Role of IgE-mediated hypersensitivity in otitis media with effusion: pathophysiologic considerations

Allergy in pathogenesis of Eustachian Tube Dysfunction

Eustachian tube dysfunction (ETD) is a condition where the Eustachian tube (ET) fails to function normally, resulting in symptoms such as aural fullness, tinnitus, autophony, and hearing loss. ETD can further lead to middle ear diseases such as otitis media effusion and adhesive otitis media, which is becoming more common in the field of otology. Although the pathogenesis of ETD remains unclear, recent animal studies and clinical experiments have found allergic reactions and allergic diseases are closely related to the occurrence of ETD. As the mucosa of the ET is continuous with that of the nasopharynx and tympanic cavity, it is reasonable to assume that the immunological basis of the ET itself is similar to that of respiratory allergic diseases. However, due to the special anatomical location and complex pathogenesis of the ET, there is still no unified diagnostic gold standard. Additionally, there is an ongoing debate regarding whether ETD can be classified as a distinct disease or even an allergic disease. Furthermore, the effectiveness of anti-allergic therapy in patients with ETD is yet to be fully understood. Therefore, this review elaborates on the possible mechanisms of allergic reactions in the occurrence and development of ETD, and explores the potential role of anti-allergic therapy in managing this condition, in order to provide new insights into the pathogenesis and prevention of ETD.

Correlation between Serum Interleukin-17 level and Serum Reactive Oxygen Species levels among Children experiencing Otitis Media with Effusion

Introduction  The detection of inflammatory mediators in the serum of children with have otitis media with effusion (OME) and their correspondence with clinical considerations may enable the use of a modern nonsurgical curative treatment for OME.

Objective  To determine the relation between interleukin-17 (IL-17) serum level and reactive oxygen species (ROS) serum levels in children suffering from OME and to disclose if any variation occurs in the level of IL-17 Will affect the ROS and antioxidant equilibrium in the serum, which indicates the entire body's reaction to OME.

Methods  The present study was a case-control study. A total of 24 children experienced OME, and 24 healthy controls were recruited.

All participants in the study were subjected to a systematic clinical investigation including otoscopic, audiometric, and tympanometric examination. Also, venous blood samples were collected from all children to determine the levels of IL-17 and ROS.

Results  The mean ± standard deviation (SD) age ranges of the patients and the control group were 6.8 ± 2.7 and 6.2 ± 3.4 years, respectively. A stylistically significant difference in the levels of serum nitric oxide (NO), catalase (CT), myeloperoxidase (MPO), and malondialdehyde (MDA) ( p  < 0.05) was detected between OME and control patients. No significant difference was found in serum levels of superoxide dismutase (SOD) and glutathione peroxidase (GPX) between OME and control patients. The serum levels of MDA, NO, and MPO positively correlated with the serum levels of IL-17 in OME patients.

Conclusion  In the present study, there is a reasonable role of the IL-17 pathway in OME pathogenesis through an increase in ROS levels.

Relationship of T lymphocytes, cytokines, immunoglobulin E and nitric oxide with otitis media with effusion in children and their clinical significances

OBJECTIVE

To investigate the relations of T lymphocytes, cytokines, immunoglobulin E, and nitric oxide with otitis media with effusion (OME) in children and their clinical significances.

METHODS

Fifty children with OME treated in our hospital were enrolled in the study (observation group). Fifty healthy children were selected as control. The percentages of CD4+ and CD8+ T lymphocyte and CD4+/CD8+ ratio in peripheral blood, and the levels of cytokine (IL)-2, IL-4, IL-6, immunoglobulin E (IgE) and nitric oxide (NO) in peripheral blood and middle ear effusion (MEE) in both groups were detected. The correlations of these indexes with OME were analyzed.

RESULTS

The percentage of peripheral blood CD4+ and CD8+ levels, CD4+/CD8 ratio, IgE, and NO levels in the observation group were significantly higher than those in the control group (P < 0.01). In the observation group, the IL-2 and IL-6 levels, and IgE and NO levels in the MEE were significantly higher than those in peripheral blood (P < 0.01). In addition, in the observation group, the MEE IL-2 and IL-6 levels were positively correlated with peripheral blood CD4+/CD8+ ratio, respectively r = 0.366, P = 0.009; r = 0.334, P = 0.018.

