TSRA Content:
Author: Roman Petrov, MD
Revision in this chapter is originally written by Roman Petrov, MD
Introduction
Endoscopic ultrasound (EUS) is a modality used for invasive evaluation and staging of esophageal and mediastinal pathology. This combined technology allows clinicians to approximate the area of interest by endoscopy and to extend visualization of lesions beyond the surface of the 1nucosa by ultrasound.
EUS also allows for tissue acquisition with the use of fine-needle aspiration (FNA) or core-needle biopsy, enhancing its diagnostic capabilities compared to routine imaging. EUS is superior to both C'T and PET for locoregional staging. CT is unable to distinguish between layers of esophageal wall, the key parameter in T staging, and demonstrates at best "esophageal wall thickening" at the site of tumor. Accuracy of T staging by EUS is approaching 70-90%. This modality is also superior to PET in assessing regional lymph nodes since regional nodes can be "outshined" by the tumor uptake. Although its value is limited in diagnostics of distant disease, evaluation and tissue sampling of adjacent organs is often possible (i.e., left lateral segment of the liver, left adrenal, retroperitoneum). The primary disadvantage of EUS is the variability noted based on operator experience. In addition, another disadvantage is that it may not be available in every institution; this is usually due to cost.
Esophageal/EUS Anatomy:
The esophageal wall is composed of three distinct layers: mucosa, submucosa, and muscularis propria. The mucosa consists of epithelium, lamina propria and muscularis mucosae. The sub mucosa, the strongest layer of the esophageal wall, is comprised of connective tissue and a rich network of blood vessels and lymphatics. The muscularis propria consists of an inner circular and outer longitudinal layer of muscle. The esophagus does not have serosa or adventitia but is surrounded by mediastinal fat. However, it is not uncommon to see the periesophageal fat referred to as "adventitia" in the EUS report. EUS displays the layers of the esophageal wall as five approximately equal and distinct alternating rings of hyperechoity (white) and hypoechoity (black). These layers are: white – mucosal surface; black - mucosa; white - submucosa and interface with muscularis propria; black - muscularis propria; and white -periesophageal tissue. High frequency probes visualize the esophageal wall as a nine-layer structure: white -superficial mucosa; black - deep mucosa; white - lamina propria; black - muscularis mucosa; white - submucosal; black - circular layer of muscularis propria; white – intramuscular connective tissue; black - longitudinal layer of muscularis propria; and white -periesophageal tissue. Normal periesophageal lymph nodes appear small (less than 1 cm in transverse dimension), oval or angular, hyperechoic, and homogeneous with indistinct borders.
Equipment:
The EUS scope is typically larger in diameter compared to a standard adult endoscope (12.7 mm vs. 9.8 mm). Mini probes, passed through the working channel of a regular scope, provide radial images at 12 to 30 MHz frequency. Due to the high frequency, tissue penetration is limited to less than 3 cm, which affects N staging.
The radial echoendoscope is the "work horse" of EUS and is used for initial imaging. A cylindrical ultrasound transducer is positioned on the tip of the endoscope and provides radial, 360-degree imaging in a plane perpendicular to the axis of the scope. A range of frequencies from 5 to 20 MHz allows tissue penetration of 3 to 12 cm in depth. A latex balloon is positioned over the transducer, which can be instilled with water for improved interface with the tissue and resulting improved visualization. Air entrapped in this balloon will compromise the view. The endoscope has oblique, forward-viewing optics allowing for some, although limited, visual control. The EUS scope has standard deflectors, identical to a regular endoscope, with the addition of balloon inflation/deflation ports being incorporated into the water flush/suction buttons.
The linear echoendoscope provides scanning in the longitudinal plane. It has oblique forward viewing optics and a working channel for biopsy coaxially located within the scanning plane, allowing for ultrasound visualization of the needle while in the tissue. The ultrasound image processor can be mounted on a separate ca1t or incorporated into an endoscopic tower and has the capability of image translation onto a separate screen or a picture-in-picture (PIP)mode. One advantage of linear is that it’s more widely available than radial
Operative Technique
The echoendoscope is prepared with placement of a latex balloon over the transducer with the aid of a special applicator. The balloon and channel must be de-aired with several flushes and aspirations. Before use, the endoscope is sprayed with silicon. The procedure is typically performed under general anesthesia.
