Key words : Hepatic artery, Liver transplantation, Portal vein, Vascular graft occlusion

Introduction
Unlike other organs, the blood supply of the liver is provided by 2 different vascular systems: the portal vein (PV) and the hepatic artery (HA).¹ Although the majority of blood flow is supplied by the PV, approximately one-fifth to one-third is supplied by the HA. Thus, blood flow of 130 mL/min per a 100-g liver is provided. Although the oxygen-rich, high-pressure blood flow from the HA and the nutrient-rich, high-pressure blood flow from the PV nourish the hepatocytes equally, the intrahepatic bile tract is generally nourished by blood flow in the HA.² Therefore, complications in the HA may also cause biliary tract complications such as biliary stricture, biloma, and biliary anastomosis leaks.³ To ensure the continuity of hepatic blood circulation, the venous hepatic system must be patent. Complications that prevent blood flow, such as stenosis in the hepatic vein (HV), can disrupt hepatic functions. Occlusion of HVs is clinically manifested by ascites, jaundice, hepatomegaly, and weight gain, whereas histopathologically diffuse sinusoidal congestion, hemorrhage, and atrophy will be shown in hepatocytes.⁴

A successful liver transplant (LT) cannot be performed without a successful vascular anastomosis. Vascular complications after LT have high morbidity and mortality rates and may result in graft loss. In studies in the literature, the rate of vascular complications after LT varies between 2% and 25%, depending on the graft type and the age of the recipient.^5,6 Therefore, early diagnosis of vascular complications and appropriate treatment are important for patient and graft survival. In this study, we evaluated early vascular complications in pediatric LT recipients treated at our center.

Materials and Methods
From November 1988 to December 2021, our team has performed 701 LT procedures, including 334 in pediatric patients. Left lobe living donor LT in children was first performed in March 1990 and in adults in April 1990 at our center. A combined liver and kidney transplant from a living related donor was the first procedure of its kind to be done anywhere in the world, which was performed in May 1992.

We reviewed the medical records of pediatric recipients for the following: primary cause of liver failure, age and weight at time of LT, type of graft, and vascular complications and their management. Among the 334 pediatric patients, 176 were male patients and 158 were female patients. Mean age was 7.34 years (range, 0.5 months to 17 years). Nineteen pediatric LT recipients (5.7%) received organs from deceased donors, and 315 (94.3%) received organs from living related donors. The main cause of liver failure in our cohort was biliary atresia (n = 169). Mean weight of recipients was 23.3 kg. Most graft types were left lateral grafts (n = 204).

In our center, we treat vascular complications after LT with 1 of the 4 following treatment options: (1) anticoagulation, (2) interventional methods, (3) surgical methods, or (4) observation. We determine the treatment option according to the general condition of the patient, the severity of the complication, and the time of the complication. Our interventional radiological treatment options include catheterization of the HA, percutaneous thrombolysis, and percutaneous angioplasty. We prefer surgical treatment methods for cases where interventional radiological methods are unsuccessful and in active bleeding. Our surgical treatment options include vascular anastomosis revision and retransplant. Late hepatic arterial thrombosis is less common, and the presentation is often indolent. Therefore, we approach these patients conservatively because hepatic functions are not negatively affected.

Results
In the 334 pediatric LT recipients analyzed in this study, vascular complications developed in only 60 recipients. These complications included 54 associated with the HA, 3 associated with the PV, and 3 associated with the HV.

Hepatic artery complications occurred in 54 patients: 31 patients had HA thrombosis, 13 patients had HA stenosis, 7 patients had bleeding from anastomosis, 2 patients had HA dissection, and 1 patient had a pseudoaneurysm. We successfully treated 43 of these patients with interventional radiological methods and 11 patients with surgical methods (Figure 1).

Portal vein complications occurred in 3 patients. One patient required surgery for hemostasis because of bleeding from the PV anastomosis. The second patient required surgical revision of the anastomosis because of thrombus formation in the PV. In the third patient, because of stenosis of more than 50% in the PV anastomosis, a stent was placed in the anastomosis region after balloon dilation using interventional radiological methods, and blood flow was successfully maintained.

