While multiple sclerosis typically affects adults, at least 2-5% of the total population with MS is composed of pediatric patients, and up to 10% of patients diagnosed with MS may have had their first symptoms before 18 years of age.
Initial presentations of MS in children at younger ages may resemble other similar idiopathic inflammatory demyelinating diseases of the nervous system (IIDD). Collectively, these IIDD have been estimated to occur in between 0.9 and 1.66 per 100,000 children per year, while the incidence in a recent large population study in southern California estimated the incidence of MS in children at 0.51 cases for 100,000 children. The number of MS diagnoses in children has been rising over the last few decades, but it is unclear if this trend represents a true increase in disease incidence, or if this is related to increased disease awareness by physicians and improved diagnostic criteria. The risk of disease occurrence varies on many environmental factors, including latitude and geographical location, UV radiation and sunlight exposure, vitamin D levels, smoking and secondhand smoke exposure, exposure to some common viral infections (especially Epstein-Barr virus), and obesity in girls.
Genetic influences are also associated with risk of MS, and may interact with environmental factors from the time of birth. Both boys and girls may be affected, but whereas adults typically show a female to male ratio of 2.5-3 to 1, this falls to 2 to 1 for children after puberty, 1.6 to 1 for children between 6 and 10 years of age, and 0.8 to 1 for children younger than 6 years old. While the majority of adults with MS are of white northern European ancestry, a higher percentage of pediatric MS patients in population studies at multiple North American centers are of African, Caribbean, Asian, Hispanic, Latino and Middle Eastern origin than in the adult population. This may reflect the influence of multiple genetic and environmental factors on early MS risk, which may be determined for all children and adults before 16 years of age.