The brain of humans consists of gray matter and white matter. The gray matter contains the nerve cells.
The white matter of the brain is composed of nerve fibers and myelin. The nerve fibers form the connections between the nerve cells. Myelin is a fatty sheath wrapped around nerve fibers. The myelin sheath has two functions: insulation and acceleration of impulse conduction. Insulation is important for the prevention of short-circuits. Through its special construction, myelin accelerates the propagation of impulses along nerve fibers.
Myelin is an essential part of the white matter. When the myelin sheath is damaged or disappears, the conduction of impulses along nerve fibers slows down or fails completely. Consequently, brain functions become hampered or be lost.
In many neurological disorders in childhood, the white matter of the brain is predominantly involved. These are called "the white matter disorders". The diagnosis is usually made on the basis of MRI findings. White matter abnormalities on MRI can have different bases on tissue level. It may be that there is a lack of myelin because the myelin was never made in sufficient amounts. It may be that the myelin was formed all right, but is now broken down and lost. It may be that innumerable vacuoles are formed within the myelin sheath. It may be that scar tissue is formed within the white matter. There may be increased amounts of water between the myelinated fibers, etc. Nerve fibers may be lost as well.
It is clear that the term "white matter disorders" comprises many different disorders which all have different consequences for brain function. For instance, loss of myelin is worse for brain function than increase of water content or formation of scar tissue between nerve fibers. It is therefore also clear that the resulting handicap for the child with a white matter disorder is highly variable, depending on what is happening at brain tissue level.
"White matter disorders" have a bad name. The first thought is usually that the disease is progressive and that the affected child is going to have an increasing neurological handicap and may die. This is true for some disorders, but by no means for all. Some disorders are progressive whereas others are not. It all depends on the cause of the white matter disease.