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Multiple sclerosis (abbreviated MS, also known as
disseminated sclerosis or encephalomyelitis disseminata) is a
disease in which the fatty myelin sheaths around the axons of the
brain and spinal cord are damaged, leading to demyelination and
scarring as well as a broad spectrum of signs and symptoms.
Disease onset usually occurs in young adults, and it is more
common in females. It has a prevalence that ranges between 2 and
150 per 100,000. MS was first described in 1868 by Jean-Martin
Charcot.
MS affects the ability of nerve cells in the brain and spinal cord
to communicate with each other. Nerve cells communicate by sending
electrical signals called action potentials down long fibers
called axons, which are wrapped in an insulating substance called
myelin. In MS, the body's own immune system attacks and damages
the myelin. When myelin is lost, the axons can no longer
effectively conduct signals. The name multiple sclerosis refers to
scars (scleroses better known as plaques or lesions) in the white
matter of the brain and spinal cord, which is mainly composed of
myelin. Although much is known about the mechanisms involved in
the disease process, the cause remains unknown. Theories include
genetics or infections. Different environmental risk factors have
also been found.
Almost any neurological symptom can appear with the disease, and
often progresses to physical and cognitive disability. MS takes
several forms, with new symptoms occurring either in discrete
attacks (relapsing forms) or slowly accumulating over time
(progressive forms). Between attacks, symptoms may go away
completely, but permanent neurological problems often occur,
especially as the disease advances.
There is no known cure for MS. Treatments attempt to return
function after an attack, prevent new attacks, and prevent
disability. MS medications can have adverse effects or be poorly
tolerated, and many patients pursue alternative treatments,
despite the lack of supporting scientific study. The prognosis is
difficult to predict; it depends on the subtype of the disease,
the individual patient's disease characteristics, the initial
symptoms and the degree of disability the person experiences as
time advances. Life expectancy of patients is nearly the same as
that of the unaffected population.
Classification
Progression of MS subtypes
Several subtypes, or patterns of progression, have been described.
Subtypes use the past course of the disease in an attempt to
predict the future course. They are important not only for
prognosis but also for therapeutic decisions. In 1996 the United
States National
Multiple Sclerosis Society standardized four subtype definitions:
1. relapsing remitting,
2. secondary progressive,
3. primary progressive, and
4. progressive relapsing.
The relapsing-remitting subtype is characterized by unpredictable
relapses followed by periods of months to years of relative quiet
(remission) with no new signs of disease activity. Deficits
suffered during attacks may either resolve or leave sequelae, the
latter being more common as a function of time. This describes the
initial course of 85–90% of individuals with MS. When deficits
always resolve between attacks, this is sometimes referred to as
benign MS, although patients will still accrue some degree of
disability in the long term.[citation needed] The
relapsing-remitting subtype usually begins with a clinically
isolated syndrome (CIS). In CIS, a patient has an attack
suggestive of demyelination, but does not fulfill the criteria for
multiple sclerosis. However only 30 to 70% of persons experiencing
CIS later develop MS
Secondary progressive MS (sometimes called "galloping MS")
describes around 65 % of those with an initial relapsing-remitting
MS, who then begin to have progressive neurologic decline between
acute attacks without any definite periods of remission.
Occasional relapses and minor remissions may appear. The median
time between disease onset and conversion from relapsing-remitting
to secondary progressive MS is 19 years.
The primary progressive subtype describes the approximately 10–15%
of individuals who never have remission after their initial MS
symptoms. It is characterized by progression of disability from
onset, with no, or only occasional and minor, remissions and
improvements. The age of onset for the primary progressive subtype
is later than for the relapsing-remitting, but similar to mean age
of progression between the relapsing-remitting and the secondary
progressive. In both cases it is around 40 years of age.
Progressive relapsing MS describes those individuals who, from
onset, have a steady neurologic decline but also suffer clear
superimposed attacks. This is the least common of all subtypes.
Atypical variants of MS with non-standard behavior have been
described. These include Devic's disease, Balo concentric
sclerosis, Schilder's diffuse sclerosis and Marburg multiple
sclerosis; and there is debate on whether they are MS variants or
different diseases. Multiple sclerosis also behaves differently in
children, taking more time to reach the progressive stage.
Nevertheless they still reach it at a lower mean age than adults
Signs and symptoms
Main article: Multiple sclerosis signs and symptoms
Main symptoms of multiple sclerosis
The person with MS can suffer almost any neurological symptom or
sign, including changes in sensation (hypoesthesia and
paraesthesia), muscle weakness, muscle spasms, or difficulty in
moving; difficulties with coordination and balance (ataxia);
problems in speech (dysarthria) or swallowing (dysphagia), visual
problems (nystagmus, optic neuritis, or diplopia), fatigue, acute
or chronic pain, and bladder and bowel difficulties. Cognitive
impairment of varying degrees and emotional symptoms of depression
or unstable mood are also common. Uhthoff's phenomenon, an
exacerbation of extant symptoms due to an exposure to higher than
usual ambient temperatures, and Lhermitte's sign, an electrical
sensation that runs down the back when bending the neck, are
particularly characteristic of MS although not specific. The main
clinical measure of disability progression and symptom severity is
the Expanded Disability Status Scale or EDSS.
