In the epidemics of past centuries the occurrence of mental disturbance as a result of influenza is recorded by various writers. But it is the last pandemic which has especially directed attention to the frequency of influenzal psychoses and to their great variety.
No other acute epidemic infectious disease-epidemic cerebrospinal fever excepted-is followed so frequently by acute and chronic psychoses, in the narrowest sense of the term. No doubt the extraordinary frequency is to a large extent an absolute one, depending upon the enormous morbidity of the disease, which in times of epidemics affects at least 50 per cent, of the population. Nevertheless, I still adhere to my former expressed opinion of the greater relative frequency of psychoses due to influenza. Even if just as many persons had been attacked by enteric fever or pneumonia as were attacked by influenza in the pandemic of 1889, not nearly so many posttyphoid and metapneumonic psychoses would have occurred as postinfluenzal psychoses actually did occur. In my influenza lectures (1890) I remarked: "It has been asserted that the psychoses of influenza depended upon the "condition of nervous exhaustion ('exhaustive psychoses'); I must take exception to this view. Enteric fever lasting several weeks surely influences nutrition in general, and also the nutrition of the brain in a much more marked degree, than an influenza attack lasting but two or three days. And yet among the 2000 cases of enteric fever, and of the more than 3000 cases of pneumonia, that I have treated in the Municipal Hospital in Cologne during the last fifteen years I have not seen as many psychoses as among the 439 influenza cases in the last pandemic, even if we admit that the post typhoid psychoses often appear late, after the patient has left the hospital, and may in this way escape the observation of the attending physician." To this we can add the experience of recent years, which shows that even up to the present time, when cases of true influenza are much rarer, we still see, from time to time, genuine post influenzal psychoses. In explanation of the conspicuous frequency of influenza psychoses I at that time (1890) remarked that "influenza, as shown also by other observations, produces a specific nervous toxin which, besides producing the conspicuous nervous prostration, in some cases by its poisonous action on the cortex of the brain, calls forth these psychoses. The influenza toxins are powerful nervous poisons."
To these statements, which I made at that time from a small amount of clinical material, I still adhere. They have been confirmed in the observation of a largely increased number of influenza psychoses, especially in two particulars: (1) The cases observed by us were conspicuous by the fact that hereditary or other neuropathic influences were not found in them; (2) the obvious frequency with which the young, even infants, were affected by these psychoses.
In contrast to this the alienists (Ladame, Mehr, Krapelin, Mispel baum, Ayer, Jutrosinski, and many others) affirm, with almost complete unanimity of opinion, that the occurrence of influenza psychoses depends upon a hereditary or acquired psychopathic condition. "It would seem," says Krapelin, "that influenza alone would not be able to produce a disturbance of the mental function in a normally constituted person." Feyr and Schiile express themselves even more definitely. According to the latter, influenza "only plays the part of an etiologic accompaniment in the production of mental disease." Ladame says: "L'influenza a elle seule ne suffit jamais a provoquer la folie."
Statements of this sort seem to us entirely wide of the mark. Without, of course, wishing to undervalue the importance of the neuropathic constitution, we must emphasize the fact that many of the influenzal psychoses observed by us occurred in entirely healthy individuals without hereditary or other stigmata.
Bossers says: " While agreeing in the main with the opinion of Krapelin, Ladame, and others, yet I must remark that cases occur in which there was not only not the slightest trace of an inherited psychopathic disposition, but also in which, in spite of every inquiry, we could not find any grounds for assuming any acquired disposition. In these cases, I think, we are forced to the conclusion that the influenzal poison is of so toxic a nature that it alone is sufficient, by poisoning the cortex of the brain, to develop a psychosis. It was not possible in my two cases to find the slightest predisposition. Althaus, Mucha, Ulliel, also admit the possibility of the occurrence of an influenza psychosis due to influenza alone, without any nervous predisposition."
Gray reported, in the epidemic of 1782: "Symptoms of mania were sometimes produced by it, where no predisposition to that complaint could be traced."
Kirn, in 22 "febrile psychoses," that is, in those which commenced during the febrile stage of influenza, could find a hereditary or acquired disposition only five times, that is, in 23 per cent.; but, on the other hand, 47 times in 51 postfebrile psychoses, that is, in 92 per cent,
It seems to me that the etiologic conception of "hereditary tendency" is extended beyond reasonable limits; for instance, when traced back to grandparents and great grandparents. The same is true of the conception "acquired disposition." Thus in the above mentioned 92 per cent, of predisposed influenza psychoses, the "acquired disposition" is referred to "injuries to the head occurring in youth or a hysteria and neurasthenia or to later chronic affections of the most varied kinds (stomach, lung, heart, and head affections). With so comprehensive an "acquired disposition," 92 per cent, of predisposed cases can readily be gotten together.
The contradiction between my own experience and that of the alienists I can explain only on the supposition that the prolonged chronic psychoses, on account of which the patients were finally handed over to lunatic asylums, occurred mostly in neuropathic individuals, especially in those with a hereditary taint.