Blood enters the liver from two sources, the portal vein and the hepatic artery [1, 2]. Hepatic veins are arranged in two groups, an upper group (the major hepatic veins) and a lower group (the minor hepatic veins). Upper group consists of right, middle and left hepatic veins which drain into inferior vena cava (IVC). The lower group varies in number and extent of distribution. They are small veins draining directly into the IVC from segment I and occasionally from segments VII and VIII [3-6].

Live donor liver transplantation (LDLT) is an emerging option in patients with end stage liver diseases due to scarcity of donor livers [5, 7]. LDLT involves two major procedures, (1) a portion of liver is removed from the donor and (2) the removed portion is transplanted to the recipient and hepatic venous reconstitution and anastomosis are done [7, 8]. In the right lobe transplantation, the hepatectomy line passes approximately 1 cm to the right side of the middle hepatic vein as a standard procedure [7].

Variations in the drainage pattern of hepatic veins have been reported from time to time [6, 9, 10]. Knowledge of surgical anatomy of hepatic veins and its associated variations is prerequisite for liver transplantation surgeries [10]. Presence of confluence and accessory vein can lead to change of entire surgical approach. If neglected may account to postoperative haemorrhage and failure of transplantation.

We have done earlier a pilot study of 100 cases on different population [11, 12]. Scarcity of data in literature, variations encountered in the pilot study and significance of this information for successful liver transplantation prompted the present study of larger sample size of 1,000 cases.

This was retrospective study of 1,000 contrast enhanced abdominal computed tomography (CT) scans. Cases with liver reported as normal were only taken into consideration. Multiplaner images were studied in the venous phase in various planes. Terrarecon software was used for measurements [11, 12].

The total number of such major hepatic veins was noted. Normally right, middle and left hepatic veins drain independently into the IVC so total number of major hepatic veins is 3. In the presence of accessory hepatic veins the total number of major hepatic veins will be more than 3 [11, 12].

When two or more hepatic veins join and drain into IVC by forming common stem then it is called as “confluence.” The confluence pattern was also noted and classified as per Diwan et al. [13] classification. Length and diameter of stem of the confluence were also measured.

As the study was retrospective. It was done on already done CT scans. It was exempted from ethics. Ethics committee approval letter number: AIIMS/BBN/IEC/JUNE/2022/162.

Number of major hepatic veins

The hepatic veins draining directly into IVC were reported as major hepatic veins. The total number of major hepatic veins varied depending upon presence or absence of the confluence and accessory hepatic veins (Fig. 1).

Figure 1. CT images with different number of major hepatic veins. (A) 2 Major hepatic veins. Confluence between LHV and MHV. (B) 3 Major hepatic veins. Absence of confluence. (C) 4 Major hepatic veins. Presence of AHV and absence of confluence. RHV, right hepatic vein; MHV, middle hepatic vein; LHV, left hepatic vein; AHV, accessory hepatic vein; CT, computed tomography.

The absence of confluence and accessory hepatic veins make the total number of major hepatic veins 3-right, middle and left (Fig. 1). The presence of confluence and absence of accessory vein reduce the total number to 2 while the presence of accessory vein and absence of confluence increase the total number to 4 (Fig. 1C), only 4% of cases showed 4 major hepatic veins. In remaining 96% of cases, 48% of cases showed 3 major hepatic veins while remaining 48% showed 2 hepatic veins.

Confluence pattern of hepatic veins

Depending upon the veins involved, author Diwan et al. [13] had classified veins into different categories (Fig. 2). Maximum cases (38%) showed confluence between left and middle hepatic veins. The 2% of cases showed confluence between middle and accessory hepatic veins which was not mentioned in any category by author Diwan et al. [13]. CT images of various categories along with their schematic presentations shown in Figs. 2 and 3.

Figure 2. CT images of categorization of hepatic veins on the basis of confluence (Category-I, Category-II, Category-III) along with schematic presentation. RHV, right hepatic vein; MHV, middle hepatic vein; LHV, left hepatic vein; IVC, inferior vena cava; CT, computed tomography.

Figure 3. CT images of categorization of hepatic veins on the basis of confluence (Category-IV, Category-V, New category) along with schematic presentation. RHV, right hepatic vein; MHV, middle hepatic vein; AHV, accessory hepatic vein; LHV, left hepatic vein; IVC, inferior vena cava; CT, computed tomography.

Length and diameter of confluence of hepatic veins

Length of confluence was measured from its commencement till the drainage into the IVC while diameter of confluence was measured at the level of commencement of confluence [11, 12].

In the present study, the mean confluence length was 0.88± 0.39 cm while mean confluence diameter was 0.57±0.2 cm.

Knowledge of total number of major hepatic veins is very significant in LDLT. It helps in deciding the plane of resection in donor liver which can prevent haemorrhage and subsequent complications [11].

