Reflux esophagitis is an esophageal mucosal injury that occurs secondary to retrograde flux of gastric contents into the esophagus. Clinically, this is referred to as gastroesophageal reflux disease (GERD). Typically, the reflux disease involves the distal 8-10 cm of the esophagus and the gastroesophageal junction. The disease is patchy in distribution.
The American College of Gastroenterology has defined GERD as “chronic symptoms or mucosal damage produced by the abnormal reflux of gastric contents into the esophagus.”  Histologically, this is referred to as “reflux esophagitis,” because it was initially thought to cause an inflammatory (~itis) response in the esophageal mucosa. Later in 1970s, it was noted that reflux esophagitis shows morphologic changes unrelated to the presence of inflammation.
The morphologic features of reflux esophagitis in the distal esophagus are variable and nonspecific, and they include basal cell hyperplasia, elongation of vascular papillae, intercellular edema, presence of intraepithelial eosinophils, intraepithelial lymphocytosis, ballooning degeneration of squamous cells and ulceration/erosions.  Reflux esophagitis also affects the gastroesophageal junctional mucosa, and the histologic features include multilayered epithelium and inflammation of gastric cardiac mucosa (carditis). 
The images below depict histology from a normal esophagus (first image) and that from a patient with reflux esophagitis (second image).
US data vs international data
Gastroesophageal reflux disease (GERD) is a very common disorder. In the late 1970s and 1980s, a US population-based study by Nebel et al  and a Gallup Organization National Survey [5, 6] revealed that 25-40% of healthy adult Americans experienced symptomatic GERD at least once a month. Furthermore, about 14% of adults suffered from heartburn or acid regurgitation on a weekly basis, and 7% noticed it once a day.
Although the reported approximate prevalence of GERD is more in Western countries (1-20%) relative to developing countries (5%), [7, 8] there is an increasing global trend in prevalence of GERD with time.  A Norwegian study reported a weekly increase in gastroesophageal reflux symptoms by 47% between years 1995-97 and 2006-09.  This study reported the average annual incidence of any or severe gastroesophageal reflux symptoms to be 3.07% or 0.23%, respectively, for the 2 time periods. Another cross-sectional study from Netherlands reported that the incidence of reflux esophagitis has doubled over a period of 10 years. 
Racial and ethnic differences in the prevalence of reflux esophagitis are also well reported in the literature. Studies from the US and Europe show that white individuals have higher prevalence of severe grades of esophagitis and Barrett esophagus (a complication of GERD) relative to Hispanics, blacks, and Asians.  A Swedish population-based, cross-sectional study compared the prevalence of gastroesophageal reflux symptoms among English and Swedish populations and reported a higher prevalence of reflux symptoms occurring at least twice weekly in English populations as compared to Swedish populations. 
With regard to sex-related differences, some studies report no sex predilection.  Other studies report a higher frequency of esophagitis in men,  whereas others report a higher frequency in women. 
As noted in adults, there is an increasing trend of GERD in the pediatric population, too. Nelson et al reported an increased incidence of GERD that ranged from 12% to 50% in children aged 0-18 years between the years 2000 and 2005. 
Interestingly, a study out of Taiwan by Chen et al showed a seasonal variation in the incidence of GERD.  The investigators evaluated a total of 76,636 ambulatory care visits for the treatment of GERD between 2001 and 2006 and reported that the incidence of GERD increased in the autumn and winter for each sex, age group, as well as combined groups. They also reported that relative humidity negatively related to monthly GERD incidence for men and the age group of those older than 64 years. 
The acidic nature of the refluxed gastric contents is predominantly responsible for the esophageal mucosal damage and subsequent development of reflux esophagitis. The frequency of acid reflux correlates more with erosive reflux esophagitis than with nonerosive reflux disease (NERD).  This is due to the proteolytic enzyme “pepsin” in the reflux contents, which becomes activated under acidic conditions and leads to disruption of intercellular cell junctions and cellular damage.  Strong acid (pH < 2), however, can cause mucosal damage independent of the presence of pepsin.
