Barrett’s Esophagus and Intestinal Metaplasia

Intestinal metaplasia refers to the replacement of the differentiated and mature normal mucosal epithelium outside the intestinal tract by the intestinal epithelium. This paper briefly describes the etiology and clinical significance of intestinal metaplasia in Barrett’s esophagus. This article summarizes the impact of intestinal metaplasia on the diagnosis, monitoring, and treatment of Barrett’s esophagus according to different guidelines. We also briefly explore the basis for the endoscopic diagnosis of intestinal metaplasia in Barrett’s esophagus. The identification techniques of goblet cells in Barrett’s esophagus are also elucidated by some scholars. Additionally, we further elaborate on the current treatment methods related to Barrett’s esophagus.


INTRODUCTION
First reported by Norman Barrett, Barrett's esophagus (BE) can be simply defined as the presence of columnar epithelium in the esophagus (1). Until 1976, Paull et al. (2) classified the presence of columnar epithelium within the esophagus based on histiological subtypes: Gastric fundic type, junctional type, and specialized type with intestinal metaplasia (IM).
IM refers to the replacement of normal mucosal epithelium with the intestinal epithelium outside the intestine, implying the transformation of a well-differentiated and mature tissue into another differentiated mature tissue under abnormal conditions. IM is common in the stomach and esophagus, but it can also occur in the gallbladder, bile ducts, uterus, bladder, pelvis, ureter, and urethra (3)(4)(5)(6)(7)(8)(9).
In recent years, there has been much controversy regarding the role of IM in the diagnosis of BE. We hereby summarize the significance of IM and the evidence supporting the fact that the presence of IM is either mandatory or not to establish a diagnosis of BE. IM in BE is often missed, and several other cells can mimic goblet cells. We cover the classification, manifestation and detection rate of IM under different endoscopic techniques, and methods to accurately identify IM in BE based on endoscopic and pathologic findings. Finally, we also discuss the principles regarding the monitoring and management of BE with and without IM.

THE SIGNIFICANCE OF IM IN BE
IM is considered a precancerous lesion and is closely related to the occurrence of cancer. According to the study of Watanabe et al (10), the presence of IM is associated with the degree of severity of the Barrett's mucosa, regardless of the size of the tumor. Therefore, they speculated that IM might be an epiphenomenon of BE extension rather than a reflection of the tumor's origin. However, differentiated mature intestinal epithelial cells can acquire additional mutations, then develop into dysplasia and cancer (11). There is also evidence that IM is associated with a higher frequency of cancer-related mutations compared with nongoblet cell metaplasia (12). Similar to the significance of IM in the stomach (13), although there was evidence that tumor cells may not directly originate from goblet cells (11,14). It has been undoubtedly suggested that the BE's mucosa is more prone to neoplastic transformation. In the replacement of the normal esophageal stratified squamous epithelium with columnar epithelium, IM is the only type that is clearly prone to malignant transformation (15).
The possible etiologies for BE include the transcommitment of stem cells or progenitor cells, transdifferentiation of differentiated cells, the expansion of residual embryonic cells located at the squamous-columnar junction (SCJ), and the differentiation of circulating bone marrow cells (16)(17)(18)(19)(20). It was not until 2017 that Jiang et al. (21) simulated a BE mouse model with real goblet cells. They described a novel transitional columnar epithelium with distinct basal progenitor cells (p63 + KRT5 + KRT7 +) at the SCJ. After the expression of the caudaltype homeobox transcription factor-2 (CDX2), these progenitor cells were differentiated into intestinal-like epithelium, including goblet cells, resulting in BE with IM. Moreover, such epithelium was found to be localized at the subject's gastroesophageal junction (GEJ). Moreover, Jin et al. (11) proposed the SPEM model of esophageal IM to explain the multiformity of the stem cells involved in esophageal IM. The differentiated cells and stem cells in this model could be transformed into SPEM-like cells (TFF2+ MUC6+); persistent inflammatory injury eventually led to IM, dysplasia, and carcinogenesis of the SPEM.