In the present study, we found 3 and 2 major hepatic veins each in 48% of cases while 4 major hepatic veins were found in only 4% of cases. Nayak et al. [3] had reported maximum cases with 3 hepatic veins while Joshi et al. [14] and Bhingardeo and Khambatta [11] had mentioned maximum cases with 2 hepatic veins indicating the presence of confluence. In the present study we found equal number of cases (each 48%) with 3 and 2 major hepatic veins. This finding is similar with Fersia and Dawson [6] but the author had not reported any case with 4 major hepatic veins in the study (Table 1).

Table 1 . Incidence of different major hepatic veins in various studies

No. of major hepatic veins	Nayak et al. 2016 [3]	Fersia and Dawson 2010 [6]	Joshi et al. 2009 [14]	Bhingardeo and Khambatta 2017 [11]	Present study
2	47%	50%	65%	51%	48%
3	51%	50%	33%	48%	48%
4	2%	0%	2%	1%	4%

When two or more hepatic veins join and form a common stem before draining into IVC then it’s called as confluence. On review of literature, maximum studies had mentioned confluence with the incidence ranging from 3% to 84% as shown in the graph. In the present study we encountered different types of confluence pattern among the hepatic veins with left and middle hepatic veins confluence being highest accounting for 38% of cases among 54% of total cases with confluence.

Wind et al. [15] and Sureka et al. [5] had reported highest incidence of confluence as 84% and 81% respectively. Many studies in literature mentioned the incidence in the range of 45% to 80% (Fig. 4) [16-22]. While Singh et al. [21] had mentioned lowest incidence of only 3.33% in his study (Fig. 4). Knowledge of pattern of confluence is necessary in deciding involvement of different veins in the graft and in venous anastomosis in transplantation surgeries [13-16].

Figure 4. Incidence of confluence of hepatic veins in different studies.

Diwan et al. [13] in his study classified hepatic confluence into six different categories (Table 2). The categorical incidence of confluence in different studies is depicted in Fig. 5. The present study is similar to study by Diwan et al. [13] and Bhingardeo and Khambatta [11] with highest number of cases belonging to Category-II.

Figure 5. The categorical incidence of confluence in different studies.

Table 2 . Categorization of hepatic veins on the basis of confluence

Category	Veins forming confluence	Total cases (%)
Category-I	MHV, LHV	38
Category-II	RHV, MHV	5
Category-III	No confluence	46
Category-IV	LHV, AHV	2
Category-V	MHV, LHV, AHV	7
Category-VI	RHV, MHV, LHV	0
Any other confluence not fitting in any category above	MHV, AHV	2

MHV, middle hepatic vein; LHV, left hepatic vein; RHV, right hepatic vein; AHV, accessory hepatic vein.

In the present study, the mean confluence length was 0.88±0.39 cm while mean confluence diameter was 0.57±0.2 cm. Liu et al. [17] mentioned the length of the common trunk 1.12±0.62 cm in their study while Cheng et al. [8] stated that it ranged from 0 to 3.5 cm with an average of 1.5 cm. Ulziisaikhan et al. [18] in his study mentioned the average length of common trunk as 7.5±2.3 mm while mean confluence diameter as 16.09±3.32 mm. When compared our study findings were close to the study by Liu et al. [17] and Cheng et al. [8].

The difference in the mean length and diameter of the confluence in different studies may be because of different methods of study and regional variation of study population groups.

The presence of accessory hepatic vein is common in liver [5, 23]. Knowledge of incidence of accessory hepatic veins is necessary in liver transplantation. Varied incidence reported in different studies in literature [23-27]. Accessory hepatic vein if present should be mentioned in the hepatic transplantation reports [5]. Their presence can affect the surgical approach in 10%–20% of cases [27]. The large accessory vein if present needs special attention and separate transplantation, if required, to avoid haemorrhage and graft rejection [22, 26].

In the present study, we found accessory hepatic veins in 15% of cases. This finding is similar with study by Fang et al. [22], Elshazly and Elkholy [26] and Bhingardeo and Khambatta [11] who had reported 21%, 20%, and 18% incidence respectively (Fig. 6). Erbay et al. [23] and Orguc et al. [25] had mentioned 47% of incidence rate in their study. This variation in incidence may be due to genetic and regional variation of different population [28, 29].

Figure 6. The incidence of accessory hepatic veins in different studies.

In conclusion, variations in hepatic veins are not uncommon. Variations can be seen in total number of major hepatic veins as well as their drainage pattern. For surgeries like liver transplantation, knowledge of such possible variations is of utmost importance to avoid postoperative complication like haemorrhage and graft rejection. In this study, we also measured length and diameter of common stem when confluence is present among hepatic veins. This surgical anatomy will surely guide and help in liver transplantation surgeries and radiological interventional procedures related to liver.

Conceptualization: AVB. Data acquisition: AVB, SJ, MC, KSR. Data analysis or interpretation: AVB, AS, SJ. Drafting of the manuscript: all authors. Critical revision of the manuscript: all authors. Approval of the final version of the manuscript: all authors.

No potential conflict of interest relevant to this article was reported.

This study was supported by Indian Council of Medical Research (ICMR).