NERVOUS DISEASES. - Nervous diseases and mental diseases together constitute the group of morbid affections that affect the nervous system. Although the former form a fairly homogeneous and characteristic group, distinguished from the rest of medicine by a special semeiotics, particular research techniques, and a different therapy, nevertheless their boundaries, both with respect to mental diseases and to medicine in general and the other specialties, are by no means clear-cut, but become blurred from case to case.
For the sake of clarity it is worth briefly returning to the functions of the nervous system. Alongside the psychic function, whose alterations form part of mental diseases, there are, simplifying to the utmost, the motor function and the sensory-sensorial function; by nervous diseases are meant those that chiefly involve these two functions. It should also be noted that it is characteristic of the nervous system to present not only a "deficit symptomatology" due to interruption of the centers or of the conduction pathways of nervous impulses, but also "irritative syndromes" that manifest themselves with an excess of function. Thus, for example, in motor disturbances, alongside a series that in order of aggravation runs from the diminished strength of a finger (asthenia) to the total loss of the possibility of movement of one or more limbs (paralysis), there is the appearance of involuntary movements that range from fine twitches limited to a single segment (Jacksonian seizure of one hand) to the generalized convulsive attack. Analogously, in alterations of the sensory-sensorial functions, alongside the disturbances that range from diminished sensitivity to touch of a restricted area of skin (hypoesthesia) to the complete loss of sensation of half the body (hemi-anesthesia), there exists a range of irritative disturbances that can go from the tingling of a finger to the intense pain of vast regions of the body. With this, only the disturbances of the principal functions have been mentioned; but it is clear that a system as complex as the nervous one interferes in the entire activity of the organism. Thus, from case to case, in nervous diseases one may find ocular, auditory, respiratory, sphincteric, endocrine disturbances, etc.
When faced with a nervous disease one must make two diagnoses: that of localization of the lesion and that of the nature of the lesion itself. The first is, for obvious reasons, almost peculiar to these diseases, which has created a particular semeiotics, ranging from the neurological examination to neuro-radiology, 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 must explain the entire symptomatology of the patient) or multiple; in this case there may be diseases that affect one system, for example only the pathways of motility (systemic diseases), or those that affect several systems (non-systemic, disseminated). The problem of the diagnosis of nature is common also to the other branches of medicine and differs in nervous diseases only because the alterations take place in a tissue that has its own particular characteristics. Thus, for example, the onset, the course, and the spread of a disease will be strictly bound to the anatomical position, to the capacity for tissue reaction, and to the functions of a tissue. It should furthermore be borne in mind that multiple causes of different nature can give a similar symptomatology, so that the anamnesis and the auxiliary investigations play a very 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 diseases, and those due to physical agents. This brief classification, without any claim to completeness, aims only to give a rapid panoramic view of the subject in question. The forms of infectious cause, provoked by pathogenic agents, whether microbes or viruses, comprise a series of diseases that, according to their localization, take the name of "meningitis" (when the meninges, that is the coverings of the brain, are invaded), "myelitis" (when it is a matter of the spinal cord), "neuritis" (when the nerves are lesioned); such diseases can be diffuse or circumscribed. One may recall cerebral abscesses, chorea of rheumatic origin, and syphilis of the nervous system (the meningo-vascular form, tabes, and general paralysis). Neoplasms or tumors comprise types of different microscopic structure, according to the cells from which they originate. Thus there are "meningiomas" (arising from the meninges), "gliomas" (deriving from the glial cells of the nervous parenchyma), and "neurinomas" (which begin from the sheaths of the cranial or peripheral nerves). Degenerative diseases, often on hereditary or familial bases, consist of a notable group of morbid forms, of very varied appearance. Thus the spinocerebellar ataxias (Friedreich's disease), cerebellar atrophies, myopathies, and Parkinson's disease.
Vascular diseases of the brain are often related to essential and malignant hypertension and to heart diseases; they can present with sudden and often transient episodes, such as hypertensive vasomotor crises, or with more serious episodes, such as hemorrhages and softenings, sometimes finally with coma. Finally, they present forms with a chronic and progressive course, and in this case psychic disturbances are also present. Deficiency diseases are due to the absence of substances necessary for normal metabolism; among these, the most common: beriberi, pellagra, and Lichteim's syndrome which accompanies pernicious anemia; they generally manifest themselves with spinal symptoms. Among physical agents one may cite traumas, responsible for cerebral and spinal lesions (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 seizures), provoked by various stimuli (mechanical, chemical, physical) acting on the brain (symptomatic epilepsy) or by unknown causes (idiopathic epilepsy).