DEBATE ON THE DIAGNOSIS OF BE
Replacement of the distal esophageal squamous epithelium by metaplastic columnar epithelium forms the pathological basis for BE. Whether IM must be present or whether the length of the columnar mucosa needs to be larger than 1 cm are the main controversial points. The American Gastroenterological Association (AGA), American College of Gastroenterology (ACG), and European Society of Gastrointestinal Endoscopy (ESGE) all believe that IM is necessary for the diagnosis of BE, while the British Society of Gastroenterology (BSG), Asia-Pacific Working Group (APWG) and Benign Barrett's and Cancer Taskforce consensus group (BOB CAT) all reckon that IM is not needed for the diagnosis of BE (15,(22)(23)(24)(25)(26). However, BOB CAT also stressed that the existence of IM should be noted when diagnosing BE (23) ( Table 1).
The main basis supporting the fact that IM is mandatory for the diagnosis of BE includes the following: a large number of population-based cohort studies have shown that the risk of esophageal adenocarcinoma is much lower in patients with columnar metaplasia without IM compared to those with IM (27). Compared with columnar metaplasia without goblet cells, tumorigenesis is most commonly seen in columnar metaplasia with goblet cells (12,28). In-depth evaluation of esophageal biopsy specimens before EMR and esophageal resection specimens after EMR found that IM was found in the columnar esophagus of all the patients with adenocarcinoma (29). Furthermore, some studies have demonstrated that it might be unwise to diagnose a disease that has a negative impact on insurance status and quality of life in non-IM-CLE (Columnar metaplasia without IM) patients (30,31). The use of IM as a prerequisite for the diagnosis of BE was conducive to the adoption of a more cost-effective approach to the care of BE patients (26).
Evidence supporting the fact that IM is not necessary for diagnosing BE includes the following: a single endoscopic examination and a small number of biopsy samples were not sufficient to rule out IM (32,33). In the study of Jankowski et al. (34), all the patients with BE who did not have IM at the beginning of the study developed IM during follow-up. Related retrospective studies have reported that patients with or without IM had a similar risk of developing cancer (35). Studies carried out on columnar mucosa resected endoscopically in early cancer subjects have confirmed that cancer may also occur in non-IM columnar epithelium (36). There was also evidence that columnar epithelium without goblet cells may contain similar molecular abnormalities to columnar epithelium with goblet cells (28,(37)(38)(39). Lavery et al. (40) reconstructed the cloned ancestor of EAC and provided direct genetic evidence for the malignant tendency of metaplastic columnar epithelium without goblet cells. This is the first direct demonstration that the clonal expansion and precancerous progression of BE are not limited to the metaplastic columnar epithelium with goblet cells.
With or without IM, it was uncertain whether columnar mucosa with a length smaller than 1 cm at the GEJ would progress to cancer. Current research has also depicted that the rate of carcinogenesis in this group of patients is significantly lower than in those with BE (Columnar mucosa more than 1 cm). Thus, most guidelines explicitly require that the diagnosis of BE must include a columnar mucosa exceeding 1 cm at the GEJ (22,(24)(25)(26) (Table 2). Although BOB CAT did not specify that the columnar mucosa at the GEJ must be more than 1 cm to establish a diagnosis of BE. BOB CAT also indicated that an irregular and ≤ 1 cm lesion at the GEJ may be a natural phenomenon (26). Besides, according to a recent prospective multicenter cohort study, patients with BE lesions smaller than 1 cm will not develop high-grade dysplasia (HGD) or esophageal cancer within 5 years of endoscopic examination (41).
In Japan, the 1 cm criteria regarding the morphological changes of BE's mucous membrane is not mandatory, and such lesions are called ultra-short-segment BE (USSBE). The true meaning of cardiac cancer refers to cancer that occurs in the intestinal metaplastic area of the anatomical cardia or esophagogastric junction (42). By definition, the anatomical position of the cardia and USSBE basically coincided. IM can occur in the cardia and USSBE, and the cardiac mucosa with IM is considered a precancerous lesion of cardiac carcinoma. Over the past few years, with the increasing incidence of cardiac cancer, more attention has been paid to USSBE. Nonetheless, due to the above reasons, the concept of USSBE is not widely used and accepted. In a large sample study in 2016, the annual cancer rate of USSBE was only 0.01% (compared with 0.22% and 0.03% for the long and short segments of BE, respectively) (43). A related etiological investigation also pointed out that BE lesion's length exceeding 1 cm was not the defining factor (44). However, IM is not required in the definition of USSBE in these studies. If we investigate USSBE with IM exclusively, there may be new results. Monitoring may be crucial for these patients' prognosis, albeit further research is required to confirm this assumption. Given the high incidence of USSBE in the population, this issue should be taken into account when calculating and comparing the incidence of BE.

