NERVOUS DISEASES. — Nervous diseases and mental illnesses together constitute the group of morbid affections that affect the nervous system. Although the former form a fairly homogeneous and characteristic group, distinct from the rest of medicine by a special semiology, particular research techniques, and different therapy, their boundaries, both in relation to mental illnesses and to general medicine and other specialties, are by no means clear-cut, but rather tend to become blurred from time to time.
For the sake of clarity, it is advisable to refer briefly to the functions of the nervous system. Alongside the psychic function, whose alterations are part of mental illnesses, there are, simplifying to the utmost, the motor and sensory-sensory functions; by nervous diseases, those are understood which chiefly involve these two functions. It must also be noted that a characteristic of the nervous system is not only a "deficit symptomatology" due to the interruption of centers or conduction pathways of nerve impulses, but also "irritative syndromes" that manifest as an excess of function. Thus, for example, in motor disorders, alongside a series that, in order of severity, ranges from diminished strength in a single finger (asthenia) to the total loss of movement in one or more limbs (paralysis), there is the appearance of involuntary movements ranging from fine tremors limited to a single segment (Jacksonian seizures of a hand) to generalized convulsive attacks. Similarly, in alterations of sensory-sensory functions, alongside disorders that range from diminished tactile sensitivity in a small area of skin (hypoesthesia) to complete loss of sensation in half the body (hemianesthesia), there exists a range of irritative disorders that can go from tingling in a finger to intense pain in large regions of the body. With this, only the disorders of the main functions have been touched upon; but it is clear that such a complex system as the nervous system interferes with the entire activity of the organism. Thus, in nervous diseases, one may at times find ocular, auditory, respiratory, sphincteric, endocrine, and other disorders.
In the face of a nervous disease, two diagnoses must be made: that of the localization of the lesion and that of the nature of the lesion itself. The former is, for obvious reasons, almost peculiar to these diseases, which has created a particular semiology ranging from the neurological examination to neuroradiology, neuro-ophthalmology, neuro-otology, and electroencephalography, all subsidiary branches of the simple physical examination of the patient. The localization may be single (and as such will have to explain all the symptomatology of the patient) or multiple; in this case, there may be diseases that affect one system, for example, only the motor pathways (systemic diseases) or those that affect multiple systems (nonsystemic, disseminated). The problem of diagnosing the nature of the lesion is common to other branches of medicine as well and differs in nervous diseases only because the alterations occur in a tissue with its own particular characteristics. Thus, for example, the onset, course, and spread of a disease will be closely linked to the anatomical position, the tissue's capacity for reaction, and the functions of the tissue. It must also be kept in mind that multiple causes of different natures can produce similar symptomatology, so that the anamnesis and auxiliary investigations play an extremely important role in the diagnosis of diseases of the nervous system.
Nervous diseases can be classified, in relation to the causal factor, into infectious, neoplastic, degenerative, vascular, deficiency, and physical-agent diseases. This concise classification, without any pretense of completeness, is intended only to provide a rapid overview of the subject in question. Infectious diseases, caused by pathogenic agents, whether microbes or viruses, include a series of diseases that, depending on their localization, take the name of "meningitis" (when the meninges, i.e., the coverings of the brain, are involved), "myelitis" (when the spinal cord is affected), "neuritis" (when the nerves are damaged); such diseases may be diffuse or circumscribed. Among these may be mentioned cerebral abscesses, rheumatic chorea, and syphilis of the nervous system (meningovascular form, tabes, and general paralysis). Neoplastic diseases or tumors include types of different microscopic structure, depending on the cells from which they originate. Thus there are "meningiomas" (arising from the meninges), "gliomas" (derived from the glial cells of the nervous parenchyma), "neurinomas" (which begin in the sheaths of the cranial or peripheral nerves). Degenerative diseases, often on a hereditary or familial basis, consist of a considerable group of morbid forms of very varied respect. Thus spinocerebellar ataxias (Friedreich's disease), cerebellar atrophies, myopathies, and Parkinson's disease.
Cerebral vascular diseases are often related to essential and malignant hypertension and to heart disease; they may present with sudden and often transient episodes, such as vasomotor hypertensive crises, or with more serious episodes, such as hemorrhages and softenings, and sometimes coma. Finally, they present forms with a chronic and progressive course, and in this case psychic disorders are also present. Deficiency diseases are due to the absence of substances necessary for normal metabolism; among the most common are beriberi, pellagra, and Lichtheim's syndrome, which is accompanied by pernicious anemia; they generally manifest with spinal cord symptoms. Among physical agents, trauma is cited, responsible for cerebral and spinal cord injuries (contusions, concussions, and commotions). Among the various diseases that have been excluded from this list, only the most important will be recalled: epilepsy (v.), which presents with attacks ranging from "petit mal" (loss of consciousness lasting an instant) to "grand mal" (generalized convulsive crises), caused by various stimuli (mechanical, chemical, physical) acting on the brain (symptomatic epilepsy) or by unknown causes (idiopathic epilepsy).