What Is Multiple Sclerosis? How MS Damages the Nervous System

What Is Multiple Sclerosis?

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) in which the immune system attacks myelin — the protective fatty sheath that surrounds nerve fibers in the brain, spinal cord, and optic nerves. When myelin is damaged, nerve signals slow or stop, causing a wide range of neurological symptoms depending on which nerves are affected.

"Sclerosis" refers to scar tissue (sclerae or plaques) that forms at sites of myelin damage. "Multiple" refers to the multiple sites of damage scattered throughout the CNS. MS affects approximately 1 million people in the United States and 2.8 million worldwide, and is more common in women than men (3:1 ratio) and more common in people of Northern European descent.

How MS Damages the Nervous System

The central nervous system is protected by the blood-brain barrier, but in MS, this barrier breaks down and immune cells (particularly T lymphocytes) cross into the CNS and attack myelin as if it were a foreign invader. The process:

1. Immune cells infiltrate the CNS and attack myelin sheaths
2. Inflammation causes edema and temporary myelin damage (explaining why symptoms often resolve during remission)
3. Repeated attacks cause progressive myelin loss and axonal damage
4. Oligodendrocytes (cells that produce myelin) attempt remyelination, but this repair is incomplete over time
5. Scar tissue (plaques) forms at lesion sites, permanently disrupting nerve conduction

What triggers the initial autoimmune response is not fully understood — genetics (HLA-DRB1 gene variants), viral infections (particularly Epstein-Barr virus), vitamin D deficiency, smoking, and geographic factors (MS is more common farther from the equator) all appear to play roles.

Types of MS

• Relapsing-Remitting MS (RRMS): The most common form (~85% of initial diagnoses). Characterized by episodes of new or worsening symptoms (relapses or attacks) followed by periods of partial or complete recovery (remission). Most relapses involve inflammation; remission occurs as inflammation subsides and partial remyelination occurs.
• Secondary Progressive MS (SPMS): Many people with RRMS eventually transition to a phase of gradual worsening without distinct relapses, usually 10–20 years after onset.
• Primary Progressive MS (PPMS): Approximately 15% of people with MS have gradual neurological worsening from onset without distinct relapses or remissions. More common in men and older age at onset. Historically harder to treat.
• Clinically Isolated Syndrome (CIS): A first episode of neurological symptoms lasting 24+ hours that may — or may not — develop into MS.

Symptoms

MS symptoms depend on where in the CNS lesions occur:

• Fatigue: The most common and debilitating symptom, often described as an overwhelming exhaustion unrelated to activity
• Numbness/tingling: Often in the face, arms, legs, or trunk
• Vision problems: Optic neuritis (inflammation of the optic nerve) causes eye pain and blurred or lost vision, often the first MS symptom
• Weakness and spasticity: Muscle weakness, stiffness, or spasms, particularly in the legs
• Balance and coordination problems: Difficulty walking, vertigo
• Bladder and bowel dysfunction
• Cognitive issues: Memory, concentration, and processing speed problems
• Pain: Neuropathic pain, the Lhermitte sign (electric shock sensation when flexing the neck)

Diagnosis

MS is diagnosed clinically using the McDonald Criteria, which requires evidence of damage in multiple areas of the CNS at multiple points in time. Key tools:

• MRI: Reveals white matter lesions in the brain and spinal cord. Gadolinium-enhancing lesions indicate active inflammation.
• Cerebrospinal fluid (CSF) analysis: Lumbar puncture to detect oligoclonal bands (abnormal proteins indicating immune activity in the CNS)
• Evoked potentials: Measure nerve signal speed along specific pathways

Treatment

MS treatment has been transformed by the development of disease-modifying therapies (DMTs) since the 1990s. Over 20 FDA-approved DMTs now exist, ranging from injectable interferons and glatiramer acetate to oral medications (fingolimod, dimethyl fumarate, siponimod) to powerful intravenous infusions (natalizumab, ocrelizumab, ofatumumab). Ocrelizumab (Ocrevus) was the first DMT approved for PPMS.

The goal of DMTs is to reduce relapse frequency and severity, slow MRI lesion accumulation, and delay disability progression. Symptom management — for fatigue, spasticity, pain, and bladder dysfunction — is equally important for quality of life.