Regional oesophageal sensitivity to acid and weakly acidic reflux in patients with non-erosive reflux disease


Dr Sara Emerenziani, Department of Digestive Disease, University Campus Bio Medico, Via E. Longoni, 83, 00155 Roma, Italy.
Tel: +39 06225411; fax: +39 0622541456; e-mail:


Abstract  The mechanisms underlying symptoms in non-erosive reflux disease (NERD) remain to be elucidated. Non-erosive reflux disease patients appear to be more sensitive to intraluminal stimula than erosive patients, the proximal oesophagus being the most sensitive. In order to assess regional oesophageal changes in reflux acidity and sensitivity to reflux, according either to the acidity or the composition of the refluxate, combined multiple pH and multiple pH-impedance (pH-MII) was performed in 16 NERD patients. According to multiple pH-metry, 29% and 12% of reflux events reached the middle and proximal oesophagus respectively, and 35% and 19% according to conventional pH-MII (P < 0.05). The per-individual analysis confirmed the difference between the two techniques. According to combined distal and proximal pH-MII, approximately 30% of distal acid reflux became weakly acidic at the proximal oesophagus. In all patients, the frequency of symptomatic refluxes, both acid and weakly acidic, was significantly higher at the proximal, compared with distal oesophagus (25 ± 8%vs 11 ± 2% for acid reflux and 27 ± 8%vs 8 ± 2% for weakly acidic reflux; P < 0.05). Compared with multiple pH-metry, pH-MII shows a higher sensitivity in the detection of proximal reflux. As approximately 30% of acid reflux becomes weakly acidic along the oesophageal body, to better characterize proximal reflux, in clinical practice, combined proximal pH-impedance monitoring should be used. In NERD patients, the proximal oesophagus seems to be more sensitive to both acid and weakly acidic reflux.


The mechanisms responsible for reflux perception are not yet fully understood. Although, of the gastric refluxate, acid plays a pivotal role in eliciting typical gastro-oesophageal reflux disease (GORD) symptoms, only a small proportion of acid refluxes are perceived by the patients; moreover, the lack of association between symptom severity, presence and/or degree, of erosive oesophagitis is well documented.1,2

At present, non-erosive reflux disease (NERD) represents a challenge in clinical practice. Several studies have demonstrated that NERD patients complain of symptoms similar to those in patients with erosive reflux disease (ERD), being, at the same time, less sensitive to proton pump inhibitor (PPI) treatment.3–5 In a recent study, we demonstrated that the presence of gas in the refluxate significantly enhances the probability of reflux perception in NERD patients, in whom weakly acidic reflux is also highly perceived.6 In keeping with these findings, it has recently been demonstrated that NERD patients show greater sensitivity to oesophageal acid infusion than ERD patients,7 thus supporting the hypothesis of hypersensitivity to intraluminal stimuli in this large group of GORD patients .

Growing evidence demonstrates that, in addition to alterations in central pain processing of visceral stimuli, the proximal extent of acid reflux is also relevant in triggering both typical and atypical symptom elicitation. Weusten et al.8 first suggested that, in GORD patients, perception of reflux might depend on the proximal extent of the refluxate. Cicala et al.9 confirmed the higher frequency of proximal reflux in GORD patients and reported an increased perception of this proximal reflux in NERD patients, particularly in those showing normal acid exposure at the distal oesophagus. Moreover, it has been demonstrated that the proximal extent of the refluxate is the only factor significantly associated with reflux perception in non-responder patients, during PPI treatment, thus confirming previous findings in patients off treatment.10 More recently, the relevance of proximal reflux has been confirmed in studies in which the pH-impedence technique was used.11–13 Finally, it has been demonstrated that, in NERD patients, with respect to ERD patients, the proximal oesophageal acid infusion induces significantly greater sensitivity than distal infusion.7

Multiple pH monitoring has been used to assess the distribution of acid reflux along the oesophagus. Classically, a proximal pH-drop across 4, following a distal acid reflux, is used to define proximal acid reflux; however, this measurement has shown low sensitivity and reproducibility.14 Oesophageal intraluminal multiple pH-impedance (pH-MII) monitoring can accurately identify less acidic or even non-acid reflux, and is also a very sensitive method for the detection of proximal extent of reflux.15 At present, using the available pH-MII catheter, proximal refluxes are classified as acid or weakly acidic according to the pH value detected at the distal oesphageal site.

In order to assess intra-oesophageal changes in reflux acidity and sensitivity to reflux, according either to the acidity or the composition of the refluxate, in this study, concurrent multiple pH and pH-MII were performed in NERD patients.