Hepatic vein complications occurred in 3 patients. In all 3 patients, stenosis was detected in the anastomosis region and was successfully treated with interventional radiological methods by placing a stent in the anastomosis region after balloon angioplasty. The decision on whether interventional radiological treatment would be performed with a jugular or femoral approach depended on the type of anastomosis and degree of occlusion.

Discussion
Liver transplant is the only curative treatment for pediatric patients with end-stage liver disease. Currently, as a result of advances in the field of transplantation, the 20-year survival rate of pediatric patients who have undergone LT is 79%.⁷ Although survival of LT recipients has increased, vascular complications remain the most important factor that negatively affects graft and patient survival.^8,9

In general, there are 4 types of arterial complications after LT. The most common and most severe of these is HA thrombosis. Other complications are HA stenosis, HA pseudoaneurysm, and arterioportal fistula. The incidence of HA thrombosis in pediatric patients has been shown to range from 8% to 30%.^10,11 Unfortunately, the mortality rate in LT recipients with HA thrombosis is 54.5%.¹² Causes of HA complications are height and diameter mismatches between graft and recipient arteries, celiac stenosis, excessive dissection of the HA wall, and traumatic manipulations to the intima. Prolonged surgery time, prolonged cold ischemia time, elevated hematocrit level, and ischemia-reperfusion injury can also contribute to the development of HA complications. Arterial complications occurring in the early period are usually technical problems; if revascularization is not achieved quickly, irreversible damage to hepatocytes can occur, resulting in graft loss.¹³ For HA complications that develop in the late postoperative period, because of the collateral circulation that usually develops, adequate circulation is provided by the PV and a conservative approach can be applied for such complications. Complications associated with the HA can usually be detected by Doppler ultrasonography. In cases where Doppler ultrasonography is not sufficient for diagnosis, computed tomography hepatic angiography is used.^14,15 After a patient is diagnosed with HA complications, there are 3 choices for clinicians: revascularization, retransplant, or observation. If the surgical team sees the possibility of radiological complications, amazingly successful results can be achieved with early interventional methods. On the other hand, the success rate of surgical revascularization is greater than radiological interventions for HA complications, but an attempt to use radiological methods can prevent the patient from having to experience another surgical trauma, if successful.

Portal vein complications are less common than HA complications (incidence of 1% to 2%), but these complications are associated with a high incidence of graft loss.¹⁶ Portal vein complications in pediatric LT recipients have an incidence rate of 3% to 17%.¹⁷ Most PV complications occur within 60 months posttransplant. The risk factors for PV complications are patients with biliary atresia, young age, small body weight (<6 kg), small PV size (<5 mm), malrotation of the vessels, previous operation history, and emergent operations. Because Doppler ultrasonography results may be misleading, any suspicion for a PV complication requires computed tomography hepatic angiography or magnetic resonance angiography for diagnosis.¹⁸ The role of percutaneous treatment for PV complications is growing. Percutaneous PV thrombolysis, thrombectomy, balloon dilatation, and stenting have reasonable results and are alternatives to surgical treatment. Venoplasty may be better than stents for pediatric PV stenosis after LT because children are expected to grow and the stent is fixed in size.¹⁹

Outflow obstruction from the anastomotic stricture of HVs after living donor liver transplant is a serious complication, especially among pediatric patients. Fortunately, HV complications occur less frequently as surgical experience increases, with overall incidence of 1% to 4%.²⁰ The surgical technique contributes to approximately one-third of cases; however, the recurrence of Budd-Chiari syndrome may also contribute to venous outflow thrombosis or stenosis. A percutaneous angioplasty via the internal jugular vein has excellent results, with improvement of symptoms in over 80% of patients; however, most patients require multiple sessions in order to maintain patency.²¹

In our transplant centers, we prefer interventional radiological techniques for treatment of vascular com-plications that develop in recipients after LT. Our principle is to keep the recipient away from complications and injuries that may develop secondary to repetitive surgeries. In conclusion, the most important step in vascular complications that may develop after LT is to make an early diagnosis. In this way, the recipient and graft can receive appropriate interventions in a timely and successful manner. During determinations of the treatment method for any developing vascular complications, the capabilities of the transplant center and the condition of the patient and graft should be considered. Vascular complications can be successfully treated in experienced transplant centers by an experienced clinician, surgical team, and radiology team.

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