Purpose of Endoscopic Ultrasound (EUS)
EUS serves as the modality of choice for characterizing subepithelial lesions (SELs) of the gastrointestinal tract and has two primary roles: (1) detailed characterization of lesions including their layer of origin, size, echogenicity, and vascular involvement, and (2) tissue acquisition through fine-needle aspiration or biopsy. 1
Primary Diagnostic Functions
Lesion Characterization
EUS excels at determining whether a lesion is intramural versus extramural and identifying its precise layer of origin within the GI wall 1. The technique visualizes five distinct wall layers:
- Layers 1-2: Mucosa and muscularis mucosa
- Layer 3: Submucosa
- Layer 4: Muscularis propria
- Layer 5: Serosa or adventitia
This layer identification has critical diagnostic and therapeutic implications. For example, lesions from layer 2 typically represent lipomas, carcinoids, or pancreatic rests, while layer 4 lesions are often GISTs or leiomyomas 1.
Echogenicity Assessment
The ultrasound characteristics provide diagnostic clues:
- Anechoic lesions: Fluid-filled structures (cysts, varices, lymphangiomas)
- Hypoechoic lesions: GI mesenchymal tumors, granular cell tumors, neuroendocrine tumors, metastases, lymphoma
- Hyperechoic lesions: Lipomas or fibrolipomas
- Mixed echogenicity: Pancreatic rests, malignant mesenchymal tumors, abscesses 1
Malignancy Risk Assessment
EUS evaluates lesion size and vascular involvement to predict malignant potential, with overall sensitivity of 64% and specificity of 80% for detecting malignancy 1. EUS is superior to cross-sectional imaging (CT/MRI) in delineating the layer of origin and narrowing the differential diagnosis 1.
Tissue Acquisition Role
When standard forceps biopsies are non-diagnostic (which occurs frequently since SELs have normal overlying mucosa), EUS-guided fine-needle aspiration (FNA) or fine-needle biopsy (FNB) provides tissue diagnosis with accuracies ranging from 46-93% 1. Accuracy improves with lesion size:
- Lesions <2 cm: 71% accuracy
- Lesions 2-4 cm: 86% accuracy
- Lesions >4-5 cm: 95-100% accuracy 1
Clinical Applications Beyond SELs
EUS has expanded applications including:
- Cancer staging: Esophageal, gastric, pancreatic, bile duct, and lung cancer (via transesophageal mediastinal staging) 2, 3
- Pancreatic disease: Detection of early chronic pancreatitis and pancreatic masses 4, 5
- Therapeutic interventions: Pancreatic pseudocyst drainage, celiac plexus blocks for pain control 4, 6
- Tissue sampling: Lymph nodes, pancreatic masses, liver lesions, adrenal glands 7
Important Caveats
EUS interpretation is operator-dependent, requiring advanced training and expertise 1. The technique's sensitivity may lead to overdiagnosis, particularly in early chronic pancreatitis, so findings should be correlated with other clinical tests 3. Technical difficulty varies by location—transesophageal and transgastric approaches are easier than transduodenal approaches 7.