Multiple sclerosis progresses through a combination of inflammatory attacks and a slower, quieter process of nerve damage that can happen even between relapses. About 85% of people with MS start with a relapsing-remitting pattern, where symptoms flare up and then partially or fully resolve. Over time, many of these people shift into a phase where disability accumulates steadily, with or without relapses. Understanding these patterns, and what drives them beneath the surface, helps explain why MS looks so different from one person to the next.
The Two Engines of MS Damage
MS progression is driven by two overlapping processes. The first is acute inflammation: the immune system attacks the protective coating around nerve fibers (myelin), creating lesions in the brain and spinal cord. These attacks cause the relapses most people associate with MS, and they tend to be most active early in the disease. The second process is more gradual. Immune cells trapped inside the central nervous system, particularly certain types of white blood cells and activated brain-resident immune cells, cause slow, ongoing damage to nerve fibers themselves. This “smoldering” damage doesn’t produce dramatic flare-ups. Instead, it chips away at nerve function over months and years.
At the cellular level, this slow damage involves a cascade of problems. Damaged mitochondria, the energy-producing structures inside cells, can no longer power the nerve fiber’s basic maintenance functions. When nerves lose their energy supply, calcium floods into the fiber and triggers enzymes that break down its internal structure. Iron released from damaged tissue amplifies this process by fueling oxidative stress. Nerve-supporting cells called oligodendrocytes, which normally provide energy and insulation to nerve fibers, fail or die, leaving axons even more vulnerable.
Relapsing-Remitting MS and Its Trajectory
Most people are diagnosed with relapsing-remitting MS (RRMS), where distinct episodes of new or worsening symptoms are followed by periods of recovery. Early in the disease, the brain can compensate for damage through its natural plasticity, which is why recovery from early relapses is often good. But each attack leaves behind some residual damage, and the brain’s capacity to compensate gradually shrinks.
Without treatment, natural history studies show that more than 50% of people with RRMS transition to a secondary progressive phase within 10 years, and about 90% do so within 25 years. More recent data from treated populations paints a somewhat better picture: roughly 10% convert at 10 years, 50% at 20 years, and 93% at 30 years. The difference likely reflects the impact of modern disease-modifying therapies, though the transition still happens for most people eventually.
Secondary Progressive MS
Secondary progressive MS (SPMS) is diagnosed when someone who originally had a relapsing-remitting course develops a steady worsening of disability that persists for at least six months to a year. There’s no single test or scan that marks the moment of transition. The diagnosis is made retrospectively, after a pattern of gradual decline becomes clear. Some people still have occasional relapses layered on top of this progression, while others simply get slowly worse without distinct flare-ups.
A key clinical threshold is reaching an EDSS score of 4, which roughly corresponds to being able to walk without aid but having noticeable functional limitations. Once someone reaches this level, further progression tends to follow a more predictable downhill course regardless of earlier relapse patterns, suggesting that the smoldering neurodegenerative process has taken over as the main driver of disability.
Primary Progressive MS
About 10 to 15% of people with MS never experience a relapsing-remitting phase. Instead, disability accumulates from the very beginning. This form, called primary progressive MS (PPMS), tends to affect the spinal cord more prominently, often causing gradually worsening difficulty with walking. PPMS is typically diagnosed later in life compared to RRMS and affects men and women at closer to equal rates, whereas RRMS is two to three times more common in women.
Smoldering Damage and Silent Progression
One of the most important shifts in MS research has been the recognition that disability can worsen even when relapses are completely controlled. This phenomenon, called progression independent of relapse activity (PIRA), occurs when people accumulate disability without new relapses and without new inflammatory lesions on MRI. PIRA is now considered a hallmark of what researchers call “smoldering MS,” and it can begin surprisingly early in the disease course, not just in later progressive stages.
