An exacerbation in multiple sclerosis, also called a relapse or flare, is a new or worsening neurological symptom that lasts at least 24 hours, happens without a fever or infection to explain it, and reflects fresh inflammatory damage to the brain or spinal cord. Relapses are the defining feature of relapsing-remitting MS and remain a major driver of disability, especially in the early years of the disease. What makes them tricky is that not every symptom flare-up counts as a true relapse, the triggers range from everyday infections to severe psychological stress, and the damage they leave behind can extend well beyond what the person first notices.
What Happens Inside the Nervous System During a Relapse
MS exacerbations are driven by the immune system crossing a barrier it normally respects. In a relapse, immune cells that were activated outside the brain manage to breach the blood-brain barrier. Once inside, they release inflammatory molecules that damage myelin, the insulating sheath around nerve fibers, and can injure the nerve fibers themselves. This disruption of the blood-brain barrier and the resulting migration of immune cells into the central nervous system are among the earliest measurable changes seen in MS brains during active disease.1PubMed. Role of the blood-brain barrier in multiple sclerosis The result is a sudden burst of symptoms that depends on where in the nervous system the inflammation lands. A patch of inflammation in the optic nerve causes blurred or lost vision. One in the spinal cord can produce numbness, weakness, or trouble walking. One in the brainstem might cause dizziness or double vision.
Optic neuritis, the acute inflammation of the optic nerve, is one of the most recognizable presentations. It typically shows up as painful vision loss in one eye and is often the very first sign that someone has MS.2PubMed Central. Optic neuritis as an early sign of multiple sclerosis Not everyone with optic neuritis goes on to develop MS, but it is common enough to be a red flag that neurologists take seriously.
What Triggers a Relapse
No one can predict exactly when a relapse will strike, but certain factors consistently raise the odds. Infections are the best-established trigger. Common upper respiratory infections, gastrointestinal bugs, and urinary tract infections have all been linked to MS exacerbations, alongside specific pathogens like Epstein-Barr virus and human herpesvirus 6.3PubMed. The role of infections in multiple sclerosis A prospective study tracking MS patients over time found that the risk of relapse rose significantly during the four-week window surrounding a common infection.4PubMed. Viral infections trigger multiple sclerosis relapses: a prospective seroepidemiological study Another study confirmed an association between systemic infections and both clinical relapses and increased activity on MRI scans.5PubMed. The risk of relapses in multiple sclerosis during systemic infections
Psychological stress is the other major trigger that keeps coming up in the research. A large study found that negative life events, including serious illness or death of a close family member, major relationship changes, and job-related upheaval, were all associated with a modestly increased risk of MS.6PubMed Central. Stressful life events are associated with the risk of multiple sclerosis Intense stress has been identified as a trigger for both first episodes and subsequent relapses.7PubMed Central. Stress as provoking factor for the first and repeated multiple sclerosis seizures A striking recent study examined MS patients during a period of active armed conflict and found increased MRI-detected disease activity during war-related psychological stress, even in patients who were not experiencing obvious clinical symptoms.8PubMed Central. Subclinical imaging activity in multiple sclerosis patients during war-related psychological stress The authors recommended that clinicians consider more frequent monitoring during periods of extreme stress.
This does not mean every stressful week will trigger a relapse. The effect sizes are modest, and plenty of relapses happen during calm periods. But the pattern is consistent enough that managing stress is a legitimate part of MS care, not just a feel-good recommendation.
True Relapses Versus Pseudo-Exacerbations
One of the most important distinctions in MS management is telling a real relapse apart from a pseudo-exacerbation. A true relapse involves new inflammatory damage. A pseudo-exacerbation is a temporary worsening of existing symptoms caused by something else: a fever, overheating, a urinary tract infection, sleep deprivation, or even just physical exhaustion. The symptoms can look identical. Someone might experience a return of old numbness or worsened vision that feels exactly like a prior relapse, but the underlying cause is not new inflammation.
