Demyelination is a term used to describe the destruction of a substance called myelin that surrounds axonal fibres. This is caused by diseases that damage the myelin sheath or the cells that form it.
Myelin is mainly composed of lipids and is formed from the processes of the oligodendroglial cells which serve to protect, develop and regenerate nerve cells. Damaged myelin leads to disruption of neuronal signal transmission which eventually leads to severe neurological symptoms.
Dysmyelination on the other hand, is a term used to describe the defective structure and function of myelin sheaths. Unlike demyelination, nerve lesions are not a feature of dysmyelination.
Demyelination is typically seen in neurodegenerative disorders such as multiple sclerosis (MS). Some known causes of demyelinating diseases include:
- Genetic predisposition
- Certain infections
- Some autoimmune disorders
- Exposure to toxic chemicals
One of most common forms of autoimmune demyelinating disease is MS. Autoimmune disorders are those caused by the body’s immune attack of its own tissues and in MS, the myelin is mistaken for a foreign body and targeted by immune cells. The immune system produces excess amounts of inflammatory markers called cytokines by raising the activities of tumor necrosis factor or interferon. These inflammatory agents damage the myelin sheath and lead to demyelination or stripping of the myelin away from the nerve fibre that it usually insulates.
Other autoimmune disorders that can cause demyelination include transverse myelitis, acute disseminated encephalomyelitis, Guillain-Barré Syndrome, chronic inflammatory demyelinating polyneuropathy, central pontine myelinosis, Charcot Marie Tooth disease and leukodystrophy.
Dysmyelination refers to malformed and defective myelin sheath as opposed to the destruction of previously normal myelin that is seen in demyelination.
Dysmyelination disorders often arise from hereditary mutations that affect the synthesis and formation of myelin. The disorders can be categorized into those that affect mainly the white brain matter and those that affect both the white and grey matter. Dysmyelination disorders are also referred to as leukodystrophies and the most common example is metachromatic leukodystrophy.