Anatomical variations and pitfalls to know during thoracoscopic segmentectomies

With growing experience with thoracoscopic anatomical segmentectomies, the surgeon becomes more and more prudent and cautious during vascular dissection as he/she realizes there is not a unique and standard anatomy with variations but there is actually no regular anatomy. In other words, there are only variations. Although surgeons are performing pulmonary segmentectomies for years, they need a better understanding of anatomy when using a closed chest approach, because the vision is more limited and they cannot stretch and expose the parenchyma and broncho-vascular elements as they used to do with hands inside the chest cavity.

Throughout this article, we will describe most of the anatomical variations we have encountered during a consecutive series of 390 full thoracoscopic segmentectomies, either at surgery—with systematic shooting of anatomical features—or preoperatively by studying the 3-dimensional (3D) modelisation (1,2). This underlines the need for a thorough preparation of these procedures with use of 3D reconstructions, whose benefit has been stressed by many authors (3-9). By reading this paper and looking at the figures, those of the younger surgeons or those just embarking in this surgery will realize that even segmentectomies taken to be straightforward, e.g., lingulectomies or S⁶ segmentectomy, can be tricky.

This article will be based not only on our own experience but also on the “Illustrated anatomical segmentectomy for lung cancer” atlas by Nomori and Okada (10) that is a unique contribution to this topic and a precious support to the surgeon. Most data reported in this paper have been borrowed to this atlas.

In this article, we will describe only significant anatomical features of most common sublobar resections (SLR) and only those which have surgical consequences.

Right apico-posterior (S¹⁺²) and posterior (S²) segmentectomies

Bronchus

As it enters the parenchyma, the upper lobe bronchus triplicates into three segmental bronchi: apical (B¹), posterior (B²) and anterior (B³). B¹ and B² can originate separately or as a common trunk (B¹⁺²) (Figure 1). During a posterior approach, B² can be mistaken for B¹⁺² if B¹ and B² are independent and if both bronchi are short.

Figure 1 Independent bronchi as seen on 3D reconstruction (A) and at thoracoscopy (B), (C) shows common B¹⁺² and independent B³.

Arteries

They arise from the truncus anterior (TA) and from the PA in the fissure, also named arterial truncus intermedius (ascending arteries). The TA duplicates into two branches: the apical artery A¹ and the anterior artery A³ that must be preserved. The posterior segment is supplied by the ascending A² (Asc.A²) that originates within the fissure from the posterior aspect of the pulmonary artery, opposite the middle lobe artery. It ascends to S² and lies posteriorly to the lobar bronchus. In most patients, there is only one artery, while there are none or two in some patients. When there are two ascending A², the most anterior one must not be mistaken for an anterior ascending branch (A³) which supplies the anterior segment (Figure 2).

Figure 2 Thoracoscopic dissection demonstrating two ascending arteries within the fissure. (A) The most anterior artery (indicated by “?”) could be a second Asc.A² or an Asc.A³; (B) as dissection is continued, it becomes clear that this artery is for S³. Asc.A²s, stump of Asc.A²; B, upper lobe bronchus.

In 72% of patients, a branch of the TA supplies S² and is named recurrent A² (Rec.A²). It usually runs along the bronchus. When present, the recurrent A² can be at risk during dissection of B¹ or B².

In about 10% of patients, the ascending A² artery originates from the superior supply of the lower lobe or forms a common trunk with A⁶.

Veins

In most cases, the segmental veins of the upper lobe are the two upper tributaries of the upper lobe vein: (I) V¹ is the uppermost branch and is found in the hilum of the upper lobe. It is the most anterior and superior vessel; (II) V²⁺³ is the central vein and runs in the parenchyma and within the fissure. It gives a large posterior branch for V² and small tributaries for V³. A small vein (V^2t) draining S² frequently cross the posterior aspect of the ascending A² (Figure 3) and must be severed before dissecting this artery (11).

Figure 3 V^2t crossing the posterior aspect of Asc.A². (A) 3D reconstruction; (B) thoracoscopic view.

Right anterior segmentectomy (S³)

Bronchus

B³ is the anterior branch of the upper bronchus. It is usually independent from the apico-posterior truncus (B¹⁺²) or from B¹ and B² and is usually easily recognized by its anterior direction, while B¹ and B² have a cephalad direction. Lymph nodes are frequently found at the origin of B³. Even for benign conditions, removal of these nodes is required for an optimal disclosure of B³ root.

Arteries

A³ is the lowermost branch of the TA. An ascending A³ is present in some patients, raising close to the ascending A² and recognizable by its anterior direction. In some cases, there are two ascending arteries in the fissure. The posterior one (Asc.A²) for segment 2 and the anterior one (Asc.A³) for segment 3 (Figure 4). The latter must not be confused with a middle lobe artery which must always be identified before any ligation or clipping of an ascending artery.

