What MS is
Multiple sclerosis is a chronic autoimmune disease of the central nervous system: the brain, the spinal cord, and the optic nerves. The immune system attacks myelin, the covering around nerve fibers, and over time damages the fibers underneath. That demyelination slows or blocks nerve signals, so the symptom depends on where the damage sits. Damaged areas show on MRI as lesions, whose pattern is much of how MS is diagnosed. It is one of the most common neurological diseases of young adults: a 2019 study in the journal Neurology estimated that nearly 1 million American adults live with it.
MS is not the only demyelinating disease. NMOSD and MOGAD look almost identical on a first scan, are treated differently, and can be made worse by some MS drugs. Our demyelinating diseases page sets the three side by side.
Visual guide
Myelin and the central nervous system
- Location: MS affects the central nervous system.
- Myelin: The immune system damages the insulation around nerve fibers.
- Signals: This damage can interfere with communication along nerves.
Symptoms
An attack builds over hours to days, lasts weeks, then improves partly or fully. The early signs matter, because treatment works best started early.
- Fatigue. Exhaustion rest does not fix, often the most limiting symptom.
- Vision. Dim vision in one eye with pain on eye movement (optic neuritis), or double vision.
- Numbness and tingling. Face, limbs, or a band of tightness around the trunk.
- Weakness or stiffness. Usually the legs, making walking and stairs harder.
- Balance problems. Unsteady walking, clumsiness, dizziness.
- Thinking and memory. Slower processing, word-finding and recall trouble.
- Bladder and bowel. Urgency, frequency, incomplete emptying, constipation.
Heat sensitivity, pain, spasms, depression, and anxiety are common too.
Relapse or emergency?
Sudden weakness or numbness on one side, trouble speaking, or sudden vision loss can be a stroke as easily as a relapse. Call 911. A symptom that builds over days, lasts more than 24 hours, and has no fever or heat to explain it is more likely a relapse: call your neurologist the same day.
Anatomy at a glanceNerve signals
What changes when myelin is damaged
A protective layer helps messages travel through the brain and spinal cord.
Myelin
An insulating covering around many nerve fibers. Short gaps between its segments are normal.
Nerve fiber (axon)
The fiber inside carries electrical messages. The covering and the fiber are different structures.
Damaged myelin
Loss of this covering can slow or block signals. In MS, the nerve fiber itself can also be injured.
Where the damage occurs helps explain which symptoms appear.
Simplified nerve fibers in the central nervous system. This diagram does not show the extent of any person's MS.
Causes and risk factors
Nobody inherits MS outright and nobody catches it. Genetic susceptibility plus environmental exposure sets off the immune response. The best-established associations:
| Risk factor | What is known |
|---|---|
| Epstein-Barr virus | A 2022 Science study of 10 million US military personnel linked prior EBV infection to a roughly 32-fold higher risk |
| Low vitamin D, distance from the equator | MS is more common farther from the equator, and low vitamin D tracks with higher risk |
| Smoking, obesity in adolescence | Both raise risk, and smoking speeds progression afterwards |
| Family history | Raises risk, though most patients have no affected relative |
If you are planning a family, read our guide to MS and pregnancy early.
How MS is diagnosed
No single test confirms MS, and plenty of other conditions look like it on MRI. Diagnosis means assembling evidence, then testing it against rules.
| Test | What it answers |
|---|---|
| History and examination | Where the lesions are and when they happened. Most of the diagnosis is made here |
| MRI of brain and spine | Lesions in MS-typical places, and whether some are older. See reading your MS MRI |
| Lumbar puncture | Oligoclonal bands or kappa free light chains: immune activity inside the nervous system |
| Blood tests | Ruling out mimics. Neurofilament light tracks ongoing nerve damage |
The McDonald criteria
The McDonald criteria turn those findings into a diagnosis. For years they asked for two things: damage in more than one MS-typical place (dissemination in space) and damage at more than one time (dissemination in time), shown by a second attack, by new lesions on a later scan, or by a mix of old and actively inflamed lesions on a single scan. Oligoclonal bands in the spinal fluid could stand in for the second time point.