ENDOSCOPIC DETECTION OF IM IN BE
Conventional endoscopy has not been satisfactory in the diagnosis of BE, especially in the detection of precancerous BE lesions. Biopsy of BE lesions is routinely performed from four quadrants superior to the GEJ at intervals of 1-2 cm. According to previous studies, only 10-79% of doctors take biopsy specimens according to this protocol. The higher the length of BE lesions, the lower the application rate of the four-quadrant biopsy principle (45)(46)(47). Also, obtaining multiple biopsy specimens and pathological interpretation both imply a significant financial burden (48). BE with IM is a precancerous lesion, which has more significant clinical significance (49). Relevant investigations conducted in Japan suggested that similar to the most common site of occurrence of esophageal adenocarcinoma (EAC), IM often appeared in the 0-3 o'clock area (50). This finding indicates that this area should be the focus of attention and that biopsy specimens taken from this area may increase the detection rate of IM. With the advancement of endoscopic techniques, there are more means for endoscopic detection of IM (Table 3, Figure 1).

Chromoendoscopy
Methylene blue is a dye that can be absorbed by IM cells giving them a blue coloration, whereas the normal esophageal mucosa does not stain with methylene blue. Studies have shown that methylene blue can improve the detection rate of IM, but subsequent multivariate analysis showed that, compared with the traditional four-quadrant biopsy, the methylene blue stain had no obvious advantage in the diagnosis of BE with IM, BE with dysplasia, and even early cancer. Methylene blue also has a carcinogenic effect (48,49,58,59). Three types of mucosal patterns were noted by Sharma et al. (51) within the columnar mucosa after spraying mucicarmine and using high magnification endoscopy: eidged/villous pattern, circular pattern, and irregular/distorted pattern. The sensitivity, specificity, and positive predictive value of the ridge/villous pattern for detecting IM were 97%, 76%, and 92%, respectively. The sensitivity (100% vs 92%) and specificity (100% vs 69%) of long-segment BE were better than those of short-segment BE. However, there is a lack of sufficient research data.