Figure 4 3D reconstruction demonstrating two ascending arteries from the main pulmonary artery, one for S² and one for S³.

Veins

There are two types of veins: (I) a large V3 that is the lowermost branch of the central vein and (II) 1 or 2 small ascending veins branching from the central vein that are easily recognized as they come directly from the anterior segment.

Right superior segmentectomy of the lower lobe (S⁶)

Bronchus

The superior segmental bronchus originates opposite or slightly above the middle lobe bronchus. It lies posteriorly to the segmental artery and separates into two main branches, rarely into 3. B⁶ is single in most patients but can seldom be double.

Arteries

The superior segment of the right lower lobe is supplied by an artery (A⁶) which originates within the fissure at the same level than the basal trunk. It is usually single, but can be double (Figure 5) and even triple. In some patients, the superior segmental artery originates from the ascending artery of the upper lobe (A²), or from the basal trunk (12) (Figure 6).

Figure 5 Double A⁶ artery.

Figure 6 Common origin of A⁶ and A².

Veins

The vein to the superior segment is the uppermost and smaller segmental tributary (V⁶) of the inferior pulmonary vein (IPV).

Right basilar segmentectomy (S^7–10)

Bronchus

The origin of the common basal trunk is found in the fissure 1 to 2 cm beyond the origin of B⁶. The basilar bronchial trunk usually separates in three branches: B⁷, B⁸ and B⁹⁺¹⁰ (Figure 7) that run posterior to the corresponding segmental arteries. In rare cases (8%), instead of the usual B⁸ and B⁹⁺¹⁰ pattern, the branching is reverse, i.e., B⁸⁺⁹ and B¹⁰ and in 6% of patients, B⁷, B⁸, B⁹ and B¹⁰ are independent.

Figure 7 Usual pattern of right basilar bronchi, with a common trunk for B⁹ and B¹⁰.

Arteries

The arterial supply of the basal segments is the termination of the pulmonary artery after the birth of A⁶. It runs anterior to the segmental bronchus and usually separates into two main trunks, one for S⁷⁺⁸ and one for the posterior and lateral segments S⁹⁺¹⁰. It can also separate into 3 to 4 segmental branches. A⁷ can be lacking (16%). A⁸ and A⁹⁺¹⁰ are usually presenting as 2 separate trunks (90%) (Figure 8).

Figure 8 Two different pattern of the arteries to the basilar segments. (A) Four independent branches to each of the segments; (B) A⁹⁺¹⁰ and one A⁸ with lacking A⁷.

A middle lobe artery can arise from the basilar trunk, sometimes at a low level (Figure 9). This means an extensive dissection or the basilar trunk and/or segmental arteries is necessary to avoid accidental stapling of a middle lobe artery.

Figure 9 Example of a middle lobe artery arising from the basilar arterial trunk and that could be damaged if dissection of the basilar arteries is insufficient.

Veins

The basal segments are drained by two venous trunks, the inferior basilar vein (IBV) for S⁹⁺¹⁰ and the superior basilar vein (SBV) for S⁷⁺⁸. In some patients, there is only one common venous trunk or, on the contrary, a multiramified vein. In most cases, the two basilar veins receive tributaries from adjacent segments. This means that—as an example—ligating the S⁹⁺¹⁰ vein at the level of the central vein when performing a segmentectomy is at risk of sacrificing a venous branch draining S⁸.

The superior vein (V⁶) must be clearly identified before stapling these two trunks. The middle lobe vein can join the IPV (Figure 10). Conversely, the basilar vein can drain into the middle lobe vein.

Figure 10 Basilar vein and middle lobe vein draining together in the inferior pulmonary vein. Bas.As, stump of the basilar arterial trunk; Bas.Bs, stump of the basilar bronchial trunk; IBVs, stump of the inferior basal vein.

Lingula sparing left upper lobectomy (S^1–3)

Bronchi

In its most common pattern, the upper lobe bronchus splits immediately into the lingular bronchus and a common stem that usually separates into an anterior bronchus (B³) and an apico-posterior bronchus (B¹⁺²). These three segmental bronchi have a short course that can make their identification and dissection difficult. Confusion between the anterior bronchus (B³) and the lingular bronchus is theoretically possible, as both B³ and B⁴⁺⁵, have an anterior course. However, when dissected in the fissure, the lingular bronchus is usually not visible if the lingular artery has not been divided. Only its origin is usually seen.

Arteries

There are two different supplies to the let upper lobe: the TA and the posterior arteries. The TA is often broad and short. It usually gives two main tributaries, the uppermost (A¹⁺²) for S¹ and S² and the lowermost (A³) for S³. The posterior arteries originate in the fissure and distribute themselves over the curve of the pulmonary artery (Figure 11). Their number varies from 1 to 5, but most often from 2 to 3.

