiate acute rejection from tubular necrosis?
No, ultrasound signs (edema, high resistive index) are non-specific. Percutaneous biopsy is required for an accurate diagnosis.
What else to study on this topic
- Surgical and Endovascular Management of Transplant Thrombosis
- Histological Criteria for Acute Renal Transplant Rejection
- Acute Tubular Necrosis: Etiology and Radionuclide Imaging
- Surgical Anatomy of Vascular Anastomosis in Transplantation
- Clinical Interpretation of the Renal Vascular Resistance Index (RI)
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Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
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