Schematic representation of the gastroesophageal junction (GEJ)
In health, the lower esophageal sphincter (LES) is approximately 4 cm long extending from just above the squamo-columnar junction (Z-line) into the proximal stomach with distinct upper and lower sections. The upper section comprises relatively thick, tonically contracted esophageal smooth muscle fibers and the lower section comprises the sling and clasp muscle fibers of the gastric cardia [11, 15]. The function of the intrinsic sphincter is modulated by vagal tone such that LES pressure is higher in expiration than inspiration. The striated muscle of the crural diaphragm, which forms the esophageal hiatus, encircles the proximal 2 cm of the LES; an anatomical arrangement that increases EGJ pressure during inspiration, coughing and abdominal straining [10, 14]. Thus, the intrinsic and extrinsic components of the EGJ have complimentary effects that provide effective reflux protection throughout the respiratory cycle and during physical exertion.
3.3 EGJ Function
On pharyngeal swallowing a vagal reflex is triggered that results in “deglutitive” relaxation of the esophagus and LES to allow bolus transit from the mouth to the stomach. Repetitive swallowing results in complete relaxation of the intrinsic LES and the extrinsic crural diaphragm to facilitate rapid intake of food and fluid. During this process, relaxation of the proximal stomach (“gastric accommodation”) ensures that the stomach can be filled without an important increase in intra-gastric pressure.
Ingestion of a meal is accompanied by gastric secretion that tends to collect immediately below the LES forming an “pocket” or layer of unbuffered acid overlying an ingested meal. In health, the transition from the acid to alkaline milieu occurs at the EGJ in the post prandial period . However, when the EGJ barrier is weak or disrupted (e.g. in presence of hiatus hernia) the acid pocket can migrate into the distal esophagus, leading to pathological acid reflux in the distal esophagus . Delayed gastric emptying  and acid hypersecretory states are additional downstream factors that can contribute to the esophageal reflux burden. At the same time, gastric filling is accompanied by a decrease in LES pressure and an increased frequency of spontaneous, transient LES relaxations (TLESRs) that allow air swallowed during the meal to be released (belching). Together, these events represent a major challenge to the EGJ reflux barrier and it has been shown that when the EGJ barrier is weak or disrupted, especially in the presence of hiatus hernia, the acid pocket can migrate into the distal esophagus, leading to pathological acid reflux and mucosal disease . A small number of reflux events during TLESRs after meals is normal in healthy individuals; however, the number of reflux events is much higher in GERD patients. Studies using magnetic resonance imaging combined with high-resolution manometry (HRM) have shown how active contraction of the clasp and sling fibers maintains an acute angle of insertion between the esophagus and the proximal stomach (termed “angle of His” in surgical studies) [20, 21]. The presence of an acute angle of insertion allows the proximal stomach to compress the EGJ and prevents reflux of gastric contents into the esophagus . This “flap-valve” effect is much less efficient if the angle of insertion is wide (obtuse) due to ineffective contraction of the clasp and sling fibers or structural disruption of EGJ anatomy, both of which are observed in GERD patients .
Another challenge to EGJ function occurs in the fasted state, especially at night, when powerful migrating motor complex (MMC III) contractions clear the stomach of undigested material. These contractions increase intra-gastric pressure and can trigger gastro-esophageal reflux; however, this does not occur in healthy individuals with an intact reflux barrier because LES pressure increases during MMC III contractions .
3.4 Mechanism of Reflux
Large studies have identified several markers from manometry studies that correlate with the severity of reflux defined by the presence of reflux esophagitis or pathological acid exposure on 24 h pH-studies. These include resting LES pressure, intra-abdominal LES length (i.e. distance between PIP and distal LES border1) and disorders of esophageal motility that impact on clearance function [3, 24–26]. More detailed observations after a test meal identify three main mechanisms that cause individual reflux events: [27–30].
