SHORT COMMUNICATION Influence of climatic factors in the incidence of multiple sclerosis relapses in a Portuguese population A. C. Fonseca a , J. Costa b , C. Cordeiro a , R. Geraldes a and J. de Sa ´ a a Department of Neurosciences (Neurology), Hospital de Santa Maria, Lisboa; and b Laboratory of Clinical Pharmacology and Therapeutics, Lisboa Faculty of Medicine, IMM, Portugal Keywords: atmospheric pressure, climatic factors, environmental factors, humidity, multiple sclerosis relapses, Portuguese population, seasons, temperature Received 9 November 2008 Accepted 28 November 2008 Background and purpose: Environmental factors are thought to be important in multiple sclerosis (MS) pathophysiology. We aimed to evaluate if there was an asso- ciation between MS relapses and some climatic factors in a Portuguese population. Methods: Four year retrospective study analyzing 414 MS relapses in 249 consecutive relapsing–remitting patients. Non-parametric statistics were used to compare the distribution of relapses across months and seasons. SpearmanÕs coefficient was determined to evaluate the correlation between relapses frequency and maximum and minimum atmospheric temperatures, humidity and atmospheric pressure. Results: The mean number of relapses was not significantly different between months or seasons. No correlation was found between relapse frequency and any climatic factor. Conclusion: Our series is one of the largest addressing the influence of specific climatic factors on MS relapses. The number of clinical MS relapses seems to be unrelated to climatic factors. Introduction Multiple sclerosis (MS) is a chronic inflammatory immune-mediated disease of the CNS that usually has an onset between 20 and 40 years of age. Relapses are a defining feature of MS and about two-thirds of the patients are categorized into a relapsing–remitting form. In relapsing-remitting multiple sclerosis (RRMS), the relapse generally lasts from a week to a month and is associated with inflammation, demyelination, axonal transection and remyelination [1]. The causes of MS are unknown, but it probably results from an interaction between genetic and exo- genous factors [2]. Environmental factors can increase the susceptibility to MS in genetic predisposed individ- uals, trigger MS relapses and influence its outcome [3]. Different factors that may contribute to predict relapses have been investigated in a search for a window of opportunity to initiate interventions before a relapse begins. These factors include infectious events, stress, puerperium, cranial irradiation and proinflammatory cytokines [4]. Included amongst environmental factors are climatic factors. Several studies worldwide have evaluated the relationship between the incidence of MS relapses and annual seasons. Some studies report a significant association [5,6] between seasons and the incidence of MS relapses but the vast majority fails to show it [7]. However, most studies were carried out before the established McDonald criteria for MS [8] and before the use of MRI. Moreover, only few analysed specific climatic factors [9,10]. In this study, we aimed to evaluate if there was a significant association between the incidence of MS relapses and specific climatic factors in a large representative Portuguese population. Methods Retrospective study enrolling 249 consecutive patients with RRMS according to McDonald criteria [8]. Relapses were collected in four consecutive years (January 2004 to December 2007) as well as their dis- tribution throughout the months of each year. Data were extracted from the Neurology ward discharge notes and the Outpatient MS Clinic of the Department of Neurology of Santa Maria Hospital in Lisbon. Relapses were defined according to established criteria [8] and confirmed by a MS expert neurologist (de Sa´ J). Daily registries of maximum (T max ) and minimum (T min ) atmospheric temperatures, humidity and atmo- spheric pressure for Lisbon were collected from the Portuguese Meteorological institute. Kruskal–Wallis test was used to compare the distri- bution of relapses frequency across months and sea- sons. SpearmanÕs rank correlation coefficient was determined for relapses frequency and each climatic Correspondence: Ana Catarina Fonseca, MD, Servic¸o de Neurologia, Hospital de Santa Maria, Avenida Professor Egas Moniz, 1649-035 Lisboa, Portugal (tel.: 351 21 7805000; fax: +351 21 7957474; e-mail: catarinagfonseca@gmail.com). Ó 2009 The Author(s) Journal compilation Ó 2009 EFNS 537 European Journal of Neurology 2009, 16: 537–539 doi:10.1111/j.1468-1331.2008.02528.x