The clinical picture of influenza is so varying, so "protean," as it is termed, that many authors have expressed the opinion that there is no typical form of influenza-"Every case presents a different picture," or, "But few cases are alike in every respect." Such statements are gross exaggerations. It is, however, true that no acute infectious disease shows such a variety of different groupings of the various symptoms or variety of complications and sequels as does influenza. Nevertheless, a few quite characteristic types can be described to which the large majority of the cases conforms.
The typical influenza consists in a sudden pyrexia of from one to several days' duration, commencing with a rigor, and accompanied by severe headache, generally frontal, with pains in the back and limbs, by prostration quite out of proportion to the other symptoms, and marked loss of appetite.
To these characteristic symptoms may be added, in most cases, as might have been expected from our knowledge of the organs to which the influenza germs first gain access, catarrhal phenomena affecting the respiratory tract, particularly the upper parts (coryza), and occasionally the lower parts-the trachea and the bronchi. These catarrhal phenomena occur so frequently that they must be regarded as belonging to a typical attack of influenza.
This general class is subdivided into groups which are characterized, apart from the fever which is common to them all, by the predominance of either the nervous, the catarrhal, or the gastric symptoms. A fourth class combines several or all of these typical influenza symptoms: Fever, nervous, respiratory, and gastric phenomena. The ancient division of influenza into nervous, respiratory, and gastric forms was retained in the last pandemic. But this classification is insufficient, for the numerous mixed varieties find no place in it, and, what is more important, neither do those innumerable cases in which the influenza arises as a simple pyrexial attack of short duration, without special nervous and gastric symptoms and without any respiratory symptoms. [In young children (four to twenty four months old) the attack commences, according to 0'Donovan, with prodromal excitement, activity, and unrest. Pyrexia and vomiting follow, the latter not stopping until inhibited by a laxative. The tongue is clean; pulse, 160 to 180; urine, markedly yellow.-Ed.] Below we give a purely pathogenetic classification, which has several advantages over a purely morphologic division. We differentiate:
1. The Purely Toxic Varieties.-To these belong:
(1) The simple influenza fever, those mild cases where, apart from a rise in temperature and the general malaise, which depends upon the fever (headache, weakness, anorexia), specific influenza symptoms, especially all inflammatory processes, are absent. In such cases we have merely the effects of the influenza toxin. The physicians of the last century already recognized an influenza without catarrhal symptoms. Gray writes in 1782: "In some the catarrhal symptoms were entirely wanting, the disorder in those cases being like a common fever." More recently Norman Kerr, Rosenbach, Baumler, and others have drawn attention to this "simple influenza fever." Thomas and Obkircher believe that it is especially frequent in children.
(2) The nervous form of influenza, those cases in which, as well as fever, but quite in disproportion to its height, marked nervous signs, such as headache, pains in the back, limbs, and joints, general nervous prostration, neuralgia, insomnia, are the principal symptoms, while inflammatory changes in the respiratory and digestive tract are absent. Here, again, we have a simple toxic action affecting especially the cerebrospinal system.
2. The toxic inflammatory varieties of influenza, in which, in addition to the above mentioned toxic phenomena (fever, nervous symptoms), inflammatory changes play a principal part. To this variety belong two principal groups:
(1) The catarrhal respiratory influenza, characterized by coryza, laryngeal tracheal, and bronchial catarrh.
(2) The gastro intestinal variety, characterized by catarrhal processes in the gastro intestinal mucous membrane, with heavily coated tongue, complete anorexia, vomiting, and diarrhea. This variety is much more uncommon than any of the previously mentioned forms.
Too much stress cannot be laid on the fact that typical influenza very frequently-perhaps in one quarter of the cases-may occur without any symptoms referable to the respiratory tract. Before the outbreak of the last pandemic probably every physician would have considered an inflammatory catarrhal condition of the respiratory mucous membrane, especially a severe coryza and bronchial catarrh, as essential to the clinical picture of influenza. On account of the numerous cases at the commencement of the pandemic of 1889 without coryza and bronchitis, some physicians considered that the arising epidemic could not be influenza. In Paris it was thought more probable that the condition was dengue (" dengue attenuee"), since in tnis disease catarrhal symptoms of the respiratory tract are entirely absent.
The forms just described constitute the principal varieties of influenza, and include the great majority of influenza cases. They represent simple uncomplicated influenza.
This picture of the disease may be confused either by the predominance of some of the typical symptoms, or by the occurrence of other complications. Thus there may be, in the febrile form, hyperpyrexia or long continued fever, with a dry tongue and sordes on the teeth; or in the nervous variety, delirium, coma, and meningeal symptoms; or in the respiratory influenza, capillary bronchitis, pneumonia, pleurisy, etc.; or in the gastro intestinal variety, meteor ism, severe colic, profuse and even bloody diarrhea, and peritoneal symptoms.
By a combination of several of these complications the disease may simulate septicemia, acute miliary tuberculosis, and very often enteric fever (the "typhoid form " of influenza). We have not, however, come across any case in which we were long in doubt regarding the diagnosis between enteric fever and influenza. The sudden onset with rigors is peculiar to influenza; and so rare in enteric fever that from this fact alone-to mention only one of the many differential signs-a definite diagnosis can be made. In addition to the varieties mentioned, other types have been described, without, however, bringing the purposed order into the chaos of complicated forms of influenza. The rare "forme cardiaque " of Huchard may be mentioned, in which, early in the acme of the attack in sthenic, robust individuals, alarming signs of weakness appear (in so called syncopal forms of the older authors) in consequence of the unusually intense action of the influenza toxin on the heart. We shall return to some of the important symptoms, in particular to the complications and sequelae of influenza, in our description of the lesions of the various organs.
We may point out here that : (1) The frequency of the several clinical varieties; (2) the numerical relation of the simple to the complicated cases; (3) the frequency of special complications varied considerably at different times and at different places. The reports from all countries show this to be the case. It will suffice to emphasize the following important fact, that the simple, typical, uncomplicated influenza which occurred so frequently in the pandemic of 1889-1890 was less frequent both absolutely and relatively in the later epidemics, while the complicated cases were much more frequent. This agrees with the ascertained greater mortality of later epidemics, to which we have already called attention. (See p. 550.)
We cannot here go into a comparison of the frequency of the clinical course of influenza in former epidemics with that of our latest pandemic. We should encounter, in doing so, considerable differences, sometimes so great that doubt might arise whether some of the former epidemics really were influenza. A study of the tabulated arrangement of the principal symptoms of influenza in the epidemics from 1510 to 1889 by E. Symes Thompson (loc. cit., p. 467) is very instructive.