Research shows that 85 percent of people with MS begin by having the relapsing-remitting form of the disease and the majority of these people later develop secondary progressive MS. A relapse is defined in the study as worsening of neurological symptoms for more than 24 hours, without a fever or infection. Because relapse-related symptoms often improve within a few weeks, it's been unclear how much disability comes from relapses, and how much from progression.

For the study, scientists reviewed the medical records of 2,477 people with MS who experienced relapses in British Columbia, Canada. The study looked at whether the participants had disability severe enough to require the use of a cane for walking and whether this was related to relapses occurring within five years, five to 10 years, or more than 10 years after onset of symptoms. The participants were followed for an average of 20 years. During that time, the group experienced 11,722 relapses.

Scientists found that people who had a relapse within five years of disease onset were at a 48 percent higher hazard (a measure of relative risk) of needing a cane to walk within five years of disease onset than those who did not have an early relapse. Importantly, the impact of the early relapse lessened over time. Those with early relapse who did not need a cane after five years were at only a 10 percent higher hazard (a measure of relative risk) of needing one 10 years after disease onset than those without early relapses.

"Our findings may represent an important message to people diagnosed with MS today. Those who have a history of relapses could potentially be offered reassurance that as time goes on, these relapses will have a diminishing effect on their everyday lives," said study author Helen Tremlett, PhD, with the University of British Columbia in Vancouver. "In addition, our study calls upon the need for new medications that target axonal degeneration, which is suspected of causing permanent disability, especially for people who have had MS for many years or who are older at diagnosis."

The impact of relapses that occurred later, either at five to 10 or more than 10 years after the start of the disease, also waned over time and became insignificant after long-term follow up. Relapses in people under 25 had a longer impact on disability compared to those over 35 years.

"There may be a longer window of opportunity for treating younger people with MS right away, changing the course of progression later on," Tremlett said.

Note: If no author is given, the source is cited instead.

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