Hidden in plain sight: unknown anatomy depiction and applications of the aorto-esophageal ligament

Introduction

The mediastinum was initially classified based on the lateral chest radiograph, but a newer system of classification developed by the International Thymic Malignancy Interest Research Group (ITMIG) has been accepted as the standard (1). This new schematic is based on cross-sectional imaging primarily computed tomography (CT) and divides the mediastinum into prevascular, visceral and paravertebral compartments (1). A unanimous system for classification of mediastinal compartments is necessary for various reasons including generating differential diagnoses for mediastinal diseases, planning surgical management and for ease of interaction with other clinicians (1). The advent of minimally invasive thoracic surgeries allowed for visualization of previously unknown tissue planes in the mediastinum such as the aorto-esophageal (AE) and aorto-pleural ligament (AP) ligaments which were confirmed by Cuesta et al. through histological study (2). Furthermore, a study done by Weijs et al. identified these structures on T2 weighted magnetic resonance imaging (MRI) through an ex-vivo cadaveric study and in-vivo study involving patients with esophageal cancer (3). Our article focuses on depicting the anatomy of the AE ligament and its applications.

While analyzing central mediastinal diseases on cross-sectional imaging, it is important to understand the pathway influencing the spread of disease at a radiological point of view. In addition to depicting the anatomy, this article will also discuss the various applications of the AE ligament such as its role in influencing the restriction or spread of disease in the mediastinum, acting as an anatomic landmark in determining the location of lymph node metastases from esophageal cancer, which will further influence the possibility of thoracic duct resection/sparing; and its role in preoperative planning of minimally invasive thoracic surgeries/potential use as a dissection plane during esophagectomy.

Anatomy of the AE ligament

The AE ligament is a portion of the mediastinal visceral fascia, which courses from the anterior aspect of the descending thoracic aorta (11 o’clock position) to the left lateral aspect of the esophagus (two o’clock position) (Figure 1A). The AE ligament consists of two connective tissue layers with blood vessels and nerves coursing in between similar to mesentery however in contrast it is not lined with mesothelial cells (3). The AE ligament can be identified on cross-sectional imaging such as CT but is best visualized on T2 weighted MRI (Figure 1B). The course of the ligament starts at the level of the aortic arch and extends till the level above the diaphragm (3). However, in Weijs et al.’s study, the most common location where it was visualized by radiologists was in the para-aortic region at the T8–T10 level (3). The AE ligament divides the posterior mediastinum into an anterior ‘peri-esophageal compartment’ compartment containing the esophagus, vagus nerve, and carinal lymph nodes and a posterior ‘para-aortic compartment’, containing the thoracic duct, azygos vein, and occasionally lymph nodes (Figure 1C).

Figure 1 Anatomy of the AE ligament. (A) Schematic depicting the AE ligament (red arrow) and AP ligament (blue arrow) at the level between the tracheal bifurcation and the diaphragm. (B) Axial T2 weighted MRI depicting AE ligament (red arrow). (C) Division of posterior mediastinum by the AE ligament into anterior peri-esophageal space (pink) and posterior para-aortic (green) space. AE, aorto-esophageal; AP, aorto-pleural; AV, azygous vein; E, esophagus; MRI, magnetic resonance imaging; TD, thoracic duct; V, vagus nerve.

Role of imaging

In order to confirm the presence of the AE ligament; Weijs et al. [2017] conducted a two-arm study. The first arm involved an ex-vivo study where Transverse T2 weighted images of the thoracic region of one cadaver was taken using a 3 Tesla MRI. These images were taken within 24 hours post-mortem to avoid tissue decay/long term fixation (3). On the other hand, the second arm involved an in-vivo study where pre-treatment MRI of 34 patients with esophageal cancer was analyzed (3). These images were independently assessed by two radiologists for their presence and location and the results showed an inter-observer agreement was k value of 0.57 which showed substantial agreement (3).

