Hysteresis of the Lower Esophageal Sphincter: Relevance to the Pathogenesis of Esophageal Achalasia and its Phenotypes

Abstract

Background & Aims:

Hysteresis is a change in strain for a given repeated stress; it is a material property of the viscoelastic tissues. We aimed to determine hysteresis of the esophagogastric junction (EGJ) in patients with esophageal achalasia and differences in EGJ hysteresis in different achalasia phenotypes.

Methods:

In a cross-sectional study design, we measured the change in EGJ distensibility index (DI) with repeated distensions (a marker of hysteresis), and the effects of atropine on the DI using functional lumen imaging probe in 40 patients with esophageal achalasia (types 1, 2 and 3).

Results:

The DI increased significantly with second distension (hysteresis) as compared to first distension, but not with subsequent ones. Atropine, which ablates active smooth muscle contraction, had no effect on the DI value. Type 1 esophageal achalasia patients and those with severe dilatation (stage III and IV disease) had a higher index DI and lower hysteresis, as compared to esophageal achalasia subtypes 2 and 3.

Conclusion:

A low DI following atropine suggests that the passive elements (viscoelastic properties) of EGJ are an important cause of low DI in esophageal achalasia. Hysteresis of the EGJ, a material property of the viscoelastic tissue, is different in different achalasia subtypes.

New & Noteworthy:

Hysteresis, a key biomechanical property of the esophagogastric junction (EGJ), may play a crucial role in achalasia pathogenesis. Using functional lumen imaging probe (FLIP) topography, we demonstrate that EGJ distensibility increases with repeated distensions, with subtype-dependent variability. Our findings suggest hysteresis is associated with achalasia progression and treatment outcomes, offering novel insights into esophageal biomechanics. These results may guide refinements in FLIP-based diagnostics and inform future therapeutic approaches targeting determinants of hysteresis.

Graphical Abstract

INTRODUCTION

The functional lumen imaging probe (FLIP) is a relatively new technique for evaluating esophageal peristalsis and esophagogastric junction (EGJ) function.^{1, 2} Unlike manometry, which detects active muscular contractions, FLIP captures both the active and passive properties of the esophageal wall and lower esophageal sphincter (LES).³ It is accomplished through controlled distension of a bag placed across the EGJ and measuring intra-bag luminal cross-sectional area and pressure.⁴ The two key FLIP-derived metrics are EGJ distensibility index (DI) and presence/absence of repetitive antegrade contractions. These parameters have shown utility in distinguishing esophageal achalasia from normal healthy controls. A DI < 2 mm²/mmHg is considered diagnostic of the esophageal achalasia.⁵

Stress and strain are foundational concepts in biomechanics and material properties of the viscoelastic tissue. Stress refers to the internal force per unit area within a tissue when subjected to an external load. Strain quantifies the relative deformation of tissue in response to a given load.^{6, 7} In biological tissues, the deformation (strain) for a given load during loading and unloading is not same, a property known as hysteresis.⁸ Strain is generally larger during unloading than during loading. Hysteresis reflects the internal rearrangement of structural elements of tissues, and it has been described in multiple organ systems, i.e., blood vessels, muscles and esophagus.^8–10

Goyal et al reported hysteresis in the rat skeletal muscle esophagus and found that it is dependent on the rate of esophageal distension.^{10, 11} Gregersen et al. found that chronic esophageal obstruction results in an increase in esophageal wall hysteresis.¹² More recently, El-Khoury et al. using functional luminal imaging probe (FLIP) found that the DI was greater during bag deflation than during inflation, across all esophageal motility disorder phenotypes.¹³ In the context of FLIP, strain is measured as a change in the intra-bag diameter during distension, while stress corresponds to the intra-bag pressure.⁹ The increase in the DI of the EGJ during inflation and deflation of the FLIP bag likely reflects hysteresis of the EGJ. Using FLIP one can also study the effect of active muscle contraction and passive/viscoelastic properties on the DI by using pharmacologic agents that inhibit active muscle contraction.^14–17

We hypothesize that the increase in the distensibility of the EGJ with repeated distensions during FLIP is an indirect marker of hysteresis, which to the best of our knowledge has never been studied in esophageal achalasia. The goals of our studies were to determine the effects of repeated distension of the EGJ on the DI and the effect of atropine, which ablates active smooth muscle contraction on the DI of the EGJ during repeated distension of the EGJ. We studied patients with three esophageal achalasia phenotypes and various radiographic clinical stages to determine if there were differences in the DI during repeated distension (hysteresis).

