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Demyelinating diseases

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Demyelinating diseases

Demyelinating disease
Classification and external resources
10 9 MeSH D003711

A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged.[1] This damage impairs the conduction of signals in the affected nerves. In turn, the reduction in conduction ability causes deficiency in sensation, movement, cognition, or other functions depending on which nerves are involved.

Some demyelinating diseases are caused by genetics, some by infectious agents, some by autoimmune reactions, and some by unknown factors. Organophosphates, a class of chemicals which are the active ingredients in commercial insecticides such as sheep dip, weed-killers, and flea treatment preparations for pets, etc., will also demyelinate nerves. Neuroleptics can also cause demyelination.[2]

The precise mechanism of demyelination is not clearly understood but there is good evidence that the body's own immune system is at least partially responsible. Acquired immune system cells called T-cells are known to be present at the site of lesions. Other immune system cells called Macrophages (and possibly Mast cells as well) also contribute to the damage.[3]

Causes

Some demyelinating diseases are caused by genetics, some by infectious agents, some by autoimmune reactions, some by exposure to chemical agents, and some by unknown factors.

Evolutionary Considerations

The role of prolonged cortical myelination in human evolution has been implicated as a contributing factor in some cases of demyelinating disease. Unlike other primates, humans exhibit a unique pattern of postpubertal myelination, which may contribute to the development of psychiatric disorders and neurodegenerative diseases that present in early adulthood and beyond. The extended period of cortical myelination in humans may allow greater opportunity for disruption in myelination, resulting in the onset of demyelinating disease.[4] Furthermore, it has been noted that humans have significantly greater prefrontal white matter volume than other primate species, which implies greater myelin density.[5] Increased myelin density in humans as a result of a prolonged myelination may therefore structure risk for myelin degeneration and dysfunction. Evolutionary considerations for the role of prolonged cortical myelination as a risk factor for demyelinating disease are particularly pertinent given that genetics and autoimmune deficiency hypotheses fail to explain many cases of demyelinating disease. As has been argued, diseases such as multiple sclerosis cannot be accounted for by autoimmune deficiency alone, but strongly imply the influence of flawed developmental processes in disease pathogenesis.[6] Therefore, the role of the human-specific prolonged period of cortical myelination is an important evolutionary consideration in the pathogenesis of demyelinating disease.

Signs and symptoms

Symptoms that present in demyelinating diseases are different for each condition. Below is a list of symptoms that can present in a person with a demyelinating disease.:[7]

  • Ocular paralysis
  • Impaired muscle coordination
  • Weakness (muscle)
  • Loss of sensation
  • Impaired vision
  • Neurological symptoms
  • Unsteady gait
  • Spastic paraparesis
  • Incontinence
  • Hearing problems
  • Speech problems

Diagnosis

Below are various methods/techniques used to diagnose Demyelinating Diseases.

Treatment

Treatment typically involves improving the patient's quality of life. This is accomplished through the management of symptoms or slowing the rate of demyelination. Treatment can include medication, lifestyle changes (i.e. quit smoking, adjusting daily schedules to include rest periods and dietary changes), counselling, relaxation, physical exercise, patient education and, in some cases, deep brain thalamic stimulation (in the case of tremors).[8] The progressive phase of MS appears to driven by the innate immune system, which may directly contribute to the neurodegenerative changes that occur in progressive MS. Until now, there are no therapies that specifically target innate immune cells in MS. As the role of innate immunity in MS becomes better defined, it may be possible to better treat MS by targeting the innate immune system.[9]

Treatments are patient-specific and depend on the symptoms that present with the disorder, as well as the progression of the condition.

Prognosis

Prognosis depends on the condition itself. Some conditions such as multiple sclerosis depend on the subtype of the disease and various attributes of the patient such as age, sex, initial symptoms and the degree of disability the patient experiences.[10] Life expectancy in Multiple sclerosis patients is 5 to 10 years lower than unaffected people.[11] MS is an inflammatory demyelinating disease of the central nervous system (CNS) that develops in genetically susceptible individuals after exposure to unknown environmental trigger(s).The bases for MS are unknown but are strongly suspected to involve immune reactions against autoantigens, particularly myelin proteins. The most accepted hypothesis is that dialogue between T-cell receptors and myelin antigens leads to an immune attack on the myelin-oligodendrocyte complex. These interactions between active T cells and myelin antigens provoke a massive destructive inflammatory response and promotes continuing proliferation of T and B cells and macrophage activation, which sustains secretion of inflammatory mediators.[12] Other conditions such as central pontine myelinolysis have about a third of patients recover and the other two thirds experience varying degrees of disability.[13] There are cases, such as transverse myelitis where the patient can begin recovery as early as 2 to 12 weeks after the onset of the condition.

Epidemiology

Incidence of demyelinating diseases vary from disorder to disorder. Some conditions, such as Tabes dorsalis appear predominantly in males and begins in mid-life. Optic neuritis on the other hand, occurs preferentially in females typically between the ages of 30 and 35.[14] Other conditions such as multiple sclerosis vary in prevalence depending on the country and population.[15] This condition can appear in children as well as adults.[11]

Types

Demyelinating diseases can be divided in those affecting the central nervous system and those presents in the peripheral nervous system, presenting different demyelination conditions.

The disorders affecting the CNS include:

These disorders are normally associated also with the conditions Optic neuritis and Transverse myelitis, which are inflammatory conditions, because inflammation and demyelination are frequently associated.

Demyelinating diseases of the peripheral nervous system include:

Research

Research is being conducted in a variety of very specific areas. The focus of this research is aimed at gaining more insight into how demyelinating disorders affect the central nervous system and peripheral nervous system,[16][17][18][19][20] how they develop and how these disorders are affected by various external inputs[21][22][23][24][25] . Much of the research is targeted towards learning about the mechanisms by which these disorders function in an attempt to develop therapies and treatments for individuals affected by these conditions.

Insights

Currently it is believed that N-cadherin plays a role in the myelination process. Experimentation has shown that N-cadherin plays an important role in producing a remyelination-facilitating environment.[16] It has been shown in animal models that there is a direct correlation between the amount of myelin debris present and the degree of Inflammation observed.[17]

Effects of environmental inputs

Experimentation has shown that manipulating the levels of thyroid hormone can be considered as a strategy to promote remyelination and prevent irreversible damage in Multiple sclerosis patients.[19] N-cadherin agonists have been identified and observed to stimulate neurite growth and cell migration, key aspects of promoting axon growth and remyelination after injury or disease.[21] It has been shown that intranasal administration of aTf (apotransferrin) can protect myelin and induce remyelination.[23]

Much of the research referenced in this section has been conducted in 2012 and represents very new information about demyelinating diseases and potential therapies for them.

In Other Animals

Demyelinating diseases/disorders have been found worldwide in various animals. Some of these animals include mice, pigs, cattle, hamsters, rats, sheep, Siamese kittens, and a number of dog breeds (including Chow Chow, Springer Spaniel, Dalmatian, Samoyed, Golden Retriever, Lurcher, Bernese Mountain Dog, Vizsla, Weimaraner, Australian Silky Terrier, and mixed breeds).[26][27]

See also

References

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