A multiple sclerosis (MS) attack, also called a relapse or exacerbation, is the sudden onset of new neurological symptoms or the reoccurrence of previous resolved symptoms. Further, true attacks typically last at minimum 24 hours, usually days to weeks, and generally are agreed to have to occur at least 30 days from a previous attack, so that they are separated in time.
The underlying cause of the MS relapse is inflammation in the nervous system. The inflammatory immune response strips the nerves of their insulation, or myelin, allowing the electrical signals to “short out” as they move about the nervous system. When inflammation causes the myelin to be damaged, the medical community refers to the process as demyelination. Demyelination occurs at focal points, termed “lesions” on MRI, resulting in many of the symptoms associated with MS.
Inflammation can vary in strength and location, which results in a wide-array of symptoms. Symptoms from an attack can vary from mild, such as a blurring of vision, to severe, such as complete loss of vision. They also vary in functional outcome by location: inflammation in the brain can cause problems with thinking whereas inflammation in the sensory tracts of the spinal cord may cause numbness. Typically, the formation of a new neurological symptom results from a new focal area of demyelination, or MS lesion. The deterioration of previous symptoms results from the reactivation, and subsequent worsening, of an old MS lesion. (http://www.nationalmssociety.org/about-multiple-clerosis/treatments/exacerbations/index.aspx).
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