METHODS & PROTOCOLS

An investigator initiated prospective, cross-sectional study (NCT04641702, Emory IRB 00001665 & 00002955) of adult patients referred to the esophageal Clinic of Emory University with a diagnosis of esophageal achalasia was conducted. A written informed consent was obtained from all subjects, prior to the study.

Inclusion / Exclusion Criteria

Consecutive adult patients evaluated in the esophageal clinic for achalasia between December 2020 and June 2024 were included in the study. They met the following inclusion criteria: 1) clinical diagnosis of achalasia based on the high-resolution manometry CCv4.0 criteria, symptom consistent with esophageal achalasia, exclusion of mechanical causes of EGJ obstruction, and 2) no history of prior intervention for esophageal achalasia (myotomy, pneumatic dilation, Botulinum toxin injection).¹⁸ Per our institutional protocol, all patients with non-obstructive dysphagia undergo comprehensive evaluation with upper endoscopy, FLIP, and high-resolution manometry. Individuals with hiatal hernia ≥ 3 cm or a prior foregut surgery including fundoplication were also excluded. Exclusion criteria for receiving atropine were: i) atrial fibrillation or any tachyarrhythmia, ii) baseline HR > 90, iii) congestive heart failure with ejection fraction <35%, iv) history of myocardial infarction, v) baseline MAP <65 or systolic BP >140, vi) asthma or chronic obstructive pulmonary disease, vii) urinary retention requiring use of Foley catheter (including intermittent use), viii) narrow angle glaucoma, ix) myasthenia gravis, and/or x) GFR <60. Any FLIP studies with intra-bag pressures <15 mmHg were also excluded based on manufacturer recommendations.

FLIP Study Protocol:

Studies were conducted in 3 cohorts. In the first cohort, the objective was to measure the effect of atropine on the EGJ distensibility after first or index distension of the EGJ. This cohort underwent a standard FLIP protocol with a single measurement of the EGJ distensibility at the 60 mL bag volume before and after atropine (dose - 15 μgm/Kg).⁵ In the second cohort, the protocol was modified to study the effect of repeated distension sequences (inflation and deflation) on the change in EGJ distensibility by repeating EGJ distensibility measurements, 4–6 times, with bag volumes of 50, 60 and 70ml, during inflation and deflation (Figure 1). Each volume was held for 20–30 seconds. After the 4^th or 6^th measurement, atropine was administered, and bag distension sequence was repeated once. In the third cohort of subjects, no atropine was administered because of the cardiopulmonary or other exclusion criteria. These subjects only underwent 4–6 measurements of the EGJ distensibility during bag inflation and deflation.

Figure 1.

FLIP protocol used in Cohort 2 for assessing EGJ hysteresis and atropine effect. Patients in Cohort 2 underwent a standardized series of 4–6 FLIP bag distensions (ramps) from 50–70 mL, with EGJ measurements at the 60 mL fill volume. The distensibility index (DI) was recorded at the point of maximal EGJ opening. Following completion of the initial series, atropine (15 mcg/kg IV) was administered to inhibit cholinergic tone, and a final FLIP ramp was performed to assess for changes in EGJ compliance. EGJ = esophagogastric junction; DI = distensibility index (mm²/mmHg); IV = intravenous.

Technique for HRM, FLIP, and barium esophagogram

High resolution manometry (HRM)

Data from HRM studies performed at our or referring institution were included if performed with the Medtronic system in accordance with the Chicago Classification v4.0 protocol.¹⁸ The medications which affect esophageal motility (e.g., metoclopramide, hyoscyamine, dicyclomine, prucalopride, nitrates, sildenafil, nifedipine, diltiazem, and narcotics) were held for 24 hours prior to the HRM studies. HRM studies were analyzed by one of the authors (ASJ).

Functional Luminal Imaging Probe (FLIP)

FLIP studies were performed at the time of upper endoscopy exam by one of two operators (ASJ or SK). Each operator has expertise in performing over 250 FLIP studies. The upper endoscopy exam was performed in the left lateral position under general anesthesia to reduce the risk of aspiration. All patients received inhaled anesthetics (most commonly sevoflurane), a short-acting neuromuscular blocker (succinylcholine), and intravenous propofol. Additional agents, fentanyl, midazolam, or ketamine were administered at the discretion of the anesthesiologist.

