The current study examined the trend of MS prevalence in Tehran, the capital of Iran, from 2006 to 2018. The results of this study suggested that the prevalence of MS in Tehran is increasing, and the unadjusted prevalence has increased from 79.3 cases per 100,000 people in 2006 to 151.7 cases per 100,000 people in 2018. Moreover, the age-standardized prevalence has increased from 73.7 cases per 100,000 people in 2006 to 137.6 cases per 100,000 people in 2018.
Similar studies have been conducted in countries surrounding Iran, countries in the Middle East, and Arab countries; however, it is important to note that as Iranians are identified as Persian, the heredity and genetic structure of Iranian population differ from those of the mentioned countries. Other studies have also been conducted in various parts of Iran and have indicated that the pattern of MS disease in Tehran may be different from that of other parts of the country. The observed pattern can be attributed to a number of differences between Tehran and other cities including the higher level of air pollution in Tehran[19].
Therefore, according to the global divisions [20], Tehran should be considered as a region with a high MS prevalence (prevalence ≥30/100,000). Moreover, Tehran has the highest MS prevalence in Asia and among the Middle Eastern countries [19]. In addition, the results revealed that the time trend of age-standardized MS female-to-male ratio in recent years has been declining to 2.14 in 2018.
A study conducted by Wallin MT et al. [3] in the United States in 2010 revealed that the prevalence of MS in the population aged over 18 years old was 309.2 per 100,000 people over 10 years. Moreover, the highest prevalence was reported for the 55-64 age group. As compared with the findings of the current study, in which most cases were observed in the age group below the age of 40 years, Wallin MT et al.’s study has a higher prevalence and a different age distribution. When coupled with prior estimates of the MS prevalence in Iran, it seems that there has been a relative increase in this regard over the last decades. In Eskandarieh et al.’s study [7], the MS prevalence in Tehran was 101.39 per 100,000 people in 2014, and the age-adjusted prevalence were 134 and 42.5 for females and males, respectively. In another review study, the MS prevalence in Iran was 85.8 per 100,000 people in 2013 [19].
According to Hosseinzadeh et al. [8], Tehran is considered as one of the high-frequency regions for MS. The high MS frequency in Tehran may be related to the rate of urbanization [21], the social and economic status [22, 23], and air pollution [24-26]. In addition, better access to health services can play an important role and in turn lead to better diagnosis and case registration [27].
Tehran has been recognized as one of the most polluted cities in the world [28]. Although some studies have not reported a significant association between MS and air pollution markers such as PM2.5, NO2, and O3 [29], several studies [24-26] have shown some evidences that air pollution can have a significant relationship with the incidence and recurrence of MS. Air pollution leads to deficiency of vitamin D, production of excessive free radical, expression of inflammatory factors, induction of chronic inflammation, dysfunction of mitochondrial, and increase of oxidative stress, all of which can be linked to MS incidence and recurrence [25, 26]. Therefore, as previous studies have shown [30], it can be hypothesized that a portion of the high prevalence of MS in Tehran may be attributable to the air pollution. To establish causality, it is recommended to compare different geographic regions with different pollution levels in terms of MS occurrence.
One finding presented in most of the pertinent studies is that women as compared with men are more susceptible to MS [31, 32]. In a study addressing a relatively large sample of Canadian MS patients, sex ratio was estimated to be 3.2, which was so similar to the findings of the current study[31]. Moreover, the sex ratio has been reported to be 3.06 in Eskandarieh et al.’s study [6]. A study conducted in Turkey [33] revealed that the prevalence of MS in Karabük and Akçakoca were 95.9 and 46.1 per 100,000 people, respectively [34]. In addition, the mentioned study indicated that the prevalence of MS in the Middle East and North Africa was 51.52 cases per 100,000 people [34]. The estimated prevalence in the current study revealed that MS is more prevalent in Tehran as compared with other cities in the Middle East countries.
It should be noted that the trend of sex ratio in the present study was rather declining from 3.20 in 2006 to 2.14 in 2018. Some reasons can be provided to justify the increase of MS incidence among males in Iran. First, a larger percentage of males than females are in the workforce, mainly in urban areas (61.3% vs. 11.6%) [35] and therefore they are more profoundly affected by the related stressors in the workplaces. Second, males are more exposed to air pollution because of outdoor work.
The obtained trend in the current study using the extracted data up to 2010 is in agreement with the world global trend, which indicates the increasing sex ratio in MS patients. However, the findings of the study examining the extracted data over recent years have demonstrated descending sex ratio changes, which are consistent with the findings of Norway, Tasmania, and Sweden researches. The mentioned studies have indicated the stable sex ratio changes and did not support the former ascending trend [31, 36, 37]. However, some studies have reported an increase in this trend [38, 39].
In terms of age groups, nearly 60% of cases in the current study were diagnosed with MS before the age of 30 years. Other similar studies have also shown a higher incidence rate of MS at an early age [7, 40]. The disease onset in the early age leads to an increase in the number of years lived with disability (YLDs) and eventually to disability-adjusted life years (DALYs) [4, 41, 42].
The present study had some limitations and strengths. Considering that Tehran is a large province in terms of both the population and geographical area, and MS registration is elective, the MS registration may be incomplete and some cases may not be registered. Therefore, the obtained prevalence may be underestimated. In addition, it should be highlighted that as IMSS registration facilitates provide health care services for patients, the mentioned point can be regarded as a strong inducement that may facilitate the registration and retention processes and thus be considered as study strength.