ANATOMICAL VARIATIONS OF PORTAL VENOUS SYSTEM: IMPORTANCE IN SURGICAL CLINIC (2024)

Language: English | Portuguese

ABSTRACT - BACKGROUND:

HEADINGS: Anatomy, Portal System, Portal Vein, Mesenteric Veins, Splenic Vein

RESUMO - RACIONAL:

DESCRITORES: Anatomia, Sistema Porta, Veia Porta, Veias Mesentéricas

INTRODUCTION

The abdominal portal vein (PV) starts at the level of the second lumbar vertebra,anterior to the inferior vena cava and posterior to the pancreatic neck. It iscomposed of the hepatic pedicle, posterior to the hepatic artery and to the commonbile duct. The PV is formed by the convergence of superior mesenteric vein (SMV) andsplenic vein (SV), measures about 6.5 cm in length and 0.8 cm in diameter onaverage6^,9^,11. Its main tributaries are the left gastric vein, which ends at its leftborder; the pancreaticoduodenal vein, superoposteriorly close to the head of thepancreas9^,11; and the veins proceeding from the small and large intestines (SMV andinferior mesenteric vein [IMV]). Other tributaries of the hepatic PV are the cysticveins, proceeding from the gallbladder; pancreatic veins; and right and leftgastroepiploic vessels, besides the short gastric veins through the splenic andright gastric veins. The IMV receives blood from the upper part of the rectum,sigmoid, and descending colon6^,9^,11^,14. IMV is predominantly ventral and to the left of the superior mesentericartery, at the level of the third portion of the duodenum together with theduodenojejunal flexure. The SV is formed by 5-15 venules, originated at the red pulpof the splenic parenchyma, which join together close to the tail of the pancreas.Then, the SV receives as tributaries the short gastric veins in variable number;pancreaticoduodenal veins, also variable in number; and posterior gastric veins,including, eventually, the left gastric vein as well as the IMV. It must beemphasized that the splenogastric vessels are independent and are not among thetributaries of the SV, despite that communicating vessels could occur among them.The SV continues in a dorsal sulcus of the pancreas toward the direction of itshead, which can be visible through the lower border of the pancreas. The bloodsupply of the SV comes from the spleen, larger curvature of the stomach, pancreas,left half of the colon, upper rectum, and retroperitoneum6.

The SMV is formed by tributaries of the small intestine, right colon, head of thepancreas, and part of the stomach - through the right gastroepiploic vein. Itsposition is predominantly ventral and is a short vein that is formed from multipletributaries as they cross the third portion of the duodenum to the right of theduodenojejunal junction, close to the uncinate process11.

This presentation of the abdominal portal system is the most commonly found; however,there are variations which are the reason of the study among the anatomists for morethan a century ago. The first work relating variations of the PV tributaries waspublished by Thomson et al., which distributed them into three types25:

• Type I - IMV as tributary of the SV

• Type II - trifurcation in the PV, formed by the union of the SMV, IMV,and SV

• Type III - IMV as tributary of the SMV

Despite that there are many anatomical works on the PV, its variations still continueto be described and, sometimes, they are unprecedented. Benninger et al. suggestedanother tributary of the PV, i.e., the splenomesenteric vein3. Recently, it was also described as modification to Thomsonclassification14, with variations of the left gastric vein as direct tributary of the PV orthe SV.

The objective of this study was to review the literature related to anatomicalvariations of the PV system and their frequency, accentuating the morphologicalknowledge and its surgical applicability, which may aid to prevent surgical adverseevents.

METHODS

Systematic review of primary studies was performed with the elaboration based on theChecklist Preferred Reporting Items for Systematic Reviews andMeta-Analyses (PRISMA)18 in the databases such as PubMed.gov, SciELO, BIREME, LILACS, Embase,ScienceDirect, and Scopus. In the search strategy, the unitermsused were as follows: Anatomy, Portal vein, Mesenteric vein,Formation, Variation, Variant anatomic, Splenomesenteric vein, Splenic veintributaries, and Confluence. The research included MeSH/DeCS/Emtree and Allfiels,excluding animals, in vitro studies, studies published in congressannals, secondary studies, and articles with inexplicit method. The includedarticles were only on humans with studies in cadavers, imaging examination, reports,and case series. Complete articles in English, Spanish, and Portuguese were studiedwithout restriction from the institution of origin nor year of publication (Figure 1).

Only 61 articles were found for reading in their entirety, as well as the tile of thearticles of their respective bibliographical references. After reading all texts, 19were selected for this review, 18 with studies discussing (n=20) and 1 studyreporting a case not described yet in the literature with a total of 2418 cases in11 countries. Only one work investigated the PV during the operative act (Table 1). The other articles were not includedin this review because they do not discuss the frequency of the variations, do notpresent anatomical descriptions of the variations found, and do not presentstatistical data of measuring the frequency of the variations of the portalsystem.

There are 12 variations described relative to PV system through the union of the SV,SMV, and IMV (Figure 2), such that Thomson(1890) classification refers to the three most frequent, with predominance of type I(28-75%, M=47%), followed by type III (15-40%, M=27.8%) and type II (1.4-28.8%,M=18.6%). The three variations described by Thomson were the only ones described in16 out of 19 articles. The rest are rarer and make a total of 5.2% of the cases2^,8^,9^,10^,12^,13^,15^,16^,17^,20^,21^,22^,25^,27.

