BACKGROUND: Pediatric-onset multiple sclerosis (MS) affects life at a stage vital for social and educational achievements and psychiatric co-morbidity is common after MS onset. Few studies have examined psychiatric morbidity before MS onset.

METHODS: In this nationwide study, detailed case ascertainment was performed in all children with pediatric MS, including chart review. For each MS patient, we selected five controls using density sampling from the entire Danish population, matching controls to children with MS by sex and birthdate. We analyzed data as a nested case-control study with psychiatric morbidity as exposure and MS as outcome, and a matched cohort study with MS as exposure and psychiatric co-morbidity as outcome. Hazard ratios (HR) including 95% confidence intervals (CI) were estimated using Cox regression.

RESULTS: We identified 212 children with MS and 1060 controls. No association between psychiatric morbidity and the rate of MS was found before MS onset. After MS onset, children with MS had two times higher hazard for psychiatric co-morbidity compared with children without MS (HR=2.0; 95% CI=1.3-3.1; p<0.001).

CONCLUSION: Psychiatric morbidity seems to commence after MS onset, making screening for neuropsychiatric conditions pertinent in newly-diagnosed children with MS.