Part 6

The most important extrapulmonary lesions in influenza are those of the hematopoietic and the vascular systems. The first are typical of a general non-suppurative, inflammatory process, and are characterized in the majority of cases by a picture not unlike that encountered in typhoid fever, although the hyperplasia of the lymphadenoid tissues, as seen in the latter disease, is not present. The more important lesions are associated with the vascular system; phlebitis and arteritis occur, but are not so frequent as hemorrhages in the skeletal system, in the parenchymatous organs, and in the mucous membranes of the hollow viscera. These hemorrhagic necroses may be etiologically associated with capillary bacterial thrombi.

IV. COMPARISON BETWEEN THE RESPIRATORY LESIONS OF INFLUENZA AND THOSE INITIATED BY THE INHALATION OF POISONOUS GASES

Immediately preceding the advent of the influenza epidemic in New Haven, there had been completed in this laboratory the experimental studies of the effects of a large number of different toxic gases upon the respiratory tract. This was begun with the support of the Bureau of Mines, and subsequently, after the formation of the Chemical Warfare Service, the work received the support, not only of this branch of the Army, but also of the Surgeon General’s Office. Exceptional opportunities for animal experimentation were offered. Every stage of the inflammatory process in the respiratory tract was studied, from the most acute, a few hours after the exposure of the animal to a high concentration of gas, to the very chronic types that resulted from sublethal concentrations and led to lesions encountered at the sacrifice of the animals many months later.

The criticism will unquestionably be introduced, that with the anatomical picture of the respiratory lesions initiated by irritating gases freshly in mind, an analogy between them and those of the pulmonary lesions of influenza may not be impartially drawn. However, such fundamental and striking similarities exist between the two processes (128) that the argument is simplified.

A. THE INFLAMMATORY RESPONSE VERSUS THE SYSTEMIC CAPACITY TO COMPENSATE

Symmers (141) and Oberndorfer (108) have already likened the influenzal pulmonary complications to plague pneumonia, and it is quite possible that others will find fundamental similarities with other fulminating, inflammatory reactions. Inflammation, a series of processes manifested after injury by a tissue which still retains its viability, is modified by another important factor usually neglected; namely, the state of the host. As a consequence, the inflammatory reaction might readily have similar manifestations even though the primary exciting agents are diverse. This has been amply demonstrated for many different reactive processes, and still it may be considered as a challenge for those who are of the opinion that specific bacterial agents necessarily produce characteristic anatomical manifestations. Consider the state of knowledge of the various reactions to the tubercle bacillus before this microorganism was isolated; and then bear in mind that we are dealing here with a disease whose causative agent, despite the present uncertainty regarding its specific nature, produces a lesion that paves the way for the invasion of the respiratory tract by many different organisms. The extent of the reaction may depend upon the systemic capacity to compensate, as well as upon the degree of primary damage in a specific system of organs. Unquestionably, these two factors must take their places in the balance, the opposite pan of which contains the one or the other secondary bacillary invader.

Scientific medicine has reached that evolutionary period where, in addition to the specific infecting microorganism, associated etiological factors find an important place in the interpretation of a disease. More and more attention is being directed toward the latter factors and the rôle they play in respiratory infections. Many different factors open the pulmonary portal for organisms and, in this way, terminate by a serious respiratory inflammatory complication what otherwise might have been a relatively mild disease. In the past two years this has been demonstrated for measles.

For the above reasons, not only the immediate action but the complications and sequelæ, too, of the inhalation of irritating gases by normal healthy animals, emphasize themselves as a framework for a more comprehensive interpretation of respiratory disease in general.

B. THE PRIMARY INJURY

With few exceptions, the gases studied have their most marked effect upon the respiratory tract. They differ, however, in the localization of the lesions and in the extent of the damage. For instance, mustard gas, in high concentrations, has a necrotizing effect upon the entire respiratory tract even to the pulmonary parenchyma itself. Inhaled in a more dilute form it involves the larynx and the trachea and has spent itself before the lung is reached. With phosgene, the opposite is true. The upper respiratory tract is only slightly involved. The outspoken lesions involve the bronchioles, the ducti alveolares, and extend to the alveolar walls of the lung. Chlorine, while it produces a less severe lesion of the upper respiratory tract, extends more often through all of the ramifications of the tracheal tree to the lung.

With vital stains (trypan blue), it may be readily demonstrated that these gases kill the epithelium of the respiratory tract and extend through this superficial cellular coat to the deeper tissues of the bronchiolar wall and to the lung tissue, killing it entirely, just as a corrosive chemical destroys the wall of the stomach.