FLIP placement -- After endoscopic exam was completed, a 16 cm long FLIP bag (EndoFLIP^® EF-322N; Medtronic, Inc, Shoreview, MN) was placed in the esophagus and stomach such that the distal 2–3 sensors were located in the stomach and remainder in the EGJ and esophagus. At this point the FLIP was inflated to 40 mL and correct location of the bag was identified by noticing the presence of a waist at the level of the EGJ.⁵

Inflation / deflation prior to atropine – After confirming placement at 40 mL, the FLIP bag was inflated in increments of 10 mL in a stepwise fashion from 50–70 ml. Each distension volume was maintained for 20–30 seconds. In the initial cohort, the DI was only measured once before and once after atropine administration. In the main cohort, the entire inflation/deflation sequence from 50–70 mL was performed 2 to 3 times, resulting in 4–6 measurements of the DI at the 60 ml bag volume.

Atropine 15 μgm/kg was administered after deflation to 50 mL via one-time intravenous push. At 90–120 seconds and once an increase in heart rate was confirmed on telemetry, the FLIP was inflated to 60 and 70 mL, with the measurement of the EGJ distensibility performed at the 60 ml volume.

Measurement of the distensibility index (DI) – FLIP studies were manually analyzed (ASJ) using an open-source software (NMGI WKlytics).¹⁹ The software identifies LES as a waist on the FLIP bag and allows for operator-selected point measurements of the DI. The DI measurement was manually recorded at the timepoint of maximum LES opening and at least 10 seconds after reaching the target intra-bag volume and with an intra-bag pressure of at least 15 mmHg.⁵ A commercially-available software, EndoVizX (MotilityViz, La Jolla, CA), was used solely for the purpose of creation of visualizations as shown in Figure 2B.²⁰

Figure 2.

Panel A shows placement of the functional lumen imaging probe catheter in vivo (left) and the topographic display (right). Panel B is the heat map of the change in the EGJ distensibility index at the 60 mL fill volume (EGJ-DI60, mm²/mmHg) over the course of 4 distensions in a patient with Type 2 achalasia. A cylindrical representation of the distal esophagus, esophagogastric junction, and proximal stomach is shown. The width at each level correlates with the measured DI as shown in the scale to the right of the cylindrical representation. Panel C shows representative studies from each achalasia subtype (type 1, type 2, type 3) showing variability in the DI over repeated measurements. To the right of each graph is raw change in the actual DI as well as % change relative to the index measurement.

Safety protocols

The atropine dose (15 mcg/kg, up to maximum dose of 1 mg) was chosen based on previous literature and guidance of anesthesiology team.^{15, 17} In all subjects, heart rate, blood pressure, and respiratory parameters were assessed prior to atropine administration and for a minimum of 60 minutes afterward (longer if needed, until resting heart rate was restored). Short-acting beta blocker administration as per the discretion of anesthesiologist was permitted.

Radiographic Achalasia Staging

Double contrast barium esophagogram or timed barium esophagogram studies performed at ours or the referring institution were analyzed to confirm diagnosis of achalasia and for measurement of the maximum diameter of the distal esophagus in accordance with the established guidelines for achalasia staging.^{21, 22} Esophageal diameter of ≤ 2 cm was classified as Stage 0 disease, 2–4 cm as Stage I, 4–6 cm as Stage II, and ≥ 6 cm as Stage III disease. Presence of sigmoid esophagus was the criteria for Stage IV disease.²³

Statistical Methods

The study sample was derived based on Wilcoxon-signed rank test sample size calculation of repeated measures. Descriptive statistics were calculated as mean (standard deviation) or count (%) as relevant. Comparisons between groups and correlations between diagnostic metrics were assessed via non-parametric methods. A linear regression model was constructed to assess the relative effect of the change in the diameter vs pressure on the change in the DI. Two and three group comparisons were tested via Mann-Whitney test, Kruskal-Wallis test, and Kendall’s tau as appropriate. Wilcoxon-signed rank test was used for paired comparisons, p value of ≤ 0.05 was considered statistically significant. Statistical analysis was performed using Graphpad Prism v9.0.2 (GraphPad Software, San Diego, CA) and R version 4.3.1 (R Foundation for Statistical Computing, Vienna, Austria).

