If the neuralgia be of some standing and a certain degree of severity, there will inevitably be found—
(9) Some of the fixed tender points of Valleix, in such situations as have been described in Chapter I.; and—
(10) Secondary affections (a) of secreting glands, or (b) vaso-motor nerves; or (c) of nutrition of tissues; or secondary localized paralyses of muscles, or localized anæsthesia of a somewhat decided though not complete kind, as described in Chapter II.; any one or any number of these various complications may be present.
I must insist that the above picture includes only the essentials for a diagnosis of neuralgia; if the painful affection will not answer to the conditions therein included, we have no right to call it a neuralgia—it belongs, for every practical purpose, to some other category of disease. Let me add one more essential characteristic, which is, that the pain begins and assumes its characteristic type before any other of the phenomena appear, with the single and partial exception of anæsthesia.
There are some special modes of diagnosis of the varieties of neuralgia, developed of late years, that require notice here; they are chiefly the result of the researches of Moriz Benedikt.
As regards the quality of the pain, Benedikt says that the curve of intensity has an intimate relation to the locus in quo of the neuralgia (i. e., whether in the periphery, trunk, or roots). An inflammatory irritation set up at the periphery of a nerve (by a joint-inflammation, for instance) produces a continuous pain; the same kind of irritation, attacking a nerve-trunk (e. g., in the bony canals), produces a paroxysmal pain; an inflammation spreading from the vertebræ to the nerve-roots or the cord-centres produces momentary lancinating pains. The latter characteristic he supposes to be especially characteristic of the centrally-produced neuralgias; and I may observe, as so far confirmatory of this idea, that this is especially the character of the pains in locomotor ataxy. There are sundry special cases to be considered, however: thus, Benedikt himself remarks that the pain set up by the pressure of a pulsating aneurism is, from the nature of things, lancinating from moment to moment. Eulenburg,[32] moreover, says that Benedikt's tests of the locality of the primary mischief only hold good under the following circumstances: (1) When the irritability and the exhaustibility of the nerves are in a normal condition during the neuralgia; (2) when the irritation that calls forth the paroxysm is either identical with the original cause of the disease, or at least operates upon the same spot. The two conditions, however, do not concur. The irritability and exhaustibility may be sometimes excessive in neuralgias, sometimes normal, and perhaps, in certain cases, beneath the normal standard; by which means the form of the curve of intensity must be considerably modified. Moreover, the irritation that provokes an attack may from the periphery attack the primary seat of the disease, even when this is central, on account (says Eulenburg) of exaggerated conductivity of the nerves (his second cause[33] of "hyperæsthesia"), as is, in fact, very frequently the case. He also thinks the distinction between paroxysmal and lancinating pains too indefinite to serve as a sufficiently reliable basis of diagnosis, especially considering the endless nuances of the form which the pain is apt to take. I agree with Eulenburg upon this point; and am convinced, from my own observations, that such a distinction as that between lancinating and paroxysmal pains is illusory, [I have taken some pains to investigate the character of the pains, not only in neuralgia, but in locomotor ataxy. It is true that the lancinating character predominates, on the whole, in the latter disease; but there are great differences in different individuals, and even in the same patient at various times, which plainly depend on subjective influences. Compare for instance, Dr. Headlam Greenhow's report on an ataxic patient, with a report on the same man by Dr. Buzzard and myself. ("Trans. Clin. Soc.," vol. i., 1868, pp. 152-162.)] the two kinds being frequently found alternate in the same case. The only useful distinction, in my opinion, is Benedikt's first one: he is probably right in saying that, where such an affection as an inflamed joint forms the source of peripheral irritation that immediately provokes a neuralgia, the pain is apt to be unusually continuous.