This distinction matters because the treatments are completely different. A true relapse may call for high-dose steroids. A pseudo-exacerbation calls for addressing the underlying trigger, whether that is treating an infection, cooling down, or getting rest. COVID-19 highlighted this challenge: SARS-CoV-2 infection can worsen MS symptoms in ways that mimic a true relapse, forcing clinicians to carefully assess whether new inflammatory damage is actually occurring.9Eastern Ukrainian Medical Journal. Differential Diagnosis of Exacerbations and Pseudo-Exacerbations Against the Background of SARS-CoV-2 by the Example of a Clinical Case of a Patient with Multiple Sclerosis
Practically speaking, if your symptoms worsen and you also have signs of infection or have been exposed to heat, your neurologist will want to rule out a pseudo-exacerbation first. This usually means checking for infection, waiting to see if symptoms resolve once the trigger is removed, and sometimes ordering an MRI to look for new inflammatory activity.
How MRI Confirms Disease Activity
MS relapses are diagnosed clinically, meaning a neurologist evaluates your symptoms and history. But MRI plays a supporting role that can be critical in ambiguous cases. When a contrast agent called gadolinium is injected during the scan, actively inflamed lesions light up on certain sequences. These contrast-enhancing lesions reflect acute or recent inflammation, while new or enlarging lesions on other sequences show the permanent footprint of a previous inflammatory event.10PubMed. Use of gadolinium-based contrast agents in multiple sclerosis: a review by the ESMRMB-GREC and ESNR Multiple Sclerosis Working Group This means an MRI during a relapse can both confirm active inflammation and provide a baseline for comparison later.
A blood-based biomarker called neurofilament light chain is also gaining traction as a way to track disease activity. Levels of this protein rise when nerve fibers are damaged, and studies have shown that people with active inflammatory lesions on MRI have significantly higher levels in their blood. Higher levels are also independently associated with disability and the occurrence of relapses.11PubMed Central. Serum Neurofilament light: A biomarker of neuronal damage in multiple sclerosis It is not yet a routine clinical tool for diagnosing individual relapses, but it is moving in that direction and may eventually allow monitoring between MRI scans.
Treating an Acute Relapse
The standard first-line treatment for an MS exacerbation is a short course of high-dose corticosteroids, most commonly methylprednisolone. A Cochrane review covering multiple trials found that corticosteroids decreased by more than 60 percent the probability of a patient remaining stable or worsening within the first five weeks after an exacerbation.12PubMed Central. Corticosteroids or ACTH for acute exacerbations in multiple sclerosis Steroids work by tamping down the inflammatory response, which shortens the relapse rather than letting it burn through on its own. They do not, however, change the long-term course of the disease.
Steroids can be given intravenously, which traditionally meant a hospital infusion center, or by mouth at equivalent high doses. A study comparing the two routes found that recovery rates were virtually identical, with about nine in ten patients recovering with either method.13PubMed Central. High-dose oral methylprednisolone for the treatment of multiple sclerosis relapses: cost-minimisation analysis and patient’s satisfaction Oral dosing is cheaper and more convenient, which matters when you are already dealing with a flare-up that makes getting to a clinic harder.
When steroids fail, the next step is therapeutic plasma exchange, a procedure that filters inflammatory molecules out of the blood. This is reserved for severe relapses that do not respond to steroids. Evidence from trials and real-world data shows that roughly 40 to 60 percent of patients experience moderate to marked improvement with plasma exchange, with the best results occurring when treatment starts within two weeks of symptom onset.14PubMed Central. Therapeutic Plasma Exchange in Corticosteroid-Refractory Multiple Sclerosis Relapses: Mechanisms, Efficacy, and Integration into Clinical Practice A large observational study found that half of patients treated with plasma exchange saw improvement in their disability scores, while about a third remained stable and a smaller fraction continued to worsen.15PubMed Central. Rehabilitation interventions for the management of multiple sclerosis relapse: a short scoping review — Section: Abstract
The Cognitive Toll of a Relapse
Most people think of MS relapses in terms of physical symptoms: vision loss, weakness, numbness. But the cognitive effects are real and often underappreciated. Researchers have identified what they call “cognitive relapse,” where processing speed and other thinking abilities decline acutely during a flare-up, with varying degrees of recovery afterward.16PubMed Central. Cognitive Relapse in Multiple Sclerosis: New Findings and Directions for Future Research
A study that tracked cognitive processing speed found a significant decline starting as early as 30 days before a relapse was clinically obvious and lasting up to a year and a half afterward. The sharpest drop occurred in the first month after relapse onset, and the decline was considered clinically meaningful for up to three months.17PubMed Central. Reduction in Cognitive Processing Speed Surrounding Multiple Sclerosis Relapse The finding that cognitive changes can precede a clinical relapse is particularly interesting because it suggests the inflammatory process may be affecting the brain before symptoms become obvious enough to report. It also underscores why reducing relapse rates matters beyond just preventing physical symptoms.
