Meningitis Health Article

Definition

Meningitis is a potentially fatal inflammation of the meninges, the thin, membranous covering of the brain and the spinal cord. Meningitis is most commonly caused by infection (bacteria, viruses, or fungi), although it can also be caused by bleeding into the meninges, cancer, diseases of the immune system, and an inflammatory response to certain types of chemotherapy or other chemical agents. The most serious and difficult-to-treat types of meningitis tend to be those caused by bacteria.

Description

Meningitis is a particularly dangerous infection because of the very delicate nature of the brain. Brain cells are among a very small number of cells in the body that, once killed, will not regenerate. Therefore, if enough brain tissue is damaged, serious, life-long handicaps may remain.

To understand meningitis, it is important to have a basic understanding of the anatomy of the brain. The meninges are three separate membranes layered together that encase the brain and spinal cord:

• The dura is the toughest, outermost layer, and is closely attached to the inside of the skull.
• The arachnoid, the middle layer, is important because of its involvement in the normal flow of the cerebrospinal fluid (CSF), a lubricating and nutritive fluid that bathes both the brain and the spinal cord.
• The pia, the innermost layer, helps direct blood vessels into the brain.

The space between the arachnoid and the pia contains CSF, which helps insulate the brain from trauma. Many blood vessels course through this space.

CSF, produced within specialized chambers deep inside the brain, flows over the surface of the brain and spinal cord. This fluid serves to cushion these relatively delicate structures, as well as to supply important nutrients for brain cells. CSF is reabsorbed by blood vessels located within the meninges. A careful balance between CSF production and reabsorption is important to avoid the accumulation of too much CSF.

Because the brain is enclosed in the hard, bony case of the skull, any disease that produces swelling of the brain will be damaging. The skull cannot expand at all; so, when any swollen brain tissue pushes up against the skull's hard bone, brain tissue may become damaged and ultimately die. Swelling on one side of the brain will not only cause pressure and damage to that side of the brain, but, because it takes up precious space within the tight confines of the skull, the opposite side of the brain will also be pushed against the skull, causing damage to that side also.

Another way that infections injure the brain involves the way the chemical environment of the brain changes in response to that infection. Brain cells require a very well-regulated environment. Careful balance of oxygen, carbon dioxide, sugar (glucose), sodium, calcium, potassium, and other substances must be maintained to avoid damage to brain tissue. An infection upsets this balance. Brain damage can occur when cells are either deprived of important nutrients or exposed to toxic levels of particular substances.

The cells lining the brain's tiny blood vessels (capillaries) are specifically designed to prevent many substances from passing into brain tissue. This is commonly referred to as the blood-brain barrier. The blood-brain barrier prevents various substances that could be harmful to brain tissue (toxins), as well as many agents of infection, from crossing from the blood stream into the brain tissue. While this barrier is obviously an important protective feature for the brain, it also serves to complicate treatment in the case of an infection by making it difficult for medications to pass out of the blood and into the brain tissue where an infection is located.