CONCLUSIONS

The levels of peripheral blood CD4+ and CD8+ lymphocytes and MEE IL-2, IL-6, IgE, and NO levels are increased in children with OME. These indexes have provided significant clues for the diagnosis of OME in children.

Histamine Applied Topically to the Nasal Mucosa Increases the Transmucosal Nitrous Oxide Exchange for the Middle Ear

OBJECTIVE

Determine if the middle ear transmucosal nitrous oxide (N₂O) exchange rate is affected by nasal inflammation caused by topical application of histamine.

METHODS

In a randomized, double-blind, crossover study, 20 adults were challenged intranasally with histamine (5 mg) and placebo on separate occasions. At each session, the subjects were fitted with a non-rebreathing mask and breathed room air for 20 minutes, 50% N₂O:50% O₂ for 20 minutes, and 100% O₂ for 10 minutes. Throughout, heart rate, blood pressure, and blood O₂ saturation were monitored, and bilateral middle ear pressure was recorded by tympanometry every minute. The primary outcome measure was the slope of the middle ear pressure-time function for the 50% N₂O:50% O₂ breathing period, which is a measure of the transmucosal N₂O exchange-constant. The effects of challenge substance, session, and period on the measured vital signs and of treatment, session, ear disease history, and test ear on the pressure-time slopes were evaluated using repeated measures ANOVAs.

RESULTS

The post-challenge total symptom score and the slope of the middle ear pressure-time function were greater after histamine when compared to placebo challenge. Of the signs, only heart rate was affected, responding to challenge substance and study period.

CONCLUSION

The transmucosal N₂O exchange rate for the middle ear is increased during inflammation caused by nasal histamine exposure.

Diseases of the middle ear in childhood

Middle ear diseases in childhood play an important role in daily ENT practice due to their high incidence. Some of these like acute otitis media or otitis media with effusion have been studied extensively within the last decades. In this article, we present a selection of important childhood middle ear diseases and discuss the actual literature concerning their treatment, management of complications and outcome. Another main topic of this paper deals with the possibilities of surgical hearing rehabilitation in childhood. The bone-anchored hearing aid BAHA(®) and the active partially implantable device Vibrant Soundbridge(®) could successfully be applied for children. In this manuscript, we discuss the actual literature concerning clinical outcomes of these implantable hearing aids.

Otitis media with effusion: an effort to understand and clarify the uncertainties

Otitis media with effusion--defined as the accumulation of middle-ear effusion behind an intact tympanic membrane without signs or symptoms of acute infection--is one of the most common causes of hearing loss in children in developed countries, potentially leading to language deficits. Although treatment of chronic or relapsing otitis media with effusion is considered imperative, none of the preventative or nonsurgical management measures currently available have proven effective. Tympanostomy tube placement remains the recommended treatment option for high-risk children or for cases of unresponsive otitis media with effusion. This can be attributed to the uncertainties surrounding its pathogenesis. Multiple factors and several possible pathogenetic models have been proposed to explain the production and persistence of middle-ear effusion; only a few of them are supported by sufficient evidence. In this review, the authors will present current knowledge on the pathogenesis, consequences, diagnosis and management of otitis media with effusion. An effort will be made to clarify those aspects sufficiently supported by evidence-based studies, and to underline those that remain unfounded.

Differential diagnosis in allergy

The otolaryngic allergist must be able to distinguish between common nonallergic diagnoses that present very similarly to allergic conditions. This article describes a few of the vast myriad of conditions that must be ruled out before a diagnosis of allergy may be made. After reading this article clinicians will be able to identify various conditions, which will enhance their ability to appropriately make correct decisions for prompt and efficient management of their patients with allergic or nonallergic diseases of the head and neck.

Atopy: pediatric ENT manifestations in children

OBJECTIVE

The aim of the present study is to discuss the basics of atopy in children in relationship to the principal ENT allergic disorders such as allergic rhinitis, rhinosinusitis and their impact on lower airways, allergic otitis media, and oral cavity focusing on their natural history.

METHODS

An updated and exhaustive review of principal literature on these topics is performed, underlining the constant but growing interest evoked by these disorders most of all the possible sequelae or complications. Considering the different districts which can be selectively or simultaneously affected by the allergic sensitisation, diagnosis can be a really hard task; in this paper, we tried to draw an integrated diagnostic approach to atopic children and some guidelines for a correct therapeutic approach.