Applications of the AE ligament

Spread of disease in the mediastinum

When assessing mediastinal diseases; whether it be gas, fluid or soft tissue masses on cross-sectional imaging, it is vital to pay attention to the factors that influence the restriction or spread of disease. Some pathologies that manifest as fluid in the mediastinum include fluid from esophageal perforation, hemorrhage from aneurysm rupture, infection originating from the deep neck or the abdomen etc. (4). On the other hand, some common etiologies for soft tissue disease in the mediastinum include fat lesions like lipoma, soft tissue lesions like thyroid masses/cysts, lymphadenopathy and primary/metastatic tumors (4). Finally, gas can manifest in the mediastinum from various sources such as visceral perforation, trauma, foreign bodies, excessive coughing/vomiting/retching, soft tissue infection etc. (4). The AE ligament plays a vital role in delineating the spread of these disease processes in the mediastinum. It serves as an axial barrier to soft tissue disease spread, while directing its spread along a longitudinal pathway (4). Thus, it inhibits the spread of soft-tissue disease in an antero-posterior direction depending on where the mass is located in relation to the AE ligament (Figure 2). However, fluid and gas have the potential to pass through the ligament (Figure 3).

Figure 2 A 65-year-old male patient with mediastinal lipoma. (A) Axial contrast-enhanced chest CT image shows lipoma being limited axially by aorto-esophageal ligament (red arrow), E: esophagus. (B) Sagittal CT chest image shows lipoma (blue arrows) extending along the peri-esophageal space, displacing the trachea (pink arrow) and esophagus (purple arrow). Lipoma continues longitudinally below carina (orange arrow). CT, computed tomography.

Figure 3 A 54-year-old male patient with distal esophageal perforation. (A) CT axial chest shows oral contrast and air (pink arrows) due to esophageal perforation passing through the AE ligament (red arrow). (B) CT sagittal chest image shows contrast material (green arrows) and air (blue arrows) in the periesophageal space and esophageal lumen (purple arrow). AE, aorto-esophageal; CT, computed tomography.

Oncology imaging

AE ligament plays a key role during pre-op imaging for esophageal cancer, which is known for its frequent lymph node metastases and tumor ingrowth to the surrounding structures. Esophageal cancer is the eighth most common type of cancer and is the sixth leading cause of cancer death, with up to 0.54 million deaths worldwide in 2020 (5). The most common locations for esophageal cancer metastases are liver, lymph nodes, lung, bone and brain (5). Lymphatic spread is an important prognostic factor for esophageal cancer and standard treatment consists of neoadjuvant chemoradiation and trans-thoracic esophagectomy (TTE) with lymph node (LN) resection (6). Thoracic duct lymph nodes (TDLN) are found along the thoracic duct and are located posterior to the AE ligament in the para-aortic space (7). They are situated in the adipose tissue in between the thoracic esophagus and the descending aorta (7). TDLN metastases occur in highly advanced stages of esophageal cancer and survival in patients with TDLN metastases is identical to those with distant metastases (7). Thus, it is important to know the presence of TDLN metastases as it indicates a strong negative prognostic factor and lymphadenectomy of the TDLN with thoracic duct resection should be considered (7).

With the guide of pre-operative MRI, the AE ligament acts as an anatomical landmark to determine the location of the lymph node metastases and tumor ingrowth. As mentioned above the AE ligament divides the posterior mediastinum into an anterior ‘peri-esophageal compartment’ and posterior ‘para-aortic compartment’. If there are lymph node metastases anterior to the AE ligament (peri-esophageal space), this may influence surgeons to perform less extensive resections possibly sparing the para-aortic space and its contents; especially avoiding the complications that come with TD resection (8). However, if there are metastases to the TDLN; which are located posterior to the AE ligament (para-aortic space), treatment will require esophagectomy with resection of the thoracic duct and TDLN (7). Some of the benefits of this surgical plan is that it will lead to increased lymph node yield, increased radicality of surgical resection, as the adipose tissue surrounding the TD and esophagus that could contain cancer cells can be removed simultaneously; and increased circumferential resection margin (CRM) (7). Transthoracic minimally invasive esophagectomy (TMIE) including thoracic duct resection was associated with improved recurrence free survival (RFS) (7). On the other hand, the complications of thoracic duct resection include postoperative leak and resultant chylothorax, edema due to the interruption of lymph flow or piling of lymphatic fluid, malabsorption and acute pancreatitis secondary to resection, negative effect on early postoperative nutrition (9). All in all, the AE ligament acts as an important landmark in determining the location of lymph node metastases from esophageal cancer and can aid in choosing the proper surgical management.