A key feature of smoldering MS is the presence of slowly evolving lesions with rims of activated immune cells at their edges. These lesions, detectable on specialized MRI sequences, expand outward over time, destroying surrounding tissue. Unlike acute lesions, they don’t involve a breach of the blood-brain barrier and are far less likely to repair themselves through remyelination. At autopsy, these lesions are found almost exclusively in people who reached the progressive phase, but on imaging, they can be spotted years earlier during the relapsing phase. People with four or more of these lesions at baseline have a 1.6-fold increased risk of clinical progression.
How the Brain Shrinks Over Time
Everyone loses a small amount of brain volume with normal aging. In MS, this process is dramatically accelerated. People with MS lose brain volume at a rate of roughly 0.5 to 1.35% per year, far exceeding what’s expected from aging alone. This atrophy reflects the cumulative loss of nerve fibers and the brain tissue that supports them.
Specific patterns of atrophy can signal trouble ahead. Shrinkage of the thalamus, a deep brain structure that acts as a relay station for sensory and motor signals, predicts future disability progression even when overall brain volume looks relatively preserved. Spinal cord atrophy is another strong predictor. People who later converted from RRMS to SPMS were losing cervical spinal cord volume at a rate of about 2.2% per year, more than double the rate seen in those who remained relapsing-remitting. This accelerated spinal cord shrinkage was detectable at least four years before the clinical transition happened.
Blood Tests That Track Nerve Damage
When nerve fibers are damaged, they release a structural protein called neurofilament light chain (NfL) into the spinal fluid and blood. Measuring NfL levels gives doctors a real-time window into how much nerve damage is occurring. Levels are higher during and after relapses, higher in progressive MS than in early relapsing MS, and higher in people who go on to accumulate more disability over time.
NfL has proven to be a more reliable predictor of disease progression than traditional MRI measures like lesion counts. It drops in response to effective treatment, providing a way to gauge whether therapy is actually protecting nerves. In people on high-efficacy therapies, NfL levels tend to decrease even when clinical symptoms stay stable. When levels remain elevated despite treatment, it often signals ongoing damage and a worse prognosis.
What Speeds Up or Slows Down Progression
Several modifiable factors influence how quickly MS progresses. Smoking increases the risk of reaching significant disability milestones by about 22 to 27%. Obesity (a BMI of 30 or above) carries an even larger effect, increasing the risk by 37 to 45%. When smoking and obesity are combined, their effects don’t simply add up. They interact, producing worse outcomes than either factor alone. People who both smoked and had obesity had an 86% higher risk of reaching moderate disability compared to people with neither risk factor. The combination also disproportionately accelerated cognitive decline.
Disease-modifying therapies remain the most impactful tool for slowing progression. Some older therapies reduced the risk of reaching significant disability by about half over the long term. Across different treatments, the reduction in disability progression risk ranges from 19 to 68%, with higher-efficacy therapies generally performing better. Starting treatment early, before significant damage accumulates, appears to offer the greatest benefit, since it’s far easier to prevent nerve damage than to repair it once it’s occurred.
Signs That Progression May Be Starting
Because the transition to progressive MS is diagnosed only in hindsight, recognizing early warning signs matters. The most common pattern is a gradual worsening of walking ability, fatigue, or cognitive function that doesn’t recover the way relapse symptoms used to. You might notice that your baseline between relapses is slowly shifting, that you’re not bouncing back as fully, or that new symptoms are creeping in without a clear relapse to explain them.
On imaging, an increase of 1 milliliter per year in the volume of shrinking older lesions was associated with a nearly fivefold increase in the risk of developing SPMS. Thinning of the retinal nerve fiber layer, measured with a quick, noninvasive eye scan, also predicts future disability. People with thinner retinal nerve fibers at baseline had a 90% increased risk of disability progression in the first three years, rising to a fourfold increase after three years. These tools, combined with NfL blood tests and spinal cord measurements, are increasingly helping doctors identify progression before it becomes clinically obvious.