A system of practical medicine. By American authors. Vol. 5

by Unknown

Every nerve-fibre has the power of conducting both centripetally and centrifugally, but the organs with which they are connected at their extremities permit the exhibition of their conductivity only in one direction. Thus, if a nerve-fibre in connection with a muscle at one end and a motor nerve-cell at the other be stimulated, although the stimulus is conducted to both ends of the fibre, the effect of the stimulus can only be exhibited at the end in connection with the muscle, causing the muscle to contract. Or if a fibre in connection with a peripheral organ of touch be stimulated, we can only recognize the effects of such stimulation by changes in the nerve-cells at its central end which give rise to a sensation.

When we consider the extensive distribution and exposed position of the peripheral nerves, their liability to mechanical injury and to the vicissitudes of heat and cold, we cannot but anticipate that they will be the frequent seat of lesions and morbid disturbances. It may be that not a few of their diseased conditions have escaped observation from a too exclusive looking to the central nervous system as the starting-point of morbid nervous symptoms. This occurs the more readily as many of the symptoms of disease of the peripheral nerves, as paralysis of muscles, anæsthesia, hyperæsthesia, etc., may equally result from morbid conditions of the brain or spinal cord, and not unfrequently the peripheral and central systems are conjointly affected in a way which leaves it doubtful in which the disease began or whether both systems were simultaneously affected.

The elucidation of such cases involves some of the most difficult problems in diagnosis, and requires not only a thorough acquaintance with the normal functions of the peripheral nerves, but also the knowledge of how those functions are modified and distorted in disease.

The symptoms arising from injuries and diseases of the peripheral nerves are referable to a loss, exaggeration, or perversion of their functions, and we often see several of these results combined in a single disease or as the result of an injury.

The fibres may lose their conductivity or have it impaired, causing feebleness or loss of motion (paralysis), or diminution or loss of sensation (anæsthesia). Or there may be induced a condition of over-excitability, giving rise to spasm of muscles and sensations of pain upon the slightest excitation, not only from external agents, but from the subtler stimulation of molecular changes within themselves (hyperæsthesia). Or diseased conditions may induce a state of irritation of the nerve-fibres, which shows itself in apparently spontaneous muscular contraction or in sensations abnormal in their character, and not corresponding to those ordinarily elicited by the particular excitation applied, as formication or tingling from simple contact, etc. (paræsthesiæ), or in morbid alterations of nutrition in the tissues to which the fibres are distributed (trophic changes).

If we could recognize the causes of all these varied symptoms and discover the histological changes invariably connected with them, it would enable us to separate and classify the diseases of the peripheral nerves, and give us a sound basis for accurate observation and rational therapeutics. But, although the progress of investigation is continually toward the discovery of an anatomical lesion for every functional aberration, we are still so far from a complete pathological anatomy of the peripheral nerves that of many of their diseases we know nothing but their clinical history. We are therefore compelled in treating of the diseases of the peripheral nerves to hold still to their classification into anatomical and functional, as being most useful and convenient, remembering, however, that the two classes merge into each other, so that a rigid line cannot be drawn between them, and that such a classification can only be considered as provisional, and for the purpose of more clearly presenting symptoms which we group together, not as entities, but as pictures of diseased conditions which may thus be more readily observed and studied.

It is well to begin the study of the diseases of the peripheral nerves by a consideration of nerve-injuries, because in such cases we are enabled to connect the symptoms which present themselves with known anatomical alterations, and thus obtain important data for the elucidation of those cases of disease in which, although their symptomatology is similar, their pathological anatomy is imperfectly or not at all known.

Injuries of the Peripheral Nerves.

If the continuity of the fibres of a mixed nerve be destroyed at some point in its course by cutting, bruising, pressure, traction, the application of cold, the invasion of neighboring disease, etc., there will be an immediate loss of the functions dependent on the nerve in the parts to which it is distributed. The muscles which are supplied by its motor fibres are paralyzed; they no longer respond by contraction to the impulse of the will. No reflex movements can be excited in them either from the skin or the tendons. They lose their tonicity, which they derive from the spinal cord, and are relaxed, soft, and flabby. As the interrupted sensory fibres can no longer convey impressions to the brain, we might naturally look for an anæsthesia, a paralysis of sensation, in the parts to which they are distributed, as complete as is the loss of function in the muscles. Such, however, is not the fact. Long ago cases were observed in which, although sensitive nerves were divided, the region of their distribution retained more or less sensation, or seemed to recover it so quickly that an explanation was sought in a supposed rapid reunion of the cut fibres. Recent investigations, moreover, show that in a large number of cases where there is complete interruption of continuity in a mixed nerve the region to which its sensitive fibres are distributed retains, or rapidly regains, a certain amount of sensation, and that absolute anæsthesia is confined to a comparatively small area, while around this area there is a zone in which the sensations of pain, touch, and heat are retained, though in a degree far below the normal condition; in short, that there is not an accurate correspondence between the area of anæsthesia consequent upon cutting a sensitive nerve and the recognized anatomical distribution of its fibres. We find the explanation of this partly in the abnormal distribution of nerves, but principally in the fact of the frequent anastomoses of sensitive nerves, especially toward their peripheral distribution, thus securing for the parts to which the cut nerve is distributed a limited supply of sensitive fibres from neighboring nerves which have joined the trunk below the point of section. This seems proved not only by direct anatomical investigation, but also from the fact that the peripheral portion of the divided nerve may be sensitive upon pressure, and that the microscope shows normal fibres in it after a time has elapsed sufficiently long to allow all the divided fibres to degenerate, in accordance with the Wallerian law. Some of the sensation apparently retained in parts the sensitive nerve of which has been divided may be due to the excitation of the nerves in the adjacent uninjured parts, caused by the vibration or jar propagated to them by the mechanical means used to test sensation, as tapping, rubbing, stroking, etc.³ It is to be observed that this retained sensation after the division of nerves exists in different degrees in different regions of the body; thus it is greatest in the hands, least in the face.

³ Létiévant, Traité des Sections nerveuses.