Transient LES Relaxation (TLESR). In health and in patients with mild-moderate GERD most reflux occurs during TLESRs characterized by a period of complete, prolonged (10–60 s) LES relaxation that is not caused by swallowing . Gastric distention, laryngeal or pharyngeal stimulation provide the afferent stimulus for the TLESR reflex, which is transmitted to the nucleus solitarius in the brainstem. A set of events from the dorsal vagal nucleus and the nucleus ambiguous mediates EGJ relaxation via the vagal efferent fibres .
Swallow-induced LES relaxations . In health about 5–10% of reflux episodes occur during swallow-induced LES relaxations . The relatively low risk of reflux events during swallow-induced LES relaxations compared to TLESRs is due to incomplete and shorter relaxation of the crural diaphragm during swallowing and immediate clearance of reflux by oncoming peristalsis [32, 33].
Manometry can identify TLESRs and other potential causes of reflux. Moreover, the presence of “common cavity pressure” (i.e. equilibration of pressure) between the stomach and the esophagus indicates the occurrence of a reflux event. The initial description of TLESRs by Dent et al. was based on the conventional water perfused manometry with an “sleeve sensor” to provide stable pressure measurements from the EGJ . TLESRs were defined by the absence of a preceding swallow, rapid rate of relaxation, low nadir LES pressure and prolonged duration of LES relaxation . Additional markers include inhibition of crural diaphragm and presence of a prominent after-contraction . The inter-observer agreement for detection of TLESRs is superior for HRM compared to conventional manometry  and criteria for identification of TLESRs were recently validated for this advanced technology . These include the occurrence of LES relaxation in the absence of swallowing 4 s before and 2 s after the onset of LES relaxation and prolonged duration of LES relaxation lasting more than 10 s with concurrent inhibition of the crural diaphragm. The introduction of combined high-resolution impedance manometry (HRiM) supports this observation by direct detection of EGJ relaxation, opening and retrograde flow of gastric contents (i.e. reflux) during TLESRs (and other events) [37, 38]. It is also possible to discriminate between reflux of air (belching) and gastric secretions based on the impedance profiles [37, 38].
Additionally, the use of HRiM facilitates the detection of rumination syndrome and supra-gastric belching in patients with persistent “reflux” symptoms on PPI medication [39, 40]. These behavioral conditions are characterized by the voluntary, albeit unconscious, contraction of abdominal and thoracic muscles resulting in the forceful return of gastric or oesophageal contents to the mouth. It is important to recognize these conditions because the mechanism of disease is not the same as GERD, cannot be corrected by standard medical treatment and can be exacerbated by anti-reflux surgery . Instead, patients can be taught to suppress these “abnormal responses” by simple exercises delivered by physiotherapists .
3.4.1 Motility and Reflux
After a reflux episode occurs, the refluxate is cleared most often by a primary peristaltic contraction that also neutralizes acid by bringing saliva from the mouth . In many GERD patients, esophageal motor function is preserved; however, ineffective esophageal motility (IEM) can impair esophageal clearance and is associated with increased likelihood of esophagitis [26, 42, 43]. The spectrum of IEM consists of fragmented peristalsis , hypotensive peristalsis and absent contractility (all can be observed in the same patients). Patients with more severe disease are characterized by a failure to respond to the physiologic challenge of multiple repeated swallows or the solid swallows (absent “contractile reserve”). It is the frequency of ineffective esophageal contractions after reflux events that impacts on esophageal clearance and the severity of acid exposure [44, 45].
