INTRODUCTION — Endoscopic ultrasonography (EUS) is a combination of endoscopy and ultrasonography. EUS can be used to visualize and sample lesions of the gastrointestinal tract, pancreas, posterior mediastinum, and retroperitoneum. EUS is often used for staging gastrointestinal malignancies such as esophageal, gastric, pancreatic, and rectal cancers.
Echoendoscopes operate across a broad range of frequencies from 5 to 20 MHz, permitting a spectrum of depth of penetration and image resolution. High-frequency catheter EUS operating at 20 to 30 MHz provides a more detailed ultrasound image of the intestinal wall, and such technique is discussed separately. (See "High-frequency catheter endoscopic ultrasonography".)
This topic will review factors associated with performing diagnostic or staging EUS including indications, contraindications, patient preparation, technical aspects, and adverse events.
PATIENT SELECTION
Clinical applications — EUS of the upper gastrointestinal tract has been performed for the diagnostic evaluation or staging of conditions such as:
●Esophageal cancer (see "Endoscopic ultrasound in esophageal cancer")
●Subepithelial lesions of the upper gastrointestinal tract (see "Endoscopic ultrasound for the characterization of subepithelial lesions of the upper gastrointestinal tract")
●Chronic pancreatitis (see "Endoscopic ultrasound in chronic pancreatitis")
●Exocrine pancreatic cancer (see "Endoscopic ultrasound in the staging of exocrine pancreatic cancer")
●Choledocholithiasis (see "Choledocholithiasis: Clinical manifestations, diagnosis, and management")
EUS-guided sampling technique and therapeutic interventions are discussed separately:
●(See "Endoscopic ultrasound-guided fine needle aspiration in the gastrointestinal tract".)
●(See "Endoscopic ultrasound-guided fine needle biopsy in the gastrointestinal tract".)
●(See "Therapeutic endoscopic ultrasound".)
●(See "Endoscopic ultrasound-guided celiac plexus interventions for pain related to pancreatic disease".)
Contraindications — There are few contraindications to EUS; however, the most common contraindications are related to upper endoscopy and include:
●Patients who cannot tolerate moderate sedation, monitored anesthesia care, or general anesthesia. (See "Anesthesia for gastrointestinal endoscopy in adults".)
●Patients who are hemodynamically unstable.
●Patients with known or suspected perforated viscus.
●Patients with gastrointestinal obstruction (eg, duodenal stricture due to a lesion in the head of the pancreas) may undergo EUS, but the imaging is limited to an area proximal to the level of obstruction.
●Patients with untreated disorders of hemostasis (eg, platelet count <50,000/microL). (See "Gastrointestinal endoscopy in patients with disorders of hemostasis".)
PATIENT PREPARATION — The preprocedure preparation for patients undergoing EUS is similar to that described for patients undergoing upper gastrointestinal endoscopy (see "Overview of upper gastrointestinal endoscopy (esophagogastroduodenoscopy)", section on 'Patient preparation'):
●Adjusting medications – The management of antiplatelet and anticoagulant therapy in patients undergoing EUS is typically individualized, managed in conjunction with the prescribing subspecialist, and is discussed separately. (See "Management of antiplatelet agents in patients undergoing endoscopic procedures" and "Management of anticoagulants in patients undergoing endoscopic procedures".)
●Sedation/anesthesia – The procedure is typically performed on an outpatient basis using monitored anesthesia care or general anesthesia. Anesthetic management for endoscopic procedures including preprocedure fasting is discussed separately. (See "Anesthesia for gastrointestinal endoscopy in adults".)
●Antibiotics – Antibiotic prophylaxis is generally not required for noninterventional EUS (table 1). (See "Antibiotic prophylaxis for gastrointestinal endoscopic procedures".)
PROCEDURE
General imaging principles — During EUS scanning, the ultrasound transducer is guided through the gastrointestinal tract and placed at the luminal site that is directly adjacent to the area of interest (figure 1). Close proximity of the transducer to anatomic structures (eg, extraluminal organs, vessels) permits the use of high-frequency, high-resolution sound waves. To bring an area of interest into focus, the balloon surrounding the transducer (located at the echoendoscope's tip) is filled with water (approximately 15 mL), thus interposing a nonattenuating acoustic coupling medium between the transducer and the surface of the gastrointestinal wall.