Symptoms of MS usually appear in episodic acute periods of
worsening (called relapses, exacerbations, bouts, attacks, or
"flare-ups"), in a gradually progressive deterioration of
neurologic function, or in a combination of both. Multiple
sclerosis relapses are often unpredictable, occurring without
warning and without obvious inciting factors with a rate rarely
above 1 and a half per year. Some attacks, however, are preceded
by common triggers. Relapses occur more frequently during spring
and summer. Viral infections such as the common cold, influenza,
or gastroenteritis increase the risk of relapse. Stress may also
trigger an attack. Pregnancy affects the susceptibility to
relapse, with a lower relapse rate at each trimester of gestation.
During the first few months after delivery, however, the risk of
relapse is increased. Overall, pregnancy does not seem to
influence long-term disability. Many potential triggers have been
examined and found not to influence MS relapse rates. There is no
evidence that vaccination and breast feeding, physical trauma, or
Uhthoff's phenomenon are relapse triggers.
Causes
Most likely MS occurs as a result of some combination of genetic,
environmental and infectious factors. Epidemiological studies of
MS have provided hints on possible causes for the disease.
Theories try to combine the known data into plausible
explanations, but none has proved definitive.
HLA region of Chromosome 6. Changes in this area increase the
probability of suffering MS.
MS is not considered a hereditary disease. However, a number of
genetic variations have been shown to increase the risk of
developing the disease.
The risk of acquiring MS is higher in relatives of a person with
the disease than in the general population, especially in the case
of siblings, parents, and children. The disease has an overall
familial recurrence rate of 20%. In the case of monozygotic twins,
concordance occurs only in about 35% of cases, while it goes down
to around 5% in the case of siblings and even lower in
half-siblings. This indicates susceptibility is partly
polygenically driven.
It seems to be more common in some ethnic groups than others.
Apart from familial studies, specific genes have been linked with
MS. Differences in the human leukocyte antigen (HLA) system a
group of genes in chromosome 6 that serves as the major
histocompatibility complex (MHC) in humans increase the
probability of suffering MS. The most consistent finding is the
association between multiple sclerosis and alleles of the MHC
defined as DR15 and DQ6. Other loci have shown a protective
effect, such as HLA-C554 and HLA-DRB
Different environmental factors, both of infectious and non
infectious origin have been proposed as risk factors for MS.
Although some are partly modifiable, only further research
especially clinical trials will reveal whether their elimination
can help prevent MS.
MS is more common in people who live farther from the equator,
although many exceptions exist. Decreased sunlight exposure has
been linked with a higher risk of MS. Decreased vitamin D
production and intake has been the main biological mechanism used
to explain the higher risk among those less exposed to sun.
Severe stress may also be a risk factor although evidence is weak.
Smoking has also been shown to be an independent risk factor for
developing MS. Association with occupational exposures and toxins
mainly solvents has been evaluated, but no clear conclusions have
been reached. Vaccinations were also considered as causal factors
for the disease; however, most studies show no association between
MS and vaccines. Several other possible risk factors, such as diet
and hormone intake, have been investigated; however, more evidence
is needed to confirm or refute their relation with the disease.
Gout occurs less than would statistically be expected in people
with MS, and low levels of uric acid have been found in MS
patients as compared to normal individuals. This led to the theory
that uric acid protects against MS, although its exact importance
remains unknown.
Infections
Many microbes have been proposed as potential infectious triggers
of MS, but none has been substantiated.
Genetic susceptibility can explain some of the geographic and
epidemiological variations in MS incidence, like the high
incidence of the disease among some families or the risk decline
with genetic distance, but does not account for other phenomena,
such as the changes in risk that occur with migration at an early
age. An explanation for this epidemiological finding could be that
some kind of infection, produced by a widespread microbe rather
than a rare pathogen, is the origin of the disease. Different
hypotheses have elaborated on the mechanism by which this may
occur. The hygiene hypothesis proposes that exposure to several
infectious agents early in life is protective against MS, the
disease being a response to a later encounter with such agents.
The prevalence hypothesis proposes that the disease is due to a
pathogen more common in regions of high MS prevalence. This
pathogen is very common, causing in most individuals an
asymptomatic persistent infection. Only in a few cases, and after
many years since the original infection, does it cause
demyelination. The hygiene hypothesis has received more support
than the prevalence hypothesis.
Evidence for viruses as a cause includes the presence of
oligoclonal bands in the brain and cerebrospinal fluid of most
patients, the association of several viruses with human
demyelination encephalomyelitis, and induction of demyelination in
animals through viral infection. Human herpes viruses are a
candidate group of viruses linked to MS. Individuals who have
never been infected by the Epstein-Barr virus have a reduced risk
of having the disease, and those infected as young adults have a
greater risk than those who had it at a younger age. Although some
consider that this goes against the hygiene hypothesis, since the
non-infected have probably experienced a more hygienic upbringing,
others believe that there is no contradiction since it is a first
encounter at a later moment with the causative virus that is the
trigger for the disease. Other diseases that have also been
related with MS are measles, mumps and rubella.
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