RESULTS

Sample

Clinical characteristics and the overall effect of repeated measurements on the FLIP DI in the full cohort [n=39] (age 52.9 ± 16.3 years, 56% female) are shown in Table 1. The Eckardt score in the sample was 7.8 ± 1.9. Achalasia subtype 2 was the most prevalent (26 / 39, 66.7%) of the 3 types of achalasia.

Table 1:

Sample Characteristics (N=39)

Clinical Variable	Value
Age (Years)	52.9 ± 16.3
% Female (N, %)	22 (56%)
BMI (kg/m2)	28.0 ± 8.0
Eckardt Score	7.8 ± 1.9
Achalasia subtype
I	8 (20.5%)
II	26 (66.7%)
III	5 (12.8%)
Achalasia stage
0	10 (25.6%)
I	7 (17.9%)
II	16 (41.0%)
III or IV	4 (10.3%)
Missing	2
Distal esophageal width (cm)	3.67 ± 1.5
Baseline LESP (mmHg) (n=36)	41.1 ± 19.8
Supine IRP (mmHg) (n=36)	29.2 ± 13.3
FLIP
DI 60 (mm2/mmHg)	1.51 ± 0.94
Minimal diameter (mm)	7.24 ± 2.05
Intrabag pressure (mmHg)	33.2 ± 11.6
Max EGJ diameter (mm)	9 ± 2.6

Count (%) or mean ± SD are shown.

Abbreviations: BMI – body mass index, DI – distensibility index, EGJ – esophagogastric junction, FLIP – functional lumen imaging probe, IRP – integrated relaxation pressure, LESP – lower esophageal sphincter pressure

DI Measurement Increases During Repeated Distensions

Figure 2 shows the placement of the FLIP catheter (Figure 2A), a visual depiction of EGJ opening in response to multiple distensions (Figure 2B) and the change in DI on FLIP studies in one representative subject of each achalasia subtype (Figure 2C). The DI was markedly higher with the second distension as compared to first in each of the achalasia patients shown. Figure 3 shows the mean change in DI in each achalasia subtype (panels A-C) with repeated distension. The DI was significantly increased with the 2^nd compared to the 1^st distension in all 3 subtypes (panels A-C) but increases thereafter were not statistically significant.

Figure 3.

Variability in the distensibility index (DI) over the course of repeated measurements based on the achalasia subtype (Panel A – Type 1, Panel B – Type 2, Panel C – Type 3. Across all subtypes, a significant increase in the DI value occurs during the 2^nd distension, with less pronounced increases thereafter. Mean values and standard deviation are shown. P values for the differences between the adjacent values are shown

Effect of Atropine on the Repeated DI Measurements

Data from atropine studies are shown in Tables 2 and 3. In 13 subjects (cohort 1), we measured the effect of atropine on the DI at 60 ml distension, before and after a single inflation (Table 2). The median DI before atropine was 1.6 mm²/mmHg [IQR 0.7 – 2.3] as compared to 2.6 mm²/mmHg [IQR 1.8 – 5.4] (p = 0.0007) after atropine. In 20 subjects (cohort 2), we performed 4–6 bag distension followed by an injection of atropine (Table 3). We compared the DI measurement before the last distension with the one after atropine injection. The median DI before and after atropine were 2.56 mm²/mmHg [IQR 2.10 – 3.74] and 2.56 mm²/mmHg [IQR 0.74 – 3.21] (p = 1.000), respectively.

Table 2 --.

Cohort 1 (N=13): Pre vs. Post-Atropine Measurements

Measurement	Baseline (Median [IQR])	Post-Atropine (Median [IQR])	Median Raw Change [IQR]	Median % Change [IQR]	P-value
Pressure (P)	26.10 [17.20–29.90]	18.50 [16.70–24.40]	−5.50 [−8.60–−2.70]	−20.13 [−29.49–−14.79]	0.013
Diameter (D)	6.70 [5.10–8.40]	8.40 [7.50–10.50]	1.70 [1.00–2.33]	22.92 [12.66–36.18]	0.005
Distensibility Index (DI)	1.60 [0.70–2.25]	2.60 [1.80–5.40]	1.10 [0.50–2.70]	72.73 [51.35–133.33]	0.0005

P-values reflect Wilcoxon signed-rank tests comparing pre- and post-atropine measurements. DI = Distensibility Index; mm²/mmHg.

