 [25].
In our study, the prevalence of EMPD varied greatly by the geographic areas of mainland China. The Northwest China, Southcentral China and Southwest China presented significantly higher prevalence of EMPD than the rest of the areas. Based on current data, it is difficult for us to determine the exact explanation for this finding. However, this finding, to some extent, excluded the influences of socioeconomic level and medical level on the prevalence of EMPD, as the Northwest China and Southwest China are relatively underdeveloped areas in mainland China. In Europe, a clear difference in the incidence of EMPD among geographic areas were also reported, but the cause remained unclear [13]. In addition, no other studies accessing regional difference of EMPD were available to confirm our findings, further studies are needed to explain this regional discrepancy.
Secondly, patients with EMPD showed a male predominance in mainland China, with a male-to-female ratio of 1.7:1 in our study. This was consistent with multicenter studies in Asian population, the reported male-to-female ratios were approximately 3.5:1 in Taiwan of China [24], 3.9:1 in South Korea [26] and 2:1 in Japan [17]. By contrast, a significant female predominance was reported in studies from Western countries based on registry database, the male-to-female ratios ranged from 1:1.6 to 1:3.6 [9,10,11,12,13]. Consistent with these gender discrepancies, Asian studies commonly identify the scrotum and penis as the most frequent sites of involvement [26,27,28,29], rather than vulva, which is the most common site in Western populations [10, 13]. The reasons for this discrepancy remained unclear, there were two possible explanations. First, ethnic disparity might play a role, considering that Asian males were more susceptible to EMPD than Caucasian males in multiracial country [1]. Moreover, conservative attitudes among elderly Asian females that might discourage them from seeking medical treatment for lesions in the genital area was also considered an explanation in a previous study [24].
Thirdly, EMPD were more prevalent in aged patients older than 60, with a peak prevalence at 70–79 years of age in mainland China. This was consistent with previous literatures—that is, EMPD were commonly occurred in older individuals aged 60–80 years [8, 12, 14, 30]. The reason for the aging of EMPD patients was unclear, a more pro-oncogenic microenvironment in aged skin might be a possible explanation [31, 32]. In addition, we found that patients with EMPD in this study were much younger than those in the United States, Europe, Japan, South Korea and Taiwan of China [9,10,11,12,13, 17, 24, 26]. These areas listed above had a relatively longer life expectancy than that in mainland China, which suggested that the age for EMPD patients might be closely related to the mean life expectancy in corresponding areas [33]. However, the influence of ethnic differences should also be noted, as patients with EMPD in other Asian areas including Japan, South Korea, Thailand and Taiwan of China were consistently younger than those in Western populations [17, 24, 26, 30]. Further studies are needed to clarify this point in depth.
The large, national representative sample of Chinese urban population in this study not only ensured the overall estimation of the prevalence of a rare disease like EMPD but also allowed us to explore age and gender patterns of the prevalence as well as regional differences in China. This study has several limitations. First, the basic medical insurance database lacked some detailed information, such as tumor stage and laboratory results. It limited the possibility to stratify the diagnosis in greater detail. Second, rural inhabitants and certain urban populations, such as military soldiers are not included in the UEBMI and URBMI system because they have different types of medical insurance. The exclusion of these groups could have affected the estimates.
This research is the first population-based study to investigate the prevalence of patients with EMPD in mainland China based on the basic medical insurance database. The prevalence of EMPD was lower in mainland China than those in Europe and the United States. Patients with EMPD in China were younger than those in developed countries, with a significant male predominance. These findings add to our understanding of the epidemiologic characteristics of EMPD in China. At the same time, as China is a country with a population of 1.3 billion, these findings can also provide important implications for further epidemiological studies of EMPD worldwide.
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