The causes of these disturbances of the nervous system, when not attributable to appreciable lesions, such as congestion, inflammation, hemorrhage, embolism, thrombosis, and softening, are not established. The following appear to be reasonable conclusions from the facts at present known:
The most constant condition, and without which these cerebral symptoms very rarely arise, appears to be some susceptibility or vulnerability of the nervous system, inherited or acquired, rendering it apt to be disturbed by influences which less susceptible centres would successfully resist. Trousseau, who has especially advocated this view,95 considered intemperance in the use of spirits to be a frequent source of this nervous predisposition. Accepting this neurotic predisposition as the factor generally present when acute articular rheumatism is complicated with disturbances of the nerve-centres, we may inquire what are the circumstances in the disease capable of developing into activity the predisposition.
95 Clin.-Med., Syd. ed., i. 513 et seq.
Unquestionably, the existence of acute pericarditis, or of endocarditis, or of inflammation of the lungs or pleura, is one of those conditions.
Probably hyperpyrexia acts in some cases as an exciting cause of the nervous phenomena, for while the delirium preceded the hyperpyrexia in 6 cases, it accompanied it in 19 and followed it in 10;96 and the nervous symptoms disappear when the hyperthermia is removed by the employment of cold, and recur with the return of high temperature. The phenomena of sunstroke and heat-apoplexy prove that a high temperature is capable of producing convulsions and coma. That these grave cerebral disturbances are so infrequent in acute rheumatism (obtaining in about 3 or 4 per cent. only) is probably owing to the usual moderate range of temperature and the rarity of hyperpyrexia in the disease. Still, while hyperpyrexia is a disturber of cerebro-spinal function, too much importance must not be attached to it, for not only does such disturbance very frequently precede the hyperpyrexia, but there are many facts indicating that the hyperpyrexia is itself very frequently, like the delirium, tremor, and coma which precede or accompany it, but a consequence of disorder, usually of a paralyzing kind, of the nerve-centres. It has been met with in lesions of the pons, in tetanus, in injuries of the cord, in some cases of non-inflammatory softening of the brain and of cerebral hemorrhage; that is, in a class of affections not belonging to the specific fevers, but to those directly disturbing or destroying the functions of the nerve-centres. And cases of acute rheumatism do rarely occur in which a very high temperature is not accompanied by cerebral disturbances. Sibson quotes two such,97 one of which, with a temperature of 110.8°, was only restless and talked when asleep, and the other, with a temperature of 106.3°, presented only vomiting and dyspnoea. Cardiac inflammation was absent in both. DaCosta relates one in his valuable paper upon cerebral rheumatism in which, although the temperature was 110°, no cerebral symptoms nor cardiac affection existed.98
96 "Abstract Report upon Hyperpyrexia in Ac. Rheum.," Brit. Med. Jour., 1882, p. 807.
97 Lib. cit., p. 264.
98 This essay contains a record of 11 cases of cerebral rheumatism and several autopsies: Am. Jour. Med. Sci., 69, 1845, p. 36, case xi.
The goodly number of instances lately published in which grave cerebral symptoms have obtained in acute articular rheumatism at ordinary febrile temperatures, while they prove that hyperthermia is not an essential condition productive of such symptoms, require to be explained. Some such, no doubt, have been instances of marked predisposition, so that a moderate febrile temperature or some complication sufficed to disturb the brain, as we see in typhoid and other fevers, in pneumonia, etc. If there be a rheumatic poison—which has not yet been proved—it may, in predisposed persons, produce the cerebral symptoms. The argument99 that such poison should produce inflammation of the nervous centres if it acted directly on them is not convincing. It need not necessarily produce similar alterations in serous or synovial membranes and in nervous tissues. Many toxic agents disturb, and even suspend, the cerebro-spinal functions, and leave no appreciable changes in them. Do these cases prove that there is something peculiar to rheumatic fever which tends to disturb the nervous centres? Hardly; for while such disturbance is comparatively rare in that disease, it is observed frequently in many other febrile affections, notably in typhus, scarlatina, and small-pox; and as in these, so in rheumatic fever, it is more often observed in the severe than in the mild cases, as though it were a part of the systemic disturbance incident to the febrile affection and largely proportionate to its severity.
99 Maclagan, Rheumatism: Its Nature, Pathology, etc., 1881, 287.