Figure 11 Multiple arterial stumps (arrowheads) after control of all arteries to segments 1, 2 and 3 of the left upper lobe.

The presence of a mediastinal lingular artery (18%) should be search on 3D reconstruction (Figure 12). If present, dissection of the TA must be conducted with caution as this artery runs anterior between the vein and the B¹⁺²⁺³ bronchial trunk and is in close contact with the bronchus. Dissecting around the bronchus should be done smoothly.

Figure 12 Mediastinal lingular artery. (A) 3D modelisation; (B) thoracoscopic view after division of the bronchial trunk B1-3. LUL, left upper lobe; LLL, left upper lobe; TA, truncus anterior.

Veins

The superior pulmonary vein has usually three major tributaries (Figure 13A). The superior branch (V¹⁺²) drains S¹ and S² segments. The middle branch (V³) drains S³ and the lowermost branch drains the lingula. In some rare cases, V¹⁺² can form a common trunk with V³. Even when 3D reconstruction demonstrates a clear pattern of the venous anatomy with a distribution of the lowermost venous branch to the lingula, V⁴⁺⁵ can be tiny (Figure 13B). It can be preferable to preserve the inferior branch of V³. In some cases, it is almost impossible to determine if the adjacent vein to the lingular one comes from the lingula or from S³. It seems prudent to preserve this vein, especially if the lingular vein is small.

Figure 13 The left superior pulmonary vein. (A) Usual pattern with three main branches; (B) tiny lingular vein at risk of torsion and/or thrombosis.

Lingulectomy (S⁴⁺⁵)

Bronchus

The lingular bronchus originates from the bifurcation of the upper lobe bronchus and has a short course before it enters the parenchyma. In rare cases, the lingular bronchus can rise from the basilar bronchial trunk.

Arteries

The main supply to the lingula comes from the lingular trunk, which is the most anterior branch of the posterior segmental arteries. It originates from the anterior aspect of the pulmonary artery within the fissure and splits into two segmental branches. In 26% of the patients, these two branches car rise separately from the main pulmonary artery.

An accessory lingular artery coming from the mediastinum, also named mediastinal or prebronchial lingular artery, is present in 18% of the patients. Its presence should be suspected when the usual lingular artery is tiny or absent. However, having 2 of these arteries with a normal diameter is not unusual.

An A³ artery can rise close to A⁴⁺⁵ or even form a common trunk with the lingular artery (Figure 14).

Figure 14 Common trunk between the lingular arteries and A³. LUL, left upper lobe; LLL, left upper lobe.

An artery to the basilar trunk or to A⁸ can rise from A⁴. This underlines the need for an extensive dissection of the lingular artery. If this variation is encountered, the branches of the lingular artery must be controlled separately.

Veins

The lingular vein is the lowermost tributary of the superior pulmonary vein. It is easily recognizable when the superior pulmonary vein has three major roots (V¹⁺², V³ and lingular). However, in some patients, there are multiple radiating venous branches. In these cases, it is safer to divide only the lowermost branch. Once the lingula will become mobile thanks to the arterial and bronchial division, the venous drainage will become more apparent.

A lingular vein can drain into the IPV (Figure 15).

Figure 15 Venous drainage of the lingula into the inferior pulmonary vein (arrow). IPV, inferior pulmonary vein.

Left superior segmentectomy of the lower lobe (S⁶)

Bronchus

The superior segmental bronchus B⁶ is the first branch of the lower lobe bronchus. It arises laterally and posteriorly and lies posterior to the segmental artery. In obese or in some kyphotic patients, the bronchus can be located deeply and remote from A⁶, so that its identification and dissection can be difficult. It may be advisable to approach B⁶ from the back and from below, after division of V⁶ (Figure 16).

Figure 16 Exposure and dissection of B⁶. Note that the bronchus can be at some distance from the artery. LN, lymph nodes; A6s, stump of A⁶; PA, pulmonary artery.

Arteries

The superior segment of the left lower lobe is supplied by a single (80%) (Figure 17A) or double (18%) (Figure 17B,C) or even triple (2%) artery that originates from the posterior surface of the pulmonary artery in the posterior portion of the fissure. When single, A⁶ artery bifurcates in 2 or 3 branches. The artery runs anterior to the segmental bronchus. A lymph node is frequently encountered close to the posterior aspect of A⁶. It can tightly adhere to the artery and exposes to a vascular tear during dissection.

Figure 17 A⁶ artery, in its most common pattern. (A) Early bifurcation in two branches; (B) double A⁶ artery (3D reconstruction); (C) double A⁶ artery (thoracoscopic view).