Image orientation — Radial EUS scanning provides images that are perpendicular to the long axis of the echoendoscope [1]. In general, the upper part of the endoscopic image is anterior, and the right part of the image reflects the anatomic structures on the patient's left side. (See 'Equipment' below.)
Intestinal wall layers — On EUS imaging, the normal intestinal wall is visualized as five alternating hyperechoic (bright) and hypoechoic (dark) bands that correspond to five distinct histologic layers of the intestinal wall as described in the figure [2] (image 1 and figure 2).
Equipment — EUS is performed with an echoendoscope (an endoscope with an ultrasound transducer engineered into its tip that emits high-frequency acoustic waves to the surrounding tissues). The echoendoscope is a flexible instrument that has biopsy channels and can obtain reliable images at a depth ranging from 3 to 80 mm from the ultrasound transducer. A water-filled balloon at the tip of the echoendoscope allows for acoustic coupling. Echoendoscopes that are commonly used operate across a range of frequencies (from 5 to 12 MHz).
Two major types of echoendoscopes exist:
●Radial echoendoscope – Radial echoendoscopes are usually oblique-viewing and have an electrical transducer that scans 360 degrees perpendicular to the long axis of the echoendoscope. The ultrasound signal is then integrated by a processor and transmitted in real-time to a video screen. The radial echoendoscope evaluates lesions for diagnosis and staging (eg, esophageal, gastric, or pancreatic cancers) [3]. Endosonographers who are experienced in radial echoendoscope technique are able to identify and stage target lesions.
●Linear echoendoscope – Linear echoendoscopes provide between 100- and 180-degree views that are parallel to the shaft of the echoendoscope, thereby permitting real-time visualization of an instrument such as a fine needle for aspiration as it is advanced into the periluminal space. (See "Endoscopic ultrasound-guided fine needle aspiration in the gastrointestinal tract".)
Technique
Initial steps — Upper gastrointestinal endoscopy (esophagogastroduodenoscopy) is often performed immediately prior to EUS to identify abnormalities that may limit the EUS examination (eg, esophageal stricture, duodenal diverticulum). Diagnostic upper gastrointestinal endoscopy is discussed separately. (See "Overview of upper gastrointestinal endoscopy (esophagogastroduodenoscopy)".)
EUS procedures of the upper gastrointestinal tract are initiated with an approach that is similar to upper endoscopy:
●Position the patient in the left lateral decubitus position.
●Introduce the echoendoscope into the mouth and advance it under direct visualization through the upper esophageal sphincter and into the proximal esophagus [2]. Continue advancing the echoendoscope until the tip of the echoendoscope has reached the target region (eg, esophagus, stomach, duodenum).
Imaging the target region
Transduodenal — Transduodenal imaging is used to assess extraluminal structures in the lower retroperitoneum (pancreas, common bile duct) (table 2). The sequence of examining the pancreaticobiliary tract can be approached using either of the following methods: starting with transduodenal imaging and then proceeding to transgastric imaging, or vice versa. (See 'Transgastric' below.)
The steps of transduodenal EUS imaging are summarized as follows:
●Wedge the tip of the echoendoscope into the apex of the duodenal bulb.
●Inflate the balloon of the echoendoscope and place the shaft of the echoendoscope along the greater curvature of the stomach (ie, the long position). This positioning results in a longitudinal view.
●In the longitudinal plane, visualize the common bile duct and pancreatic duct converging into the ampulla (surrounded by the pancreatic head) (image 2). Alternatively, this view can be achieved from the duodenal bulb or by slowly advancing the tip of the echoendoscope beyond the duodenal bulb and into the second portion of the duodenum.
●Examine the region of the major ampulla and pancreatic head for polypoid lesions or masses.
●Identify the "stack sign," an anatomic configuration consisting of the common bile duct, pancreatic duct, and the portal vein positioned almost parallel to one another.
●Deflect the tip of the echoendoscope upward as it passes beyond the major ampulla.
●Gradually withdraw the echoendoscope with subtle left and right tip deflection.
●Advance the echoendoscope tip to the second or third part of the duodenum and identify the inferior vena cava and aorta, which may be seen in cross-section.