Incomplete Recovery and Long-Term Disability
Not every relapse resolves completely. A study of people with relapsing-remitting MS who experienced severe relapses found a high rate of incomplete recovery at 12 months, suggesting that residual damage from individual relapses may accumulate over time and contribute to long-term disability.18PubMed. Residual disability after severe relapse in people with multiple sclerosis treated with disease-modifying therapy A large analysis confirmed that relapses contribute to the accumulation of disability, primarily in the early years of the disease.19PubMed Central. How patients with multiple sclerosis acquire disability
This is why neurologists push so hard for effective disease-modifying therapy from the outset. Each relapse that does not fully resolve leaves the person a little worse off than they were before, and those small increments add up. The early years of MS are the window where preventing relapses makes the biggest difference to where someone ends up decades later.
The Role of Disease-Modifying Therapies in Preventing Relapses
The entire philosophy of MS treatment has shifted over the past two decades toward preventing exacerbations rather than just treating them after they happen. Disease-modifying therapies work by suppressing or modulating the immune system to reduce the frequency of relapses and slow disability accumulation. A network meta-analysis comparing available therapies found that the most effective options for reducing annualized relapse rates were alemtuzumab, natalizumab, and ocrelizumab, each cutting relapse rates by roughly two-thirds compared to placebo.20PubMed. Disease-Modifying Therapies for Relapsing-Remitting Multiple Sclerosis: A Network Meta-Analysis
Newer therapies continue to show strong results in real-world settings. A study of ofatumumab, a newer anti-CD20 therapy, found a 75 percent reduction in annualized relapse rate after patients started the medication compared to the year before.21Multiple Sclerosis and Related Disorders. Real-world change in annualized relapse rate and healthcare resource utilization following initiation of ofatumumab in people with multiple sclerosis These numbers are encouraging, though no therapy eliminates relapses entirely. The goal is to push the relapse rate as low as possible to minimize the accumulation of residual damage described above.
Pregnancy and Postpartum Relapse Risk
Pregnancy creates a distinctive pattern of relapse risk that every woman with MS should understand before planning a family. A landmark study found that relapse rates dropped substantially during pregnancy itself, falling lowest in the third trimester. But the first three months after delivery brought a sharp spike, with the relapse rate climbing well above the pre-pregnancy baseline before settling back down.22PubMed. Rate of pregnancy-related relapse in multiple sclerosis
A systematic review and meta-analysis confirmed this pattern and added important nuance. The postpartum relapse rate in the first three months was roughly double the pre-conception rate. Two factors appeared to significantly influence this risk: stopping high-effectiveness disease-modifying therapy before conception was associated with higher postpartum relapse rates, while exclusive breastfeeding was associated with a meaningfully lower risk.23PubMed. Postpartum relapse risk in multiple sclerosis: a systematic review and meta-analysis By 10 to 12 months postpartum, the relapse rate actually dipped below pre-conception levels. The practical takeaway is that pregnancy planning in MS should involve a conversation with a neurologist about which therapies can be continued, paused, or switched, and what the monitoring plan will look like in the months after delivery.
Pediatric MS and Higher Relapse Rates
Children and adolescents who develop MS tend to relapse more frequently than adults. A study comparing pediatric-onset to adult-onset MS found that the annualized relapse rate in the younger group was nearly three times higher, and this difference held up even after accounting for time spent on disease-modifying therapy.24JAMA Neurology. Increased Relapse Rate in Pediatric-Onset Compared With Adult-Onset Multiple Sclerosis Relapses in pediatric MS tend to cluster more densely in the early years of the disease compared to what is seen in adults.25PubMed Central. Pediatric multiple sclerosis: Clinical features and outcome
Despite the higher relapse frequency, children tend to recover more completely from individual relapses than adults do, likely because younger brains have a greater capacity for repair. The challenge is that pediatric MS patients have decades of disease ahead of them, so even small amounts of residual damage from each relapse can compound over a much longer timeline. This is why early, aggressive treatment is generally favored in pediatric MS despite the understandable reluctance to put children on potent immunosuppressive medications.