Enhanced-Magnification Endoscopy (EME)
EME is a diagnostic method combining acetic acid with magnifying endoscopy. Guelrud (52) first used EME to study the relationship between various histological types and fine structures of mucous membranes and divided the mucosal image into four types: I, round pits; II, reticular; III, villous; and IV, ridged. They found that the IM corresponding to round pits, reticular, villous, and ridged types were 0%, 11%, 87%, and 100%, respectively. In 2003, Toyoda et al. (53) found that pattern III included both villous and slit like patterns, and a new classification was devised as follows: type 1, small round pits of uniform size and shape ("corpus" type); type 2, slit reticular pattern with horizontally elongated mucosal pits ("cardia" type); and type 3, gyrus, villous, or mixed gyrus-villous patterns ("IM" type). The sensitivity, specificity, and diagnostic accuracy of type 3 in predicting IM were 85.5%, 92.2%, and 90.0%, respectively. Even though the specific mucosal classification is different, a large number of studies have demonstrated that EME can accurately identify IM (52,53,60). Eight prospective studies in a recent meta-analysis provided data for the diagnosis of IM (54). The combined sensitivity, specificity, positive likelihood ratio (LR), and negative LR for IM were 96%, 69%, 3.0, and 0.06, respectively. This finding suggested that EME is an adequate screening method for IM, and histological confirmation remains key considering its low specificity.
Narrow-Band Imaging Magnification Procedure (ME-NBI) ME-NBI allows for detailed examination of mucosal morphology without the use of stains (61). ME-NBI is widely used to diagnose and monitor BE, not only by expert endoscopists but also by non-experts (62   Simplified classification of capillary pattern in BE by ME-NBI. The capillary pattern was divided into two types: type I: (A, B) a branched or rattan-shaped pattern that was clearly shaped and could be tracked smoothly, and type II: (C, D) a curly or spiral pattern whose shape was disordered and could not be fully tracked. All dysplasia regions were type II, but there was no significant difference in intestinal phenotype between type I and type II. 7 antigen present at the brush border, which is also expressed within the LBC-positive mucosa. Researchers speculate that the LBC's morphology can be explained by the difference in the reflectance of the light at the surface of the ciliated tissue structure (63). Hence, if LBC was detected on NBI, we would always find evidence of IM on histology. However, if LBC was not found on NBI, IM could not be excluded. A meta-analysis (57) elucidated that the targeted biopsy performed by NBI had high sensitivity and specificity for IM and HGD per patient and each lesion. Targeted biopsies by NBI have the same IM detection rate as high-definition white light endoscopy under the Seattle protocol, but require fewer biopsies (64). NBI targeted biopsy could detect more atypical areas of hyperplasia, and the normal surface pattern did not include HGD or EAC. Based on this fact, a biopsy could be avoided in these normal areas. Similar results were obtained from the study conducted by Pascarenco et al. (65), which found that the villus type (According to Shama et al.) had higher sensitivity and specificity for the diagnosis of IM.
To establish a simplified classification, Goichi Uno et al. (61) proposed a new capillary pattern (CP) classification based on the shape and microvessel occupied areas. They divided CP into the following two categories: type I, uniform branched or vine-like pattern with a clear shape that is able to be traced smoothly, and type II, coiled or spiral pattern with a nonuniform shape that cannot be traced sufficiently and with increased vascularity. In this study, all the regions of dysplasia were type II, but there was no significant difference in the intestinal phenotype between type I and type II.
The combination of magnifying endoscopy and NBI makes it easier to detect IM. Although the detection rate of IM has not been greatly improved compared with random biopsy under white light, it has greatly reduced the number of biopsies, which also reduces the number of specimens needed for pathological observation, generating a certain financial benefit. Compared to chromoendoscopy, NBI does not require additional staining, and many large-capacity endoscopy centers use it for routine esophageal observations at no additional cost. This finding indicates its application prospects in the diagnosis and monitoring of BE, but it is still necessary to establish a uniform and reliable standard.

Linked Color Imaging (LCI) and Blue Laser Imaging (BLI) Systems
In recent years, numerous reports have been published on the application of LCI and BLI in gastrointestinal examinations. Despite the fact that some medical centers have used it to observe the esophagus, there are very few reports on BE. These studies concluded that compared with WLI, LCI improved the visibility of short-segment BE lesions (Esophageal columnar epithelium was less than 3 cm, more than 1 cm, and did not require IM), especially for trainees (66). BLI was deemed extremely helpful for the early detection of synchronous adenocarcinoma in BE patients (67). Reports on the observation of IM in BE with LCI and BLI have not been retrieved. Upper gastrointestinal examination entails continuous observation. Moreover, LCI and BLI have obvious advantages over WLI with respect to the detection of lesions within the stomach To optimize the system for complete detection of lesions in the upper digestive tract, the detection rate for esophageal lesions requires more clinical data.

Multiphoton Microscopy
Multiphoton microscopy (MPM) based on 2-photon excitation fluorescence and second-harmonic generation allows for simultaneous visualization of cellular details and extracellular matrix components of fresh, unfixed, and unstained tissue (68). MPM can easily identify several cells in the GEJ. For this purpose, the mucosa was classified into a squamous type, columnar stomach type, and metaplastic columnar intestinaltype/BE based on the type of cells identified. Chen et al. (68) showed that goblet cells were identified in 10 out of 25 patients examined by MPM, of which only 7 were diagnosed pathologically as IM. Of the 35 biopsy specimens, 3 (9%) delineated clear goblet cells by MPM observation, which could not be seen in histopathological sections. This study showed that MPM can accurately identify goblet cells, opening a new door for the identification of IM and diagnosis of BE, but its diagnostic efficiency in combination with endoscopy and clinical application still necessitates more exploration.

