During HRM analysis, EGJ pressure is dynamically monitored during normal respiration with defined axial resolution (usually 1 cm) and without artifacts attributable to swallow-induced sphincter movement [14] or to EGJ conformational changes that may spontaneously occur [15]. However, even within the domain of EPT, there are still a number of variables regarding the methodology for assessing EGJ relaxation, morphology, and competence (barrier function). Progress in the understanding of the optimal methodology for assessing the EGJ among these functional domains has been considerable with the widespread adoption of HRM into clinical practice.

With HRM and Clouse plots, the relative localization of the two constituents of the EGJ, the lower esophageal sphincter (LES) and the crural diaphragm (CD), defines EGJ morphologic subtypes [16]. The EGJ morphology was simply classified in three types: type I EGJ morphology, in which there is complete overlap of the CD and LES with no spatial separation evident on the Clouse plot and no double peak on the associated spatial pressure variation plot; type II EGJ morphology, in which the LES and CD are separated (double-peaked spatial pressure variation plot), but the nadir pressure between the two peaks does not decline to the gastric pressure; type III EGJ morphology, in which the LES and CD are clearly separated as evidenced by a double-peaked spatial pressure variation plot and the nadir pressure between the peaks equal to or less than the gastric pressure; with type IIIa the pressure inversion point remains at the CD level, while in type IIIb, it is located at the LES level [11]. Recently Tolone and coworkers [17] evaluated, by means of HRM and impedance with pH monitoring, 130 consecutive patients and identified 46.2 % type I EGJ, 38.5 % type II, and 15.4 % type III patients. Patients with type III EGJ had a higher number of reflux episodes (61 versus 45, p < 0.03, versus 25, p < 0.001), a greater mean AET (12.4 versus 4.2, p < 0.02, versus 1.5, p < 0.001), and a greater positive symptom association (75 % versus 72 %, p = 0.732 versus 43.3 %, p < 0.02) compared to patients with types II and I, respectively. They concluded that increasing separation between LES and CD could cause a gradual and significant increase in reflux. Thus, they demonstrated that EGJ morphology assessment may be useful to predict an abnormal impedance-pH testing in gastroesophageal reflux disease (GERD) patients [17]. Similarly, the same group [18] evaluated the vigor of EGJ and its relationship with GERD by adopting a new HRM metric, namely, the contractile integral (CI). The EGJ-CI was calculated using the distal contractile integral toolbox during three consecutive respiratory cycles. They observed that patients with a defective EGJ-CI had more frequently a positive impedance-pH monitoring or esophageal mucosal breaks at endoscopy (p < 0.05) than patients with a normal EGJ-CI and concluded that a defective EGJ-CI at HRM is associated with evidence of GERD at reflux monitoring or endoscopy [18]. These data reinforced the need of performing HRM to better understand the mechanisms of GERD and suggested a potential diagnostic application of HRM for GERD diagnosis, at least as complementary test, and not only for positioning the pH electrode before reflux monitoring or for excluding achalasia in case of gastroesophageal surgery, in particular anti-reflux surgery.

During swallowing, EGJ relaxation is evaluated using the integrated relaxation pressure (IRP). This has been and will continue to be defined as the mean of the 4 s (contiguous or non-contiguous) of maximal deglutitive relaxation in the 10-s window beginning at deglutitive UES relaxation. The IRP is referenced to gastric pressure. The IRP represents a realistic alternative to the “nadir LES residual pressure” obtained during a standard manometry. Lin et al. [19] evaluated in a large group of patients the difference between single-sensor-detected EGJ relaxation and IRP to diagnose achalasia. They observed that the single-sensor method of assessing EGJ relaxation had a sensitivity of only 52 % for diagnosing achalasia. The 4-s IRP using a cutoff of 15 mmHg performed optimally with 98 % sensitivity and 96 % specificity in the detection of achalasia. This is important because failing to detect impaired EGJ relaxation in these patients would result in giving them a wrong diagnosis.

