Endoscopic ultrasound

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Endoscopic Ultrasound (EUS)

What is EUS?

Endoscopic ultrasound combines traditional mucosal endoscopy with a separate ultrasound probe mounted on the echoendoscope tip. Because the pancreas, bile duct, and mediastinal structures lie within a few millimeters of the GI lumen, placing the transducer intraluminally allows use of higher ultrasound frequencies (7.5-20 MHz) than transcutaneous scanning - yielding far superior resolution at the cost of reduced penetration depth. This makes EUS uniquely able to characterize the five-layer wall structure of the GI tract and adjacent organs in real time.

Bailey and Love's Short Practice of Surgery, 28th ed., p. 182
Clinical Gastrointestinal Endoscopy Expert Consult, 3e

Instrument Types

Type	Ultrasound Arrangement	Optics	Primary Use
Radial echoendoscope	Radially arranged (360° scan)	Forward-viewing	Diagnostic staging (esophagus, stomach)
Linear echoendoscope	Linearly arranged (alongside needle path)	Side-viewing	Guided biopsy (FNA/FNB), therapeutic procedures

The linear scope has a working channel like an ERCP scope, allowing real-time needle visualization during EUS-FNA. The radial scope gives a more intuitive anatomical cross-section but cannot guide needles in real time.

Bailey and Love's, p. 182

GI Wall Layers on EUS

EUS resolves the GI wall into five distinct layers:

Layer	Appearance	Tissue Correlate
1	Hyperechoic	Interface/superficial mucosa
2	Hypoechoic	Deep mucosa (muscularis mucosae)
3	Hyperechoic	Submucosa
4	Hypoechoic	Muscularis propria
5	Hyperechoic	Serosa/adventitia

Identifying which layer a lesion arises from directs both the diagnosis (e.g., lipoma from layer 3, GIST from layer 4) and the T-stage.

Indications

Diagnostic

Staging of esophageal/gastric/rectal malignancy (T and N staging)
Staging of hepatobiliary malignancy (pancreatic cancer, cholangiocarcinoma)
Subepithelial lesions (SELs) - characterizing intramural masses (lipoma, GIST, carcinoid, varices, cysts)
Diagnosis of choledochal microlithiasis / biliary sludge
Evaluation of chronic pancreatitis - parenchymal and ductal criteria
Idiopathic recurrent acute pancreatitis (IRAP) - 7-point checklist including bile duct stones, pancreatic masses, pancreas divisum, SOD, CP, AIP
Suspected pancreatic masses - EUS detects small masses missed by CT

Therapeutic / Interventional

EUS-FNA/FNB - tissue acquisition from pancreatic masses, lymph nodes, submucosal lesions, left adrenal, portal adenopathy
Transgastric drainage of pancreatic pseudocysts and abscesses (EUS cystgastrostomy)
Coeliac plexus neurolysis/block for pain management in pancreatic cancer
Biliary interventions - EUS-guided biliary drainage (increasingly performed)
EUS-guided gastroenterostomy for gastric outlet obstruction
Bailey and Love's, Table 9.6, p. 182

EUS in Tumor Staging

Esophageal Cancer

EUS is the most accurate pre-operative staging modality for esophageal cancer:

T staging accuracy: 87-92% (vs. CT: 51-74%)
N staging sensitivity: 92-95%, specificity 50-54%
EUS is superior to CT for primary tumor and regional nodal status
EUS-FNA improves N-staging accuracy to >90% for celiac axis nodes
CT or MRI should be performed before EUS to exclude distant metastases
Post-chemoradiotherapy: a ≥50% reduction in tumor cross-sectional area correlates with treatment response, though EUS cannot reliably distinguish residual cancer from inflammation/fibrosis

EUS image - esophageal tumour with adventitial invasion:

EUS image of oesophageal tumour showing 'ragged' edge suggesting invasion of adventitia and muscularis propria

EUS image of an oesophageal tumour. The 'ragged' edge at the top indicates invasion into the adventitia (T3/T4 disease). The intact muscularis propria layer is visible at the bottom. - Bailey and Love's, Fig. 9.20

Yamada's Textbook of Gastroenterology, 7e, p. 976

EUS-FNA (Fine Needle Aspiration)

EUS-FNA is performed using the linear echoendoscope with real-time needle visualization. A needle (typically 22G or 25G; 19G for core biopsy) is passed through the working channel and directed into the target lesion under continuous ultrasound guidance.