Confocal Laser Endoscopy (CLE)
CLE is a combination of traditional video endoscopy and confocal laser microscopy. The addition of confocal laser microscopy enabled us to dynamically identify different cell structures in real-time and make histological observations in vivo. In 2006, Kiesslich et al. (69) divided the distal esophagus into three types according to the appearance of different vessels and cell structures: Gastric-type epithelium, Barrett's epithelium, and neoplastic changes. The predictive sensitivities of BE (with IM) and related tumors were 98.1% and 92.9%, respectively, whereas the specificities were 94.1% and 98.4%, respectively. Studies performed by Richardson et al. (70) also revealed that significantly more IM patients were detected by early CLE users compared with the conventional Seattle protocol. Regarding the detection of BE-related neoplasia, both a clinical randomized controlled trial and meta-analysis showed that CLE combined with targeted biopsy was superior to high-definition white light endoscopy combined with four-quadrant biopsy (71,72). A recent meta-analysis concluded that CLE is superior to NBI in the detection rate of single lesions (73). However, due to the need to inject a fluorescent agent, the high cost of equipment, and the need for additional learning, the clinical application of CLE is still limited.

Ultrathin Transnasal Endoscopy
Sugimoto et al. (74) believed that, although present endoscopies used high vision or magnified endoscopy, these endoscopes were of larger caliber and needed sedation. Thus, these endoscopes were not suitable for endoscopic examination. Consequently, they used a newly developed ultrathin transnasal endoscope to closely observe the mucosal structure and investigated the incidence of BE and the usefulness of mucosal structural pattern classification. The study suggested that the new ultrathin transnasal endoscopy is a useful technique for monitoring BE, especially SSBE. The pit pattern was divided into closed (Oval/round and long straight patterns) and open (Villous, cerebriform, and irregular patterns) types. Their results revealed that IM was more prevalent in the open type.

Computer-Aided Identification System
With the emergence of artificial intelligence, the use of computer-aided identification has increased in the field of endoscopy. The use of artificial intelligence and machine learning techniques have become major aids to cope with pattern prognosis associated with BE, and there have been significantly more research projects on automatic stage classification of BE using different endoscopic techniques (75). Recently, Ghatwary et al. (76) proposed an automatic classification method for the stage classification of BE, which focused on improving the classification of IM. This classification method divides the mucous tissue into four types: Normal scaly tissue, gastric metaplastic tissue, IM, and tumor. This method's sensitivity and specificity for IM detection were as high as 0.97 and 0.96, respectively. The sensitivity and specificity for the diagnosis of dysplasia and tumors were also over 90%, and the overall accuracy was 96.05%. These aided systems can be used as a second opinion to assist physicians in the diagnosis, as well as training beginners to improve the detection rate of IM. Withal, it should be noted that a huge image data set must be constructed in order to enhance the computer-aided identification system's overall diagnostic accuracy.

IDENTIFICATION OF GOBLET CELLS IN THE PATHOLOGICAL DIAGNOSIS OF IM OF BE
The presence of goblet cells in the CLM is the pathological diagnosis of IM in BE. However, some non-goblet columnar epithelial cells can mimic goblet cells and impair the diagnosis of IM. Panarelli et al. (77)delineated the characteristics of these cells and their differences from goblet cells. Damaged foveolar epithelial cells containing a copious amount of cytoplasmic mucin can mimic the globular structure of goblet cells and are therefore known as "pseudo-goblet cells," which can be distinguished from goblet cells due to their pink cytoplasm. A foveal cell that produces enough acid mucin to cause the blue discoloration of the cytoplasm can mimic the blue tone of goblet cells and become a "columnar blue cell." These cells are cylindrical and can be distinguished from goblet cells due to their difference in shape. The multilayered epithelium is composed of immature-appearing squamoid cells and superficial clusters of columnar cells containing acid mucin. The multilayered epithelium can also simulate goblet cells. Furthermore, all three cell types simulating goblet cells continually fill the surface epithelium diffusely. In contrast, goblet cells are typically individually dispersed in the foveal epithelium background, which is a distinctive feature.
Immunohistochemical staining is commonplace nowadays. Alkaline blue, high iron diamine, Alcian blue, periodic acid Schiff (PAS), CDX2, MUC 2, Heppar1, etc. can all stain goblet cells. However, numerous studies have shown that these types of stains can also stain non-goblet cells, entailing a decrease in specificity (78)(79)(80)(81)(82)(83)(84)(85)(86). Although the expression of some immuno-labels in non-goblet columnar epithelium may suggest that IM is more likely to be found elsewhere, it is still not possible to identify goblet cells and non-goblet cells. As a result, the relationship between the staining characteristics of non-goblet epithelium and the risk of dysplasia and/or cancer remains unclear as of yet (78).
Besides, Maskacci et al. (87) demonstrated the combined use of descriptive endoscopy, the suspected esophageal metaplasia range of the Prague standard, and diagnostic charts all conveniently improved the consistency of the CLE interpretation of the esophageal biopsy, which resulted in improved consistency in the diagnosis of IM.

