**Background:** Eosinophilic esophagitis (EoE) is an allergic inflammatory condition characterized by esophageal eosinophil infiltration. Unchecked inflammation is believed to lead to fibrostenotic disease, but assessing the degree of fibrostenosis before frank strictures develop is challenging. Symptom assessment is confounded by lifestyle adaptations such as food avoidance and prolonged eating, while endoscopy, radiography, and biopsies provide limited sampling. The endolumenal functional lumen imaging probe (FLIP) offers novel information on esophageal distensibility. This study aimed to evaluate the relationship between histologic, endoscopic, and symptomatic findings and esophageal distensibility in adult EoE patients, and to determine the utility of EndoFLIP in identifying a 'slender' esophagus missed on routine endoscopy.
**Methods:** Adult EoE patients were prospectively recruited at the Hospital of the University of Pennsylvania. Exclusion criteria included anatomic esophageal abnormalities unrelated to EoE, chest radiation, esophageal surgery, motility disorders, or inflammatory bowel disease. Symptom assessment was performed on the day of endoscopy. Control patients with normal esophageal biopsies were included, most undergoing endoscopy for reflux, dyspepsia, nausea, or vomiting. The FLIP EF-322 catheter (Medtronic) was placed transorally to the esophagogastric junction. Distensibility plateau was defined as the minimal esophageal body diameter at maximum distension at an intrabag pressure of 40 mm Hg. Standard biopsies were obtained after FLIP measurements, and eosinophil counts were assessed. Lamina propria (LP) scores from the EoE-histology scoring system were used; samples with <35 μm LP thickness or technical artifact were scored as 'not applicable/evaluable.' Data were presented as mean ± SEM or mean ± SD and analyzed using 2-tailed Student's t-test, ANOVA, or χ² test, with P < 0.05 considered significant.
**Key Results:** Forty-eight adult EoE patients and 17 controls were enrolled. Patients were predominantly White (92%), male (56%), and younger than 50 years (88%). Both active and inactive EoE patients had significantly lower distensibility index compared with controls (P < 0.05 for active vs control and inactive vs control). Distensibility index did not correlate with eosinophil counts (R² = −0.06, P = 0.0502). Patients with a history of stricture requiring dilation did not have significantly different distensibility compared with those without (mean 15.2 mm vs 16.59 mm, P = 0.31). Patients with a history of food impaction requiring endoscopic removal also showed no difference in distensibility (mean 16.17 mm vs 16.79 mm, P = 0.4175), nor did those with dysphagia in the preceding 30 days (mean 16.68 mm vs 16.23 mm, P = 0.486). Patients with critical narrowing (<10 mm) requiring dilation during the procedure were excluded. Thirteen of 48 EoE patients had an esophageal diameter less than 15 mm. Among these, 6 had no dysphagia, 6 had no prior food impaction, and 11 had no prior stricture. Comparison of patients with <15 mm vs >15 mm diameter showed no significant differences except for disease activity (eosinophil count <15: 15% vs 51%, P = 0.02); the presence of rings trended toward significance (81% vs 46%, P = 0.058). LP fibrosis score showed no relationship with distensibility (R² = 0.0017, P = 0.8103).
**Clinical Implications:** This study identifies a previously unrecognized subgroup of EoE patients with a 'slender esophagus' (diameter 10–15 mm) who lack obvious dysphagia, narrowing, inflammation, or complications, making them clinically indistinguishable from patients in deep remission. The findings demonstrate that symptom assessment alone does not capture the true status of the esophagus, and that histologic activity does not correlate with distensibility in adults—a striking contrast to pediatric populations. Furthermore, LP fibrosis scoring, even when adequately sampled, may not be sufficient to evaluate subepithelial remodeling. Impedance planimetry with FLIP provides additional information beyond symptomatic, histologic, and endoscopic assessments, and may identify patients who could benefit from dilation and optimization of medical management to improve quality of life and potentially alter remodeling.