●Identify the uncinate process of the pancreas between the ultrasound transducer and the aorta.
●At the level of the uncinate process, identify the ventral anlage of the pancreas which appears hypoechoic (dark) and can be seen in most patients. This triangular area represents the embryologic ventral bud of the pancreas that later forms the posterior and inferior aspect of the head of the pancreas. The ventral anlage of the pancreas appears hypoechoic due to reduced fat content compared with the dorsal pancreas.
Transgastric — Transgastric imaging is used to assess the extraluminal structures in the upper retroperitoneum (pancreatic body and tail, retroperitoneal lymph nodes, and left lobe of liver) (table 2). The sequence of examining the pancreaticobiliary tract can be approached using either of the following methods: starting with transduodenal imaging and then proceeding to transgastric imaging, or vice versa. (See 'Transduodenal' above.)
The steps of transgastric imaging are summarized as follows:
●Wedge the tip of the echoendoscope in the antrum of the stomach and gradually pull it back toward the body of the stomach using subtle movements.
●From the antrum, identify the gallbladder as an anechoic (dark) structure with a three-layered wall (image 3). The gallbladder may also be viewed from the first and second portions of the duodenum. Gallbladder stones appear as round hyperechoic (bright) structures that cause posterior shadowing.
●Identify the confluence of the splenic vein and the portal vein as the transducer is pulled back from the antrum; it is also referred to as the "club head" because of its resemblance to a golf club (image 4).
●Identify the cross-section of the superior mesenteric artery by its hyperechoic (bright) walls that lie posterior to the confluence of the splenic and portal veins.
●Identify the pancreas which is located between the splenic vein and the transducer and has a grainy "salt and pepper" pattern that appears more hyperechoic (bright) than the liver (image 5). The neck and body of the pancreas and glimpses of the pancreatic duct can be seen anterior to the splenic vein.
●Slowly withdraw the echoendoscope into the gastric body and visualize of the body of the pancreas.
●Identify the pancreatic duct which appears anechoic and measures approximately 2 mm in cross-section. Based upon clinical experience, the diameter of the pancreatic duct approximately follows the 3-2-1 rule (ie, normal pancreatic duct diameter measures 3 mm in the head, 2 mm in the body, and 1 mm in the tail of the pancreas).
●Identify the splenic artery by its tortuous shape. In contrast to the shape of the splenic artery, the splenic vein appears as a long continuous structure.
●Follow the splenic vessels to the patient's left side and continue to the hilum of the spleen, which marks the leftward limit of the tail of the pancreas.
●Withdraw the echoendoscope toward the fundus of the stomach and identify the celiac trunk bifurcating into the hepatic artery and the splenic artery (referred to as the whale's tail sign) (image 6).
Transesophageal — The sequence of transesophageal imaging typically begins with the tip of the echoendoscope positioned distal to the esophagogastric junction (EGJ) (ie, in the gastric cardia) and then proceeds proximally (table 2):
Steps of the examination distal to the EGJ are summarized as follows:
●Identify the aorta which appears as an oval-shaped, approximately 2 cm hypoechoic structure, positioned between five and six o'clock. If it is not seen, gentle advancement or withdrawal of the echoendoscope will usually bring it into view.
●Trace the path of the aorta distally to the celiac trunk, which is the first major vessel branch below the diaphragm.
●Next, trace the aorta to the celiac axis and examine this region for enlarged lymph nodes (>1 cm).
●Identify the hepatic artery which courses to the right and superiorly, and identify the splenic artery which courses to the left. Confirm the vascular nature of these structures with color Doppler imaging.
Steps of transesophageal imaging in the distal esophagus include (figure 2):
●Identify the five-layer wall pattern of the esophagus that comes into view as the echoendoscope is slowly withdrawn through the EGJ and into the distal esophagus (image 1). Esophageal wall thickness normally measures 3 to 4 mm.
●For patients with a hiatal hernia, recognize that additional layers of the distal esophageal wall (>5 layers) may be visible. This probably occurs as a result of simultaneous imaging of the hiatal hernia and cardia of the stomach. To open the mucosal folds of the hiatal hernia, water instillation can help overcome these technical issues and improve visualization. (See "Hiatus hernia".)
●Identify the spine in the seven o'clock position and just adjacent to the aorta.