Krumm et al. 16 presented six variants that were not previously described: type IV(n=70-2%), in which an accessory mesenteric vein (AccMV) enters at the angle ofportal confluence as in Thomson type II; type V (n=28-1.1%), similar to Thomsonvariant I with two equal SMVs and the introduction of the IMV into the PV; type VI(n=10-0.4%), similar to variant V, that is, the IMV enters in one of the SMV; typeVII (n=8-0.3%), similar to variant I, in which an AccMV enters into the PV at theIMV introduction site; type VIII (n=8-0.3%), in which the IMV is absent; type IX(n=6-0.2%), similar to Thomson type I, with an AccMV which discharges in the PVbetween the IMV and the confluence of the SMV and PV; and type X (n=2-0.08%), withtwo equal mesenteric trunks at the confluence of the PV. Gorantla et al. 10 reported type XI, in which the left gastric vein is introduced into the SMV(n=1-0.04%). Purcell et al. 20 described 16 cases of type XII, where there were 2 IMVs: a tributary of theSMV and another from SV (0.6%).

According to Krumm et al. 16, the variations can be distributed into three IMV introduction groups asfollows:

• Group 1: IMV is tributary of SV (variants I, V, VII, and IX).

• Group 2: IMV is tributary of SMV (variants III, VI, and XI).

• Group 3: It does not fit into group 1 or 2 (variants II, IV, VIII, andX).

Variant XII is an exception because there are two IMVs, which can be included ingroups 1 and 2.

DISCUSSION

Thomson variants are the most well known in the surgical practice, inasmuch as thesum of the three junctions are found in over 90% of the patients. Less than 7% ofthe cases form the set of other nine variations. Even though rare, these variationsare possible to be found during operations such as gastroduodenopancreatectomy(Whipple surgery)1^,7^,24, colectomies28, venous bypasses due to portal hypertension (PH)5^,19, hepatectonies19 and liver transplants26, as well as in diverse operations on the pancreas and extrahepatic biliarypathways.

In minimally invasive surgeries, and, most recently, in those performed with the aidof remotely guided robotic devices, perfect knowledge of the anatomical structuresand their variations became indispensable in abdominal operations, mainly those whohave visceral venous times, all pertaining to the portal system28. Variations in the vascular architecture are the common causes of operativeaccidents with consequent increasing in surgical time and of the postoperativehospitalization period27.

Portal hypertension is one of the diseases with multiple complications, includingcirrhosis, schistosomiasis, retroperitoneal and biliopancreatic tumors, as well asadjacent arterial aneurysms and right heart failure4^,5^,19. With the increase in pressure, the PV system is reorganized, with theincrease in caliber of the veins, such as PV (>13 mm), SMV, and SV (>10 mm),associated with splenomegaly5^,19.

The anatomy of the portal system is also important in portal thrombosis (PT)26. The disease is classified into four types in accordance with the strickenvenous system and clinical manifestations as follows: type I: asymptomatic isolatedSV thrombosis; type II: asymptomatic intrahepatic PV thrombosis without PH; typeIII: asymptomatic diffused PT; and type IV: isolated or diffused symptomatic PT26. It occurs, in general, without known cause and is transitory; however, ithas been more widely studied after splenectomy. Usually, it is asymptomatic, andthere is no drug treatment yet that may prevent it or promote vascular rechanneling.Eventually, its clinical practice is associated with fever PT, abdominal pain,diarrhea, ileodynamic, ascites, and bleeding of esophageal varices23^,26^,29.

The knowledge of the abdominal portal system anatomy allows for the planning ofvenous bypasses to alleviate PH, mainly when associated with upper digestivehemorrhage. The bypasses include the right portocaval or with prosthesis,mesentericocaval, centralized and distal splenorenal, and left gastric caval.

The knowledge of portal anatomical variations helps also to understand thehepatofugal blood flow in the cases of PH1. More than 20 pathways have been described; for example, the reflux forinferior mesenteric collateral vessels that are connected through the hemorrhoidalplexus1.

CONCLUSION

Thomson classification of variants is the most well-known, accounting for over 90% ofPV variant found in clinical practice, inasmuch as the sum of the three junctionsare found in over 93% of the patients. Even though rarer and accounting for lessthan 7% of variants, the other nine reported variations will occasionally be foundduring many abdominal operations.

¹How to cite this article: Prado Neto EV, Petroianu A. ABCD Arq Bras Cir Dig.2022:35:e1666. https://doi.org/10.1590/0102-672020210002e1666

Central Message

⁴Although rare, the anatomical variations of portal venous system are possible tobe found during operations such as gastroduodenopancreatectomy (Whipplesurgery), colectomies, venous bypasses developed due to portal hypertension,hepatectomies and hepatic transplants, as well as in several surgeries on thepancreas and extrahepatic biliary pathways.

Perspectives

⁵A total of 12 anatomical variations of the portal vein have been published. Themost common is type I described by Thomson (1890), which consists of the portalvein constitution by the confluence of splenic and superior mesenteric veins,and having the inferior mesenteric and left gastric veins as tributaries.

REFERENCES