3.4.2 Obesity and Reflux
A high body mass index (BMI) and, especially, a high waist circumference is associated with an increased risk of GERD . Epidemiological studies suggest that the prevalence of GERD symptoms and reflux esophagitis is significantly increased in patients with BMI ≥ 25 compared to normal weight subjects . Obesity has effects on EGJ structure and function that increase the risk of reflux by all the mechanisms discussed above [6, 47–49]. The mechanical hypothesis proposes that obesity results in increased mechanical stress at the EGJ due to increased intra-gastric pressures and disruption of EGJ morphology (i.e. increased separation of the LES and crural diaphragm) which favors reflux . Other hypotheses include the release of metabolic and humoral mediators from visceral adipose tissue that have effects on vagal activity and may well impact on the frequency of TLESRs and other key aspects of EGJ function [50–52]. Of interest, although obese subjects complain of heartburn and acid regurgitation more frequently than normal weight controls,  this effect is weaker than that expected and many obese subjects with GERD have only mild or occasional reflux symptoms. This lack of sensitivity and failure to seek treatment could, in part, explain the relatively high risk of GERD complications, including adenocarcinoma, in this group.
3.4.3 Hiatus Hernia
In patients with a small hiatus hernia, separation of the LES and crural diaphragm results in a twofold increase in reflux events  and the risk of GERD increases with the size of the hiatus hernia [3, 53, 54]. The presence of a hiatus hernia has multiple effects on EGJ structure and function. First, the wide esophageal hiatus impairs the ability of the crural diaphragm to contribute to reflux protection . Indeed, in HRM studies, reduced augmentation of EGJ pressure during inspiration is an independent risk factor for GERD . Second, contraction of the crural diaphragm can trap gastric contents in the hiatal sac that can then pass into the esophagus through the (weak) LES due to negative thoracic pressure during inspiration . Third, the frequency of reflux events of all kinds is increased in patients with hiatus hernia [53, 54] due to mechanical effects on EGJ and proximal gastric function. Whilst TLESR remains an important cause of reflux events in this patient group, other mechanisms appear to be more important in the presence of a hiatus hernia . Additionally, once reflux has occurred, ineffective esophageal motility and impaired esophageal clearance are common in patients with a large hiatus hernia [58, 59]. This results in prolonged exposure of the distal esophagus with acid gastric secretions that are thought to be a major cause of reflux esophagitis, Barrett esophagus and other complications; all of which are more common in GERD patients with hiatus hernia than those without.
3.5 Hiatus Hernia: Diagnosis
The diagnosis of a hiatus hernia is most often made on barium esophagogram or endoscopy. Features on radiology include the presence of a herniated B-ring at the squamocolumnar junction or rugal folds traversing the diaphragm . On endoscopy, the distance between the squamocolumnar junction and the diaphragmatic pinch is measured to determine the length of the hiatus hernia. Recent studies indicate that both investigations lack sensitivity for small hiatus hernias and, moreover, distention of the stomach during endoscopy often triggers TLESR with EGJ opening potentially leading to false positive diagnosis or overestimation of hiatus hernia size .
In health and in many patients with mild-moderate GERD separation of the intrinsic (LES) and extrinsic (crural diaphragm) components of the EGJ reflux barrier is not constant but occurs intermittently . HRM studies identify this spatial dissociation of the intrinsic LES and diaphragmatic sphincter, as a double peak pressure profile at the EGJ . HRM provides a more prolonged and detailed analysis of EGJ pressure and this increased spatial and temporal resolution has excellent sensitivity and specificity for hiatus hernia . Compared to barium fluoroscopy or endoscopy which each have a sensitivity of 73% for detection of a hiatus hernia, the sensitivity and specificity of HRM for hiatus hernia detection was 92% and 93% respectively .
The Chicago Classification of hiatus hernia by HRM is based on spatial separation of the two “high pressure zones” produced by the LES and diaphragmatic crus [56, 61, 64]. An EGJ pressure morphology with a single pressure peak during inspiration and expiration, indicating that the axial position of the LES coincides with the CD, is classified as EGJ Type 1 (Fig. 3.2a). In the latest iteration of Chicago Classification v3.0,  when LES-CD separation is observed that is <3 cm the hiatus hernia is classified as EGJ Type II (Fig. 3.2b). When there is a marked LES-CD separation ≥3 cm (typically with a nadir pressure between the two pressure peaks less than gastric pressure) then the hiatus hernia is classified as EGJ Type III. Large type III hernias can be further sub-classified according to the position of the pressure inversion point (PIP) (Fig. 3.2c, d).
