The key finding of our study was that duodenal eosinophilia was independently associated with symptomatic erosive GERD and the presence of co-morbidities but not with undifferentiated functional dyspepsia. The relationship between symptomatic GERD and increased eosinophils in the duodenal mucosa was evident on two measures; the rate of duodenal eosinophilia (as defined by >15 counts per HPF) and the absolute duodenal eosinophil counts. In contrast there was no detectable relationship between functional dyspepsia and eosinophils in the duodenal mucosa based on either of the two measures. In this context, it should be noted that normal ranges for duodenal eosinophil counts among healthy adults have not been clearly established. The cut off value of eosinophils per HPF used to define duodenal eosinophilia in previous publications has been variable, ranging from 15 to 63 per HPF4,6,8,11. The arbitrary choice of 15 per HPF in our study as proposed by Chaudhari et al,11 was based on the premise that the degree of eosinophilia in conditions such as functional dyspepsia and GERD is likely to be subtle and a relatively lower cut-off would have greater sensitivity in detecting a positive signal. A number of studies have used a cut-off value of >22 counts per 5 HPF to define eosinophilia3,4,6. When we reanalysed our data using a cut-off value of > 22 counts per 5 HPF, the association between duodenal eosinophilia and concomitant co-morbidities was preserved but the associations with symptomatic erosive GERD, ethnicity and age were no longer detectable (data not shown).
The failure to find a relationship between functional dyspepsia and duodenal eosinophilic infiltration irrespective of whether it was measured in terms of absolute counts or rate of pre-defined eosinophilia, is concordant with the findings of some studies8-10 but not with others3-5. We are cognisant however of a limitation of our retrospective study that restricts the conclusions that can be drawn with regard to the relationship between duodenal eosinophilia and functional dyspepsia. This relates to the fact that while the diagnosis of functional dyspepsia per se was determined with a high level of confidence, the retrospective nature of the study precluded the precision needed to divide the group into the subtypes of epigastric pain syndrome and post-prandial distress syndrome respectively as prescribed by the Rome 4 classification12. In previous studies, duodenal eosinophilia was most frequently associated with the subtypes of patients with post-prandial distress and early satiety although it has to be noted that even then the association with these two sets of symptoms were not consistent.6-9 It may be that the number of subjects with post-prandial distress syndrome in our sample was small and our study was therefore not sufficiently powered to detect an association. Furthermore, in at least one other study, functional dyspepsia was not associated with total duodenal eosinophil counts but was positively associated with eosinophilic degranulation, the latter being a marker of eosinophil activation8. Hence the absence of an association between functional dyspepsia and duodenal eosinophil counts in our study does not necessarily negate the role of duodenal eosinophils in the pathogenesis of functional dyspepsia.
The finding in our study of an independent association between symptomatic erosive GERD and elevated duodenal eosinophil counts is intriguing. The diagnosis of symptomatic GERD in our study was relatively robust as it was made on the basis of unequivocal endoscopic findings and compatible symptoms. This observation resonates with the findings of a recent study that duodenal eosinophilia in patients with functional dyspepsia predicted the development of GERD 10 years later.14 The authors of that study postulate that subsets of GERD and functional dyspepsia may be part of the same disease spectrum and that duodenal eosinophilia could well be the link. Our finding lends credence to this hypothesis. Distinguishing with precision between the subtypes of functional dyspepsia and GERD on the basis of the history even with validated questionnaires can be challenging. It is quite conceivable that there is an overlap between patients with symptomatic erosive GERD in our study and subsets of patients with functional dyspepsia.
Assigning subjects to the diagnostic groups in our study was predicated on the ability to make the diagnosis with a high degree of certainty from the available information. A substantial number of patients in our cohort would have had non-erosive GERD but we did not assign these subjects to a defined diagnostic group because of difficulties in distinguishing true non-erosive GERD from functional oesophageal disorders in the absence of routine ambulatory pH or impedance testing.
The independent association observed between the presence of co-morbidities and duodenal eosinophilia in our study is perhaps not surprising as there already exists a body of evidence linking systemic illness, abnormal intestinal permeability and impaired intestinal barrier function.15 The definition of significant co-morbidities was necessarily broad and included a multitude of illnesses as we wanted to investigate the relative impact of these conditions on duodenal eosinophil counts in comparison to the putative effect of functional dyspepsia.
Another notable observation of our study was that Chinese ethnicity was a more significant predictor of duodenal eosinophilia than Indian or Malay ethnicity. We are unaware of any other study that detected ethnic differences in duodenal mucosal eosinophil counts. The ethnic composition of our sample is not representative of that of Malaysia as a whole and is more a reflection of the local demography, health care seeking behaviour and referral patterns. This does not however invalidate our observations of the observed association between ethnicity and duodenal eosinophilia. The quantifying of duodenal eosinophils on histological examination does not require sophisticated technology and has potential as a widely available marker of subsets of functional dyspepsia. However, our observation of the possible influence of co-morbidities, ethnicity and even age on duodenal eosinophil counts suggests that the utility of duodenal eosinophilia alone as a diagnostic biomarker of functional dyspepsia is likely to be limited.
The fact that female gender and symptoms of anxiety or depression independently predicted the diagnosis of functional dyspepsia is consistent with the general experience1 and suggests that the population of patients with functional dyspepsia attending our institution is unlikely to be substantially different from that of other populations.
An inherent limitation of a retrospective observational study such as ours is the absence of matched healthy controls. We mitigated this weakness in two ways. Firstly, by using the subset of patients with IBS as a compromise control group based on a previous report that duodenal eosinophilia was not a feature of IBS13 and secondly by using logistic regression analysis to adjust as far as was possible for confounding factors. Another limitation relates to the sample size that is particularly relevant to the gall stone disease group that may not have been sufficiently powered to detect an association with duodenal eosinophilia.