THE MONITORING AND TREATMENT OF BE WITH OR WITHOUT IM
Although there is a difference in the need for IM in BE diagnosis, only some of the guidelines are specified in the relationship between BE monitoring and treatment strategies and IM ( Table 2). To optimize the detection rate of IM, ACG (24) suggested that patients who were suspected of having BE (but lacked IM) should undergo a repeat endoscopy within 1 to 2 years, patients with suspected BE should have at least eight random biopsies, patients having short (1-2 cm) segments of suspected BE in which eight biopsies may be unfeasible should have at least four biopsies per cm of circumferential BE, and one biopsy per cm in tongues with BE. BSG (22) suggested that surveillance should consider the presence of IM and the length of BE. For BE, without IM or dysplasia and BE shorter than 3 cm, repeated endoscopic biopsies are recommended to confirm the diagnosis, and if no IM is confirmed repeatedly, de-surveillance is indicated. For patients with BE shorter than 3 cm and accompanied with IM, endoscopic monitoring should be performed every 3-5 years. For patients with BE ≥ 3 cm, endoscopy should be performed every 2-3 years.
The treatment of BE mainly includes drug therapy, anti-reflux surgery, esophagectomy, endoscopic treatment, etc. The main advantage of drug treatment and anti-reflux surgery is that they can alleviate the symptoms of reflux. For simple BE, with or without IM, drug therapy is recommended (23,24), mainly to bring reflux symptoms under control. If the reflux symptoms are severe and the curative effect of drugs is lacking, anti-reflux surgery can also be considered. Routine endoscopic treatment of BE without dysplasia is not recommended unless the length of BE exceeds 10 cm (26) or is accompanied by nodules (24).
Some breakthroughs in the drug therapy of BE have been made. Initial studies suggest that proton pump inhibitors (PPIs) have a protective effect on the malignant progression of BE (87)(88)(89). However, a case-control study accompanied with the followup of 9883 patients with BE for up to 10 years showed that the use of PPIs increased the risk of developing EAC or HGD (90). But, a recent large-scale randomized controlled trial of patients with BE showed that high doses of PPIs protect against a composite endpoint of all-cause mortality, EAC, and HGD (34). The study also showed a coupled treatment of PPIs and Aspirin was more effective than PPIs alone, and high doses of PPI (40 mg twice daily) combined with aspirin were the best combination. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) have been reported to reduce the risk of malignant progression of BE, and studies have shown that aspirin is more effective than nonsteroidal anti-inflammatory drugs in reducing the risk of malignant progression of BE (69). Recently, Huo et al. (91) provided a theoretical basis for this. Their work showed that aspirin exerts a unique anti-BE protective effect by acting on IKKb, inhibiting the action of the nuclear factor-k-gene binding (NF-kB) pathway and inhibiting the expression of CDX2. Also, some auxiliary drugs that are often used in the treatment of gastroesophageal reflux disease (GERD), including medicines that promote gastric motility, neutralize bile acids, and inhibit transient relaxation of the lower esophageal sphincter, which reduces the progression of GERD to BE at varying degrees (92)(93)(94)(95)(96) (Table 4).
Surgery is still the primary method employed in anti-reflux treatment. Furthermore, laparoscopy gave rise to numerous surgical techniques. The surgical methods mainly include gastric fundus folding, magnetic sphincter augmentation (MSA), electrical stimulation of the lower esophageal sphincter (LES), and bariatric surgery. These procedures all carry merits and shortcomings, but the difficulty of the operation and related complications remain a matter of concern (97-104) ( Table 5).
