The New International Standard Medical & Health Encyclopedia

I f any part of the brain has developed abnormally, the usual function of that structure would be expected to be altered. Abnormalities present at birth are called congenital defects. If the nervous system does not receive a normal blood supply because of the obstruction of a blood vessel, or if there is a tear in the vessel with subsequent hemorrhage, the cells are deprived of blood and will die. These lesions are vascular or cerebrovascular accidents. Cerebral injury, or trauma , can destroy brain tissue, with consequent loss of normal function; infections of the nervous system may also permanently injure tissue. Finally, brain function can be altered by metabolic , toxic , or degenerative changes in normal body chemistry. It is not surprising, then, that such a beautifully organized nervous system is vulnerable to the hazards of living.

Diagnostic Tests

A patient who has been referred to a neurologist , a physician who specializes in diseases of the nervous system, will be asked to tell the history of the problem in great detail, for that history will describe the nature of the disorder; the neurological examination will help locate the site of the problem. After the neurological examination has been completed, the physician may order radiographs (X-ray pictures) of the skull and spinal column and an electroencephalogram. The electroencephalogram (EEG), or brain wave recording, assists the physician in locating such brain abnormalities as tumors and in determining the characteristics of convulsive disorders.

It may also be necessary to perform alumbarpuncture ( spinaltap ) in order to obtain a specimen of the cerebrospinal fluid (CSF), the fluid that bathes the brain and spinal cord. For this test a needle is inserted through the patient's lower back into the spinal canal. A sample of CSF is then withdrawn for analysis.

Occasionally, other specialized tests are used to provide the physician with an image of the brain, spinal cord, or veins. For example, ultrasonography , a test using high-frequency sound waves to produce images, can be used to examine major veins in the neck for damage or blockages. Ultrasound tests are non-invasive and can also be used to examine other areas of the body, such as the liver, heart, or kidneys.

Another specialized test is the cerebral arteriography , also known as an angiography. In this test a material that is opaque to X rays is injected into the blood vessels that supply the brain. The vessels can then be seen plainly on X-ray film, and abnormalities in the vessels can be diagnosed. In some cases this test has been replaced by noninvasive tests, such as computed tomographic scanning and magnetic resonance imaging.

Myelography is another invasive test using radiopaque liquid dye similar to that used in angiography. The dye is injected with a spinal needle into the sac-enclosed space around the spinal cord. X rays are then taken that can show problems with the spine, such as compression or disc injuries, or tumors in the lower brain.

Computed tomographic scanning , also known as CT scanning , is a non-invasive procedure in which X-ray beams from many angles are directed at a patient lying on a special table. As the X rays pass through the body, different tissues absorb different amounts of the X rays. The strength of the beams emerging from the patient's body is measured by an X-ray detector. The measurements are processed by a computer that then makes an image of the interior of the patient's body. One function of this test is to generate pictures that look like slices of the brain. CT scans can be used to locate such problems as tumors and fluid accumulation in the brain. CT scans can also be used to diagnose problems in other parts of the body.

Magnetic resonance imaging , also known as MRI and nuclear magnetic resonance ( NMR ) imaging , is a noninvasive process using a magnetic field instead of X rays to create images of the inside of a patient's body. For this test a patient lies on a special table that passes into a tube-like structure that contains a large magnet. Atoms in the body react to the magnetic field and this reaction is recorded by a scanner that sends the information to a computer. The computer then creates images of the interior of the patient's body. This test can be used in diagnosing problems in the structure of the brain or other parts of the body. The functional MRI shows brain function as well as its structure.

Another test that can show brain function is positron emission tomography , also known as PET . In this test very small amounts of radioactive substances are injected into a patient's body. When these substances decay they emit particles called positrons. A scanner records this process and sends information about the emission of positrons to a computer. The computer then creates an image from the information. Besides showing brain function, PET scans can be used on other parts of the body to show such problems as muscle damage.