Multiple sclerosis (MS) is an inflammatory, chronic, degenerative disorder that affects nerves in the brain and spinal cord. This neural system works efficiently unless there is a disease process affecting the pathways in the spinal cord and brain. Multiple sclerosis is one of the diseases that can affect these pathways.
Myelin, the fatty substance that surrounds and insulates nerves and facilitates the conduction of nerve impulses is the initial target of inflammatory destruction in multiple sclerosis. MS is characterized by intermittent damage to myelin , called demyelination. Demyelination causes scarring and hardening (sclerosis) of nerve tissue in the spinal cord, brain, and optic nerves. Demyelination slows conduction of nerve impulses, which results in weakness, numbness, pain, and vision loss. Ultimately, this may result in deterioration of the nerves themselves, a process that's not reversible.

Because different nerves are affected at different times, MS symptoms often worsen (exacerbate), improve, and develop in different areas of the body. Early symptoms of the disorder may include vision changes (e.g., blurred vision, blind spots), numbness, dizziness, and muscle weakness. MS can progress steadily or cause acute attacks (exacerbations) followed by partial or complete reduction in symptoms (remission). Most patients with the disease have a normal lifespan.

Symptoms vary widely, depending on the amount of damage and which particular nerves are affected. People with severe cases of multiple sclerosis may lose the ability to walk or speak. Multiple sclerosis can be difficult to diagnose early in the course of the disease, because symptoms often come and go — sometimes disappearing for months.
Although multiple sclerosis can occur at any age, it most often begins in people between the ages of 20 and 40. Women are more likely to develop multiple sclerosis than are men.