When chlorine and phosgene reach the lung, an intense reactive process follows immediately. The congestive changes, the hemorrhage into the pulmonary parenchyma both in the alveoli and in the subpleural and interstitial tissues, and the albuminous rich, serous exudates occupy the foreground of the acute picture and are associated clinically with intense cyanosis, great dyspnœa, hemorrhage, bronchorrhea, and also with pulmonary and subcutaneous interstitial emphysema. The microscope reveals a hyalinization of the walls of the bronchioles or ducti alveolares which are distended during this acute period and form a picture that is very unusual in the more frequent types of respiratory inflammation. It shows also a hyalinization of the alveolar walls, usually those in direct continuity with the larger air passages. The interstitial edema and hemorrhage, often perivascular in distribution, is only overshadowed by the thick, almost colloid-like material within the alveolus itself. Fibrin stains at this stage show this exudative element in surprisingly large amounts. Fibrin not only covers the alveolar wall, but crosses this structure and often forms intracapillary plugs. In fact, besides the red blood cells, the serum and the fibrin, one often sees, not only an excess of polymorphonuclear leucocytes in the vessels of the lung, but these appear in process of migration within a few hours after the animal has been exposed.

Animals surviving this stage often succumb later with a typical pneumonic process. Although frequently pseudolobar, the process may be lobar in extent, and as time goes on more definite lobular involvement is frequently encountered. With the localization of the inflammatory process, the general edema, congestion and milder reactions in other portions of the lung tend to subside, and may be entirely absent in a few days. The clinical picture, in the experimental animal at least, also assumes a more typical expression of respiratory infection,—leucocytosis, chlorine retention, etc., appear. Often the pneumonias, both lobar and lobular, are complicated, grossly, by softening, and histologically, by necrotization of the bronchiolar and alveolar walls. Recrudescence of the active pulmonary infection is not uncommon, and perhaps is associated with these focal necrotizing areas.

C. THE TENDENCY TO ORGANIZATION OF BRONCHIOLAR AND ALVEOLAR EXUDATES

The sequelæ after exposure to gas may be markedly delayed. They occur, not only in animals which have exhibited characteristic acute symptoms, but also, and especially with phosgene, where no serious immediate clinical effects followed exposure to the gases. They are dependent upon the fact that the pneumonic process has a very striking tendency to undergo organization, a process which involves not only the exudate in the alveoli, but also that within the bronchioles. The organizing process may be present in both these portions of the lung or may be confined to the bronchiole alone, and lead to a progressive interference with the ingress and egress of alveolar air, so that atelectasis and emphysema become permanent, and in turn lead to narrowing of the vascular bed, ultimately producing right-sided cardiac decompensation. Bronchiectases also occur; they may be tubular, associated with organization of the pneumonic process in their vicinity, or saccular if death has not followed the destruction of the bronchiolar wall.

Compare this picture with that of influenzal pneumonia and its sequelæ; in both there is, first the acute diffuse involvement of the lung, initiated by, or occurring simultaneously with, an acute tracheobronchitis and presenting clinically dyspnœa, cyanosis, blood-stained, abundant sputum, and even interstitial emphysema; next, the tendency for the pneumonic process to localize and to necrotize; and finally, if the acute period is survived, organization of the pneumonic and bronchiolar exudates with resulting bronchiolitis and bronchiectasis. With even more minute comparison, the resemblance is sustained. The initial or early dilatation of the ducti alveolares, so characteristic in gas poisoning, also occupies the foreground of the histological picture in influenza. These dilated structures are the more prominent on account of the red, ribbon-like strands that cover the surface and often involve the walls. Similar necrotization, varying in its depth even to involve the whole of the alveolar walls, occurs in both of these conditions. Similar hemorrhages in the early stage, rupture of the alveolar wall with interstitial and subcutaneous emphysema, necrotization, gangrene, and organization in the same localities, characterize the two lesions, and are not found with the same uniformity in any other type of respiratory disease.