Different types of EGJ morphology identified by esophageal pressure topography across the EGJ. Different types of EGJ morphology identified by esophageal pressure topography across the esophagogastric junction. (a) EGJ Type I: the LES and CD overlap both in inspiration and expiration, and the PIP is located directly above the LES. (b) EGJ Type II: minimal separation of LES and CD, but a nadir pressure between the peaks that is higher than gastric pressure. (c) EGJ Type IIIa: LES and CD separated but PIP is located at the proximal margin of the diaphragm. (d) EGJ Type IIIb: LES and CD separated but PIP Is located above the LES
This classification system for hiatus hernia has been validated against endoscopy and ambulatory pH-measurement. There is a positive correlation between increasing disruption of EGJ morphology with the prevalence and severity of reflux esophagitis [55, 57, 65]. In addition, esophageal acid exposure is higher in GERD patients with compared to those without hiatus hernia [66, 67]. Notwithstanding the above, other metrics derived from HRM measurements of EGJ function may provide better sensitivity and specificity for the diagnosis of GERD (see below).
3.6 EGJ Measurement
Advances in esophageal motility diagnostic tests have provided further insights into the anatomy and function of the EGJ reflux barrier [68–70]. Unlike “conventional” esophageal manometry systems with up to 12 pressure sensors, HRM systems acquire pressure measurements from closely spaced sensors and display this information as topographic (Clouse) plots that integrate time, position and pressure data. Assessment of esophageal motility by the Chicago Classification for HRM is based on objective metrics derived from these measurements . The intraluminal pressure at the EGJ is referenced to the intra-gastric pressure in clinical studies. The presence of adequate resting pressure provides an indication of resistance to retrograde flow of gastric content across the reflux barrier. As discussed above, the intact EGJ barrier consists of superimposed LES and CD. The intrinsic LES can independently have a low resting tone, with values <8 mmHg during the end expiratory phase consistently abnormal [56, 64]. Inspiratory augmentation of the CD provides adjunctive barrier function when intrathoracic pressures are at their lowest;  an element of the EGJ barrier that is not well assessed by basal and end expiratory LES pressure measurements. The EGJ contractile integral (EGJ-CI) may overcome these drawbacks by combining EGJ anatomy, basal tone, and variation with respiration into a single metric assessing vigor of the EGJ [72, 73]. A further improvement can be achieved by calculating the “total-EGJ-CI ”, a parameter that summarizes EGJ barrier function during the entire HRM protocol compensating for variation in morphology and pressure over time . Normative EGJ-CI and total EGJ-CI values have been described, and available data indicate that the risk of GERD is increased in the setting of abnormal results [74, 75]. Notwithstanding this finding, GERD has a complex aetiology and the ability of any measurement of EGJ function to provide a definitive GERD diagnosis is limited (although a robustly normal value may rule GERD out) . Therefore, an abnormal EGJ barrier can be hypotensive (with reduced resting tone that can be overcome by events that increase intra-abdominal pressure), disrupted with separation of the two components of the EGJ barrier (i.e. hiatus hernia) or both. In the presence of a hiatus hernia, the resting tone of the intrinsic LES is typically hypotensive, with esophageal reflux burden higher than with either abnormality alone. Therefore, abnormalities of EGJ structure and function can coexist, and both can contribute to abnormal reflux burden.
3.7 Ambulatory Reflux Monitoring
The presence of “typical reflux symptoms” including heartburn and acid regurgitation on validated questionnaires are unreliable in the diagnosis of GERD . In general, typical reflux symptoms are initially managed with an empiric PPI trial; however, this approach is also unreliable, with a specificity of only 50–60% despite sensitivity of approximately 80% in predicting erosive esophagitis or an abnormal pH study . Indeed, only half of patients referred for physiological investigations have pathological acid exposure on ambulatory pH-studies . These findings emphasize the need for objective evaluation of reflux prior to antireflux surgery . This includes esophageal manometry to rule out achalasia and other, clinically relevant motility disorders that appear in almost every published series.
