Nervous system disorders

Malformation

The central nervous system develops as a hollow neural tube by the fusion of the crests of the neural groove, beginning in the cervical area and progressing rostrally and caudally, the last points to close being termed the anterior and posterior neuropores. If the anterior neuropore fails to close (about 24 days of fetal age), anencephaly develops. The poorly organized brain is exposed to amniotic fluid and becomes necrotic and hemorrhagic, with death usually within hours after birth. See Congenital anomalies

If the posterior neuropore fails to close (about 26 days of fetal age), the lumbosacral neural groove is exposed to amniotic fluid. The nervous tissue becomes partially necrotic and incorporated in a scar. Such a meningomyelocele is readily infected unless buried surgically within a few hours after birth. In addition, in about 95% of such infants hydrocephalus occurs, which usually can be adequately treated by shunting the ventricular fluid into the venous system or peritoneal cavity.

Other developmental disorders of the nervous system may appear as hypoplasia or hyperplasia (decrease or increase in growth of cells, respectively) or as a destruction of otherwise normally developing tissues. Rapidly growing tissues such as the embryonic nervous system are generally rather easily damaged by many toxic agents. The time of onset and the extent of repair rather than the nature of the agent determine the resulting pattern of abnormal development.

Infection

Infections of the nervous system may occur through a defect in the normal protective coverings caused by certain congenital malformations, as mentioned above, but also through other defects as the result of trauma, especially penetrating wounds or fractures opening into the paranasal sinuses or mastoid air cells. Subsequent infection of the nervous system may be the major complication of such “open head” injuries.

Infections may also spread directly from adjacent structures, as from mastoiditis, sinusitis, osteomyelitis, or subcutaneous abscesses. Such infections usually spread along venous channels producing epidural abscess, subdural empyema, leptomeningitis, and brain abscess. All of these infections are characteristically caused by pyogenic (pus-forming) bacteria. Other pyogenic bacteria may metastasize by way of the bloodstream from more distant infections, such as bacterial endocarditis, pneumonia, and enteritis.

Infections of the nervous system must be treated promptly as medical emergencies. The diagnosis is easily established by spinal puncture; the microorganisms can be visualized with special stains.

Many other microorganisms can infect the nervous system: Mycobacterium tuberculosis (the organism causing tuberculosis), Treponema pallidum (the organism causing syphilis), several fungi and rickettsiae, and many viruses.

Viral infections vary widely geographically, generally related to the necessity for intermediate hosts and vectors (animal reservoirs) by which the virus is spread. Poliomyelitis, now largely prevented by effective vaccination of most children, is primarily an intestinal infection which occasionally spreads to the nervous system, infecting and destroying motor nerve cells, thereby producing weakness of certain muscles. Herpes zoster has a similar preference for infecting sensory nerve cells and producing an acute skin eruption in the distribution of the affected sensory cells. Herpes simplex is closely related to herpes zoster, resides in the trigeminal or sacral sensory nerve cells, and intermittently produces eruptions in the distribution of these cells: “fever blisters” in and around the mouth in type I herpes, or similar blisters in the genital area in type II herpes. The latter is increasingly being recognized as a venereal disease. Rabies virus also affects certain nerve cells in the temporal lobe of the brain, as well as in the cerebellum, and is transmitted through the saliva of animals that bite other animals or humans; rabies is the single exception to the rule that immunization must precede infection to be effective, and the immunization must begin promptly after the bite. See Animal virus, Herpes, Poliomyelitis

Inflammation

Certain viruses frequently produce a meningitis in humans from whose cerebrospinal fluid the virus is relatively easily grown. Other viruses, such as measles and varicella, occasionally produce meningitis or encephalomyelitis, but the cerebrospinal fluid does not contain the virus. See Meningitis

Allergy to one's own tissue elements is an interesting possibility that has evoked many experimental approaches. Two human diseases, multiple sclerosis, a demyelinating disease affecting the central nervous system, and the Landry-Guillain-Barré syndrome, a demyelinating disease affecting the peripheral nervous system, are considered likely candidates to be related to experimental allergic encephalomyelitis and experimental allergic neuritis, respectively. See Autoimmunity

Vascular disease

Vascular diseases of the nervous system are commonly called strokes, a term which emphasizes the suddenness of onset of neurological disability. Such a cataclysmic onset is characteristic of vascular diseases, since the nerve cell can function without nutrients for only a matter of seconds and will die if not renourished within several minutes.

Two main types of hemorrhage occur: hemorrhage into the subarachnoid space from rupture of an aneurysm (a focal weakening and dilatation) of a large artery; and hemorrhage into the brain from rupture of an aneurysm of a small artery or arteriole. Both types of hemorrhage occur more commonly in hypertensive adults.

Nerve cells require oxygen and glucose for functional activity, and can withstand only brief periods of hypoxia or hypoglycemia. Even a few seconds of hypoxia can block the nerve cell's function, and more than 10 min is almost certainly fatal to most nerve cells. Transient ischemic attacks may result, with temporary impairment of blood flow to a part of the brain and consequent focal neurological dysfunction. These attacks may also be successfully treated with drugs or surgery and the disastrous major stroke prevented. Myocardial infarction, postural hypotension, and stenosis or narrowing of the carotid or vertebral arteries greater than 60% are common causes of cerebral ischemia. If the ischemia is not rapidly reversed, the neurons undergo selective necrosis; if the ischemia is more severe or prolonged, the glia and blood vessels in the gray matter also undergo necrosis; and if the ischemia is still more severe or prolonged, all the gray and white matter in the ischemic zone becomes necrotic, a condition known as cerebral infarction or encephalomalacia. One of the common ways the brain reacts to small or large hemorrhages or ischemic episodes is by swelling. Such swelling itself may be fatal within a few days to a week or so by a process known as transtentorial herniation, compressing the brainstem, where there are important neural circuits for vital functions, such as breathing and maintenance of blood pressure.

Degenerative and other diseases

Degenerative, metabolic, toxic, and deficiency states include the largest numbers of both common and rare diseases of the nervous system. Since neurons in the brain may be destroyed after birth and cannot be replaced, mental deterioration, deafness and blindness, incoordination and adventitious movements, and other neurologic signs that are so typical of these disorders are generally not reversible even if the basic metabolic defect can be corrected. Advances have been made in the early diagnosis and treatment of several diseases usually manifest in infancy with mental retardation. Three examples are phenylpyruvic oligophrenia (phenylketonuria or PKU), which is treatable with a phenylalanine-deficient diet; galactosemia, requiring a galactose-free diet also as early as possible to avoid cataracts and mental retardation; and cretinism, which requires treatment with thyroid.

Neoplasm

Neoplasms of the nervous system can be divided into primary and metastatic, the primary into gliomas and others, and the metastatic into bronchogenic and others. These four groups each account for about 25% of all intracranial neoplasms.