Brain reserve and timing of clinical onset in multiple sclerosis

Background: A latent period of variable length elapses between multiple sclerosis (MS) biological onset and the occurrence of the first clinical episode reflecting a central nervous system (CNS) demyelinating event. Factors affecting the duration of such interval are unknown. Objective: To explore whether brain reserve, which moderates the impact of structural damage along MS course, could also affect the timing of MS clinical onset. Methods: We conducted a time-to-event analysis in 326 relapsing-onset multiple sclerosis patients to ascertain the effect of brain reserve, that is, larger maximal lifetime brain growth (MLBG) estimated as intracranial volume, on the risk of an earlier disease onset. For this purpose, we carried out a Cox proportional hazards regression model stratified by sex and adjusted by site and pre-morbid MS risk factors. All patients reached the event (i.e. the disease onset) with no censored case; the age (years) at disease onset was set as the main time variable. Results: We identified a protective effect of brain reserve on the time to disease onset (HR = 0.11, 95% CI = 0.02-0.83, p = 0.032), unchanged when accounting for MS risk factors. Conclusion: Brain reserve might counteract the pathological mechanisms ongoing after biological initiation, thus delaying the disease overt clinical manifestation.