CONCLUSIONS

Atopic disorders could expose young patients to years of chronic diseases that interferes with their development and with many important aspects of their lives. For these reasons, and considering the high social and medical costs of this disease, it is extremely important to adequately treat allergic pathologies from the early phases of its natural history. Moreover, we cannot forget that an appropriate therapy of allergic pathologies should not be only able to decrease symptoms but, it should also be able to improve patients health related quality of life.

Immune modulatory oligonucleotides in the prevention and treatment of allergen-induced eustachian tube dysfunction in the animal model

This article reviews the current literature investigating the applications and success of immune modulatory oligonucleotides as immunotherapy to treat and prevent allergen-induced eustachian tube dysfunction in animal models. Synthetic DNA-based immunotherapy agents composed of unmethylated cytosine-guanine dinucleotides (CpG ODNs) that bind to Toll-like receptors have been found to have tremendous potential as therapeutic agents and adjuvants. CpG ODNs can induce a shift in the cytokine profile and immune response that favors the T-helper type 1 pathway and suppresses the T-helper type 2 pathway. This makes CpG ODNs promising candidates for treating allergic diseases. Current CpG ODN studies have demonstrated prevention and treatment of acute allergen inflammation of the eustachian tube in an animal model of otitis media. Immune modulatory oligonucleotides in immunotherapy, administered systemically or topically, have been shown to be safe and effective in the animal model.

Clinical and demographic risk factors associated with chronic suppurative otitis media

BACKGROUND

The incidence of chronic suppurative otitis media (CSOM) is high worldwide but increasing occurrence of complications appear peculiar to West Africa. However, knowledge of associated risk factors is sparse, we report the sociodemographic risk factors of CSOM with the aim of control of the disease and complications; and possible preventive strategies.

METHOD

This is a survey of children with CSOM in five sites spread in two suburban cities in two states in Nigeria. Questionnaire was administered on the informants followed by examination of the children.

RESULT

Of the 189 children, 114 (60%) had developed ear suppuration before 6 months of age, the number of attacks within the previous 18 months ranged between 2-12 with average of 7. Sociodemographic risk factors included low socioeconomic class in 153 (81%), 136 (72%) live in congested houses with more than 10 people and 79 (42%) belonged to families with more than 5 children. Indoor-cooking and infant daycare attendance were 117 (62%) while supine bottlefeeding was 115 (61%) and 34 (18%) of subjects had smoking father. The clinical risk factors were upper respiratory tract infection (URI) 85 (45%), allergy 53 (28%), adenoid 54 (28%) and malnutrition 65 (34%). The univariate analysis revealed that low social class (OR=7.33, CI=4.18-12.83, P=0.0001), malnutrition (OR=3.57, CI=1.88-6.76, P=0.00001), bottlefeeding (OR=2.93, CI=1.63-5.28, P=0.0001), indoor-cooking (OR=1.35, CI=0.88-2.10, P=0.161) and high number of people in a household (OR=0.59, CI=0.34-0.98, P=0.04) are significant in development of OM; while multivariate logistic regression analysis showed malnutrition (OR=3.48, CI=1.633-7.425, P=0.001), low social status (OR=7.74, CI=4.15-14.43, P=0.0001) and indoor-cooking (OR=2.34, CI=1.18-4.66, P=0.014), second table. Parental smoking, daycare attendance, allergy, adenoiditis/adenoidal hypertrophy and upper respiratory tract infection were not found significant.

CONCLUSION

Low socioeconomic class, malnutrition, congestion from high number of children in the household and bottlefeeding constitute significant risk factor. The early onset of disease (<6 months) may suggest a prenatal predisposition. We need further research for the understanding of the biologic effect of these factors while this remains a database for prevention and control of disease.

The role of allergic rhinitis in the development of otitis media with effusion: effect on eustachian tube function

PURPOSE

The pathogenesis of otitis media with effusion (OME) is considered multifactorial, with viral upper respiratory tract infection and eustachian tube dysfunction. Allergy may be related to the pathogenesis of OME or to another etiological factor. We investigated the role of allergic rhinitis (AR) in children with OME and evaluated eustachian tube function in patients with AR.

