Clinical and Health Affairs
Endoscopic Ultrasound
By Federico T. Rossi, M.D.
Abstract
Endoscopic ultrasound (EUS) is being used to diagnose and evaluate a number of conditions of the gastrointestinal tract. This article describes the type of EUS equipment that is being used in clinical practice, the type of conditions for which it is used to diagnose and evaluate, and its therapeutic uses.
Endoscopic ultrasound (EUS) was developed in the 1980s, following the widespread acceptance of gastrointestinal endoscopy as an easily tolerated diagnostic tool. Standard endoscopy allows for evaluation of the gastrointestinal tract’s lumen. Researchers discovered that the gut lumen also provides an excellent ultrasound window to several complex intrathoracic and intra-abdominal organs and structures, which are otherwise impossible to image in fine detail with standard ultrasound or other imaging techniques. An echoendoscope—an endoscope with an ultrasound probe at the tip—could “open” this window, providing high-resolution ultrasound images with detail and information that was previously unattainable. This article describes the type of endoscopic ultrasound equipment that is used in clinical practice, the type of conditions for which it is used to diagnose and evaluate, and its application as a therapeutic tool.
Endoscopic Ultrasound Basics
As with standard endoscopy, EUS is performed with patients in the left lateral decubitus position. After the patient is sedated, the echoendoscope is advanced into the gastrointestinal tract. Once the instrument reaches the desired location, as much air as possible must be suctioned out of the gastrointestinal lumen through the endoscope; air is the primary cause of interference in obtaining ultrasound images. Latex balloons mounted over the endoscope’s transducer tip can be inflated and suctioned as needed to displace air trapped between the transducer tip and the bowel wall. Once the air is removed, attention shifts to the interpretation of ultrasound images.
Echoendoscopes have gone through several transformations during the past 20 years. Three main types are currently used: radial, linear, and mini-probe. These instruments use high ultrasound frequencies ranging from 5 MHz to 20 MHz, leading to a depth of penetration from 1 cm to 6 cm. The most common frequency is 7.5 MHz for radial and linear echoendoscopes; the higher frequencies are used with mini-probes.
Radial echoendoscopes have a transducer tip that produces a 360-degree view that is perpendicular to the shaft of the echoendoscope. These views provide circumferential images that allow complete evaluation of a given area of the gut wall and its surrounding structures. This orientation is especially helpful in examining the esophagus, as it allows a complete, circumferential evaluation of the esophageal wall layers as well the lymph nodes and other surrounding structures.
Linear echoendoscopes provide a 120-degree ultrasound image parallel to the axis of the endoscope. This linear orientation allows for the passage of a fine aspiration needle through the endoscope’s working channel and into targeted tissues. The advantage of doing fine-needle aspiration (FNA) using a linear echoendoscope is that the course of the needle can be directly visualized via the ultrasound image. This allows for accurate and safe sampling of the tissues in question, which would not be possible with a radial echoendoscope.
Endoscopic ultrasound mini-probes are small, rotating ultrasound probes that can be passed through the working channel of a standard endoscope in the same way one would a biopsy forceps or other accessory. These probes are only 2 mm to 3 mm in diameter and are ideal for the evaluation of small, submucosal lesions within the bowel wall. Their range of frequency tends to be high, between 12 MHz and 20 MHz. Once inside the gastrointestinal lumen, the mini-probe is placed over the lesion in question. The ultrasound images produced do not provide much depth penetration, but the high frequency allows for a very detailed examination of the layers of the bowel wall. Mini-probes have also been passed into bile and pancreas ducts to provide information about potential lesions within these structures.
Diagnostic Applications
Endoscopic ultrasound has been used to evaluate disease or lesions involving the pancreaticobiliary tree, the esophagus, the stomach, the rectum, the left adrenal gland, the liver, and the posterior medistinum. Endoscopic ultrasound is particularly effective in the locoregional staging of cancer involving these organs and structures. In addition, EUS allows FNA sampling of questionable lesions and lymph nodes for diagnostic and staging purposes. The following are some of the ways EUS is being used in clinical practice for diagnosing and evaluating various conditions.
♦ Submucosal Gastrointestinal Lesions
Before the development of EUS, submucosal gastrointestinal lesions were difficult to accurately evaluate. Because the gastrointestinal tract tends to collapse, CT and MRI imaging frequently fail to identify these lesions; even when lesions are identified, the information these imaging techniques produce is incomplete.
Endoscopic ultrasound imaging allows for detailed evaluation of all 5 echosonographic layers of the gastrointestinal wall. It identifies the layer from which a given lesion originates as well as all of the layers that are affected.
The echofeatures of a given lesion may also suggest its etiology. For example, a hypoechoic lesion suggests a gastrointestinal stromal tumor or carcinoid. Conversely, lipomas tend to be hyperechoic, while cysts and blood vessels tend to be anechoic. Echofeatures, along with FNA sampling, allow for the accurate diagnosis and staging of a given submucosal lesion.