Table 3 --.

Cohort 2 (N=20): Baseline, Repeat, and Post-Atropine Measurements

Measurement	Timepoint	Median [IQR]	Δ from Prior	%Δ from Prior	P-value
Pressure (mmHg)	Baseline	32.90 [29.82–39.45]	–	–	–
	Repeat	37.70 [28.40–43.13]	4.64 [–4.73–−5.62]	14.20% [–8.61–20.45]	0.42
	Atropine	24.70 [23.30–29.27]	–13.00 [–14.26–−4.25]	–28.50% [--36.34––12.21]	0.0005
Diameter (mm)	Baseline	7.29 [4.29–9.43]	–	–	–
	Repeat	11.73 [7.86–14.43]	4.44 [2.12–5.38]	56.60% [36.85–89.60]	0.00003
	Atropine	8.70 [7.43–11.71]	–1.66 [–2.85–−0.37]	–16.10% [–22.55––3.99]	0.001
Distensibility Index (mm2/mmHg)	Baseline	1.43 [0.61–2.66]	–	–	–
	Repeat	2.56 [2.10–3.74]	1.43 [0.67–−2.04]	141.00% [87.17–238.71]	0.00003
	Atropine	2.56 [0.74–3.21]	--0.06 [–0.47–−0.60]	--0.75% [–6.66–20.62]	1.00

P-values reflect Wilcoxon signed-rank tests comparing sequential timepoints. “Repeat” refers to the final FLIP ramp prior to atropine administration. DI = Distensibility Index; mm²/mmHg.

The DI value and percent change in DI following single distension vs multiple distension protocols are shown in Figure 4. The median change in the DI was +1.1 mm²/mmHg [IQR 0.4 – 2.75] vs 0.14 [IQR −0.19 – 0.91] (p = 0.013) with single vs multiple distensions, respectively. The percent gain was +72.73% [IQR 45.68 – 141.3] vs +8.48% [IQR −9.48 – 43.42] (p = 0.002). Thus, the effect of atropine disappears after multiple measurements.

Figure 4.

Effects of atropine (ATR) measured after a single distensibility index (DI) measurement vs repeated DI measurements. Panel A shows the effect of ATR on the change in the DI value; Panel B shows the effect of ATR on the % change in the DI value relative to the baseline. The effect of atropine on increasing the DI disappears after multiple distensions suggesting that the effect was due to hysteresis and not cholinergic influence on the LES tone. Point values for each subject with median (line) are shown.

Contribution of the Diameter vs Pressure to the Change in the DI

Hysteresis is measured as a change in diameter for a given pressure.¹⁰ In our experiments, intra-bag volume, not pressure was controlled during the FLIP bag distension. We assessed whether the change in DI with repeated distensions truly reflects hysteresis by comparing the relative contribution of the diameter and pressure to the DI. In cohorts 2 and 3 subject (n=27), we determined the relationship between % change in DI after multiple distensions with % change in diameter and pressure. The % diameter change accounted for the majority of the explained variance in the DI (partial R² = 0.798, i.e., 80%), while pressure change contributed less (partial R² = 0.152, i.e., 15%). These observations prove that the change in the DI with repeat distensions is indeed a marker of hysteresis.

Inverse Correlation Between Initial DI Measurement and Amplitude of Change

27 subjects in our study had at least 4 DI measurements and HRM data. In these subjects, we assessed correlations between the index DI (during filling) with the 2^nd DI, the baseline LES pressure and integrated relaxation pressure. There was an inverse correlation between the initial DI and the 2^nd DI measurement, i.e., patients with a higher initial DI had a smaller change in DI with second distension: r = −0.651 (95% CI −0.830 to −0.350, p = 0.0002). There was no correlation between DI with baseline LES pressure: r = −0.099 (95% CI −0.471 to 0.303, p = 0.625) or integrated relaxation pressure: r = −0.169 (95% CI −0.524 to 0.237, p = 0.399) [Figure 5].

Figure 5.