Rehabilitation Alongside Steroid Treatment
Steroids shorten a relapse, but they do not address the functional loss that a person experiences during it. Starting rehabilitation during a relapse, rather than waiting until it resolves, appears to add real benefit. A study found that patients who began a structured rehabilitation program alongside their steroid course showed significantly greater improvements in physical activity and self-efficacy compared to those who received steroids alone, though the effect on fatigue was less clear.26PubMed. Effectiveness of rehabilitation in multiple sclerosis relapse on fatigue, self-efficacy and physical activity A scoping review of multidisciplinary rehabilitation during relapses found consistent benefits for reducing impairment and disability.27PubMed Central. Rehabilitation interventions for the management of multiple sclerosis relapse: a short scoping review
The types of rehabilitation that help depend on what the relapse affects. Vision rehabilitation for optic neuritis, physical therapy for mobility problems, occupational therapy for hand coordination, and cognitive rehabilitation for thinking difficulties all have roles to play. The key message is that rehabilitation is not something to save for after the acute phase is over; there is evidence that starting early leads to better outcomes.
Sleep, Fatigue, and the Relapse Cycle
Fatigue is the most common complaint in MS overall, and relapses make it dramatically worse. A meta-analysis looking at sleep disturbance and fatigue in MS found a strong connection between poor sleep quality and fatigue severity. Insomnia and self-reported sleep quality had the largest effect sizes, while objective sleep measures like total sleep time and number of awakenings did not correlate as strongly.28PubMed Central. Sleep disturbance and fatigue in multiple sclerosis: A systematic review and meta-analysis This suggests that how people perceive and experience their sleep matters more than the raw numbers on a sleep tracker.
During and after a relapse, fatigue can become severe enough to be the most disabling symptom, even when the visible neurological deficits are modest. High-dose steroids themselves can disrupt sleep, creating a frustrating cycle: the treatment for the relapse worsens the sleep problems that fuel the fatigue. Addressing sleep hygiene, managing restless leg syndrome when present, and treating insomnia directly can all make a meaningful difference in how a person feels during recovery from a relapse, even if they do not change the underlying inflammation.
Conditions That Look Like MS Relapses but Are Not
Several other autoimmune conditions attack the central nervous system in ways that mimic MS, and telling them apart matters because the treatments differ and the wrong therapy can cause harm. Neuromyelitis optica spectrum disorder and myelin oligodendrocyte glycoprotein antibody-associated disease are the two most important conditions on this list.29PubMed. MRI to differentiate multiple sclerosis, neuromyelitis optica, and myelin oligodendrocyte glycoprotein antibody disease Both can cause optic neuritis and spinal cord inflammation that looks very much like an MS relapse.
MRI features can help distinguish these conditions. MS tends to produce many small white matter lesions with a characteristic pattern, including a feature called the central vein sign, where a tiny vein runs through the middle of each lesion. One study found that the proportion of lesions showing this sign was far higher in MS than in neuromyelitis optica spectrum disorder, and combining this feature with cortical lesion counts achieved very high diagnostic accuracy.30PubMed Central. Differentiating Multiple Sclerosis From AQP4-Neuromyelitis Optica Spectrum Disorder and MOG-Antibody Disease With Imaging Blood tests for specific antibodies also help, but the imaging differences are valuable when antibody results are ambiguous or pending.
Gut Bacteria and Emerging Research
An area of active investigation is the relationship between the gut microbiome and MS disease activity. A study comparing the gut bacteria of treatment-naive MS patients to healthy controls found that people with MS had significantly lower levels of certain anti-inflammatory bacterial species, including Bacteroides fragilis and Bifidobacterium.31Scientific Reports. Gut dysbiosis in multiple sclerosis patients: a comparative analysis in fecal samples The idea is that an imbalanced gut microbiome might contribute to the overactive immune response that drives MS relapses. This research is still in its early stages, and no one can yet say whether altering gut bacteria through diet, probiotics, or other interventions would reduce relapse rates. But the consistency of findings across multiple research groups suggests this is more than a dead end, and it may eventually open new avenues for prevention that complement traditional immune-suppressing therapies.