The presence of bile in reflux contents is increasingly thought to contribute to reflux disease in a subset of cases, as observed in patients on proton pump inhibitor (PPI) therapy.  Bile refluxate contains bile acids (both conjugated and unconjugated) and trypsin. Animal studies have shown that the conjugated bile acids induce mucosal damage in an acidic environment, and the unconjugated bile acids and trypsin are responsible for mucosal damage at more neutral pH values (pH 5–8).  At an acidic pH, the conjugated bile acids diffuse through the mucosal cells. Subsequently, the detergent effect of bile acids leads to the dissolution of the lipid content of cell membranes.  This causes disruption of the esophageal mucosal barrier, with accumulation of more bile acids in the mucosa. At the molecular level, bile acids stimulate squamous esophageal cells to produce inflammatory mediators, cause oxidative stress, DNA damage, and increased apoptosis. 
Occasional reflux of gastric contents into the esophagus is a universal phenomenon, and the majority of people do not develop reflux disease because of intact antireflux mechanisms, which clear the refluxed contents back into stomach before damage is done. Symptomatic reflux occurs when these antireflux mechanisms become impaired, such as in the cases of weak lower esophageal sphincter (LES) function, impaired esophageal clearance due to esophageal dysmotility or the presence of hiatal hernia, and poor gastric emptying. 
Poor/abnormal LES function
Poor or abnormal LES function is the most common cause of gastroesophageal reflux disease (GERD). This is also the most easily correctable cause and forms the basis of antireflux surgery. The LES function can be assessed manometrically; however, a normal manometric study does not preclude an abnormally functioning LES undergoing transient relaxations and causing reflux disease.
Poor esophageal clearance
Poor esophageal clearance due to motility disorders is common in elderly population or in patients with achalasia, stroke, or collagen vascular disease such as scleroderma. The presence of a large paraesophageal hernia can cause slowed emptying of esophageal contents into stomach due to obstruction and can lead to altered esophageal motility over a time period.
Delayed gastric emptying
Delayed gastric emptying can lead to the spillage of the retained gastric contents of a full stomach back into esophagus, thereby causing reflux disease. Poor gastric emptying could be due to anatomic obstruction of the gastric outlet — as seen in pyloric stenosis — or due to neuromuscular dysfunction — as in gastroparesis. Gastroesophageal reflux caused by impaired LES function can also lead to delayed gastric emptying.
Other risk factors
Obesity is an important risk factor for development of GERD.  Increased abdominal fat causes GERD due to increased intragastric pressure, increased transient LES relaxations, and increased esophageal acid exposure. Other factors such as smoking, alcohol, high dietary fat, or drugs play a minor role in the causation of GERD.  The protective role of Helicobacter pylori infection in the development of GERD remains controversial. [26, 27, 28, 29, 30]
A report by Yang et al alluded to the role of microbiome alterations in the pathogenesis of reflux esophagitis and subsequent progression to Barrett esophagus.  Type I microbiome, which has a predominance of gram-positive bacteria, is associated with healthy esophagus. Esophageal mucosal damage secondary to reflux results in the change of type I to type II microbiome, which is rich in gram-negative bacteria. The gram negative bacterial components such as lipopolysaccharide (LPS), derived from the bacterial outer membrane, can upregulate the gene expression for inflammatory cytokines via activation of toll-like receptors -4 and nuclear factor (NF)-kappa B. The LPS exacerbates reflux esophagitis by relaxing the LES via inducible nitric oxide synthetase and by delaying gastric emptying via cyclooxygenase 2. 
Clinical Features and Imaging
The clinical symptoms of gastroesophageal disease (GERD) can be divided into 2 groups: esophageal and extra-esophageal. The typical presentation of GERD includes esophageal symptoms such as heartburn, acid dyspepsia, regurgitation, and chest pain. The extra-esophageal symptoms are also referred to as “atypical” symptoms of reflux disease and include, but are not limited to, cough, asthma, throat pain, aspiration pneumonia, globus sensation, and hoarseness due to pharyngitis, laryngitis, or sinus problems.