Pre-operative planning

Good regional anatomical knowledge is essential especially during minimally invasive thoracic surgeries as it requires working in a confined space with many important structures. Surgeons need to be aware that tissues such as the AE ligament exist and that it can be used as a landmark to differentiate the beginning and end of the peri-esophageal and para-aortic space and its contents (8). In addition, connective tissue layers are frequently used as dissection planes during operation in other regions, such as ‘the white line of Toldt’ (in the mesocolon) or ‘the holy plane of Heald’ (the mesorectum); which has been instrumental in achieving complete resections for rectal cancer (3). Similarly, the AE ligament can potentially be used as a dissection plane during esophagectomies (8).

Conclusions

AE ligament is a recently discovered portion of the mediastinal visceral fascia, which courses horizontally from the anterior aspect of the aorta to the left lateral aspect of the esophagus. It can be visualized on cross-sectional imaging such as CT and MRI from the level of the aortic arch to the level of the diaphragm. Some of its applications include acting as an axial barrier to the spread of soft tissue disease in the mediastinum, while allowing gas and fluid to pass through. It also acts as an important anatomical landmark in determining the location of esophageal cancer lymph node metastases, which will further influence the possibility of thoracic duct resection/sparing. Finally, it is vital in preoperative planning of minimally invasive thoracic surgeries and can potentially be used as a dissection plane during esophagectomy. Although the AE ligament seems like an inconspicuous feature in the grand scheme of the mediastinum, it plays a pivotal role. We believe that this novel topic has potential for further research into its applications.

Acknowledgments

None.

Footnote

Peer Review File: Available at https://med.amegroups.com/article/view/10.21037/med-24-31/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://med.amegroups.com/article/view/10.21037/med-24-31/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The patients have given consent and their identities have been protected. Ethics approval was not required by our institution for the purposes of this review article.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Carter BW, Benveniste MF, Madan R, et al. ITMIG Classification of Mediastinal Compartments and Multidisciplinary Approach to Mediastinal Masses. Radiographics 2017;37:413-36. [Crossref] [PubMed]
2. Cuesta MA, Weijs TJ, Bleys RL, et al. A new concept of the anatomy of the thoracic oesophagus: the meso-oesophagus. Observational study during thoracoscopic esophagectomy. Surg Endosc 2015;29:2576-82. [Crossref] [PubMed]
3. Weijs TJ, Goense L, van Rossum PSN, et al. The peri-esophageal connective tissue layers and related compartments: visualization by histology and magnetic resonance imaging. J Anat 2017;230:262-71. [Crossref] [PubMed]
4. Ponnatapura J, Oliphant M, Holbert JM. The Mediastinal Visceral Space: The Central Pathway for the Spread of Mediastinal Disease. Indian J Radiol Imaging 2022;32:365-71. [Crossref] [PubMed]
5. Liu CQ, Ma YL, Qin Q, et al. Epidemiology of esophageal cancer in 2020 and projections to 2030 and 2040. Thorac Cancer 2023;14:3-11. [Crossref] [PubMed]
6. Matsuda S, Takeuchi H, Kawakubo H, et al. Clinical outcome of transthoracic esophagectomy with thoracic duct resection: Number of dissected lymph node and distribution of lymph node metastasis around the thoracic duct. Medicine (Baltimore) 2016;95:e3839. Erratum in: Medicine (Baltimore) 2016;95:e5074. [Crossref] [PubMed]
7. Matsuda S, Kawakubo H, Takeuchi H, et al. Prognostic impact of thoracic duct lymph node metastasis in esophageal squamous cell carcinoma. Ann Gastroenterol Surg 2021;5:321-30. [Crossref] [PubMed]
8. Weijs TJ, Ruurda JP, Luyer MDP, et al. New insights into the surgical anatomy of the esophagus. J Thorac Dis 2017;9:S675-80. [Crossref] [PubMed]
9. Schurink B, Defize IL, Mazza E, et al. Two-Field Lymphadenectomy During Esophagectomy: The Presence of Thoracic Duct Lymph Nodes. Ann Thorac Surg 2018;106:435-9. [Crossref] [PubMed]