The extent to which the pain of neuralgia spreads into different termini of the same nerve has been made the basis of distinctions as to the seat of the original mischief. For example, it has been said that pain in the mental branch of the third division of the trigeminus, which does not invade the auriculo-temporal branch, can hardly depend on an irritation operating on the trunk of the inferior dental; it must be distinctly peripheral, or else it must act upon limited portions of the central origin of the fifth nerve. But the fact seems rather to be that, whether the neuralgia was excited by lesions at the periphery, in the nerve-trunk, or in the centre, it is equally possible that either a small or a large part of the peripheral expanse of the nerve may become the seat of the pain: this almost necessarily follows from the entire independence of individual fibres in nerves.
As regards the evidence afforded by the motor, vaso-motor, and trophic complications, there is this very positive diagnostic value in them—that they enable us to say, with greater assurance than we could otherwise do, that the disease is a real neuralgia. But, the only evidence that they afford as to the situation of the mischief is, that they uniformly point to the central end of a particular nerve; and accordingly I have already shown, in the chapter on Pathology, that the attentive study of these very complications furnishes us with some of the most powerful arguments upon which rests my theory that in neuralgia there is always centric mischief. What share in the production of the malady, in any given case, has been taken by the centric disease, and what if any by a peripheral irritation, the existence of these complications in no way helps us to determine; far less does it enable us to localize a peripheral lesion which may have acted as a concomitant cause; on the contrary, I believe that there is no more fertile source of erroneous judgment on this very point, than some of these complications, especially the vaso-motor and trophic. I suspect that it has happened, in hundreds of instances, that a localized congestion or inflammation, which is a mere secondary phenomenon, produced in the centrifugal manner already so fully explained, has been taken for the veritable fons et origo of the malady: hence the neuralgia has been confidently reckoned as one peripherally produced, and, what is even worse, the whole energy of treatment has been directed to a mere outlying symptom, under the idea that the primary source of mischief was being attacked.
The application of electricity as a test of the nature of a neuralgia has been employed by Benedikt,[34] who lays down certain laws as the result of his researches. He says that (a) in idiopathic peripheral neuralgias the nerves are not sensitive to the current; (b) in neuralgias dependent on neuritis or hyperæmia of the nerve-sheath there is general electric tenderness of the nerve; (c) in cases where the pain has been set up by morbid processes in tissues surrounding the nerve, there is electric tenderness only at the site of these changes. I may, in general terms, express concurrence in these statements; but I must add that, as diagnostic rules they apply only to the early stages of neuralgia; for the occurrence of secondary complications may and does altogether change the condition of electric sensitiveness. It need hardly be said that the above remarks on diagnosis apply for the most part only to the superficial neuralgias, which, however, include an immense majority of the cases of neuralgias. The diagnosis of visceral neuralgias is, it need hardly be said, in most cases, a far more difficult and complicated matter. In these diseases we have often little more to guide us, in the actual symptoms, than (a) the intermittence of the pain, and (b) the absence of commensurate constitutional disturbance, especially the complete freedom from sense of illness in the intervals between the pains. We shall be obliged to rely greatly on such historical facts as the presence or absence of neurotic tendencies in the patient and his family; the possibility of his having been exposed to blood-poisoning (e. g., from malaria or chronic alcoholic excess, or extreme over-smoking); the circumstance that he has been habitually overworked, or greatly exposed to agitating psychical influences; perhaps that he has been subject to a combination of several of these morbific momenta. To say truth, the diagnosis of visceral neuralgias must, at the best of times, be a difficult and anxious matter, and we can hardly ever thoroughly satisfy ourselves until we have procured some decided results from treatment; fortunately, however, it happens tolerably often that we can do this, and sometimes in a very striking way.
Prognosis.—The prognosis of neuralgia varies exceedingly, according to the form and situation of the disease, and many other considerations. There are, of course, in the first place, certain neuralgias in which the prospect is perfectly hopeless as to cure; such are the cases in which the nerve is involved in a continuously growing tumor (especially within a rigid cavity, like the skull), or a slow but persistent ulcerative process.