Correlations between the initial distensibility index (DI) and, i) % change with 2^nd DI (top), ii) baseline LES pressure (BLESP) measured by the high resolution manometry (HRM) (middle), and iii) integrated relaxation pressure (IRP) measured by the HRM (bottom) in 27 achalasia subjects are shown. The initial DI and the % change with 2^nd measurement show a modest negative correlation. Spearman’s rho values with 95% confidence limits and p values are shown.

Index DI and Hysteresis Correlate with Achalasia Subtype and Achalasia Stage

We compared the 4 measures (index DI, % change in DI with 2^nd distension, BLESP, and IRP in the 3 achalasia subtypes, 1, 2, and 3. (Figure 6A). We also compared above measures in the radiographically defined achalasia stages (Stage 0: esophageal width <2 cm, Stage I: 2–4 cm, Stage II: 4–6 cm, Stage III: >6 cm, Stage IV: sigmoid esophagus) (Figure 6B). The index DI and the % change in DI were different between the 3 subtypes based on the overall test of significance. The index DI was higher and the change with 2^nd distension was lower in achalasia type 1 as compared to types 2 and type 3. The type 3 achalasia showed a higher % distensibility change compared to type 2 achalasia. The initial DI measurement and the % distensibility change were also different between the achalasia stages as determined by X ray barium esophagogram. The index DI value was lower in stage 0 as compared to stage I, which was lower in the stage II as compared to stage III and stage IV disease. There was no correlation between the achalasia subtypes or the degree of esophageal dilatation with the baseline LES pressure or the swallow-induced LES relaxation (IRP).

Figure 6.

Comparison amongst achalasia subtypes with i) index DI measurement, ii) % change from the index DI, iii) baseline LES pressure, and iv) integrated relaxation pressure are shown based on the achalasia subtype (Panel A) and the achalasia stage (Panel B). Kentall’s T and P values assessing the test of trend are shown for each metric. These suggest that the initial DI measurement is Type 1 > Type 2 > Type 3, whereas the % change (hysteresis) is Type 3 > Type 2 > Type 1. A similar trend is seen for radiographic achalasia stage -- initial DI measurement is Stage III-IV > Stage II > Stage I > Stage 0 and measured hysteresis is Stage 0 > Stage I > Stage II > Stage III-IV.

Abbreviations: DI – distensibility index, BLESP – baseline lower esophageal sphincter pressure, IRP – integrated relaxation pressure

DISCUSSION

Our study demonstrates that the esophagogastric junction (EGJ) hysteresis, reflected as an increase in the distensibility index (DI) with repeated FLIP distensions, can be measured in vivo in patients with the esophageal achalasia. The key findings of our study are: (1) the DI increases significantly with second distension, following which it plateaus; (2) the increase in DI is independent of the active LES muscle tone; it is related to the passive biomechanical (viscoelastic) properties of the EGJ; (3) increase in compliance/hysteresis varies in achalasia subtype and esophageal dilation stage; it is greater in achalasia type 3, as compared to type 2 and least in type 1. A similar pattern was seen across the esophageal dilation stages; greater increase in compliance/hysteresis was seen with the normal-caliber esophagus and a progressive decline with increasing esophageal dilation. Together, these findings suggest that the passive properties of the EGJ (low DI values after atropine) plays an important role in the pathogenesis of the esophageal achalasia. Furthermore, the magnitude of hysteresis in different achalasia subtypes and achalasia stages suggests differences in the viscoelastic properties of the EGJ in different achalasia phenotypes.