The most fundamental assessment of deglutitive contractility in the Chicago Classification is of whether or not an EGJ outflow obstruction is present as defined by an IRP > 15 mmHg. Disorders of the EGJ outflow are subdivided into achalasia subtypes and EGJ outflow obstruction based on the contractile and pressurization patterns in the body of the esophagus. Three clinically relevant subtypes of achalasia have been defined in the different versions of the Chicago Classification [9–11]: type I achalasia was characterized by 100 % failed contractions and no esophageal pressurization; type II achalasia was defined as 100 % failed contraction and panesophageal pressurization for at least 20 % of swallows; and type III achalasia was defined as the presence of preserved fragments of distal peristalsis or premature contractions for at least 20 % of the swallows [10, 11]. Some studies showed that the adoption of the Chicago Classification can improve our capability to diagnose and treat patients with achalasia. However, recent data highlighted that the use of a specific rigid cutoff (15 mmHg) to define normal from abnormal should be considered with caution. Indeed, the last iteration of the CC (v3.0) suggested assessment of EGJ relaxation by means of the median instead than by the mean value of IRP with ten swallows in order to minimize the effect of occasional outliers. Moreover, Lin et al. [19] recently showed that the critical IRP threshold may vary among achalasia subtypes and might range between 10 and 17 mmHg, specifically in type I achalasia, suggesting that IRP threshold might be reduced [19]. Similarly, Salvador and coworkers [20] observed that in a larger group of 139 patients with endoscopic, radiological, and manometric characteristics of achalasia, 10.9 % of the cases had an IRP value lower than 15 mmHg. To note, the authors showed that all patients had a positive outcome after laparoscopic Heller myotomy. Therefore, they suggested that some patients might also be correctly classified as a different type of achalasia deriving on clinical, radiological, and manometric pattern even if they had a borderline IRP [20]. Finally, another important consideration is that the cutoff for the upper limit of normal is technology specific ranging from a low value of 15 mmHg for the Sierra design transducers to as high as 28 mmHg for the Unisensor design. Thus, the diagnostic accuracy for detecting EGJ outflow obstruction for each device varies and further emphasizes the need of caution when applying a rigid cutoff value.

A different condition characterized by an impaired EGJ relaxation is defined EGJ outflow obstruction (EGJ-OO). The EGJ-OO exhibits not only an IRP greater than 15 mmHg but also a preserved peristalsis and elevated intrabolus pressure above the EGJ during peristalsis [21]. The finding of an elevated intrabolus pressure proximal to the sphincter is important because it validates the physiological significance of impaired EGJ relaxation. From a physiological perspective, elevated intrabolus pressure is the consequence of the impaired relaxation. A recent work suggested that when EGJ outflow obstruction occurs as a consequence of incomplete relaxation, it is accompanied by a relative increase in the ratio of peristaltic amplitude in the distal part of the esophagus, whereas this is not the case with mechanical obstruction [22]. With the term EGJ-OO, the CC includes a heterogeneous group of patients with some individuals having an incomplete phenotype of achalasia or an undetected mechanical cause of EGJ-OO such as hiatus hernia, esophageal stenosis, or eosinophilic esophagitis. Consequently, it is a patient group that merits further evaluation with mucosal biopsies and imaging studies to exclude inflammatory or malignant etiologies, be that with computerized tomography or endoscopic ultrasound. Only after these possibilities have been fully explored should it be accepted as atypical achalasia [23]. On this topic, van Hoeij et al. [24] evaluated 34 patients with primary EGJ-OO. They concluded that EGJ-OO is an unclear motility disorder with poor clinical significance. Indeed, the authors observed that 10 % of patients had unrelated symptoms and 15 % had spontaneous symptom relief. Moreover, one hundred percent of patients showed no stasis during esophageal radiogram, whereas treated patients showed a beneficial response to botox injections. Finally, less than 10 % of patients developed achalasia during follow-up [24].

The main HRM deglutitive peristaltic metrics used to evaluate esophageal contractile function are the distal contractile integral (DCI) and the distal latency (DL) (Fig. 13.1, Table 13.1) [10, 11]. They are used to characterize each of the ten 5-ml test swallows in order to obtain the final diagnosis. In particular, the DL physiologically represents an indirect measurement of deglutitive inhibition and thus of normal peristalsis. The DL is measured as the interval from UES relaxation to the contractile deceleration point (CDP) [10, 11]; a value less than 4.5 s defines a premature contraction. The contractile vigor is measured by using the DCI. This metric applies an algorithm to quantify the contractile pressure exceeding 20 mmHg for the region spanning from the transition zone to the EGJ [10, 11]. As described in Table 13.2, the integrity of the contraction associated with each swallow describes how completely that contraction is propagated from the upper sphincter to the EGJ, irrespective of the vigor of the contraction or latency. These qualifiers fall under the contraction pattern that is subsequently characterized.

Major motility disorders are defined as patterns of motor function that are not encountered in controls in the context of normal EGJ relaxation. The hierarchical Chicago Classification v3.0 is reported in Table 13.3 [11].