RT Journal Article SR Electronic T1 Hospital diagnosed pneumonia before age 20 years and multiple sclerosis risk JF BMJ Neurology Open JO BMJ Neurol Open FD BMJ Publishing Group Ltd SP e000044 DO 10.1136/bmjno-2020-000044 VO 2 IS 1 A1 Kelsi A Smith A1 Ayako Hiyoshi A1 Sarah Burkill A1 Shahram Bahmanyar A1 Johan Öckinger A1 Lars Alfredsson A1 Tomas Olsson A1 Scott Montgomery YR 2020 UL http://neurologyopen.bmj.com/content/2/1/e000044.abstract AB Introduction Respiratory inflammation has been proposed as a risk factor for MS. This study aims to determine if hospital-diagnosed pneumonia in adolescence (before age 20 years) is associated with subsequent multiple sclerosis (MS).Methods This case-control study included incident MS cases after age 20 years identified using the Swedish national registers. Cases were matched with 10 general population controls by age, sex and region. Pneumonia diagnoses were identified between 0–5, 6–10, 11–15 and 16–20 years of age. Conditional logistic regression models adjusted for infectious mononucleosis (IM) and education calculated ORs with 95% CIs. Urinary tract infections (UTIs), a common complication of MS, before age 20 years were included as a control diagnosis for reverse causation.Results There were 6109 cases and 49 479 controls included. Pneumonia diagnosed between age 11–15 years was associated with subsequent MS (adj OR 2.00, 95% CI 1.22 to 3.27). Although not statistically significant, sensitivity analyses showed similar magnitude associations of pneumonia between age 11–15 years and MS. No statistically significant associations with MS for pneumonia at other age groups were observed. Adjustment for IM had no notable effect on associations, but was statistically significantly associated with MS. UTIs were not associated with MS.Conclusion Pneumonia at 11–15 years of age was associated with MS, suggesting a possible role for inflammation of the respiratory system in the aetiology of MS during a period of susceptibility in adolescence. Further research on respiratory infections prior to MS onset should be conducted to replicate this finding and determine explanatory causal mechanisms.