Temporal Changes in Multiple Sclerosis onset age: Importance of Controlling for Equal Observation Time

Mihaela Dana Pirvoaica, Elaine Kingwell, Afsaneh Shirani, Feng Zhu, Yinshan Zhao, John D. Fisk, Virender Bhan, Robert Carruthers, Ruth Ann Marrie, Helen Tremlett



 Background: Previous studies have examined whether changes in the age at multiple sclerosis (MS) onset have occurred over time, but findings have been inconsistent.

Objective: We investigated temporal trends in MS onset age in three Canadian provinces, and assessed the effect of controlling for equal observation time between birth year groups.

 Methods: We included 9459 MS patients from MS clinic databases housed inBritish Columbia (BC, 5423),Manitoba (MB, 1419) andNova Scotia (NS, 2617). Birth years were grouped into five-year blocks and analysed via ANOVA and linear regression to assess temporal trends in onset age. The complete cohort included all MS patients. The restricted cohort allowed comparable observation times for each birth year group and included patients who had reached age 40 and had MS onset at age 40 or younger.

Results: The complete cohort showed a steep decline in onset age (averaging 2.0 years between birth year groups), from 37.0±10.8 years (1941-1945 births) to 28.0±6.4 years (1966-1970 births), p<0.001. In the restricted cohort (n=6003), only BC patients showed a significant decrease in the mean onset age (averaging 0.3 years between birth year groups): 29.6±6.5 years (1941-1945 births) and 27.4±5.8 years (1966-1970 births), p<0.001. No significant decrease in onset age was evident in the NS or MB restricted cohorts.

Conclusion: If the age at MS symptom onset has changed in the last four decades, shifts have been rather small. Temporal changes in age at MS onset between birth cohorts can be inflated without due consideration to comparable observation time.

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