Multiple sclerosis in the Southern and Northern hemispheres: the month of birth at different latitudes has the same influence on the prevalence and progression of the disease in the Northern and Southern hemispheres?

The etiopathogenesis of multiple sclerosis (MS) remains unclear in spite of the several attempts to predict the risk of acquiring the disease. Genetic and environmental factors related to disease susceptibility, suggested by many recent studies that configured a multifactorial etiology to MS, reflect its geographical distribution and changes in risk with migration. The incidence of MS seems to decrease with migration in early life, from high-risk to low-risk areas1,2. It’s already known that specific haplotypes represent a genetic risk factor for MS. Exposure to sunlight and production of vitamin D appear to be important factors for MS susceptibility. Vitamin D seems to promote Th2 function and to inhibit the proinflammatory cytokines IL-1a, IL 2β and TNFα. Its role in immunological response was the focus of many studies concerning autoimmune diseases, including MS3. Vitamin D was also identified as a promoter of some alleles related to the month of birth and risk of MS4. According to latitude, Kurtzke5 and Bharanidharan6 stratified the prevalence of MS in three groups, and the Northern parts of Europe and North America presented high range (>30/100,000), Southern Europe and USA had medium range (5–30/100,000), South America presented low to medium range and Asia had low range (<5/100,000 inhabitants). Many studies suggested that less exposure to sunlight in high latitudes increased the risk of MS1. Several countries in the Northern hemisphere analyzed the mother’s exposure to sunlight during pregnancy, the month of birth and the risk of developing MS, and found a higher number of MS patients born in winter time and in higher latitude regions7,8. Conversely, recent studies have not found a significant variation in the prevalence of MS with the latitude in Latin America. Risco et al.9 considered specific environmental factors and different ethnicity also as being related to the lower influence of latitude in the prevalence of MS in Latin America, compared to Europe and North America. In this issue of Arquivos de Neuro-Psiquiatria, Fragoso YD et al.10 published the first study to evaluate the risk of MS according to latitude and month of birth, which included many different regions in the same continent. The authors analyzed 1,207 MS patients and 1,207 controls in four countries of South America: Argentina, Brazil, Chile and Peru, and compared the month of birth in a large range of latitudes (zero to 10° South; 11° to 20° South; 21° to 30° South; and 31° to 40° South) and the prevalence of MS. In spite of the relatively low number of patients due to the lower prevalence of MS in the Southern hemisphere compared to the Northern hemisphere, children born in high latitudes in the Southern hemisphere did not present higher incidence of MS in adult life. There is not a major variation of exposure to daylight between different seasons in the Southern hemisphere as there is in the Northern Head of Clinic, Neurology Outpatient Clinic, Santa Casa da Misericórdia do Rio de Janeiro, Rio de Janeiro RJ, Brazil. Correspondence Maria Lucia Vellutini Pimentel Rua Santa Luzia 206, Centro 24a / 25a Enfermarias 20020-022 Rio de Janeiro RJ Brasil E-mail: [email protected] Conflict of interest There is no conflict of interest to declare.