The diagnosis of GERD depends upon the appropriate clinical scenario. Various modalities have been utilized for diagnosis of GERD, and these include esophagogastroduodenoscopy (EGD) (or, upper gastrointestinal [GI] endoscopy) with biopsy, 24-hour pH study, manometry, barium contrast study, and gastric emptying study. However, none of these modalities is the “gold standard” for diagnosis, and each is recommended in specific clinical situations.
According to the diagnostic guidelines established by American College of Gastroenterology (ACG) for GERD, if a patient’s history is typical for uncomplicated GERD, an initial trial of empirical therapy is appropriate, without further investigation. 
EGD allows for the direct visualization of the esophageal mucosal surface and for the obtainment of a mucosal biopsy for pathologic evaluation. The sensitivity of endoscopy for the diagnosis of GERD is low, because 50-70% of persons with GERD have a nonerosive reflux disease.  Therefore, endoscopy is not the first-line study for diagnosis of GERD.
Rather, endoscopy and biopsy are recommended by the ACG to identify complications of GERD, such as Barrett esophagus, dysplasia, or malignancy. Endoscopy is performed especially in older age groups (>50 y) or in patients with a prolonged duration of symptoms (>5 y).  Endoscopy is also performed to confirm the presence of esophagitis, its nature (whether it is due to reflux), and its severity, when the symptoms are less than unequivocal or atypical, and the response to treatment is not adequate, or when patient has an onset of new symptoms, such as dysphagia or hematemesis. A negative endoscopy does not exclude a diagnosis of GERD.
The endoscopic findings in gastroesophageal reflux disease (GERD) range from normal esophageal mucosa to erosions and ulcerations. Based on the presence or absence of mucosal changes seen on endoscopy, GERD is classified into 2 groups: nonerosive reflux disease (NERD) and erosive esophagitis.
NERD is defined as the presence of typical symptoms of GERD in the absence of esophageal mucosal injury on conventional upper gastrointestinal (GI) endoscopy.  The majority of patients with GERD have NERD. Erosive esophagitis is severe reflux esophagitis characterized by mucosal breaks, such as erosions or ulcerations on endoscopy.
There is little correlation between endoscopic and histologic findings in patients with GERD. Genta et al assessed the endoscopic grades of reflux esophagitis and the histopathologic findings in a large cohort of patients and found that only a minority of esophageal mucosal biopsies showed pathologic evidence of “breaks” or ulceration/strictures/Barrett mucosa, as had been reported on endoscopy.  No satisfactory explanation is available as yet for such discrepancies between endoscopic and histologic findings.
The normal histology of esophageal mucosa in patients without reflux esophagitis shows a nonkeratinizing stratified squamous epithelium, lamina propria, and muscularis mucosa. The basal cell layer is 1-3 cell layers thick and occupies about 10-15% of the epithelium. The vascular papillae, which are extensions of the lamina propria, extend less than two thirds of the distance from base to surface. Intraepithelial lymphocytes are also a normal component of the esophageal squamous mucosa.
The histology of reflux esophagitis is nonspecific, as the histologic pattern may also be seen in other pathologic conditions of the esophagus such as in mucosa adjacent to esophageal cancer.  However, there are certain histologic features that are frequently associated with clinical and/or endoscopic gastroesophageal reflux disease (GERD). These features include squamous (basal) cell hyperplasia, elongation of vascular papillae, presence of intraepithelial inflammatory cells, dilated intercellular spaces (intercellular edema), ballooning degeneration of squamous cells (due to accumulation of intracellular plasma proteins), vascular lakes (dilated small blood vessels in superficial lamina propria/vascular papillae), acanthosis, mucosal erosions and ulcerations. See the following images.
Such features are not diagnostic of reflux injury, but in the appropriate clinical and endoscopic setting, they may be consistent with reflux esophagitis.  Increased mitotic activity and decreased surface maturation may be seen in reflux esophagitis and are indicative of regenerative epithelial activity.