♦ Esophageal Cancer Staging
Studies have demonstrated the importance of accurate esophageal cancer staging. Staging not only predicts prognosis but also dictates how the disease should be managed. Whether a patient will be best served surgically or with neoadjuvant therapy depends on accurate staging.1,2 When there is no evidence of metastatic disease, EUS is an important staging tool.
Endoscopic ultrasound has been shown to be superior to CT for locoregional staging of esophageal cancer.3,4 In studies of EUS, T staging has been found to be 85% accurate and N staging 73% to 90% accurate.5,6 CT scan locoregional LN-staging accuracy has been found at 66%.7 Endoscopic ultrasound N-staging accuracy improves with FNA sampling. In addition, the echosonographic features of certain lymph nodes have been identified as being suggestive of malignant involvement. These features include a lymph node being greater than 1 cm in size, marked hypoechoity, and having sharp borders and a round shape.8 Endoscopic ultrasound has not been found to be as accurate when restaging cancer after therapy. Fibrosis and inflammation are thought to significantly decrease its accuracy.
♦ Gastric Cancer Staging
Endoscopic ultrasound has been found to be 85% accurate for gastric cancer T staging and 75% accurate for N staging.9 In this application, it is complementary to CT, which best detects distal metastasis. Endoscopic ultrasound has also been shown to be a very sensitive (87%) method for detecting ascites in gastric cancer patients, and has been found to be more sensitive than CT in this setting.10 Ascites is an important predictive factor for the presence of peritoneal metastasis in gastric cancer patients. Stomach wall folds, the significant amount of air in the gastric lumen, and the ulceration often associated with gastric cancer frequently make obtaining adequate echoendosonographic images challenging.
♦ Rectal Cancer Staging and Rectal Disease
Endoscopic ultrasound has played an important role in improving rectal cancer staging. It has been found to be superior to CT at overall tumor staging.11 Endoscopic ultrasound has been shown to be 91% accurate for T staging and 82% accurate for N staging.12 CT has been found to be 71% accurate for T staging and 76% accurate for N staging. When used with FNA, EUS has been found to be helpful in diagnosing and evaluating anastamotic cancer recurrence.
Endoscopic ultrasound also has been shown to be particularly helpful in the evaluation of fecal incontinence, allowing for the identification of sphincter defects and for preoperative sphincter mapping.13 For patients with perianal Crohn’s disease, EUS has been shown to be more accurate than CT and fistulography and as accurate as MRI in determining fistula anatomy.14
♦ Pancreatic Cancer Staging and Pancreatic Lesions
Endoscopic ultrasound has greatly contributed to the evaluation and management of diseases of the pancreas. When compared with CT, EUS has been shown to be more sensitive in detecting pancreatic masses (98% versus 56%) and more accurate in tumor staging (67% versus 41%).15 Endoscopic ultrasound has been found to be equivalent to CT in nodal staging and in determining resectability. It appears to be especially helpful in the evaluation of pancreatic cancer lesions smaller than 2 cm; these lesions are often difficult to evaluate with CT. Endoscopic ultrasound with FNA was shown to be 84% accurate in diagnosing pancreatic malignancy when a previous CT-guided biopsy failed to yield a diagnosis.16
Endoscopic ultrasound with FNA has a theoretical advantage over CT because of its shorter needle tract, which may reduce the chances of needle-tract tumor seeding. It can often determine the extent of tumor vascular involvement, although recent evidence has suggested a lower degree of accuracy than previously believed.17 In the assessment of a potentially resectable pancreas malignancy, CT and EUS appear to be complementary and should be part of a thorough preoperative evaluation.
With sensitivity and accuracy of 93%, EUS has also established itself as the most reliable way to localize suspected pancreatic neuroendocrine tumors prior to surgery.18 It has proved to be the most cost-effective initial imaging study in this setting, and as a result it is considered the primary imaging modality.19
Over the last few years, there has been an increase in the detection of pancreatic cysts. This phenomenon is most likely the result of improved accuracy and increased utilization of abdominal imaging with CT and MRI scans. Although the vast majority of these cysts are not cancerous, many are premalignant lesions, such as intrapapillary mucinous neoplasms or mucinous cystadenomas. Therefore, such lesions need to be accurately identified and appropriately managed.
There currently are no screening recommendations for pancreatic cancer in the general population, and our success in treating it remains very low. Thus, detection of these pancreatic cysts provides an opportunity to potentially intervene early in the pathogenesis of pancreatic cancer. Endoscopic ultrasound has been pivotal in enabling us to further characterize these pancreatic cysts and, thus, treat them in their early stage.
Echosonographic features provide insight into the type of cyst in question.20 Furthermore, FNA sampling has allowed for the relatively safe and accurate analysis of cyst fluid. Fluid analysis has helped in better classifying and risk-stratifying pancreatic cysts.21,22 Once a cyst is found to be premalignant, appropriate recommendations can be made; these may include surgical resection or subsequent imaging to monitor them. Conversely, if the evaluation of a cyst leads to a benign etiology, such as a pseudocyst or a serous cystadenoma, appropriate conservative management may be implemented.