Material and methods

A total of 20 consecutive patients [11 male, 9 female, mean age 43 years (range 24–79)] complaining of typical GORD symptoms – heartburn and/or acid regurgitation – lasting more than 6 months, with a history of a favourable response to PPI treatment (>50% symptom improvement at standard or double doses) and no evidence of erosive oesophagitis at previous (3–5 years) endoscopy, entered the study. All patients underwent upper endoscopy, stationary oesophageal manometry and ambulatory concurrent multiple pH (three pH sensors) and pH-MII monitoring (six impedance channels; Femu, Aachen, Germany). Following upper endoscopy, four patients were excluded from the study due to duodeno-gastric peptic disease or to the presence of erosive oesophagitis. None of the patients had undergone previous gastrointestinal surgery or were taking medication known to influence oesophageal motor function. Patients on antisecretory drugs stopped the medicament at least 2 weeks prior to the study. The study protocol was approved by the Ethics Committee of University Campus Bio Medico of Rome and written informed consent was obtained from all individuals.

Intraluminal impedance and pH

Intraluminal electrical impedance was recorded with a 2.3-mm diameter polyvinyl assembly containing a series of cylindrical electrodes, each 4 mm in axial length, spaced at 2 cm intervals (Femu). Each pair of electrodes formed a measuring segment, 2 cm in length, corresponding to one recording channel. The impedance catheter also included a pH sensor, located 5 cm above the lower oesophageal sphincter (LOS). Oesophageal pH monitoring was performed using a single probe, with three antimony sensors, with a separate skin reference (Zinetics Medical Inc., Salt Lake City, UT, USA). The pH-probe was especially designed for the purpose of this study, to place the pH and impedance sensors at the same level. Signals from both ambulatory systems were digitized, stored and synchronized for further analysis using a dedicated software (Review; K.U. Leuven, Leuven, Belgium). Following stationary manometry, the pH and impedance catheters were tied together with steri-strips and positioned in order that one pH sensor and one impedance segment were located at 5, 15 and 20 cm above the LOS (Fig. 1). The pH electrodes were calibrated using pH 4.0 and pH 7.0 buffer solutions before the beginning and at the end of the recording. Patients were studied after an overnight fast of at least 10 h. Patients were asked not to lie down during the day, but to lie down only at their usual bedtime. Furthermore, patients were instructed to consume three meals and two beverages at fixed times during the 24-h measurement period. Event markers, on the monitor, recorded meal times and posture changes. All patients tolerated the introduction of the catheter very well and were able to pursue their everyday activities on the day of the study.

Figure 1.

 Concurrent multiple pH-impedance assembly.

Analysis of data

In the analysis of impedance tracings, liquid reflux was defined as a retrograde 50% drop in impedance starting distally (above the LOS) and propagating to at least the next two more proximal impedance measuring segments. Gas reflux was defined as a rapid (3 kΩ s−1) increase in impedance >5000 Ω, occurring simultaneously in, at least, two oesophageal measuring segments, in the absence of swallowing. Mixed liquid–gas reflux was defined as gas reflux occurring immediately before or during a liquid reflux. Changes in oesophageal pH during reflux, detected by impedance, allowed classification of reflux in: (i) acid reflux: refluxed gastric juice with a pH <4, which can either reduce the pH of the oesophagus to below 4 or occur when oesophageal pH is already below 4; (ii) weakly acidic reflux: reflux events that result in an oesophageal pH between 4 and 7; and (iii) weakly alkaline reflux: reflux episodes during which nadir oesophageal pH does not drop below 7.

Proximal extent of reflux was assessed according to both conventional pH-MII and multiple pH: in the pH-MII tracing, a drop of impedance, in the mid- and upper oesophagus, indicated the proximal extent of liquid refluxate, whereas in the multiple pH tracing, a pH drop below 4 units lasting ≥4 s in the middle and proximal oesophagus, occurring simultaneously with a similar pH drop in the distal oesophagus, indicated the proximal extent of reflux. The percentage of reflux episodes that reached the impedance-pH in the middle oesophagus (15 cm above LOS) and proximal oesophagus (20 cm above LOS) was calculated. Acid exposure time (AET) was defined as pathological if the time at pH <4 exceeded 5% of the total recording time. Heartburn and acid regurgitation were considered in the analysis of symptoms. Reflux episodes were classified as symptom-related if they occurred ≤2 min before the onset of the symptom. The Symptom Association Probability (SAP) Index was calculated according to the formula described elsewhere.16

Statistical analysis

Data were expressed as mean, range or standard errors when required. The proportion of reflux events in the middle and proximal oesophagus was compared using Fisher’s exact test. Unpaired Student’s t-test was used for intra-subject comparison of regional oesophageal sensitivity. A P-value <0.05 was considered to be significant.


