This is a condition of considerable frequency, and it arises under a variety of different circumstances.

(A) Causation

In most cases the main agent in causing dilatation of the bronchi is the air-pressure acting excessively and very often acting on bronchi whose tissue is weakened by disease. In regard to the air-pressure we can recognize here as in the case of emphysema cases in which the abnormal pressure is during inspiration and others in which it is 'during expiration. Non-expansion or shrinking of the lung alveoli is a cause of bronchiectasis from the action of the forces of inspiration. To such cases the term Complementary Bronchiectasis may be applied, the term having a similar significance to complementary emphysema. This is illustrated in certain interesting cases of congenital atelectasis, in which portions of the lung do not expand at birth and remain unexpanded (see Fig. 354). In such cases the inspiratory expansion of the chest is not able to act on the lung alveoli, and so it exercises a dilating force on the bronchi, which may be expanded into a congeries of sacs. (See also under Hypertrophy of the lung.) Localized atelectasis due to other causes may have a similar result. A more frequent cause is shrinking of the lung by fibroid contraction. This occurs in the various forms of chronic inflammation of the lungs, and more particularly in fibroid phthisis. The shrinking of the lung tissue is often very great and a complementary bronchiectasis is usually associated with emphysema, the latter occurring in the parts of the lung less affected with the fibrous development. In cases of this kind the dilatation of the bronchi is partly due to the contraction of the fibrous tissue of the lung. The adventitia of the bronchi is directly continuous with the interstitial tissue of the lung, and the shrinking of the latter will act on the bronchial walls, but will do so irregularly according to the relations of the fibrous tissue. This is in part the explanation of the irregular expansion of the bronchi resulting in the saccular form.

Complementary bronchiectasis from old congenital atelectasis.

Fig. 354. - Complementary bronchiectasis from old congenital atelectasis. The bronchi in the apical region are expended into rounded sacs. The collapsed tissue around contained no carbonaceous pigment as if it bad never been in use for respiration.

Weakness of the bronchial wall leads to bronchiectasis chiefly from its association with coughing, in which the expiratory forces are in creased. In this respect bronchiectasis is comparable with substantive emphysema. In chronic bronchitis there is frequently, as we have seen, atrophy of the bronchial wall. This atrophy may be from fibrous development, and in that case the wall can scarcely be weakened. But if the wall, as frequently happens, is for prolonged periods in a state of more constant inflammation, so that its tissue is like granulation tissue, then it is liable to expand. In such cases there is abundant secretion and therefore frequent coughing, and the forced expirations with closed glottis, acting on the weakened wall, induce dilatation. The weakening or softening of the wall is even more manifest in septic bronchitis where there may be extensive bronchiectasis. Grainger Stewart regards the atrophy as of primary importance and the probable result of a constitutional, if not hereditary, defect.

Accumulation of the secretion of the bronchi is also an occasional cause of bronchiectasis. This may act in two ways. The accumulated secretion is apt to decompose and produce septic inflammation, and so weaken the wall. On the other hand, if there be a complete obstruction of a bronchus the accumulation in the parts distal to the obstruction may dilate the tube. As atelectasis results from such bronchial obstruction we have a further cause of bronchiectasis.

(B) Structural Changes

The various forms of bronchiectasis may be divided into two, namely, the cylindrical and the sacculated. The Cylindrical form occurs mainly where the causal condition has been more or less of a general one, such as a prolonged bronchitis. The bronchi, especially the middle-sized ones, are unduly wide, and their walls are thinned. The dilatation may be quite regular, but very often there are little bulgings or pouches. In the Sacculated form there has usually been a more local agent at work. The most typical sacculated dilatations are found in fibroid phthisis, but may be produced also by any cause which induces shrinking or non-expansion of the lung tissue (see Fig. 354).

In the sacculated form the dilated tubes assume the characters of cavities, which may be difficult to distinguish from cavities taking origin in destruction of lung tissue. This difficulty arises in cases of tuberculosis, which form the largest proportion of the cases of sacculated bronchiectasis. As a general rule the bronchiectatic cavity has a distinct lining membrane, but this is not always the case, and no cavity should be definitely distinguished as bronchiectatic unless a direct continuation with an open bronchus is distinguishable.

The microscopic structure of the wall of a bronchiectasis varies considerably according to the causation. Where there is much inflammation, and more particularly in the cylindrical form, the structures of the wall will be largely replaced by a highly cellular tissue, in the midst of or beneath which some of the normal structures may remain, such sis muscular bundles. In the cases of complementary dilatation there is much greater persistence of the normal tissues, mostly thinned by the stretching and chronic inflammation. The basement membrane is, for the most part, preserved in cases of this sort. The presence of epithelium lining the cavity is not a good criterion for the diagnosis of bronchiectasis, because on the one hand epithelium may grow over the surface of cavities of other origin, and on the other hand the epithelium of a true bronchiectasis may be entirely destroyed and the mucous membrane replaced by granulation tissue.

As the secretion stagnates in dilated bronchi whether cylindrical or sacculated there is commonly, during life, at intervals an abundant expectoration, which is often of a highly putrid character.