D. THE IMPORTANCE OF THE TRACHEA AND ITS RAMIFICATIONS AS A PROTECTIVE MECHANISM AGAINST INFECTION OF THE PULMONARY PARENCHYMA

In gas poisoning it has been demonstrated that the initial damage to the epithelium of the larger air passages is followed by an invasion of the pulmonary parenchyma by the bacteria of the mouth. Repeated cultures from the mouth before the exposure of the animal to gas have been followed by the recovery of the same organisms, including the pneumococci, the streptococci, the staphylococci, and a gram-negative, hemoglobinophilic, small bacillus from the pneumonic lung. They find their way into the lung after the destruction or incapacitation of the protective mechanism (70) of the upper respiratory tract,—into a lung which has been so damaged by the irritative gas that bacteria innocuous in the normal pharynx now find a favorable medium for their development. The inflammatory reaction which develops into pneumonia, perhaps necrotizing, perhaps later organizing, can only be explained by the combined action of the corrosive gas and the organisms saprophytic in the normal mouth, but now pathogenic in varying degrees in the lung whose vital reactions have either been inhibited or impaired by the gas.

There is no reason why this analogy should not be drawn, no reason why we should not consider that the unknown etiological agent in influenza produces a similar injury to, or even destruction of, the protective mechanism of the respiratory tract. Similarly, gas and influenza damage the pulmonary parenchyma itself, so that the bacteria of the air and of the mouth which find their way into the damaged lung[16] initiate processes and produce complications which may not be distinguished.

V. PECULIARITIES OF THE HISTOLOGY OF INFLUENZAL PNEUMONIA

A. THE EXTENT OF THE INITIAL PULMONARY LESION

One of the features differentiating the pneumonic process in influenza from the usual types of inflammation of the lung, is diffuseness (90). In the early cases especially, or in cases which terminate fatally at an early period, both lungs are often involved, and, on histological examination, only a small portion of the pulmonary parenchyma is found unaffected. The exudate, largely acellular, presents serum as its most conspicuous feature. The picture is one of a patchy pneumonia with intermediary areas of what might be called edema, although fibrin is often demonstrable in the coagulated, albuminous material. So little attention has been paid to earlier stages of the usual types of pneumonia, for example lobar, that it is impossible to say whether or not diffuse involvement of the pulmonary parenchyma initiates the process which later becomes localized in one or more lobes. The initial edema of influenzal pneumonia is the expression of a widespread irritation. If the injury has not extended deeply, the edema may disappear within a relatively short time, and exfoliated lining cells fill the alveoli; those remaining in their normal position are frequently in process of division (Fig. XLVII). It is conceivable—and the view has already been announced—that this edema is a disseminating factor and perhaps responsible for the diffuseness of the pneumonic process which may follow. If the fluid is simply a serous exudate, it may play no essential rôle in the severe acute symptoms manifested by these patients, for it has been shown, both in the experimental lesions induced by pulmonary irritating gases and by pulmonary irrigation through which extensive artificial edema of the lungs may be attained, that the presence of fluid in the lung in itself is not harmful (161).

B. THE HEMORRHAGIC EXUDATE

Another striking feature of the inflammatory process in this disease is the extensive hemorrhagic exudate expressed clinically in the fresh, red blood of the abundant sputum. In the tissues the blood is always fairly well preserved. It may be scattered diffusely through the cellular exudate (Fig. XXIV) or so abundantly that the area resembles an infarct (Fig. XXV). These hemorrhagic foci, which vary considerably in size, are found not only in the cases that terminate fatally within a few days, but may occur at any time during the acute manifestation of the disease. They are an exaggerated form of red hepatization and it is difficult to see how such red foci could ever change to areas of grey pneumonia. It is a widely accepted statement, in textbooks of Pathology at least, that the stage of red hepatization in pneumonia follows the period of engorgement and precedes the grey form. This interpretation is open to question concerning the lesions that are encountered in influenza, as well as in those that are seen after gas inhalation. Unquestionably, in the stage of engorgement the lung has a red appearance, enhanced by the acellular, serofibrinous exudate in the alveoli through which the greatly congested vessels are seen. At this stage, the lung has a translucency on gross examination, which is not the case when the cellular content of the alveoli is increased. This picture is not the one spoken of most commonly as red hepatization; for, although it appears as a relatively red lung in the gross, difficulty is encountered in its histological correlation, for the exudate is composed, not of red cells, but largely of serum and fibrin. The red color may persist even when numerous polymorphonuclear leucocytes and desquamated alveolar wall cells are within the alveoli, the walls of which are markedly engorged. Later, as the circulation in the pneumonic zone is impaired, the alveolar exudate determines the tone of the gross color, and a considerable number of red blood cells may be overshadowed by the larger percentage of white ones.

