The current study examined the trend of MS prevalence in Capital of Iran, Tehran, from 2006 till 2018.The results of this study suggested that the prevalence of MS in Tehran is growing, and unadjusted prevalence proportion has increased from 79.3 cases in 2006 to 151.7 cases (per 100,000) in 2018 and also age-standardized prevalence proportion has increased from 73.7 cases in 2006 to 137.6 cases (per 100,000) in 2018.
Therefore, According to the global divisions [17], Tehran is considered as a region with high prevalence (prevalence ≥ 30/100,000) and the prevalence of MS in Tehran is the highest in Asia and among Middle Eastern countries [13]. In addition, the results revealed that the time trend of age standardized MS female to male sex ratio in recent years has been declining and it has fallen to 2.14 in 2018.
According to Wallin MT et al study [3] in United States in 2010, the prevalence of MS in the population over 18 years, cumulated over 10 years was 309.2 per 100,000 and the highest prevalence was in the 55- to 64-year age group. Compared to our study findings which most cases were observed in the age group below 40 years, it has a higher prevalence and different age distribution. When coupled with prior estimates of MS prevalence in Iran, it seems that there has been a relatively increase during the last decades. In Eskandarieh et al study [4] in Tehran, the MS prevalence was 101.39 per 100000 in 2014 and age-adjusted prevalence proportion were 134 and 42.5 for females and males respectively. In another review study, MS prevalence for Iran in 2013 was 85.8 per 100,000 [13].
According to Hosseinzadeh et al [12], Tehran is considered as one of the high-frequency provinces for MS. It sounds that the high MS frequency in Tehran may be related to the level of urbanization, their social and economic status and air pollution.
On the other hand, better access to health services can play an important role, which in turn leads to better diagnosis and cases registration.
Tehran is known as one of the most polluted cities in the world [21]. Although some studies have not reported a significant association between air pollution markers (PM2.5, NO2, and O3) and MS [22], several studies had shown some evidences that air pollution can have a significant relationship with the incidence and recurrence of MS [23–25].
The possible association between air pollution and MS incidence and recurrence has been justified through several mechanisms, most likely being that air pollution leads to vitamin D deficiency, excessive free radical production, inflammatory factors expression, induction of chronic inflammation, mitochondrial dysfunction, and increased oxidative stress, which can be linked to MS incidence and recurrence [24, 25].
One issue that has been shown in most studies is that women are more susceptible to MS than men [26, 27]. In a relatively large study on Canadian patients with MS, sex ratio was estimated 3.2, which is so similar to our result [26]. Also, it has been reported as 3.06 in Eskandarieh et al study [8]. In a study conducted in Turkey [28], the prevalence of MS in Karabük and Akçakoca were 95.9 and 46.1 per 100,000 and the prevalence of MS in the Middle East and north Africa was 51.52 cases per 100,000 [29]. The estimated prevalence proportion in our study revealed that MS is more prevalent in Tehran in compared to the Middle East countries.
It should be noted that the trend of sex ratio in our study was somewhat declining (from 3.20 in 2006 to 2.14 in 2018). However, some studies have reported an increase in this trend [30, 31], However, no significant trend was observed in a study in Sweden [32].
In terms of age groups, in our study, nearly 60% of cases were diagnosed before age 30 years. Other similar studies have also shown a higher incidence of disease at an early age [4, 5]. The disease onset in the early age leads to an increase in the number of years lived with disability (YLDs) and eventually to a Disability-adjusted life years (DALYs) [33–35].
There are some limitations and strengths to our study. Because the province of Tehran is both large in 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 proportion maybe is underestimated. On the other hand, it should be highlighted that IMSS registration facilitates access to health care and services for patients. This is a strong inducement that likely facilitates registration and retention and thus, it is considered as study strengthens.