What are the layers of the abdomen visualized during Endoscopic Ultrasonography (EUS)?

Layers of the Abdomen Visualized During Endoscopic Ultrasonography (EUS)

Endoscopic ultrasonography (EUS) typically reveals 5 distinct layers of the gastrointestinal wall, which are essential for characterizing lesions and determining their origin within the abdominal structures. 1, 2

The Five Layers of the GI Wall on EUS

1. First Layer (Innermost):

   • Appears as a hyperechoic (bright) line
   • Represents the interface echo between the superficial mucosa and the acoustic coupling medium 1
   • Sometimes referred to as the mucosal surface 1

2. Second Layer:

   • Appears hypoechoic (dark)
   • Represents the deep mucosa including the muscularis mucosae 1
   • Lesions arising from this layer often include pancreatic rests, carcinoids 1, 2

3. Third Layer:

   • Appears hyperechoic (bright)
   • Represents the submucosa plus the acoustic interface between the submucosa and muscularis propria 1
   • Common lesions from this layer include lipomas, varices, and cysts 1, 2

4. Fourth Layer:

   • Appears hypoechoic (dark)
   • Represents the muscularis propria 1
   • Common lesions from this layer include GISTs and leiomyomas 1, 2

5. Fifth Layer (Outermost):

   • Appears hyperechoic (bright)
   • Represents the serosa or adventitia 1
   • Important for determining if a tumor has penetrated through the wall 1

Clinical Significance of Layer Identification

• Accurate T-staging: The depth of tumor invasion can be determined by identifying which layers are affected 1:

  • T1: Dark expansion of layers 1-3 (mucosa and submucosa)
  • T2: Dark expansion of layers 1-4 (extending into muscularis propria)
  • T3: Expansion beyond the muscularis propria with irregular outer border
  • T4a: Loss of the bright line representing the serosa
  • T4b: Extension into surrounding organs

• Lesion characterization: The layer of origin helps narrow the differential diagnosis 1, 2:

  • Anechoic (fluid-filled) lesions: Usually cysts, varices, or lymphangiomas
  • Hypoechoic lesions: GISTs, neuroendocrine tumors, lymphoma
  • Hyperechoic lesions: Typically lipomas or fibrolipomas
  • Mixed echogenicity: May represent pancreatic rest or malignant tumors

Technical Considerations

• Higher frequency transducers can sometimes visualize additional sublayers, resulting in 7 or 9 layers in some examinations 1, 3
• The muscularis mucosae may appear as a thin hypoechoic layer within the second layer 3
• The fourth layer (muscularis propria) may sometimes show a thin hyperechoic line separating the circular and longitudinal muscle layers 3

Limitations and Challenges

• EUS interpretation is highly operator-dependent 1, 2
• The overall sensitivity and specificity of EUS in predicting malignant potential of subepithelial lesions are 64% and 80%, respectively 1, 2
• Obstructing tumors may increase the risk of perforation during EUS examinations 1
• Stenotic tumors may require dilation or the use of wire-guided probes for complete staging 1

Understanding these five distinct layers is crucial for accurate diagnosis, staging, and treatment planning for abdominal lesions visualized during EUS.