The 2024 revision keeps that logic but loosens the rule about time. The optic nerve now counts as a fifth typical location, and when lesions sit in four or five of the typical locations, MS can be diagnosed at a first attack without waiting for a second. Highly specific findings can also carry the diagnosis: kappa free light chains in the spinal fluid as an alternative to oligoclonal bands, and on advanced MRI the central vein sign and paramagnetic rim lesions. In some circumstances a diagnosis can now be made in people whose lesions were found by chance and who have never had an attack.
What has not changed is the third requirement: no better explanation. Migraine, small vessel disease, NMOSD, MOGAD, and several other conditions produce white spots, and the criteria only apply once those have been considered. The aim is speed with accuracy, so therapy starts before more damage accumulates, without labeling someone with MS who does not have it.
The types of MS
MS is classified by its course, not its severity. The type matters because some therapies are approved only for relapsing forms.
| Type | Defining pattern |
|---|---|
| Radiologically isolated syndrome | MS-typical lesions found on a scan done for another reason, no symptoms |
| Clinically isolated syndrome | A first episode not yet meeting full criteria, often transverse myelitis or optic neuritis |
| Relapsing-remitting MS | Relapses with recovery between them. The most common form |
| Secondary progressive MS | Relapsing MS that has shifted into steady accumulation of disability |
| Primary progressive MS | Steady worsening from the start, without distinct relapses |
Treatment
MS is not curable, but treatment changes its course, in three layers.
Disease-modifying therapy reduces relapses, prevents new lesions, and slows disability. It comes as a self-injection, a pill, or an infusion every few months, including the B-cell therapies and natalizumab; our treatment guide compares the classes. The direction of the field, and of this practice, is to start an effective therapy early rather than escalate after damage has accumulated. Whichever you start, plan on regular MRI and blood monitoring. A therapy that is not controlling the disease should be changed, not tolerated.
Symptom management is separate, because disease-modifying therapy does nothing for symptoms you already have: rehabilitation for walking, balance, and hand function, medication for pain, fatigue, spasticity, and bladder problems, mental health support.
Lifestyle complements medication rather than replacing it: exercise, diet, sleep, not smoking, and vitamin D if a test shows it low.
Visual guide
Different parts of MS care
- Disease activity: Disease-modifying therapy aims to reduce new inflammatory activity.
- Symptoms: Specific problems are treated alongside the underlying disease.
- Monitoring: Follow-up examinations and MRI help assess the plan.
When to see a neurologist
See one for a new neurological symptom lasting more than 24 hours, or if an MRI mentions demyelination or unexplained white matter lesions. If you already have MS, a visit is due for a suspected relapse, before pregnancy planning, and whenever you are unsure your therapy is working.
An MS evaluation here is a 60-minute visit with Dr. Ntranos, who trained in MS and demyelinating diseases at Mount Sinai and researched MS at Johns Hopkins. He reads your MRI images himself, not just the report, so bring or upload them through our patient tools page. Ongoing MS care, therapy decisions, relapses, and second opinions run through our Multiple Sclerosis Clinic. MRI, blood work, and any lumbar puncture are arranged at outside facilities, never performed in our office. You can request a visit in Beverly Hills or West Los Angeles, or by video anywhere in California.
Frequently asked questions
References
- Wallin MT, et al. The prevalence of MS in the United States: a population-based estimate using health claims data. Neurology, 2019.
- Bjornevik K, et al. Longitudinal analysis reveals high prevalence of Epstein-Barr virus associated with multiple sclerosis. Science, 2022.
- Montalban X, et al. Diagnosis of multiple sclerosis: 2024 revisions of the McDonald criteria. The Lancet Neurology, 2025.
- National Institute of Neurological Disorders and Stroke. Multiple sclerosis.
- Evolving the diagnosis of multiple sclerosis: a new landscape in light of the 2024 McDonald criteria. Biomedicines, 2025.
- MedlinePlus, U.S. National Library of Medicine. Multiple sclerosis.