Initially, standard endoscopy is performed to document the precise location of all landmarks and lesions, as well as assess whether a bulky tu1nor or stricture will preclude passage of the larger echoendoscope. Pre-procedural evaluation of imaging such as CT or barium esophagram aids in orientation. Once routine upper endoscopy is completed, the scope is changed out to the radial echoendoscope.
The echoendoscope is advanced into the pharynx and deflected anteriorly once the posterior wall is reached. The jaw thrust maneuver is frequently needed to allow passage of the large scope and intubation of the proximal esophagus. Deflection is somewhat limited with the echoendoscope, and intubation of the esophagus can be the most difficult part of the procedure. Care must be taken to avoid iatrogenic perforation. Once in the lumen, the scope is advanced to the predetermined level of interest and rotated to position the aorta posteriorly on the screen (hypoechoic, pulsatile structure) to assist with orientation. After assessment of the primary lesion, the scope is advanced into the stomach. The celiac artery is identified as the first ventral branch of the aorta. Left adrenal, left lateral segments of the liver, and celiac lymph nodes can be interrogated. On the withdrawal of the scope, mediastinal level 2, 4, 7, and 9 lymph nodes can be assessed as well. If suspicious lymph nodes are identified, location from the incisors is noted. If biopsy is to be performed, the radial scope must be changed out for the linear scope to perform FNA or core-needle biopsy. Only lymph nodes outside the limits of the primary tumor should be biopsied, as the specimen can be contaminated by the primary tumor.
Bulky tumors or strictures can be troublesome during EUS examinations. However, several options exist. Pre-dilatation may be performed during routine upper endoscopy prior to passage of the larger echoendoscope. However, the risk of perforation is significant (up to 24%), especially with non-circumferential tumors where the normal, thin wall is particularly at risk. If available, mini probes provide an additional option, although the depth of penetration is limited. Aborting the procedure with an incomplete evaluation may be the safest option in some cases.
EUS in Esophageal Cancer:
The primary advantage of EUS over other imaging modalities used for staging of esophageal cancer is that tissue can be acquired using the linear echoendoscope. Accuracy of T staging of esophageal carcinoma by EUS is approaching 80-90%. Accuracy of N staging by EUS approaches 85%. Malignant lymph nodes appear large (>1 cm in transverse dimension), round, hypoechoic, and non-homogeneous with sharp borders. EUS staging of esophageal cancer is reported using the 1N.lv1 classification with the preceding modifier "u" (e.g., uT3N2). EUS has also been used for restaging after neoadjuvant therapy, although this has not become widely accepted. This is reported with the preceding modifier "y "and "u" (e.g., yuT1N0) EUS in Benign Conditions: EUS can aid in the management of benign esophageal lesions as well, with diagnosis being made based on the layer of origin and characteristics of the lesion.
Fibrovascular polyps, retention cysts, and squamous papillomas can be found in the first and second layers (mucosa); lipomas, fibromas, neurofibromas, hemangiomas, granular and cell tumors in the third layer (submucosa); and leiomyomas, leiomyosarcomas, and gastrointestinal stromal tumors in the fourth layer (muscularis propria). Cysts may be intramural or extra-esophageal and appear as round, sharp-bordered, hypoechoic structures. Solid lesions are hyperechoic. Esophageal varices appear as serpiginous, hypoechoic structures with positive Doppler flow.
EUS in Other Conditions:
EUS can be complementary to endobronchial ultrasound (EBUS) in the mediastinal staging of lung cancer or other thoracic malignancies. In addition to level 2, 4, and 7 level mediastinal lymph nodes, EUS provides access to level 8 and 9 lymph nodes, the left lateral segment of the liver, and the left adrenal gland, all of which cannot be accessed via EBUS. The esophagus also provides an access route to posterior mediastinal tumors for tissue acquisition.