♦ Biliary Tree Lesions
Endoscopic ultrasound provides excellent imaging of the extrahepatic biliary tree. As a result, EUS has played a significant role in the detection of choledocholithiasis, where it has been found to be 91% sensitive and 97% accurate.23,24 Comparison studies have shown it to be at least as accurate as magnetic resonance cholangiopancreatography (MRCP) and endoscopic retrograde cholangiopancreatography (ERCP) in identifying choledocholithiasis.25,26 Endoscopic ultrasound allows the endoscopist to assess the bile duct prior to ERCP, and, should no choledocholithiasis be detected, one can avoid the risk of pancreatitis and infection associated with ERCP. Conversely, if choledocholithiasis is present, ERCP can be performed while the patient is still under
sedation.
Gallbladder evaluation and the detection of sludge or microlithiasis has been enhanced with EUS. Studies have yielded encouraging results with regard to the use of EUS to detect cholelithiasis in patients with pancreatitis of unknown etiology or biliary colic after a negative transabdominal ultrasound.27,28 As a result, EUS can be particularly helpful in patients who are suspected of having such biliary pathology, but have a negative initial transabdominal ultrasound.
Endoscopic ultrasound alone and with FNA has been effectively used to evaluate extrahepatic biliary strictures and portal lymphadenopathy. Studies have shown rather high specificity and accuracy in the evaluation of potentially malignant biliary strictures and lymphadenopathy.29-31 Hepatic parenchymal lesions—in particular, those in the left lobe of the liver—can also be adequately evaluated and sampled with EUS and FNA.32
♦ Posterior Mediastinum and Lung Cancer Staging
From the esophageal lumen, EUS provides excellent views of the posterior mediastinum and its associated structures. In particular, the subcarinal space and the AP window can be carefully evaluated and pertinent lymph nodes or masses sampled. Because the posterior mediastinal lymph nodes are often involved in lung cancer, EUS has been found to be helpful in lung cancer staging.33
Lung cancer staging is challenging, and despite the imaging modalities available, up to one-third of all thoracotomies following staging procedures are futile.34 Lymph-node metastasis and mediastinal involvement seem to be the most difficult cases to evaluate. Endoscopic ultrasound has been found to be more accurate than CT in the detection of malignant mediastinal lymph node involvement.35
Used alone, EUS does not effectively visualize the pretracheal space and the anterior mediastinum, but when used in combination with mediastinoscopy, the preoperative staging of lung cancer improves significantly.33 In recent years, endobronchial ultrasound has shown promise when used in combination with EUS for the complete evaluation of the mediastinum in lung cancer staging.36 Such an approach may eliminate the associated morbidity and expense of mediastinoscopy.
Endoscopic Ultrasound Therapy
Although EUS has been primarily used as a diagnostic tool, it also has therapeutic applications. Two of the more commonly used EUS therapies are celiac plexus neurolysis and celiac plexus block. With EUS, the celiac plexus can be readily identified and injected with a mixture of bupivicaine and alcohol for neurolysis, or bupivicaine and triamcinolone for a block.
Neurolysis is used to control pain in patients with pancreatic cancer; the block has been used for pain control in chronic pancreatitis.37,38 These procedures were initially performed by anesthesiologists and radiologists using the posterior approach. Endoscopic ultrasound avoids the complications of paraplegia and pneumothorax associated with the posterior approach.
In recent years, endoscopic management of large, symptomatic pseudocysts using cystgastrostomies and cystduodenostomies has become first-line therapy. Endoscopic ultrasound is used for drainage of these pseudocysts.39 Initially, endoscopic approaches did not utilize EUS and instead relied on a visible endoscopic bulge in the gastrointestinal lumen to identify the cyst prior to puncture. Bleeding was one of the more common and serious complications of this approach. With EUS guidance, bleeding is reduced, as vessels clearly can be identified prior to cyst drainage.
Endoscopic ultrasound also allows for evaluation of the cyst wall, making it easier to locate a puncture site with the least distance between the cyst lumen and the gastrointestinal lumen. Finally, EUS allows for evaluation of the cyst’s contents prior to drainage. Should an overwhelming amount of debris be noted, the procedure can be aborted and surgery considered in order to reduce the risk of infection.
Conclusion
Endoscopic ultrasound has allowed for the detailed imaging and sampling of structures and organs with accuracy that was not possible with previous technology. Initial therapeutic applications are now being implemented, and new therapeutic applications of EUS are being developed. Investigators are now evaluating EUS-guided brachytherapy, the injection of radioactive seeds into tumors, and the injection of alcohol into pancreatic tissue for the potential ablation of pancreatic lesions.40,41 Others are studying EUS-guided gastrointestinal anastamosis. In addition, hepaticogastrostomies and gastrojejunostomies have been performed in animal models.42 The eventual goal is that more diseases will be evaluated and treated more accurately and less invasively, leading to more targeted therapy and shorter hospital stays with lower morbidity and complications. MM
Federico Rossi is a board-certified gastroenterologist with Minnesota Gastroenterology, P.A. He specializes in endoscopic retrograde cholangiopancreatography and endoscopic ultrasound.
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