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therapy when contralateral LNs and metastases are encountered. Unfortunately, cross‐sectional imaging with computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET) alone is not adequate to confirm a diagnosis; thus, a tissue sample is preferred. Recently, it has been suggested that the use of endoscopic ultrasound‐guided fine needle aspiration (EUS‐FNA) associated with endobronchial ultrasound‐guided transbronchial fine needle aspiration (EBUS‐TBNA) can adequately sample LNs in the mediastinum, avoiding the need for a futile surgery.

      The purpose of this chapter is to provide the basic anatomical information as well as technical maneuvers used to investigate the mediastinum successfully.

Anatomical definitions

The LNs in the mediastinum were classified in different stations based on surgical and anatomical landmarks for the purpose of staging lung cancer but this schema is now widely used in other chest diseases (Figure 3.1). The LNs with their respective stations and corresponding anatomical locations are described in Table 3.1.

      EUS‐FNA is usually best suited to sample LNs adjacent to the esophagus which runs posterior to the trachea. Because of ultrasound artifacts created by the air‐filled trachea, lesions immediately anterior to the trachea are not well seen. EUS‐accessible stations include 2L, 2R, 4L, 4R, 5, 7, 8, 9, and, sometimes depending on the size, station 6. On the other hand, EBUS‐TBNA can target LNs either anterior or lateral to the trachea to the level of the carina, and alongside the left and right bronchial tree including stations 2L, 2R, 4L, 4R, 7, 10, and 11. Although both procedures overlap in stations 2 L/R, 4 L/R, and 7, in other stations they are complementary, and in combination allow nearly complete mediastinal access.

Equipment

Radial and curvilinear array echoendoscopes are available (Figure 3.2), with scanning radius ranging from 270–360 degrees for radial to 100–180 degrees for the linear echoendoscope. These scopes have standard accessory channels (2.0–2.8 mm) and larger accessory channels (3.7 mm) capable of delivering needles and other therapeutic devices such as a 10 French (Fr) plastic stent.

      EUS can use several types of needles: 19 gauge (G), 22 G, and 25 G for FNA, as well as Tru‐cut needles for core biopsy. The needle is occluded with a stylet during passage through the gastrointestinal tract wall and bronchial wall to minimize contamination from passage through those structures.

      EBUS equipment comprises a curvilinear array echoendoscope with an outer diameter of 6.7 mm and a biopsy channel of 2 mm. The ultrasonic frequency is 7.5 MHz with a penetration depth of 4–5 cm, making it well suited for FNA of LNs and lung masses through the trachea and bronchi. A 22 G needle is used to perform TBNA in the same manner as in EUS. Both systems have integrated oblique‐viewing optics to guide intubation and limited inspection.

Figure 3.1 Mediastinal lymph node stations.

Endoscopic ultrasound technique

      The initial examination can be performed with either the radial or linear array echoendoscope; however, the linear scope is required to perform FNA. Given the obvious efficiencies, we prefer to use a single linear echoendoscope for both imaging and FNA.

      Linear scanning

The balloon should be deflated or inflated only slightly to provide good acoustic coupling with the tissue. The mediastinum is imaged by first finding the descending aorta starting at the cardia. The examination can be performed by rotating 360 degrees from the cardia, then withdrawing the shaft 4–5 cm and performing another rotation. Alternatively, one can survey from the cardia to the cervix, then rotating 90 degrees and repeating the maneuver until the whole mediastinum is examined. It is useful to use the following five stations as described by Deprez (Videos 3.1.1–3.1.3). For radial examination, see Video 3.2.

Table 3.1 Mediastinal lymph node stations with their anatomical correlations.

Level	Anatomical correlation
Superior mediastinal lymph nodes
1	Highest mediastinal
2	Upper paratracheal
3	Prevascular and retrotracheal
4	Lower paratracheal (including azygos nodes)
Aortic lymph nodes
5	Aortopulmonary (AP) window or subaortic
6	Para‐aortic (ascending aorta and phrenic)
Inferior mediastinal lymph nodes
7	Subcarinal
8	Paraesophageal (below carina)
9	Pulmonary ligament
N1 lymph nodes
10	Hilar
11	Interlobar
12	Lobar
13	Segmental
14	Subsegmental

Figure 3.2 Types of echoendoscopes: (a) linear probe; (b) endobronchial probe; (c) radial probe.

      Inferior posterior mediastinum

The descending aorta is a large echo‐poor longitudinal structure on linear array with a bright deep wall due to the air interface with the left lung. Clockwise rotation will sequentially image left lung, left pleura, left atrium, right lung, right pleura, azygos vein, and spine. The azygos vein can be localized by rotating approximately 30 degrees counterclockwise from the descending aorta. It is a thin echo‐poor structure that can be followed proximally to its union with the superior vena cava. This is the area of LN stations 8 and 9 (Figure 3.3).

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