What lesions are accessible by Endoscopic Ultrasonography (EUS) but not by Abdominal Ultrasonography (USG) in adult patients?

Lesions Accessible by EUS but Not by Abdominal Ultrasonography

EUS provides superior visualization of small intramural gastrointestinal wall lesions, subepithelial masses, mediastinal structures, pancreatic lesions, and peri-gastrointestinal lymph nodes that are poorly visualized or completely inaccessible by transabdominal ultrasound. 1

Key Anatomic and Technical Advantages of EUS

Gastrointestinal Wall Lesions

• Small gastric wall masses (<30mm) are poorly visualized by transabdominal ultrasound, with only 50-61% visualization rates for lesions ≤30mm, making EUS essential for these lesions 1
• EUS identifies the specific histologic layer of origin (mucosa, submucosa, muscularis propria, serosa) which transabdominal ultrasound cannot distinguish 1
• Subepithelial lesions (SELs) including GISTs, leiomyomas, lipomas, pancreatic rests, and varices are optimally characterized by EUS, particularly those arising from the muscularis propria (fourth layer) 1

Mediastinal and Esophageal Structures

• Mediastinal lymph nodes and masses are accessible via EUS but not transabdominal ultrasound due to anatomic location 2, 3
• Esophageal wall lesions and depth of cancer invasion are evaluated by EUS, which transabdominal imaging cannot assess 4, 5
• Periesophageal structures and lymph node staging for esophageal malignancies require EUS 3

Pancreatic and Biliary Lesions

• Small pancreatic masses, particularly in the head and uncinate process, are better visualized by EUS than transabdominal ultrasound 1
• Distal and proximal biliary strictures causing extrinsic compression are optimally sampled via EUS-guided techniques 1
• Pancreatic cystic lesions and solid masses benefit from EUS characterization and tissue acquisition 1, 2

Peri-Gastrointestinal Lymph Nodes

• Perigastric, celiac, and peri-biliary lymph nodes are accessible by EUS with reported diagnostic sensitivity around 90% for lymphadenopathy of unknown origin 1
• EUS-guided FNA of abdominal lymph nodes provides tissue diagnosis where transabdominal ultrasound-guided biopsy is not feasible 2, 5

Specific Lesion Types Better Characterized by EUS

Intramural Masses

• Hypoechoic fourth-layer masses (GISTs, leiomyomas) require EUS for layer identification and malignancy risk assessment 1, 6
• Duplication cysts appear as anechoic structures with well-defined walls on EUS, distinguishing them from pancreatic pseudocysts 1
• Pancreatic rests with characteristic heterogeneous echotexture in the submucosa are identified by EUS 1
• Varices are confirmed by EUS Doppler showing flow within hypoechoic/anechoic submucosal structures 1

Extramural Compression

• Lesions causing extrinsic compression of the bile duct (pancreatic head masses, lymphadenopathy) are optimally sampled by EUS with sensitivity of 85% for pancreatic neoplasms 1
• Adrenal gland masses adjacent to the stomach are accessible via EUS-FNA but not transabdominal ultrasound-guided biopsy 2

Clinical Implications

Diagnostic Accuracy

• EUS-FNA provides 89% sensitivity and 88% specificity for diagnosing GI tract neoplastic lesions, particularly when endoscopic forceps biopsy fails 5
• Transabdominal ultrasound has 69-93% visualization rates for gastric wall lesions overall, with significantly lower rates for smaller lesions that are most clinically relevant 1

Critical Limitations of Transabdominal Ultrasound

• Cannot identify histologic layers of the gut wall, making it unable to distinguish between different causes of intramural masses 1
• Visualization is inversely proportional to lesion size, with poorest performance for small lesions that require surveillance 1
• Limited utility for deep structures including mediastinum, distal esophagus, and structures obscured by bowel gas 4, 2

Common Pitfalls to Avoid

• Do not rely on transabdominal ultrasound for surveillance of small (<30mm) gastric wall lesions, as inadequate visualization may miss malignant transformation 1
• Do not assume transabdominal ultrasound can adequately assess subepithelial lesions, as layer of origin determination is essential for management decisions 1
• Recognize that 43% of patients with GI tract lesions have unsuccessful endoscopic forceps biopsy, making EUS-FNA the preferred diagnostic approach 5