Demyelination refers to the loss or damage of myelin in the nervous system. Myelin plays an important role in the transmission of neuronal signals by electrically insulating nerve fibers (axons). For this reason, demyelination without treatment leads to multiple impairments in the long term; however, prognoses differ for different underlying causes.
What is demyelination?
Demyelination refers to the loss or damage of myelin in the nervous system. Figure shows neuron with myelin sheath. Demyelination is also known as demyelination and can affect both the central and peripheral nervous systems. Myelin is a biological membrane that contains numerous lipids. Various body cells can produce myelin, for example Schwann’s cells or cells of the peripheral and central nervous system. The name myelin is derived from the Greek word for marrow or brain (“myelòs”). Because myelin reflects light well, it appears white when viewed under a microscope. This is also where the term “white matter” comes from, which refers to a specific type of neuronal tissue: This tissue consists mainly of nerve cells whose nerve fibers (axons) are surrounded by myelin. Myelin is of great importance for the proper functioning of the human nervous system.
Function and purpose
As an insulating sheath, it surrounds the axons of nerve cells, thereby promoting the transmission of electrical impulses. Electrical insulation increases the speed of conduction and enhances the reliability of signal transmission. The disturbed neuronal communication therefore leads to rather diffuse complaints. Examples are fatigue, motor disturbances, feeling of weakness and visual disturbances. Demyelination is a pathological damage or loss of myelin. It occurs primarily in the context of a demyelinating disease such as multiple sclerosis. Another possible cause of demyelination is direct damage to the nerve cells; medicine refers to this form of demyelination as primary neuronal damage. In these cases, defects in the cell bodies or axons lead to destruction of the myelin. However, the effects on signal transmission are largely the same in both variants. In addition, physicians assume an influence of the individual lifestyle in almost all forms of demyelination. Diet, smoking and obesity are just a few factors that may play a role in this context. Depending on the spatial distribution of the affected nerve cells, experts speak of diffuse or focal demyelination. In focal demyelination, the demyelinated nerve cells are located in spatial proximity to each other and form hotspots. Multiple such foci are also possible. In progressive demyelinating diseases, the foci gradually spread as the disease progressively damages new neurons. Unlike focal demyelination, diffuse variant does not form contiguous areas of demyelinated neurons: In this case, myelin damage does not follow a known pattern.
Diseases and symptoms
Diseases associated with demyelination may be a consequence of inflammatory and degenerative processes. As medullary decay, degenerative-metabolic demyelination potentially occurs, for example, after damage to the brain, which can manifest after infections and (in rarer cases) vaccinations. In the majority of cases, however, demyelinating diseases are primarily diseases such as multiple sclerosis. This form of nervous degeneration leads to the destruction of the medullary sheaths that electrically insulate the axons of the nerve cells. The cells of the central nervous system, i.e. the brain and the spinal cord, are affected. Multiple sclerosis is a progressive, chronic inflammatory disease whose exact origins are still unknown. Possible causes include inflammation, metabolic impairment, infection, nutrition, poisoning, and various malfunctions within the immune system. Multiple sclerosis progresses in relapses, between which the disease may temporarily stabilize. Another demyelinating disease is leukoencephalitis.Leukoencephalitis is a form of brain inflammation that affects the white matter of the brain and gradually reduces it. Leukoencephalitis is a variant of panencephalitis along with polioencephalitis (an inflammation of the gray matter). Another disease that leads to demyelination is neuromyelitis optica (NMO) or Devic syndrome. Demyelination in NMO occurs in a focal pattern. Recurrent inflammation of the optic nerve and long-standing inflammation of the spinal cord (myelitis) are the most critical risk factors for NMO. Visual disturbances, weakness, paralysis, and bladder dysfunction, among other symptoms, may occur as signs of NMO. Permanent damage from NMO is within the realm of possibility, although treatment often yields good results and can prevent long-term impairment. While inflammation causes demyelination in multiple sclerosis and leukoencephalitis, a metabolic disorder is responsible for myelin breakdown in leukodystrophy. Various underlying metabolic diseases can be considered as triggers, which in turn usually have genetic causes. Leukodystrophy also leads to rather diffuse symptoms. A demyelinating disease that may already be noticeable in newborns and young children is Alexander disease. The brain of affected children shows an insufficient mass of myelin membranes at an early age. As a result, in addition to the usual motor symptoms, conspicuous developmental delays also manifest in comparison with children of the same age. However, Alexander disease may also manifest for the first time in adulthood. The disease is progressive at any age and is not curable. The cause of Alexander disease is a very rare genetic abnormality.