Spatiotemporal characteristics of reflux

Of the 16 NERD patients, nine (five male, four female, mean age 40 years) showed a pathological AET at the distal oesophagus (NERD pH-positive, mean AET 18%, range 4.9–29.2%) and seven patients (three males, four females, mean age 35 years) showed a normal pH-metric profile (NERD pH-negative). As no differences were observed in the sensitivity between the distal pH sensor of the multiple pH and the pH sensor of the pH-impedance catheter, we analysed pH data from the multiple pH catheter. A total of 640 reflux events (mean 40, range 14–64) were detected at the distal oesophagus. Of these, 516 (80%) were acid reflux, and 124 (20%) were weakly acidic refluxes. Liquid–gas reflux accounted for 54% of events. Of the 516 acid reflux events detected in the distal oesophagus (mean 33, range 11–47), according to the pH-MII analysis, 35% reached the middle oesophagus (15 cm above LOS) and 19% reached the proximal oesophagus (20 cm above LOS), compared to 29% and 12% respectively, according to multiple-pH analysis (Fisher test P < 0.05). The per-individual analysis confirmed the different proportion of reflux reaching the middle and proximal oesophagus observed in the pooled data from all patients (34% ± 3 and 20% ± 5, according to impedance analysis, and 26% ± 2 vs 12% ± 3, according to multiple-pH analysis).

When combining pH and impedance monitoring, both at the distal and proximal oesophageal site, 30 out of 96 (31%) of the distal acid refluxes [mean nadir pH 2.2 (range 1–3.7)] reaching the proximal oesophagus became weakly acidic, with a nadir of 4.4 (range 4.1–5.4) (Fig. 2). In particular, in all patients except one, in whom only fiv3 proximal refluxes were observed, the conversion of acid, to weakly acidic, reflux, along the oesophagus, was observed in 26 ± 4% of events.

Figure 2.

 Example of acid reflux converting into weakly acidic.

Gas was present in 49% of acid refluxes, both in the distal and proximal oesophagus, and in 63% of those converting into weakly acidic.

Symptom-reflux analysis

Of the 16 NERD patients, 14 reported 63 symptoms during the study (mean 4.5, range 2–12). Heartburn accounted for 68%, regurgitation for 32%, of symptoms. Of the 14 patients presenting symptoms during the study day (eight pH-positive, six pH-negative), 11 presented a positive SAP index for acid reflux (six pH-positive, five pH-negative), with five being also positive for weakly acidic reflux (three pH-positive, two pH-negative).

Of the symptomatic reflux events, 82% were acid refluxes whereas 18% were weakly acidic. Weakly acidic reflux was more often associated with regurgitation (70%) than with heartburn (30%). The frequency of symptomatic reflux events, according to the acidity at each oesophageal site, is shown in Table 1. The frequency of symptomatic refluxes, either acid or weakly acidic, was significantly higher among proximal refluxes compared with distal refluxes in all patient groups (25 ± 8 vs 11 ± 2 of acid reflux and 27 ± 8 vs 8 ± 2 of weakly acidic reflux; P < 0.05).

Table 1.   Proportion (%) of symptomatic events according to the acidity at each oesophageal site
 All NERDNERD pH+ (n = 8)NERD pH− (n = 6)
Acidic reflux (%)
 5 cm above LOS11 ± 211 ± 310 ± 5
 15 cm above LOS18 ± 5*17 ± 6*18 ± 7*
 20 cm above LOS25 ± 8*20 ± 8*30 ± 19*
  1. *P < 0.05 vs distal.

Weakly acidic reflux (%)
 5 cm above LOS8 ± 27 ± 48 ± 4
 15 cm above LOS19 ± 7*16 ± 11*22 ± 8*
 20 cm above LOS27 ± 8*23 ± 12*32 ± 9*

Of the proximal reflux events, 54% of refluxes were acidic both in the distal and proximal oesophagus (76% heartburn, 24% regurgitation), 29% of acidic refluxes at distal oesophagus converting to weakly acidic in the proximal oesophagus (66% heartburn, 34% regurgitation) and 19% of weakly acidic refluxes both at the distal and proximal oesophagus (16% heartburn, 84% regurgitation) were associated with symptoms.