Branches to segments 9 and 10 can be close to A⁶ and should not be confused with a branch of A⁶ (Figure 18).

Figure 18 Common rise of A⁶and A¹⁰.

Vein

The superior segment is drained by the superior branch of the IPV (V⁶).

V⁶ can receive a venous branch from the basilar segments (Figure 19). In this case, only the uppermost tributary of V⁶ must be clipped.

Figure 19 Bifurcation of the V⁶ vein: upper tributary for S⁶ and lower tributary for the basilar segments (white arrow). SPV, superior pulmonary vein; IPV, inferior pulmonary vein; SBV, superior basal vein; IBV, inferior basilar vein.

Left basilar segmentectomy (S^8–10)

Bronchus

The origin of the common basal trunk is found in the fissure 1 to 2 cm beyond the origin of B⁶ and the lingular bronchus.

Arteries

The arterial supply of the basal segments is the termination of the pulmonary artery after the birth of the lingular artery and A⁶. It runs anterior to the segmental bronchus and usually divides into two main trunks, one for S⁸ and one for the posterior and lateral segments S⁹⁺¹⁰. It can also separate into 3 or 4 segmental branches.

A lingular artery can originate from the basilar artery, sometimes at a low level (Figure 20). It is thus essential to dissect the basilar arteries on a sufficient length to avoid inadvertent clipping of the lingular artery.

Figure 20 Lingular artery originating from the basilar arterial trunk. LUL, left upper lobe.

Veins

The basal segments are drained by two venous trunks, the inferior basilar vein (IBV) for S⁹⁺¹⁰ and the superior basilar vein (SBV) for S⁸. The superior vein (V⁶) must be clearly identified before stapling these two trunks.

The lingular vein can meet up with the IPV (Figure 21).

Figure 21 Lingular vein joining the inferior pulmonary vein. Bas.As, stump of the basilar arterial trunk; Bas.Bs, stump of the basilar bronchial trunk; IPV, inferior pulmonary vein.

Left S⁸ segmentectomy (S⁸)

Bronchus

In most patients (80%), the basilar bronchial trunk usually separates in two branches: B⁸ and B⁹⁺¹⁰ which lie posterior to the segmental arteries. In rare cases (16%), the three bronchi are independent.

Arteries

The basilar arterial trunk branches separate in most cases (74%) in two arteries: A⁸ and A⁹⁺¹⁰, more seldom (10%) in three independent arteries or in two trunks A⁸⁺⁹ and A¹⁰ (16%). But all arteries to the lower lobe must be clearly identified to avoid misidentification, such as a low A^8a being mistaken for an A⁹⁺¹⁰ (Figure 22) When in doubt during dissection, it is preferable to control only the anterior branch of A⁸ (A^8b), then the bronchus and eventually check the direction of the second branch. The branching of the arteries and bronchi can be difficult to understand without help of a 3D modelisation. Instead of common B⁹⁺¹⁰ trunk, B⁹ can have a common birth with B⁸. Same as for the corresponding arteries. There is a risk of dividing B⁹ with B⁸ or A⁹ together with A⁸.

Figure 22 Common bronchial B⁸ and B⁹ trunk and common arterial A⁸ and A⁹ trunk. (A) 3D modelisation; (B) thoracoscopic view after clipping of A⁸. A8s, stump of A⁸.

Vein

The superior basal vein (SBV), which is the middle root of the IPV, does not always represent the venous drainage of segment 8. Its posterior branch can drain segment 9. It is advisable to divide only the anterior branch of the vein (V^8a) which runs immediately behind the bronchus, rather than controlling V⁸ centrally in the IPV.

Left S⁹⁺¹⁰ segmentectomy (S⁹⁺¹⁰)

Bronchus and arteries

These have been described in the previous paragraph. Instead of A⁸ and A⁹⁺¹⁰ (Figure 23), the following branching pattern can be encountered in 20% of patients: A⁸⁺⁹ and A¹⁰ (Figure 22A,B) or totally independent basal arteries (A⁸, A⁹ and A¹⁰). The same distribution is valid for bronchi. Preoperative modelisation is helpful to avoid anatomical misjudgment.

Figure 23 Arteries to left segment S⁸: common pattern.

Vein

The inferior basal vein (IBV), which is the lower root of the IPV, does not always represent the venous drainage of S⁹ and S¹⁰. One of its branches can drain S⁸. If any doubt, only the lowermost branch of the vein must be divided. It is however safer to control veins in the parenchyma. There are frequently no basal veins but just a branching into a common basal vein and V⁶. This stress the need to favor venous control in the parenchyma rather than centrally in the IPV.

Acknowledgements

Three-dimensional modelisations used for the illustrations of this work were made possible thanks to a grant from the Ligue contre le Cancer-Comité de Paris.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

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