EUS-FNA image - pancreatic head mass:

EUS-FNA of pancreatic head mass showing EUS needle entering the hypoechoic mass with a stent visible in the CBD

EUS-guided FNA of a pancreatic head mass. The EUS needle is clearly visible entering the mass. A biliary stent is also visible within the CBD (common bile duct). - Bailey and Love's, Fig. 9.21

Key EUS-FNA targets:

Pancreatic masses and cysts
Paraesophageal and coeliac lymph nodes
Submucosal GI lesions
Portal lymphadenopathy
Left adrenal gland and left hepatic masses
Mediastinal lesions

Newer fine-needle biopsy (FNB) needles (e.g., fork-tip, Franseen design) provide tissue cores with preserved architecture, improving histological diagnosis and enabling immunohistochemistry - particularly useful for autoimmune pancreatitis (IgG4 staining, obliterative phlebitis).

EUS in Chronic Pancreatitis

EUS is exquisitely sensitive for early chronic pancreatitis (CP) due to proximity to the pancreas, allowing high-frequency scanning. EUS criteria are divided into:

Parenchymal criteria:

Inhomogeneity
Hyperechoic foci
Hyperechoic strands
Lobularity
Pseudocysts

Ductal criteria:

Ductal dilation (≥3 mm head; ≥2 mm body; ≥1 mm tail)
Hyperechoic main duct margins
Irregular main duct margins
Visible side branches

A quantitative score using 9 criteria is most reliable when clearly normal (≤2 criteria) or clearly abnormal (≥5 criteria) - predictive value 85% at these thresholds. Intermediate scores (3-4 criteria) are diagnostically uncertain and may represent early disease.

EUS image - chronic pancreatitis with intraductal stone:

EUS showing a pancreatic duct (PD) stone causing obstruction with upstream dilation, demonstrating normal PD upstream and dilated PD downstream

EUS showing a calcified intraductal stone in the pancreatic head (PD stone, arrow) with upstream normal duct and downstream dilation. - Clinical GI Endoscopy, Fig. 59.4

EUS in Subepithelial Lesions (SELs)

Key EUS features assessed for any intramural mass:

Feature	What to Assess
Location	Organ + position
Size	3 dimensions
Background echogenicity	Hypo/hyper/anechoic
Focal echogenicity	Foci present/absent
Shape/margins	Round, oval; smooth vs. irregular
Margin definition	Well- vs. poorly-defined
Wall layer of origin	Mucosa / submucosa / muscularis propria
Invasion	T-stage

Characteristic appearances:

Lipomas: Brightly hyperechoic, uniform, well-demarcated, layer 3 (submucosal), easily deformed by transducer
Varices: Anechoic, vermiform, nearly always in groups
GISTs: Hypoechoic, layer 4 (muscularis propria) origin, malignant risk ↑ with size >3 cm, irregular margins, echogenic foci
EUS sensitivity/specificity for distinguishing lesion types: 64-92% / 80-100%
Clinical GI Endoscopy Expert Consult, 3e, p. [block5]

EUS vs. Other Modalities

Modality	Advantage of EUS
CT scan	Better T-staging, detects small pancreatic masses, more sensitive for biliary sludge/stones
MRCP	Superior for biliary sludge/stones, luminal visualization, tissue acquisition; combined yield ~50% in IRAP (complementary)
Transcutaneous US	Far higher resolution due to intraluminal positioning and higher frequencies
ERCP	Non-invasive alternative for diagnosis (ERCP reserved for treatment)

Training Requirements and Complications

EUS requires dedicated training in both scope manipulation and image interpretation. The wide, relatively inflexible echoendoscope typically requires sedation - many units use propofol-based anaesthesia for complex therapeutic procedures.

Complications:

Diagnostic EUS: Oversedation, esophageal perforation (rare)
Therapeutic EUS (FNA/drainage): Haemorrhage, perforation, pancreatitis (rare, <1% for pancreatic FNA), infection
All patients undergoing therapeutic EUS require a normal coagulation screen beforehand
Bailey and Love's Short Practice of Surgery, 28th ed., p. 182

Summary

EUS is a versatile tool occupying a unique niche - it provides high-resolution, real-time imaging of structures adjacent to the GI lumen combined with the ability to obtain tissue under direct ultrasound guidance. Its main clinical roles are:

Cancer staging (GI, pancreatic, hepatobiliary)
Pancreatic disease (masses, chronic pancreatitis, cysts)
Tissue acquisition (FNA/FNB)
Biliary stone detection (especially microlithiasis)
Therapeutic drainage (pseudocysts, biliary)
Pain palliation (coeliac plexus block)

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