MATERIALS AND METHODS

We prospectively analyzed the prevalence of AR, serum eosinophil count, and serum total IgE concentrations in 123 children with OME and in 141 controls. IgE concentration in middle ear effusion was compared in children with OME with and without AR, and eustachian tube function after a nasal provocation test was compared between patients with AR and controls.

RESULTS

The prevalence of AR in children with OME (28.4%) and control subjects (24.1%) did not differ significantly. These 2 groups also showed no differences in total eosinophil count and serum and middle ear effusion IgE concentration. Abnormalities in eustachian tube function were the same in patients with AR and controls.

CONCLUSIONS

Allergic rhinitis may not be related to the development of OME in children.

Glucocorticosteroids in allergic inflammation: clinical benefits in otitis media with effusion

Allergy and otitis media with effusion are often considered as comorbidities. Evidence points to a role for allergy in the pathogenesis of otitis media with effusion in reports of increased prevalence of allergy in patients with otitis media with effusion and presence of late-phase allergic-response inflammatory mediators and cytokines in the middle-ear effusion of allergic subjects. However, the exact mechanism remains unknown. Many speculations have been made, but well-controlled studies to prove the efficacy of anti-allergic medication in the treatment of otitis media with effusion are lacking. Therefore, we do not recommend the use of glucocorticosteroids in allergic patients with otitis media with effusion.

Total serum IgE and IgE antibodies specific to house dust mite found in two aged-matched cohorts of children with and without otitis media with effusion

OBJECTIVES

To determine whether otitis media with effusion (OME) is associated with elevated serum immunoglobulin E (IgE) and IgE specific for house dust mite.

DESIGN

Forty-seven children who had evidence of bilateral OME, both otoscopically and on tympanometry, on two separate occasions, 3 months apart were admitted for ventilation tubes. Forty-eight children admitted for minor eye surgery who had otoscopically normal ears and no history of middle ear problems were used as controls. Bloods samples were taken under anaesthesia. Total IgE and IgE radioallergosorbent test (RAST) to house dust mite was measured by the Pharmacia Unicap 100 system. The results from the two groups were compared.

SETTING

Birmingham Children's Hospital.

PARTICIPANTS

Children between the ages of 3 and 10. Children with Down's syndrome, cleft lip and palate, ciliary abnormalities, known immunodeficiencies and cardiac abnormalities were excluded.

MAIN OUTCOME MEASURES

Total IgE and RAST to house dust mite. A RAST of >0.35 was taken to be positive.

RESULTS

There was no statistical difference between the control and study groups for the total IgE. Six children from both study and control groups had a raised house dust mite RAST. There was no difference in the levels between either group.

CONCLUSIONS

Our findings indicate that there is no direct relationship between OME and biochemical evidence of allergy, specifically to house dust mite.

The immunoregulatory and allergy-associated cytokines in the aetiology of the otitis media with effusion

Inflammation in the middle ear mucosa, which can be provoked by different primary factors such as bacterial and viral infection, local allergic reactions and reflux, is the crucial event in the pathogenesis of otitis media with effusion (OME). Unresolved acute inflammatory responses or defective immunoregulation of middle inflammation can promote chronic inflammatory processes and stimulate the chronic condition of OME. Cytokines are the central molecular regulators of middle ear inflammation and can switch the acute phase of inflammation in the chronic stage and induce molecular-pathological processes leading to the histopathological changes accompanying OME. In this review we present cytokines identified in otitis media, immunoregulatory [interleukin (IL)-2, IL-10, transforming growth factor-beta]) and allergy associated (IL-4, IL-5, granulocyte-macrophage colony-stimulating factor), as crucial molecular regulators, responsible for chronic inflammation in the middle ear and the chronic condition of OME.

The role of rhinitis in chronic otitis media

OBJECTIVE

We investigated the role of allergic rhinitis in chronic otitis media (otitis media with effusion [OME] and chronic perforation of the tympanic membrane) in São Paulo, Brazil and whether there is any association between these diseases.

STUDY DESIGN AND SETTING

We studied 51 patients followed in the otologic group of the Otorhinolaryngology Division of the University of São Paulo Hospital. The patients were divided into 3 groups: allergic rhinitis, nonallergic rhinitis with eosinophils syndrome (NARES), and patients with types of rhinitis or without rhinitis. We analyzed the age, gender, intensity of the nasal disease, surgical procedure, and surgical results in each group.