●Identify the left lobe of the liver between the 6 and 12 o'clock positions.
●Identify the inferior vena cava and other hepatic structures (ie, hepatic veins and bile ducts) which will be seen coursing through the left lobe of the liver.
●Slowly withdraw the echoendoscope and visualize the heart, specifically the left atrium, at the 12 o'clock position.
●Visualize the left pulmonary artery as it heads posteriorly to the left of the ascending aorta, and visualize the pulmonary veins entering the left atrium. The mitral valve leaflets can sometimes be imaged; the valves are seen opening from the left atrium into the left ventricle. The left ventricle, right atrium, and right ventricle are more difficult to image and may not be seen distinctly in all cases since they are situated posterior to the left atrium.
●Continue to withdraw the echoendoscope and identify the left lung that appears in the two o'clock position, while the right lung can be seen in the nine o'clock location. The lungs appear as hyperechoic rings that represent air within the lung parenchyma.
●Withdraw the echoendoscope into the mid-esophagus.
Steps of transesophageal imaging in the mid-esophagus include:
●Identify the aorta in the five to six o'clock positions, which then evolves into the aortic arch.
●Identify the spine in the seven o'clock position.
●Identify the azygous vein in the eight o'clock position and to the right of the aorta. The azygous vein courses anterior to the spine and upwards along the right lung.
●Identify the thoracic duct which appears as a small, round anechoic structure to the left of the aorta.
●Withdraw the echoendoscope further and visualize the azygous vein moving forward and merging into the superior vena cava.
●Visualize the aortic arch on the left as it arises from the aorta and then curves towards the right.
●Visualize the left and right bronchi as small hyperechoic rings that represent cartilage and air, located at the 1 o'clock and 11 o'clock positions, respectively.
●Continue to withdraw the echoendoscope and visualize the left and right bronchi forming the trachea at the level of the hilum, which is usually about 26 to 28 cm from the incisors.
●Examine the area distal to the trachea (ie, subcarina) for mediastinal lymph nodes. Malignant-appearing lymph nodes are >1 cm; have distinct, sharp borders; and a round shape. Benign-appearing mediastinal lymph nodes typically appear as hyperechoic (bright) structures with indistinct margins and irregular shapes (ie, oblong, rectangular, and triangular).
●Continue to withdraw the echoendoscope into the proximal esophagus.
Steps of transesophageal imaging in the proximal esophagus include:
●Visualize the blood vessels of the neck as the aortic arch falls below the imaging plane. Identify the subclavian artery posteriorly while the left common carotid artery and sometimes the brachiocephalic trunk are seen anteriorly.
●Recognize artifacts arising from the trachea in the mid-plane such as air within lumen of the trachea, which can limit imaging at this station.
●Visualize the thyroid gland that appears as a hypoechoic structure in the center of the image; however, the thyroid cannot always be seen.
●Visualize the thymus gland, which lies distal to the thyroid near the clavicle; however, the thymus cannot be seen in most adult patients.
●Continue to withdraw the echoendoscope to the area just distal to the upper esophageal sphincter (approximately 18 to 20 cm from the incisors).
●Visualize the carotid arteries and internal jugular veins, although these structures cannot be seen in all patients.
●Deflate the balloon, stop the ultrasound transducer, and gently remove the echoendoscope from the patient.
POSTPROCEDURE CARE — After the procedure, patients are recovered from anesthesia, and this is discussed separately. (See "Anesthesia for gastrointestinal endoscopy in adults", section on 'Postanesthesia care'.)
ADVERSE EVENTS — EUS for diagnosis or staging is a generally safe procedure. Some complications are due to the effect of procedural sedation (eg, hypotension), while others are due to the endoscopy itself. (See "Adverse events related to procedural sedation for gastrointestinal endoscopy in adults".)
Reported complications of noninterventional EUS include [4]:
●Bleeding (rare)
●Infection (rare) (see "Preventing infection transmitted by gastrointestinal endoscopy", section on 'Overview of endoscope reprocessing')
●Perforation (rare)
In a series of 4894 EUS examinations, cervical esophageal perforation was reported in three patients (0.06 percent), and all were recognized during the procedure [5].