The advent of red blood cells where grossly the exudate is red can hardly be explained by the simple process of diapedesis. Indeed, there is ample evidence that they escape by rhexis through lesions of the vascular wall. This phenomenon cannot be demonstrated in areas where a compact mass of red cells obliterates the alveolar space (Fig. XXV), but in the less firmly consolidated alveoli where red blood cells predominate (Fig. XXVI) the picture of the alveolar wall is very instructive. The capillaries may be prominent and contain red cells almost exclusively. Often the epithelium of the air space is exfoliated so that there is nothing to minimize the prominence of the engorged vessels. These capillaries, covered by such a delicate wall that rupture seems imminent, may protrude like saccular aneurysms (101) into the alveolar space (Fig. XXVI). In all probability, these sacs do rupture and this result would be one explanation for the escape of large numbers of red blood cells. In several instances such a picture was encountered, where with little reaction at the point of rupture, red cells within the vessel were continuous with an accumulation of similar cells in the alveolus. Further evidence for such rupture is offered where the vessel is collapsed. Here there is accumulation of polymorphonuclear leucocytes in the area of destruction in contrast to the well preserved red corpuscles in the remainder of the vessel (Fig. XXVI). The above pictures may be utilized in the interpretation of the outspoken foci of red hepatization which may assume infarct-like proportions. As has been said, it is impossible to conceive that these hemorrhagic areas where the alveoli are packed with red cells ever change to a grey type of consolidation. Consequently, it seems more probable that the color of red hepatization in the usual types of pneumonia depends upon the marked engorgement of the vessels seen through a relatively acellular, transparent, serofibrinous, alveolar mass and not upon the number of red cells in the exudate.

C. THE APLASTIC EXUDATE

The absence of cellular elements in the alveolar exudate is frequently observed in influenzal pneumonia (Figs. XXI, XXII, XXIII). This picture has been reproduced experimentally in animals which have been rendered aplastic with benzol, especially with reference to their myeloid elements (160). Pneumonia produced by intratracheal insufflation is more rapidly fatal in aplastic animals, and it is conceivable that the absence of cellular reaction is an explanation for the lack of resistance demonstrated by the high mortality of influenzal pneumonia. Frequently the fibrinoserous mass scattered diffusely throughout the lung is rich in bacteria. In the absence of cells of the polymorphonuclear series, the bacterial development seems to be unrestricted. The aplastic exudate is associated clinically with an absence of a myeloid reaction in the peripheral circulation. The leucocytic count may be definitely decreased, even though the tissues have been invaded by pyogenic organisms to which the usual response is a definite leucocytosis. The only explanation is that the myeloid structures have been injured, probably by the unknown virus of the disease.

D. THE HYALINE NECROSIS OF THE PULMONARY TISSUE

The hyalinization of the epithelium lining the ducti alveolares (47, 48) also merits special attention (Figs. V, XV, XVI). This process may extend through the wall of the duct and is often seen in the alveolar walls throughout the involved lung. The entire alveolar wall may be homogeneous in appearance, but, occasionally, the thrombus alone, which has formed in its vessels (41), presents this change (Fig. XVII). The alveolar as well as the bronchiolar wall is thickened by a homogeneous material in which cell-body and exudate cannot be differentiated. This acute necrosis, as has been mentioned, is encountered in gas poisoning but is unusual in other known types of respiratory infection. Doubtless, it is a precursor to the more destructive lesions commonly found in later stages of the disease—abscesses which extend through the bronchiolar walls (Fig. XXXI), necrotizing areas of pneumonia in which huge clumps of bacteria are found (Fig. XXXII), and true gangrene (Figs. XXXIII, XXXIV, XXXV). The destruction of the alveolar wall in the early stages of the disease plays a causal rôle in the production of subcutaneous emphysema (Figs. XVIII and XIX). This important phase of the histological change in influenzal pneumonia has received but little attention and, with one or two exceptions, is not mentioned in the literature (8, 162).

In the interpretation of this necrotization, the only helpful analogy is offered by the acute respiratory lesions following the inhalation of poisonous gases. With the aid of vital stains, it has been demonstrated that chlorine quickly initiates necrosis due to the direct action of the gas. Since necrosis also occurs with phosgene,—in the decomposition of which hydrochloric acid is probably liberated,—there is presumptive evidence that the halogen is responsible for the process. Studies are now in progress to determine the relation of the acid-producing properties of the different strains of organisms to the type and fate of the pneumonic exudate.

E. THE ORGANIZATION PROCESS