The Clinical Usefulness of Functional Luminal Imaging Probe in Esophageal Dysmotility Disorder

Functional luminal imaging probe (FLIP) is being increasingly used in the diagnosis of functional gastrointestinal (GI) disorders and has advantages of detection of distensibility in GI tract.^1,2 The previously used technique, manometry, could measure the inner active contractility of GI tract.³ However, without active swallowing of bolus, manometry cannot generate a significant signal. To overcome this, the measurement of bolus transit from high-resolution impedance manometry has been attempted, especially in patients with functional dysphagia without major motor esophageal dysmotility.^4-8 However, FLIP can measure the passive outer distensibility of the GI tract, thus complementing the conventionally used manometry. FLIP assesses the “esophageal response to distension,” with a degree of active distension from the FLIP filling protocol. Moreover, it can measure some degree of active esophageal muscle contraction and tone that occur as the esophagus’s response to the distension. Moreover, FLIP measures the “distensibility of the esophageal wall” (from inside the lumen, assessing the contribution of the mucosa and inner and outer muscle layers) (Figure). As regards the pathophysiology of achalasia, the most representative disease of functional GI disorder, impaired “relaxation” of lower esophageal sphincter is the pathognomic finding.^9,10 Therefore, FLIP, which can measure the outer passive distensibility of the sphincter tone, may be a more appropriate diagnostic tool than manometry, which can measure the inner active contractility of the sphincter. Previously published papers suggested that the esophagogastric junction distensibility index (EGJ-DI) is more suitable in the discrimination of achalasia compared with previously used integrated relaxation pressure, which is affected by contraction of the esophageal body.^11-14 Therefore, the recently published Chicago 4.0 criteria recommended the use of FLIP in case of an inconclusive diagnosis of achalasia or esophagogastric junction (EGJ) outflow obstruction.^15-17

Figure.

The illustration of measurement of manometry and functional luminal image probe (FLIP) in the gastrointestinal (GI) tract of humans. Conventionally used manometry can measure the active inner contractility of GI tract. Without active swallowing of bolus, manometry cannot produce a significant signal. However, FLIP can measure the passive outer distensibility of GI tract. FLIP assesses the “esophageal response to distension,” with a degree of active distension from the FLIP filling protocol. Moreover, it can measure some degree of active esophageal muscle contraction and tone that occur as the esophagus’s response to the distension. Moreover, FLIP measures the “distensibility of the esophageal wall” (from inside the lumen, assessing the contribution of the mucosa and inner and outer muscle layers). Therefore, these 2 tests are complementary to each other.

Since FLIP can measure the outer passive distensibility of the GI tract, it can broaden our understanding of motility disorders. In case of epiphrenic diverticulum, the concurrent motility disorder may be hidden.^18,19 Manometry may miss abnormal findings in the patients with epiphrenic diverticulum.^18,19 Surgeons conventionally cut the EGJ muscle during diverticulectomy for epiphrenic diverticulum to prevent recurrence. With the development of endoscopic technology, diverticulum is treated with peroral endoscopic myotomy (POEM), In diverticulum POEM (D-POEM), the measurement of EGJ-DI using FLIP is necessary.²⁰ When FLIP detects impaired relaxation of EGJ, POEM of EGJ as well as D-POEM of epiphrenic diverticulum should be considered. The abnormal thickening of the esophageal layer, which impairs the outer relaxation of the esophageal body due to a highly thickened esophageal muscle layer, may also be detected by FLIP.¹ Because manometry may miss abnormal esophageal motility findings in this case, FLIP may be beneficial for measuring the impaired distensibility of the thickened esophageal wall. However, more data is necessary to validate this hypothesis.

Additionally, FLIP can measure secondary peristalsis of the esophageal body.^21,22 When the balloon of FLIP is distended inside the esophageal body, it can show secondary peristalsis. Studies of FLIP panometry suggested its role in the prediction of outcome after treatment in patients with achalasia.^22,23 The Chicago group suggested a FLIP panometry pattern and categorized patients with dysphagia based on the value of EGJ-DI and patterns of secondary peristalsis.^1,24 From 2016 to 2022, the criteria of FLIP panometry has evolved from its prototype.^1,24,25 The most recently published criteria are based on a multicenter study with a large number of cases.²⁴ Prior research suggested the clinical usefulness of FLIP panometry in the diagnosis of EGJ obstruction.²⁶ The recently published manuscript in this issue of Journal of Neurogastroenterology and Motility used the machine learning technique in the identification of EGJ obstruction.²⁷ With the incorporation of this machine learning technique in the near future, we can effectively diagnose and expand our understanding of esophageal dysmotility disorders.