EndoFLIP Parameters and Clinical Interpretation
EndoFLIP (Endoscopic Functional Luminal Imaging Probe) measures esophagogastric junction (EGJ) distensibility and esophageal body contractility using impedance planimetry, with key parameters including distensibility index (DI), cross-sectional area (CSA), and pressure measurements that distinguish achalasia from GERD and guide treatment decisions.
Core EndoFLIP Measurements
EndoFLIP uses impedance planimetry to measure 16 cross-sectional areas along a 4.6-cm cylindrical segment of a fluid-filled bag, with simultaneous intrabag pressure measurement 1. The device is passed through the endoscopic instrumentation channel and positioned across the EGJ during routine endoscopy 2.
Primary Parameters
- Distensibility Index (DI): Calculated as CSA divided by pressure (mm²/mm Hg), measured at standardized fill volumes 1, 2
- Cross-Sectional Area (CSA): Measured in mm² at the narrowest point of the EGJ 2, 3
- Intrabag Pressure: Measured in mm Hg during volume-controlled distensions 1, 2
- Maximum Diameter: The largest diameter achieved at specific fill volumes, measured in mm 1
Standard Testing Protocol
Volume-controlled distensions are performed sequentially from 10 mL to 40-70 mL 1, 2, 3. The 16-cm catheter (model 322N) is most commonly used for comprehensive assessment of both EGJ metrics and esophageal body topography 1.
Normal Reference Ranges
EGJ Distensibility in Healthy Controls
- Normal DI at 30 mL: Approximately 9-10 mm²/mm Hg 3
- Normal DI at 40 mL: Mean 9.1 mm²/mm Hg in control subjects 3
- Normal CSA at 40 mL: Mean 249.78 ± 144.76 mm² 3
The least distensible locus at the EGJ in healthy individuals is typically at the hiatus 2.
Interpretation in Achalasia
Diagnostic Thresholds for Achalasia
In achalasia and EGJ outflow obstruction, EndoFLIP demonstrates markedly reduced EGJ opening with distensibility typically <2.0 mm²/mm Hg and maximum diameter <12 mm at 60-70 mL fill volumes 1.
Specific Achalasia Findings
- Reduced Distensibility: DI of 1.2 mm²/mm Hg is characteristic of achalasia (compared to normal >9 mm²/mm Hg) 1
- Decreased Maximum Diameter: 11.2 mm at 70 mL volume (normal would be significantly higher) 1
- Elevated Pressure: 37 mm Hg at 60 mL fill volume indicates impaired relaxation 1
- Absent Contractility: Lack of peristaltic activity in the esophageal body topography 1
Contractile Patterns in Achalasia Subtypes
EndoFLIP can identify sustained occluding contractions (repetitive antegrade or retrograde contractions), which are indicative of spastic-reactive contractile responses seen in Type III achalasia or other spastic motility disorders 1, 4. This information complements high-resolution manometry findings and helps guide treatment decisions 1.
Clinical Utility in Achalasia Diagnosis
EndoFLIP can detect achalasia patients with relatively low integrated relaxation pressure (IRP) on manometry, which may be difficult to diagnose using high-resolution manometry alone 4. The American Gastroenterological Association recommends EndoFLIP as a useful adjunct test, particularly when diagnosis is equivocal after standard workup 1, 5.
Interpretation in GERD
Diagnostic Thresholds for GERD
GERD patients exhibit two- to threefold increased EGJ distensibility compared to controls, with a diagnostic cutoff DI of 10.95 mm²/mm Hg distinguishing GERD from normal subjects 2, 3.
Specific GERD Findings
- Erosive Reflux Disease (ERD): Mean DI of 13.98 mm²/mm Hg and CSA of 291.03 ± 160.77 mm² at 40 mL 3
- Non-Erosive Reflux Disease (NERD): Mean DI of 11.42 mm²/mm Hg and CSA of 285.87 ± 155.47 mm² at 40 mL 3
- Optimal Measurement Volume: 20-30 mL distention volumes show the greatest discrimination between GERD patients and controls 2
Important Caveats for GERD Diagnosis
There is significant controversy regarding EndoFLIP's utility in GERD diagnosis. While some studies show clear discrimination between GERD and controls 2, 3, other research demonstrates that EGJ measurements do not reliably distinguish between healthy volunteers and GERD patients when defined by pH monitoring 6. Body mass index (BMI) has an important confounding effect, with an inverse association between BMI and both CSA and distensibility 6. The endoscopic flap valve grade correlates poorly with EndoFLIP measurements 2.
Intraoperative Use During POEM
Real-Time Assessment of Myotomy Adequacy
EndoFLIP can be used intraoperatively during per-oral endoscopic myotomy (POEM) to tailor or confirm adequacy of the myotomy 1. For each 1 cm increase in gastric myotomy length, the DI at 30 mL increases by approximately 2.0 mm²/mm Hg and the minimum diameter increases by 2.4 mm 7.
Target Post-Myotomy Values
This linear relationship enables EndoFLIP to calibrate the desired gastric myotomy length to achieve optimal DI and minimum diameter, though specific target values for post-myotomy success are not definitively established in guidelines 7.
Clinical Integration Algorithm
When to Order EndoFLIP
- Equivocal manometry findings: When high-resolution manometry shows borderline IRP or unclear diagnosis 1, 5, 4
- Persistent dysphagia despite histological remission: In eosinophilic esophagitis patients with normal endoscopy 1
- Intraoperative guidance: During POEM to assess myotomy adequacy 1, 7
- GERD evaluation: When considering anti-reflux surgery, though utility remains debated 1, 2, 3
Limitations and Pitfalls
EndoFLIP should not be used in isolation to justify permanent interventions 1. The British Society of Gastroenterology notes that while EndoFLIP may assess esophageal compliance, it currently remains an experimental tool with unclear impact on therapeutic decision-making in routine clinical practice 1. Obesity significantly confounds measurements, particularly in GERD assessment 1, 6.