The purpose of the present study was to estimate the prevalence of MGD in Japan, using diagnostic criteria that would permit direct comparison between our results and data from a previous study using a Caucasian population.
3 This population-based study was conducted in Spain,
3 and found the prevalences of symptomatic and total MGD in Spanish Caucasians were 8.6% and 30.5%, respectively. In contrast, the present study estimated the prevalences of symptomatic and total MGD in Tokyo as 11.2% and 74.5%, respectively, which suggests that the prevalence of MGD is higher in Japan. However, in the present study, we only included participants aged 50 years or older because the participants were scheduled for cataract surgery and few patients less than 50 years old were expected. However, the Spanish study analyzed participants aged 40 years and older. From the Spanish data,
3 the prevalences of symptomatic and total MGD for subjects aged 50 years and older were calculated as 10.9% and 32.3%, respectively. Thus, the prevalence of symptomatic MGD was similar in Japan and Spain; however, asymptomatic MGD was much higher in Japan than in Spain. Consequently, total MGD prevalence was also 2.3 times higher in Japan. From these results, it could be predicted that the prevalence of MGD is higher in Asians than Caucasians. This prediction is consistent with findings from previous studies, in which the prevalence of MGD in Asian countries was higher than that of Western countries.
2–8 However, various diagnostic criteria for MGD were used in these studies; hence, their data cannot be compared directly. Therefore, we sought to obtain data that could be compared to that of an existing study, by utilizing identical diagnostic criteria; therefore, the higher prevalence of MGD in Asians was more convincingly demonstrated by the present study. The primary underlying mechanism for the development of MGD is age-dependent hyperkeratinization of the meibomian glands.
12 Thus, it is likely that racial differences in MGD prevalence are related to the differences in the composition of meibomian oil, and the oxidation of these secretions. This hypothesis needs further investigation in future studies.
There are some important points that need to be considered regarding our comparisons between the present study and the Spanish study. First, these studies were conducted in different settings: our study involved participants from an urban population (Tokyo), in contrast to the rural population of O Salnés in Spain. It is possible that local climates or sociodemographic factors had some impact on the prevalence of MGD. Second, many diagnostic markers for MGD are subjective, which could contribute to differences between populations when comparing studies conducted by different investigators. Hence, the slit-lamp diagnostic criteria required standardization, to allow more accurate comparisons between studies.
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In the present study, we found that the prevalence of total MGD increased with increasing participant age. This finding is also consistent with those of the Spanish study.
3 However, the prevalence of symptomatic MGD was similar for all age ranges in our Japanese participants, which was not supported by their findings. One possible explanation for this inconsistency is that MGD may have developed at a younger age in Japanese participants, therefore an age-dependent increase was not observed in participants aged 50 years or older. This reasoning is supported by our finding that the prevalence of both symptomatic and total MGD for the younger age ranges (50–59 and 60–69 years) was higher in our study, compared to the Spanish study (
Table 3). Another possible explanation for this inconsistency is that the symptom questionnaire we used was originally designed for diagnosing dry eye, and therefore not optimized for diagnosing MGD, especially in Asians. However, another population-based study in Singapore also found no differences in the prevalence of MGD between different age groups.
8 The inconsistency between the findings in this Asian population and our Japanese study group would likely be due to differences in the diagnostic criteria; this highlights the importance of applying consistent diagnostic criteria when comparing disease prevalence between different populations.
In the present study, the prevalence of total MGD and symptomatic MGD did not differ significantly between men and women. For both of the previously mentioned population-based studies in Spain and Singapore, an increased prevalence of MGD was identified for men. Further population-based studies are required to determine the cause of this inconsistency for our Japanese study group.
Intriguingly, there was a large discrepancy between the prevalence of symptomatic and asymptomatic MGD. One possible explanation for this is that differences in symptoms may reflect differences in MGD severity. However, our comparison of ocular parameters between symptomatic and asymptomatic MGD groups found no significant differences between these groups (
Table 3). Alternatively, this discrepancy could reflect differences in the rates of dry eye. There was a significantly higher percentage of cases diagnosed with dry eye in symptomatic MGD group (80.7%), compared to the asymptomatic group (32.2%). Thus, our findings suggest that cases with MGD likely develop symptoms when dry eye occurs.
As expected, the total MGD group presented with higher fluorescein score, shorter BUT, and higher meibo-score, compared to the non-MGD group. Higher fluorescein scores and shorter BUTs were likely due to reduced expression of meibum, and subsequently an increased rate of tear evaporation. Notably, meibomian gland loss was increased in the MGD group; this finding suggests that meibomian glands are lost in the process of MGD deterioration.
In addition to these implications for MGD pathogenesis, we observed that a greater proportion of participants diagnosed with dry eye also suffered from MGD (150 of 193 cases, 77.7%), compared to fewer MGD cases that were also diagnosed with dry eye (150 of 380 cases, 39.5%). This result suggests that MGD is potentially a major contributor to dry eye, for patients of 50 years and older. However, this dry eye was not evident in all cases of MGD; the severity of MGD varies, and subjects with more severe MGD are likely more prone to developing dry eye.
There are several limitations to this study. First, this was a clinical study and was therefore subject to selection biases; for example, all participants were scheduled for cataract surgery and hence may not represent the general population. Moreover, some participants may have come to our clinic with symptoms caused by ocular surface disorders that were not related to vision, and eventually scheduled for cataract surgery, which would result in an overestimation of MGD prevalence. Second, the diagnostic criteria seemed too loose, and therefore may need to be adjusted to improve diagnostic specificity and provide more accurate estimation of the population prevalence of MGD in Japan. Hence, the prevalence of MGD determined in this study (74.5%) would likely differ if stricter diagnostic criteria were used. Third, the symptom questionnaire used to assess whether patients symptomatic or asymptomatic was not actually designed for MGD, but for dry eye. A more appropriate questionnaire would likely provide a more accurate determination of symptomatic MGD prevalence. Fourth, the slit-lamp diagnostic criteria were subjective, and require standardization, to allow more accurate comparisons between studies.
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In conclusion, based on our results it is highly likely that the prevalence of MGD is much higher in the Japanese population than in Spain. These findings will ideally be further supported by future population-based studies in other Caucasian and Asian populations. Furthermore, standardized diagnostic criteria will allow researchers to more accurately compare MGD prevalence between different populations, therefore providing more insight into the risk factors for its pathogenesis.