Multiple Sclerosis (MS): causes, mechanisms, symptoms and treatments

In France Multiple Sclerosis affects approximately 1 person in 1000, nearly 70 000 people.
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Multiple sclerosis (MS) is the 2nd leading cause of acquired disability in young adults, after trauma. It currently affects 120,000 people in France, with 3,000 new cases diagnosed each year. The disease is a major public health issue, as it affects an active population in the process of building a life plan, with an average age of onset of 30.


Multiple sclerosis is an inflammatory disease of the central nervous system (brain, spinal cord and optic nerves). Most patients present their first symptoms between the ages of 25 and 35, with a high proportion of women (3 women for every man).


It is characterised by lesions known as plaques, in which the protective sheath of neurons known as myelin is destroyed, leading to degeneration of the nerve cells known as neurons and a loss of communication between the brain and peripheral organs. Myelin provides a protective sheath for nerve cell extensions (axons). It also promotes faster conduction of nerve impulses. This disease is known as autoimmune because the immune system, whose role is to protect the body against pathogenic agents (viruses, bacteria), in this case attacks a component of the body itself, known as ‘self’, in this case myelin.

Multiple sclerosis usually evolves in 2 phases. The first, characterised by transient attacks of symptoms, is known as the relapsing-remitting phase. It begins around the age of 30 and accounts for 85% of initial forms. The second, which generally begins 10 to 20 years after the onset of the disease, is known as the progressive phase. It is characterised by the continuous onset of permanent residual symptoms leading to cognitive or motor functional disability.


The different forms of multiple sclerosis progression

Various variations are possible between these two forms.

  • Relapsing forms: This is the most common form at the start of the disease (80 to 85% of cases). This form is characterised by acute phases, known as relapses, during which patients present numerous, intense symptoms, alternating with periods of complete remission, with no symptoms. Relapses can last from a few days to a month, with no disability developing between symptomatic phases.
  • Secondarily progressive forms: This phase appears after 5 to 20 years of relapsing-remitting disease in more than half of patients. It is characterised by the onset of permanent and progressive disability.
  • Immediately progressive forms: These forms affect 10 to 15% of patients from the start of the disease and progress very rapidly. Disability progresses steadily with no remission phases. These forms of MS generally appear in older people, on average around the age of 60.

All the functions of the nervous system can be affected, to varying degrees depending on the individual: vision, motor skills, balance and coordination, sensations, memory and intellectual capacity, etc.


What causes multiple sclerosis? Is it hereditary?

Multiple sclerosis is a multifactorial disease, developing in people who are genetically predisposed after exposure to one or more as yet unidentified environmental factors.

There are no hereditary forms of the disease, and the monozygotic twin (identical twin) of an affected individual, i.e. having the same DNA, has only a 25% risk of developing the disease. This observation confirms a genetic component to the disease, but proves that it is not hereditary.


The biological mechanisms of multiple sclerosis

Multiple sclerosis is an inflammatory disease of the central nervous system (brain and spinal cord). It is characterised by an immune attack on a protein, myelin, which protects the neurons and facilitates the passage of nerve impulses.


This is called an autoimmune disease because the immune system, whose normal role is to protect the body against viruses and bacteria, attacks a component of the body.

The disappearance of myelin in the lesions characteristic of multiple sclerosis (the plaques) makes it impossible for the nerve signal to reach the peripheral organs, particularly the muscles. Some patients are able to reform the myelin sheaths, particularly at the start of the disease, which means that the disease progresses more slowly and the onset of disability is delayed. Current research into therapies is based on this potential in order to determine the physiological mechanisms involved.


Diagnosis and symptoms of multiple sclerosis

The diagnosis of multiple sclerosis is based on the combination of neurological symptoms associated with the presence of inflammatory plaques on MRI scans, which respond to spatial dissemination (brain, spinal cord, optic nerve) and temporal dissemination (inflammatory plaques of different ages or which appear over time).

Treatments for multiple sclerosis

To date, there is no cure for multiple sclerosis. Therapies aim to reduce the inflammatory reaction and thus slow the progression of disability. Observation of the intrinsic individual capacity to repair lesions caused by immune attack is opening up new therapeutic avenues.


Bruno Stankoff, neurologist (AP-HP / Sorbonne University) and co-leader of a research team at the Institut du Cerveau, explains the causes, symptoms and treatments of multiple sclerosis:

Discover our latest conference dedicated to multiple sclerosis:

Online conference – Multiple sclerosis – Institut du Cerveau (

Listen again to Professor Catherine Lubetzki, who talks about the challenges facing multiple sclerosis research:

Braincast #9 with Catherine Lubetzki – Repairing neurons: the ultimate challenge in multiple sclerosis – Institut du Cerveau (

Last updated May 2024.