Historically, reflux-associated esophageal (squamous) epithelial changes were first described in 1970 by Ismail-Beigi et al.  They reported that the histologic features of basal cell hyperplasia with increased thickness of the basal cell layer to greater than 15%, along with elongation and extension of the vascular papillae into the upper one third of the epithelium, in the absence of inflammation, were seen in 85% of biopsies of patients with reflux symptoms.  These features reflect the repetitive tissue damage and secondary regenerative changes in reflux disease. Subsequent studies confirmed the consistent presence of these histologic changes in clinical setting of reflux disease. 
Intercellular edema (or dilated intercellular spaces) is recognized as an early histologic change that is seen in GERD.  This change has been shown to regress following treatment with proton pump inhibitors (PPIs). 
It appears that no set of histologic features are invariably associated with reflux esophagitis, and features invariably associated with reflux esophagitis may exist but are nonspecific, as these can also be seen in other pathologic conditions of the esophagus.  When Takubo et al studied the presence of the various histologic features mentioned above in esophageal biopsies from 16 normal controls (normal upper esophageal mucosa from autopsies), 69 patients with GERD, 39 patients without GERD but with a suspicion of other pathology, and 49 patients with esophageal carcinoma, they found that the presence of individual changes ranged from 23% to 71% in the GERD group, but no single change was noted in 100% of these patients with GERD. 
The histologic feature of dilated intercellular spaces was seen in 48% of cases, balloon cells in squamous epithelium in 49%, intrapapillary vessel dilation in 71%, papillary elongation in 61%, basal cell hyperplasia in 57%, acanthosis in 41%, intraepithelial eosinophils in 23%, and intraepithelial Langerhans cells in 48%. Of these, only the histologic features of dilated intercellular spaces and papillary elongation were significantly higher in frequency in the GERD group relative to the other groups. 
Intraepithelial inflammation in reflux esophagitis includes lymphocytes, neutrophils and eosinophils. In untreated cases, the inflammation is typically mild and consists of scattered eosinophils. The presence of a large number of eosinophils raises the possibilities of other lesions, such as primary eosinophilic esophagitis, drug reaction, pill-induced esophagitis, collagen vascular disease, or very rarely, parasitic infection. The presence of a few scattered intraepithelial neutrophils may be seen in active erosive esophagitis; however, in the presence of significant intraepithelial neutrophils, a coexisting infectious esophagitis needs to be considered. Similarly, increased intraepithelial lymphocytosis has been observed in reflux esophagitis and in other conditions, such as Crohn disease and lymphocytic esophagitis. 
In patients with hiatal hernia, which is a frequent cause of GERD, endoscopic examination shows an irregular Z line that occurs due to the proximal displacement of the histologic squamocolumnar junction in relation to the anatomic gastroesophageal junction (GEJ).  Histologically, this short segment of columnar mucosa proximal to anatomic GEJ resembles gastric cardiac mucosa.
The origin and histologic features of the true gastric cardia is controversial. Most authors [44, 45, 46, 47] believe that gastric cardiac mucosa is a universal native component of the GEJ mucosa and that it is a narrow band of mucosa that measures 1 to 4 mm in length. GERD leads to an increase in the length of this narrow band of gastric cardiac mucosa. This belief is based on the results of autopsy studies done in fetuses and pediatric and adult age groups.
Against this belief, the studies of Chandrasoma et al showed that the true cardia is composed of oxyntic-type gastric mucosa, and that the GEJ separates the squamous-lined esophagus from the stomach lined by gastric (oxyntic) mucosa. [48, 49] According to these authors, the sheer presence of pure mucous or mixed mucous/oxyntic glands in the GEJ mucosa is a histologic feature of GERD, and this is believed to be a metaplastic process secondary to mucosal damage by recurrent reflux.
Inflammation of the gastric cardiac mucosa (also referred to as carditis) could also be due to gastric H pylori infection or reflux esophagitis. Isolated carditis, in the absence of inflammatory changes elsewhere in the stomach, is also thought to be a sensitive marker for reflux esophagitis. 