Ambulatory reflux monitoring is performed when there is a need to document esophageal reflux burden, or to define the relationship between symptom events and reflux episodes. The most common settings consist of persisting esophageal symptoms despite seemingly adequate acid suppressive therapy, i.e. a failed PPI test, or atypical symptoms (e.g. chest pain, cough, laryngeal symptoms) that may not directly implicate GERD, but could improve with GERD therapy if esophageal reflux burden is pathological. In the typical clinical scenario, ambulatory reflux monitoring has either ‘rule in’ or ‘rule out’ value in defining abnormal esophageal reflux burden. In settings where there is no independent evidence for reflux (unproven GERD), testing is performed off anti-secretory therapy for 7–10 days. Ambulatory reflux monitoring prior to anti-reflux surgery is also performed off anti-secretory therapy. When there is strong evidence for GERD (proven GERD), such as EGD evidence of severe erosive esophagitis, peptic stricture or long segment Barrett’s mucosa (or prior abnormal ambulatory reflux monitoring), testing can be performed on anti-secretory therapy, where the objective is to determine if ongoing symptoms can be explained by abnormal esophageal reflux burden or linked to reflux episodes. In this setting, pH impedance monitoring needs to be employed for reflux monitoring, as pH testing alone is insufficient in describing weakly acid reflux episodes that predominate in patients on PPI therapy . If suspicion for GERD is strong in the setting of negative 24 h reflux monitoring, repeating monitoring using a prolonged pH measurement can be considered, as day-to-day variation in esophageal reflux burden is present, and the finding of abnormal reflux burden can impact management direction [80–82].
The best-established method for objective diagnosis of GERD is 24-h pH-measurement with the pH-sensitive electrode mounted on an naso-esophageal catheter placed in the esophagus 5 cm above the proximal border of the LES as determined by manometry . An alternative technique using wireless pH probes is also available, and are particularly useful in patients that fail to tolerate ambulatory catheter based testing . These single sensor probes are positioned 6 cm proximal to the squamo-columnar junction during endoscopy and can record and transmit distal esophageal pH data for up to 96 h. Catheter or probe placement is performed after an overnight fast, and after withholding anti-secretory therapy for at least 7 days when testing off PPI is performed (essential in pH-only studies). Patients are recommended to maintain normal activities and meals, and keep a diary of meals, recumbency periods and, crucially, symptoms. Careful instruction to record symptoms promptly ensures that not only the objective severity of disease but also the subjective association between symptoms and reflux can be assessed.
The percentage of total recording time with pH < 4 (i.e. acid exposure time) is the single most important parameter used in GERD diagnosis . This is calculated as the percent time the pH is <4.0 in the distal esophageal (5 cm above the LES), for the duration of the ambulatory study. Total AET is considered physiologic when <4%, and pathologic when >6%; values in between are borderline and require additional clinical or physiological evidence to confirm GERD . Additional information can be obtained by separately calculated AET for upright and supine periods; elevated supine AET can implicate a disrupted EGJ barrier, as TLESRs are generally suppressed during sleep. However, proximal esophageal and pharyngeal reflux monitoring have limited value in directing anti-reflux therapy [86, 87]. AET is marginally higher with the wireless probe, but the same thresholds can be employed for both modes of reflux monitoring. AET is considered more reliable and better reproducible than the composite Johnson-DeMeester score that is no longer recommended in clinical practice .