RESULTS

We found about 50% of patients presenting with nasal disease and nasal eosinophilia (33.33% of allergic and 15.69% of NARES).

CONCLUSION

Nasal disease has an impact on otologic middle ear disease, considering that the normal nasal mucosa do not have eosinophils.

The link between allergic rhinitis and otitis media

Past studies implicated allergy in the pathogenesis of otitis media. However, definitive proof of the assertion is lacking. Epidemiologic studies, including one recently published by Rylander and Megevand, consistently identify allergy as a risk factor for otitis media. Also, higher than expected frequencies of allergy are reported for patients with otitis media, but it is unclear if this is an epiphenomenon associated with referral bias. This possibility is exemplified by the results of two reviewed papers where Velepic and colleagues reported a 25% prevalence of allergy in otitis media patients while Alles and colleagues reported a prevalence of 89%. Other studies documented biochemical components of allergic inflammation in the effusions recovered from children with otitis media, and Hurst and colleagues published a recent paper continuing on that theme. However, it is not known if a local allergic reaction is required for the production of these components and if they are produced exclusive of other inflammatory products. Three published studies explored mechanisms by which allergy could cause otitis media. Tikkanen and colleagues reexamined at age 10 a group of 56 subjects with previously diagnosed milk allergy and showed that despite decreased reactivity to milk protein a high prevalence of otitis media was retained. This suggests that it is the allergic condition, not the specific allergy to milk that increases otitis media risk. Downs and colleagues exposed the middle ears of rats to histamine and provoked changes in the pressure regulating and clearance functions of the Eustachian tube. Those changes could prolong an otitis media episode, but middle ear histamine is not limited to the allergic condition. Gentile and colleagues challenged the nose of allergic and nonallergic subjects with histamine followed by interleukin-6 or placebo and measured nasal, pulmonary, Eustachian tube and middle ear functions. Interleukin-6 did not change the Eustachian tube or middle ear functions but did cause increased secretions in allergic patients.

Clinical characteristics of so called eosinophilic otitis media

OBJECTIVE

Although "eosinophilic otitis media" is not as uncommon a condition as was previously believed, its cause and pathogenesis are not yet fully understood. The purpose of this study was to describe the clinical characteristics in patients with "eosinophilic otitis media" to clarify its pathogenesis.

METHODS

Seven adult patients with persistent and intractable otitis media with viscous middle ear effusion containing many eosinophils, who were also under treatment for bronchial asthma, were studied. The following examinations were conducted: nasopharyngeal endoscopy, pure-tone audiometry, eustachian tube function test, temporal bone CT scan, blood analysis, bacterial and fungal culture of middle ear effusion, histological study of the middle ear and nasal specimens, and measurement of eosinophilic cationic protein (ECP) in middle ear effusion.

RESULTS

Some patients had persistent perforation with papillomatous granulation tissue arising from the mesotympanic mucosa, and all the patients had nasal polyposis. The pure-tone audiometry showed the mixed-type of hearing loss in all the patients, and the hearing level deteriorated progressively during the course in some patients. The eustachian tube function was not always poor but was patulous in some cases. The most severely diseased areas were in the eustachian tube and mesotympanum by temporal bone CT images. All the seven patients had the high levels of total serum IgE, but the RAST scores were negative in three patients and low grade in three patients. The accumulation of eosinophils was observed in middle ear effusion, middle ear mucosa and nasal polyps, and the eosinophils were highly activated with degranulation. High level of ECP was also recovered from middle ear effusion.

CONCLUSIONS

Active eosinophilic inflammation occurs in the entire respiratory tract, including the middle ear in these patients. From our present investigation, we propose the criteria and clinical characteristics of "eosinophilic otitis media".

Compositional differences between bilateral middle ear effusions in otitis media with effusion: evidence for a different etiology?

The aim of this study was to clarify the site of primary pathology in otitis media with effusion. Effusions were collected from 64 children with bilateral effusions at the time of myringotomy. The rheological properties and biochemical compositions of effusions were measured for 23 pairs of effusions, and the levels of the inflammatory mediators TNF alpha, IL-1beta, and IL-8 were measured in 41 pairs using specific enzyme-linked immunosorbent assays (ELISAs). Measurements from paired ears were compared using analysis of variance (ANOVA) tests and significant differences were found for reduced specific viscosity, mucin content, protein content, and levels of IL-8. The results demonstrate that the two ears have different immunological processes or rates of processes which might explain the significantly different rheological properties of effusions. This suggests that each ear undergoes pathological changes independently and has implications for using the opposite ear as a control in clinical trials.

