The myelin sheath is a fatty layer that protects nerve cells, extending to cover the nerve throughout its length, except in small, unmyelinated gaps known as the nodes of Ranvier. Myelin aids in accelerating the speed at which an impulse travels along a nerve by causing the impulse to jump from one node of Ranvier to the next. Damage to the myelin sheath, called demyelination, compromises the nerve's ability to conduct impulses.
Diseases that cause nerve inflammation can also damage the myelin sheath. Multiple sclerosis, an autoimmune disorder in which the body's immune system attacks its own nerves, is the most common form of demyelinating disease. Certain types of encephalitis, or brain inflammation, can also damage myelin.
Mayo Clinic neurologist Dr. Jerry W. Swanson lists Devic's disease, also known as neuromyelitis optica, as another inflammatory demyelinating disease. It affects the optic nerve and spinal cord and can cause permanent blindness.
Dr. Seth Love of the Department of Neuropathology at the University of Bristol Institute of Clinical Neurosciences states in an article in the Journal of Clinical Pathology that progressive multifocal leucoencephalopathy, or PML, is the most common viral demyelinating disease. This disease causes symptoms of nerve damage such as impaired motor function, speech, brain function and vision.
Other viruses such as human immunodeficiency virus, or HIV, the measles virus, and herpes zoster, the virus associated with chickenpox, can also damage myelin.
Love notes that liver damage from alcohol abuse can cause a demyelinating condition called central pontine myelinolysis, or CPM. This disease is characterized by the rapid onset of confusion, arm and leg weakness, and speech impairment. This condition can also occur in patients who have undergone liver transplants.
Inadequate Blood Supply
In most cases lack of blood supply causes tissue death in the brain, but sometimes when the myelin sheath doesn't received enough oxygen from the blood, demyelination can ensue. This condition can occur in patients who have severe cerebrovascular disease—any condition related to the blood supply in the brain—or in cases of heart attack, suffocation, drug overdose or carbon monoxide poisoning, according to Love.
When a tumor, artery or vein places pressure on the fifth cranial, or trigeminal nerve, demyelination can occur in the area of compression. The result is trigeminal neuralgia, a condition characterized by shooting, electrical shock–like pain that radiates over the facial area on the affected side. The Merck Manual for Healthcare Professionals states that the episodes of pain are brief, lasting from a few seconds up to 2 minutes, but can be incapacitating in their severity.