SPECIAL POPULATIONS
Older patients — Age-related changes of the pancreas have been reported in studies of EUS and other imaging modalities [6-8]. These changes have included pancreatic atrophy, fatty replacement of the parenchyma, and increasing diameter of the main pancreatic duct. Recognizing age-related changes in the pancreas has been particularly important when using EUS in the diagnostic evaluation of suspected chronic pancreatitis, and this is discussed separately. (See "Endoscopic ultrasound in chronic pancreatitis".)
Patients with surgical changes — Radial EUS imaging of the entire pancreas can be challenging in patients who have undergone upper gastrointestinal (GI) surgery such as partial gastrectomy because anatomic structures have been resected or because postsurgical inflammation and surgical staples may cause artifacts on ultrasound. Performing diagnostic upper endoscopy prior to EUS may help to define the postsurgical anatomy and evaluate for anastomotic stricture.
Prior to attempting EUS in patients with surgically altered anatomy (eg, Roux-en-Y anatomy), the advanced endoscopist also reviews the operative note or discusses the expected anatomy with the surgeon. Specifically, the endoscopist reviews details about the anatomic resection, the type of reconstruction, the length of the efferent and afferent limbs, and the types of anastomoses. In addition, postoperative imaging studies such as computed tomography scanning should be reviewed, if available. (See "Gastrointestinal endoscopy in patients who have undergone bariatric surgery".)
Data on the safety and feasibility of EUS imaging for patients with surgically altered anatomy are limited. In a study of 188 patients who had upper GI surgery, EUS examination was generally successful, except for patients who had undergone Roux-en-Y surgery because inability to reach the proximal duodenum precluded imaging of the head of the pancreas and common bile duct [9]. EUS examination was successful in patients who had undergone Billroth II surgery provided that the afferent limb was intubated.
SUMMARY AND RECOMMENDATIONS
●Endoscopic ultrasonography (EUS) is a combination of endoscopy and ultrasonography. EUS can be used to visualize and sample lesions of the gastrointestinal tract, pancreas, posterior mediastinum, and retroperitoneum. EUS is often used for staging gastrointestinal malignancies such as esophageal, gastric, and pancreatic cancers. (See 'Introduction' above.)
●EUS of the upper gastrointestinal tract has been performed for the diagnostic evaluation or staging of conditions such as (see 'Clinical applications' above):
•Esophageal cancer (see "Endoscopic ultrasound in esophageal cancer")
•Subepithelial lesions of the upper gastrointestinal tract (see "Endoscopic ultrasound for the characterization of subepithelial lesions of the upper gastrointestinal tract")
•Chronic pancreatitis (see "Endoscopic ultrasound in chronic pancreatitis")
•Exocrine pancreatic cancer (see "Endoscopic ultrasound in the staging of exocrine pancreatic cancer")
•Choledocholithiasis (see "Choledocholithiasis: Clinical manifestations, diagnosis, and management")
●There are few contraindications to EUS which are generally related to upper endoscopy itself including hemodynamic instability, inability to tolerate anesthesia/sedation, gastrointestinal obstruction, and untreated disorders of hemostasis. (See 'Contraindications' above.)
●EUS is performed with an echoendoscope (an endoscope with an ultrasound transducer engineered into its tip that emits high-frequency acoustic waves to the surrounding tissues). Pre- and postprocedural care is similar to that for patients who undergo upper gastrointestinal endoscopy. (See 'Patient preparation' above and "Overview of upper gastrointestinal endoscopy (esophagogastroduodenoscopy)".)
●During EUS scanning, the ultrasound transducer is guided through the gastrointestinal tract and placed at the luminal site that is directly adjacent to the area of interest (figure 1). Close proximity of the ultrasound transducer to the anatomic structures (eg, extraluminal organs, vessels) permits the use of high-frequency, high-resolution sound waves. (See 'General imaging principles' above.)
●On EUS imaging, the normal intestinal wall is visualized as five alternating hyperechoic (bright) and hypoechoic (dark) bands that correspond to five distinct histologic layers of the intestinal wall (image 1 and figure 2). (See 'Intestinal wall layers' above.)
●EUS for diagnosis or staging is a generally safe procedure, and complications are infrequently reported. Some complications are due to the effect of procedural sedation (eg, hypotension), while others are due to the endoscopy itself (eg, perforation). (See 'Adverse events' above.)
