Incidence of optic neuritis in Stockholm, Sweden, 1990-1995 - II. Time andspace patterns

Objectives: To describe the time and space patterns of patients with monosy mptomatic optic neuritis (MON) in Stockholm County, Sweden, and to explore the role of environmental factors in the etiology of MON and multiple scler osis. Design: Population-based and prospective incidence survey. Setting: Census based on referrals from 1.68 million inhabitants of Stockho lm County. Patients: One hundred forty-seven new patients with MON were consecutively referred by ophthalmologists and neurologists from January 1, 1990, through December 31, 1995. A standardized questionnaire was used for data collecti on. Main Outcome Measures: Evaluations consisted of annual incidence, statistic al significance of temporal aggregation, Knox test, likelihood score test a pplied to the ratio of the highest to lowest seasonal proportion of registe red events, and standardized morbidity ratio for municipalities. Results: We observed a seasonal pattern of MON incidence, with the highest incidence (31%) in the spring and the lowest (17%) in the winter (ratio of highest to lowest seasonal proportion, 1.84; 95% confidence interval, 1.13- 3.01; P = .007). The seasonal monthly incidences were correlated with the a verage number of sunny hours and the temperature. The presence of positive immune activity markers (ie, mononuclear pleocytosis and oligoclonal IgG ba nds in the cerebrospinal fluid) seemed to be linked to the onset of MON in winter. No aggregation by time, space, or month of birth was detected. Conclusions: Monosymptomatic optic neuritis in Stockholm County occurred at an uneven frequency across the seasons, with the highest incidence in spri ng and the lowest in winter. This seasonal pattern is compatible with that described in most previous reports. Environmental and probable infectious f actors unevenly distributed by season may play a role in the etiology and e arly clinical course of MON.