Pulmonary hyaline membranes, aspiration and pneumonia.

The structures now called hyaline membranes were first described in the lungs of the newborn baby by Hoccheim (1903), and were considered by him to be aspirated vernix caseosa. At present there is little evidence that hyaline membranes arise as a direct result of aspiration, although aspirated substances may be included within them; many conflicting views of their pathogenesis have been held, but opinion is now in favour of their origin within the lung itself, from material, probably protein, lost through the local capillaries. Until fibrin was identified within the membranes the exact mechanism by which they developed remained uncertain, for it was difficult to understand how they could be formed from soluble serum proteins, unless these were in the process grossly denatured. A rational explanation of the formation of membranes became possible, however, after the demonstration with fluorescent antibody of their fibrin content (Gitlin and Craig, 1956); further studies using orthodox histological methods confirmed the observations of these workers and showed that the membranes arose as a result of incomplete resorption of fibrin-containing oedema fluid, and suggested that fibrin was indeed their essential component. An important conclusion drawn from this finding is that the membranes found in the lungs of newborn babies do not differ structurally from those of older patients, in whom membranes may form in a variety of diseases. In all of these there is a temporary increase in capillary permeability which is great enough to allow the escape of fibrinogen as well as of other plasma proteins; the conditions include not only inflammatory lesions and neoplastic or other infiltrations of the lung itself, but also cardiovascular disturbances, in particular severe left ventricular failure, with or without uraemia. Membrane formation in these cases is merely one common end result of a number of different pathological processes, and the possibility that this may be so in the lung of the newborn must be considered. In older children and adults membrane formation sometimes complicates pneumonia (Farber and Wilson, 1932). In the newborn these two processes are often found together; the relation between the two conditions is often, however, difficult to determine. Before the past decade hyaline membranes in the newborn were most frequently discussed in papers concerned primarily with pneumonia; although at that time origin of the membranes from vernix was generally accepted, Steinharter (1937) recognized structures in the newborn which he considered to differ from 'vernix membranes' and to be the result of pneumonia. Potter (1952) believed pneumonia was an important complication in many babies with hyaline membranes, particularly in those who died after the end of the second day. More recently the concept of the membranes forming as the result of a specific pathological process or 'hyaline membrane disease' has been widely held, so that less attention has been paid to the possibility of their origin in other ways. In this paper cases are presented in which membrane formation in the lungs of the newborn is secondary to pneumonia, and an attempt is made to define histological criteria for the recognition of such membranes; the observations recorded show that in about one-fifth of all cases in which membranes are present they are the result of an inflammatory process. The membranes found in other babies, in whom their origin is independent of pneumonia, are, to distinguish them, referred to as 'primary' hyaline membranes.