The EGJ is composed of several structures, i.e., smooth muscle LES surrounded by skeletal muscle crus of diaphragm and tissues in the esophageal hiatus, all of which are likely to contribute to the DI value in-vivo human setting.²⁴ Furthermore, both the active and passive properties of the LES and crus of diaphragm may impact the DI values. In our first series of patients where we injected atropine after the first FLIP bag distension and found significant increase in the DI, we thought that the active properties of the LES were the reason for the increase in DI with second distension. However, in our second series of patients where we performed 4–6 distensions first and then gave atropine, we found that atropine had a negligible effect on the DI value. We take these data to suggest that hysteresis, not the removal of cholinergically-mediated muscle contraction, contributed to the increase in DI value with second distension. We also observed that the DI values remain below normal in our achalasia cohort, despite repeated distensions and after atropine, suggesting some residual cause of EGJ restriction. Taken together, these observations imply that the passive properties of the EGJ are a significant contributor to the DI values in esophageal achalasia. Studies in achalasia have shown an increase in the fibrous contents of the muscularis propria of the LES and replacement of the hiatal fat with fibro-connective tissue, both of which we suspect contributes to the passive elements of the DI.^25–28 The current understanding is that the impaired inhibitory innervation of the LES is the basis of the esophageal achalasia.^{29, 30} We propose that the passive/viscoelastic properties of the EGJ likely play an important role in the pathogenesis of esophageal achalasia. Esophageal achalasia patients may have normal or even low LES pressure as measured by manometry, but most patients have a low DI.³¹ Furthermore, the LES pressure is not the best predictor of the treatment response in esophageal achalasia.^{32, 33} In an earlier study we found no relationship between the DI values and baseline LES pressure or integrated LES relaxation pressure.²⁰ All of the above arguments support that the passive/viscoelastic properties of the EGJ play an important role in the restriction at the level of EGJ in esophageal achalasia pathogenesis and treatment outcomes.

Goyal et al, in the 1970s, used pressure-diameter relationship in the rat skeletal muscle esophagus to describe hysteresis. They found that the magnitude of hysteresis is dependent on the rate of inflation of esophagus.^{10, 11} Hysteresis is higher with the lower rate of inflation as compared to the higher rate. In other words, the esophagus is less elastic at the higher rate of inflation. Gregersen et al, using impedance manometry, studied hysteresis in the opossum esophagus, before and 2 weeks after surgically induced partial distal esophageal obstruction. They found an increase in the hysteresis in obstructed esophagus, which correlated with esophageal dilation and increase in collagen content of the esophageal wall.¹² They suggested that tissue remodeling contributes to hysteresis. To the best of our knowledge, there are no studies of hysteresis and tissue remodeling of the human EGJ in the esophageal achalasia. We identified two distinct patient groups where the DI profiles suggested low or absent hysteresis, i.e., patients with type I and some with type II esophageal achalasia. Furthermore, patients with advanced radiographic stages of esophageal dilation (stages III and IV) also had a distensibility profile suggestive of lower hysteresis. In our cohort, our indirect measure of hysteresis declined across achalasia subtypes (type III > II > I), consistent with progressive tissue remodeling. Assessing hysteresis may help with precise phenotyping of inconclusive achalasia subtypes. For instance, type II patients with low hysteresis might more closely resemble type I in terms of structural remodeling. Ultimately, if the drivers of reduced hysteresis, whether fibrotic, inflammatory, or other—can be defined, they may guide future therapeutic strategy.

Our study has several limitations. We did not include non-achalasia control group for comparison with the achalasia group. The advantage of studying achalasia is the absence of esophageal contractions in achalasia, which permits more reliable assessment of the EGJ DI. Second, the sample size was small, particularly of achalasia subtypes I and III, which limits the power of subgroup comparisons. For this reason, we use Kendall’s tau as a test of trend for our subgroup comparisons, rather than comparing the individual subgroups. Third, there was significant variability in the FLIP protocol across study cohorts. When we first initiated this trial, our primary aim was to assess the contributions of neurogenic tone on the EGJ distensibility; the potential impact of hysteresis was not yet anticipated in the DI. We present here the full set of observations across all cohorts to maximize data yielded. A weakness of the experimental design is that since the DI measurements did not return to baseline after the multiple distensions, a head-to-head effect of atropine vs hysteresis cannot be assessed in the same subjects. However, the absence of any consistent additional effect of atropine towards further increasing the DI across the range of post-hysteresis DI values supports our conclusion that atropine does not further influence compliance once maximal hysteresis effect has been achieved. Another potential limitation is the use of general anesthesia in all subjects, including inhaled anesthetics known to relax airway and esophageal smooth muscle.^{34, 35} This may have attenuated the effect of atropine on the EGJ distensibility. However, because inhaled agents were used uniformly, they are unlikely to confound trends based on achalasia subtype or achalasia stage. In fact, their smooth muscle relaxing effects may have improved the reliability of hysteresis measurements. Despite these weaknesses, our study represents the first in vivo investigation of the EGJ hysteresis in humans.

In conclusion, passive elements of the EGJ are a significant contributor to the low DI seen in the esophageal achalasia. The magnitude of hysteresis which is related to tissue remodeling decreases as the disease subtype and esophageal dilatation progresses.