Generally, surgery or endoscopic treatment is only indicated in BE that is more than 10 cm in length or is accompanied by nodules, intraepithelial neoplasia, or cancer (24,26).  Esophagectomy can achieve complete eradication, but the quality of life is seriously impaired, and its mortality rate is high. There are many kinds of endoscopic therapy, including endoscopic mucosal resection (EMR), endoscopic submucosal dissection (ESD), radiofrequency ablation (RFA), argon plasma coagulation (APC), contact cryo-balloon focal ablation system (CbFAS), and so on ( Table 7). Compared with other methods, endoscopic treatment possesses the merits of high safety, conserving the normal function of the esophagus and eradicating lesions simultaneously. Patients suffering from esophageal intramucosal cancer who have undergone EMR and esophagectomy had a similar longterm recurrence rate and mortality (118). RFA is a mature endoscopic technique for the eradication of IM with dysplasia (116). EMR and RFA are usually combined in the treatment of early tumors of BE. According to a recent study (113), the focal EMR followed by RFA has a similar efficiency level as EMR but is safer than EMR in patients with HGD/EAC. However, due to the decreased access to specimens, the uncertainty of the damage to the anatomical layer, and high costs, RFA is not commonplace in Asia. For larger lesions, EMR can only be segmented and resected multiple times. The Japanese view is that the single resection of ESD is the preferred choice regardless of the size of the lesion due to the blind resection line in patients suffering from adenocarcinoma (119). However, due to the long-term learning  curve and the high rate of adverse events, the use of ESD in the West is still limited. A study has shown that the complete remission rate from neoplasia of ESD is the same as that of EMR at 3 months. ESD can potentially cause severe adverse events (two cases of perforation) because of its long operation time (114). However, according to the results of a multicenter study from the West, ESD is effective and safe and can achieve a good level of proficiency after approximately 30 operations (120). A recent meta-analysis also shows its effectiveness and safety (115). The remission rate of APC in the treatment of BE ranged from 64.9%-94.7%. Like previous endoscopic treatments, esophageal stricture was still the most commonly observed complication (121). To prevent postoperative esophageal stricture, hybrid APC was developed based on APC, that is, submucosal fluid injection before APC treatment to reduce the depth of coagulation which in turn reduces the incidence of esophageal stricture (122). In vitro animal experiments showed that the coagulation depth of hybrid APC was shallower than that of conventional APC (123). In the esophageal wall level analysis, the two methods were consistent in the injury of the epithelium, lamina propria, and muscularis mucosae. The number of submucosal injury cases by conventional APC was more significant, and only conventional APC damaged the proper muscle layer. In humans, it was found that the common observation remission rate of hybrid APC for BE was 96%, the pathological observation remission rate was 78%, and the postoperative stenosis rate was only at 2% (122).
Recently, CbFAS has been developed for esophageal mucosal ablation, which freezes the target mucosa to -85 degrees Celsius to achieve ablation of the BE mucosa (124). According to a recent prospective trial (117), the overall 1-year complete eradication of all dysplasia (CE-D) and complete eradication of intestinal metaplasia rates of BE patients with neoplasia was 95% and 88%, respectively. However, the CE-D rate of ultra-long BE, with length ≥ 8 cm, was only at 67%. The main complications were esophageal stricture (9.7%) and upper gastrointestinal bleeding without treatment (2.4%). Based on the current data, CbFAS can be used in the primary or emergency treatment of BE-related tumors and IM, but there is a lack of large-scale clinical research data to evaluate this technique.
This review summarizes BE and IM from the aspects of the definition, endoscopic recognition, pathology, diagnosis, and treatment. It is believed that there will be more detection and treatment in the future.

AUTHOR CONTRIBUTIONS
LZ and BS drafted this review. XZ and QW assisted in table making and sketching. SL and GL edited the grammar, text format, and framework, etc. TL put forward some essential suggestions on the content of the article. ML provided the direction and ideas of writing and made repeated revisions and guidance in the whole process of writing. All authors contributed to the article and approved the submitted version.