Suppression of in vivo cell-mediated immunity during experimental influenza A virus infection of adults

A variety of recent evidence documents that otitis media is a frequent complication of upper respiratory tract viral infections. This relationship has been attributed to the interaction of a number of virus-provoked host responses, including eustachian tube dysfunction, changes in nasopharyngeal bacterial flora and suppressed immune function. The present study examined the effect of experimental influenza A virus infection on immune function as assessed by delayed skin test reactivity to candida, tetanus, and diphtheria/tetanus antigens in healthy adults with (n = 12) and without (n = 15) allergic rhinitis. All subjects became infected with the challenge virus as evidenced by viral shedding into nasal secretions and/or a four-fold rise in convalescent serum antibody titers compared to baseline. Intradermal skin tests were placed at baseline and 2, 4, 17, and 24 days after intranasal influenza A inoculation, the reactions were imaged and recorded 48 h after placement, and response areas were calculated by computerized digitization. The average combined areas for the three antigens (+/- S.T.D.) on each of the 5 study days were 1.4 +/- 1.4, 0.7 +/- 0.7, 0.6 +/- 0.6, 1.4 +/- 1.4, and 1.2 +/- 1.2 cm2, respectively. The responses to candida, but not tetanus and diphtheria/tetanus, returned to baseline levels by day 17. Repeated measures ANOVA documented significant effects of study day and antigen, but not allergy status. These results show that experimental influenza A infection suppressed delayed hypersensitivity skin tests in both allergic and non-allergic subjects, and suggest that alterations in immune function may contribute to otitis media.

Role of Allergy in Eustachian Tube Blockage and Otitis Media with Effusion: A Review

Recent studies continue to support a role for allergy in the pathogenesis of otitis media with effusion. Although a variety of mechanisms have been proposed to relate these two disease conditions causally, none has been completely validated by experimental or clinical studies. This review suggests that the observed relationship between allergy and otitis media with effusion is caused by mediators of inflammation and cytokines and colony-stimulating factors released by mucosal mast cells and other inflammatory and epithelial cells in the nose and nasopharynx. These mediators produce blockage of the eustachian tube through a number of mechanisms, which may include local injury or vascular- or neural-mediated changes in the eustachian tube opening pressure and in middle ear perfusion. It is likely that the nasal allergic response in patients predisposes to eustachian tube blockage and, if prolonged, causes changes in gas absorption in the middle ear space. This gas exchange primarily involves nitrogen absorption, which may take several days to develop. This persistent underpressure will then lead to middle ear effusion. Irrespective of the theoretical mechanism, the relationship between allergy and otitis media with effusion will remain controversial until well-controlled clinical studies are conducted documenting that in select populations antiallergy therapy is efficacious in preventing or limiting the duration of otitis media with effusion.

Interferon production by leukocytes in children with otitis media with effusion

We have previously shown that leukocyte cultures of children suffering from recurrent respiratory tract infections produce less interferon (IFN) than those of healthy children. In the present study this tentative marker of recurrent infections was used to study the pathogenetic background of otitis media with effusion (OME). Altogether 57 consecutive children, aged 2-11 years, who came for tympanostomy and/or adenoidectomy were divided into three subgroups: 25 of them had OME and a history of recurrent acute otitis media (rAOM/OME+), 20 had OME without an infectious background (inf-/OME+), and 12 had a history of recurrent upper respiratory infections (inf +/OME-) without OME. All the children were free of acute illness at the time of sampling. Differences between the groups were seen in IFN yields when leukocyte cultures were stimulated with adeno-, rhino-, corona-, respiratory syncytial or influenza A viruses. Leukocytes from inf-/OME+ children produced more IFN than those of the other two groups. Though no sex differences in the IFN responses were seen among rAOM/OME+ and inf +/OME- children, leukocytes from inf-/OME+ girls produced significantly higher amounts of IFN than those of inf-/OME+ boys, or rAOM/OME+ and inf +/OME- children. These differences between clinically different groups of children support the view that the etiology of OME can be heterogeneous.