More recently multiple intra-luminal impedance (MII) monitoring has been combined with pH-measurement and is currently regarded as the gold standard for reflux detection [85, 88]. By measuring differences in resistance to alternating current between pairs of adjacent electrodes, MII can detect bolus movement through the esophagus and the retrograde flow of refluxate from the stomach into the esophagus. Moreover, the combination of pH and MII differentiates between acid and non-acid reflux events and the conductivity of esophageal contents can distinguish between liquid and gas reflux (belching)  (Fig. 3.3a, b). Thus, combined pH-impedance monitoring permits the detection of (1) anterograde and retrograde bolus movement (Fig. 3.4) (2) characterizes the nature of the refluxate (air reflux, liquid reflux or mixed air-liquid reflux) and (3) determines the pH of the refluxate (acidic [pH < 4], weakly acidic [4 ≤ pH < 7] and weakly alkaline pH ≥ 7 [89, 90] (Fig. 3.5). The number of acid and non-acid reflux events over 24-h is highly variable and is a less reliable marker of GERD than AET;  however, the presence of <40 reflux events is considered physiologic, and large numbers >80 are likely to be pathologic; as for AET, values in between are borderline and require additional clinical or physiological evidence to confirm GERD . For example, a low median nocturnal baseline impedance (e.g. MNBI <2000 ohms) and the presence of a low post-reflux swallowed induced peristaltic wave (PSPW) index2 derived from pH-impedance studies have been shown to predict response to medical and surgical GERD therapy .
(a) A combined impedance-pH catheter consisting of six impedance channels and two pH sensors. (b) Example of an acid reflux event detected on impedance-pH monitoring. Upper 6 channels display impedance readings in the esophageal body. The lower channel displays pH at 5 cm above the lower esophageal sphincter. Illustrative example of retrograde movement of liquid bolus during an acid-reflux event
Comparison of impedance tracings during a swallow event and a reflux event
Impedance patterns during swallow and reflux (liquid, gas, mixed liquid-gas) events
Ambulatory reflux studies are also used to confirm (or exclude) reflux events as the cause of patient symptoms. Symptom-reflux association requires reporting of symptoms during the ambulatory study, typically using an event monitor button on the reflux monitoring device worn by the patient. Reflux episodes are identified by reflux software, using pH drops below 4.0 or impedance-detected retrograde movement of gastric contents. A symptom event is designated as associated with a reflux episode if the symptom occurs within 2 min following the reflux episode. Studies have shown that ongoing non-acid reflux accounts for up to 50% symptoms in patients with an incomplete response to PPIs [8, 93]. A simple ratio of associated symptoms to all symptoms defines the symptom index (SI; abnormal if >50%). Symptom reflux probability (SAP) takes into account 2 min periods with and without reflux episodes and symptom events, and applies a statistical test (Fisher’s exact test) to assess the probability that symptoms and reflux episodes could have co-occurred by chance (SAP positive if >95% (1-p < 0.05)). The yield and diagnostic value of symptom reflux association is highest when many symptoms are recorded, with the patient recording the symptom promptly upon occurrence. A positive, symptom reflux association can augment the strength of a GERD diagnosis based on borderline AET or number of reflux events. Reflux hypersensitivity is present when AET is normal but a significant symptom reflux association is present (ideally both SAP and SI are positive) [92, 94].
The metrics described above have been shown to predict reflux outcome when testing is performed off PPI therapy in unproven GERD [91, 92]. In the clinical setting, the results provide guidance as to whether reflux management should continue (if pathological reflux is identified) or if alternate mechanisms should be sought for persisting symptoms. However, as discussed above, thresholds defining pathological from physiological reflux burden are not precise, and a ‘grey area’ exists (“borderline reflux burden”) where the clinical presentation and alternate evidence from physiological investigations (HRM, pH-impedance studies) could complement ambulatory reflux monitoring findings to support or reject GERD [85, 95] diagnosis. Reflux hypersensitivity is defined by a positive symptom-reflux association in the setting of physiologic reflux, is part of the GERD spectrum and can respond to optimal acid and reflux suppression therapy. In contrast, functional heartburn or functional chest pain is defined by normal ambulatory reflux monitoring with negative symptom reflux association and are functional conditions that are not caused by reflux events.