Results of this study demonstrated that, compared with multiple pH-metry, conventional pH-MII shows a higher sensitivity in the detection of the proximal extent of reflux. As far as concerns the proximal reflux, at present, using the available combined pH-MII catheter, it is assumed that the acidity at the proximal oesophageal site is equal to that detected at 5 cm above LOS. Our results show that approximately 30% of distal acid reflux becomes weakly acidic when spreading to the proximal oesophagus. A not negligible portion of these refluxes, nearly one-third, were associated with symptoms. It is tempting to suggest that the conversion of the reflux acidity is due to: (i) buffering, especially if only slight change around 4 is observed (i.e. passing from 3.9 to 4.1); (ii) the contact with wet mucosa and (iii) heterogeneity of the refluxate that touches the pH sensors. Moreover, as in the present investigation 63% of reflux events with change of chemical composition were gas containing, it could also be hypothesized that the presence of gas may, in part, explain the observed phenomenon due to the dilution of the refluxate and a decreased H+ concentration. These refluxes would not have been considered at multiple pH monitoring or would have been classified as proximal acid reflux by means of conventional pH-MII monitoring. It should be taken into consideration that, due to the design of the assembly, aimed at obtaining pH and impedance data exactly at the same levels, the proximal pH sensor, in some patients, could have been located in the very proximal part of the oesophagus.

Even if, in our series, 11 out of 14 patients presented a positive SAP, all showed a favourable response to PPI and, therefore, according to the Rome III criteria belonged to the population of GORD. Of interest, in our series, although the large majority of symptoms were related to acid reflux, the proximal reflux, both proximal acid and proximal weakly acidic, was perceived significantly more than the distal reflux. Heartburn and regurgitation may be elicited by chemical or mechanical stimulation of the oesophageal wall. Intra-oesophageal acid perfusion induces heartburn in a pH-dependent manner.17 The enhanced perception of proximal reflux may result from a larger area of chemical stimulation and/or mechanical distension and recruitment of more sensory receptors.18 Dilation of the intercellular space diameter, which is known to be involved in the pathogenesis of symptoms in NERD patients, occurs also in the proximal oesophageal epithelium, and might account for the enhanced perception of proximal acid reflux.19

Growing evidence demonstrates the relevance of the hypersensitivity of the proximal oesophagus. Indeed, Fass et al.18 reported a gradient of perception thresholds for discomfort during rapid phasic distension from the proximal to the distal oesophagus in GORD patients. In keeping with this finding, it has been demonstrated that NERD patients are more sensitive to proximal than to distal acid infusion of the oesophagus.7 Even if, of the gastric refluxate, acid plays a pivotal role in eliciting typical GORD symptoms, weakly acidic reflux seems to play a role in typical reflux symptoms, particularly in NERD patients and in non-responder patients on PPI treatment.20 The presence of gas in the refluxate significantly enhances the probability of reflux perception in NERD patients who also appear to highly perceive weakly acidic reflux.6 In a recent study, the stimulus response to acid and saline infusion was quantified and it was demonstrated that NERD patients, in particular, exhibit oesophageal hypersensitivity not only to acid but also to saline perfusion, suggesting that hyperalgaesia to acid and other factors may play a role in symptom generation, in these patients.21 Of interest, in our study, weakly acidic reflux was responsible for a not negligible proportion of symptoms, at each oesophageal site, both in NERD pH-positive and in NERD pH-negative patients, the highest proportion of symptomatic refluxes being found among proximal refluxes. These findings support the concept of the higher sensitivity of NERD patients to weakly acidic reflux, previously reported by our group, and show an enhanced chemosensitivity at the proximal oesophagus. Taking together these aspects, it might be worthwhile considering, in the treatment of these patients, the use of not only PPIs but also non-medical treatments aimed at increasing the resistance of the oesophago–gastric junction and to decrease the contact of the gastric content, whatever the acidity or composition, with the oesophageal mucosa. The enhanced oesophageal sensitivity to minimal acid changes and/or to other intraluminal stimuli might account for the lower response to PPI treatment reported in NERD with respect to oesophagitis patients.22 It should be considered that the patients enrolled in the present investigation, reporting an average of 4.5 typical symptoms during the study day, are very symptomatic, likely representing the standard of patients referring to a specialized centre more than the general NERD population. In conclusion, compared with multiple pH-metry, conventional pH-MII shows a higher sensitivity in the detection of the proximal extent of reflux. Approximately 30% of acid reflux becomes weakly acidic along the oesophageal body. In order to more accurately characterize proximal reflux, in clinical practice, adding a proximal pH sensor to the available pH-impedance monitoring would be helpful. In NERD patients, independently of distal oesophageal acid exposure, the proximal oesophagus appears to be more sensitive both to acid and weakly acidic reflux.