Based on histomorphology, the major differential diagnosis of gastroesophageal reflux disease (GERD) includes eosinophilic esophagitis (EoE), lymphocytic esophagitis, and carditis due to gastric H pylori infection. Esophagitis due to other causes such as infectious esophagitis, pill esophagitis, ingestion of corrosive agents, radiochemotherapy, and esophageal involvement in certain systemic diseases can sometimes morphologically resemble GERD. Correlation with a patient’s clinical history and endoscopic findings is essential in such situations for establishing an appropriate diagnosis.
EoE is a chronic inflammatory disorder characterized by eosinophilic infiltration of the esophageal mucosa associated with a history of atopy or allergy. The common presenting symptom is dysphagia. Distinguishing EoE from reflux esophagitis requires a comprehensive evaluation of clinical, endoscopic, and histologic features.
Histologically, intraepithelial eosinophils are useful to differentiate the 2 entities. The presence of more than 15 eosinophils per high power field (hpf) is suggestive of EoE along with the presence of eosinophilic microabscesses, which are small clusters in the superficial portion of squamous mucosa just below the luminal surface. 
LE is a relatively recently described, albeit rare, histologic entity that has been reported to be more common in older women. This condition clinically presents as dysphagia, odynophagia, or an esophageal motility disorder. 
Rubio et al first introduced LE in 2006 as a novel subset of chronic esophagitis.  Histologically, it is characterized by dense intraepithelial lymphocytosis in the peripapillary region along with spongiosis and basal cell hyperplasia of esophageal squamous mucosa. LE shows a histologic overlap with reflux esophagitis. However, in addition to above described features of LE, reflux esophagitis also shows interpapillary intraepithelial lymphocytosis and the presence of intraepithelial eosinophils or neutrophils. These features combined with the clinical and endoscopic presentation help to distinguish these 2 entities.
Carditis due to H pylori infection
Inflammation of the gastric cardiac mucosa (carditis) can occur either due to GERD or gastric H pylori infection. The presence of significant number of plasma cells and reactive lymphoid follicles suggest H pylori carditis/gastritis. Additional biopsies from the stomach corpus and antrum that reveal pangastritis are helpful in differentiating the 2 entities. [3, 53]
After evaluating the prevalence of multilayered epithelium in mucosal biopsies from the gastroesophageal junction in patients with GERD with and without Barrett esophagus, Glickman et al studied have suggested that the presence of a multilayered epithelium in biopsies from the junctional mucosa may be considered as a marker of reflux disease. 
The intraepithelial lymphocytes seen in reflux esophagitis are T lymphocytes, which express CD3, helper/inducer CD4, and cytotoxic-suppressor CD8 immunophenotype.
Reflux esophagitis has been found to be associated with certain molecular changes. The caudal-related homeobox genes CDX1 and CDX2 are intestinal transcription factors that play an important role in intestinal epithelial differentiation. An increased level of CDX2 mRNA has been reported in esophageal squamous tissues from patients with gastroesophageal reflux disease (GERD),  and this is thought to be an early event leading to the development of Barrett esophagus.
There is also increased gene expression of the proinflammatory enzyme cyclooxygenase-2 (Cox-2) in the esophageal squamous mucosa, with increased acid exposure in patients with GERD.  Similarly, expression of the cytokine interleukin-8 (IL-8) is increased in injured esophageal mucosa secondary to reflux; IL-8 expression was found to be significantly reduced secondary to antireflux surgery. 
GERD also induces oxidative DNA damage in esophageal squamous mucosa, and this has been shown to persist despite adequate acid suppressive drug therapy, thereby suggesting a role of bile reflux in its pathogenesis. 
Prognosis and Predictive Factors
The natural course of gastroesophageal reflux disease (GERD) is variable, and treatment typically involves the use of acid suppressor drugs. Surgical intervention is required in patients with hiatal hernia or an incompetent lower esophageal sphincter (LES).
The nonerosive reflux disease is thought to be nonprogressive in its clinical course. Erosive esophagitis, however, can become complicated by the development of ulcers and formation of strictures due to fibrosis. About 10% of patients with symptomatic reflux develop Barrett esophagus, which is a precursor lesion for adenocarcinoma of the esophagus.
Carditis due to Helicobacter pylori infection