Unilateral pulmonary agenesis.

Hitherto descriptions of some 200 cases of this condition have been published. In the majority of these reports a single case has been presented in detail together with the investigations showing how the diagnosis was made. Some authors have recorded a small series of cases, and in an effort to increase the number there has often tended to be indiscriminate use of the term agenesis. As a result of this considerable confusion has arisen. In 1912, Schneider classified the condition according to the extent of the defect. In Group 1 he included only those cases with a complete absence of the lung and bronchus. In Group 2 there was no evidence of any lung tissue, but the beginning of a bronchus was noted as a blind pouch or pit. There has been little difficulty in classifying cases into either of these two groups. In Group 3 he described the bronchus as being essentially normal and ending in a fleshy structure. Many authors have had difficulty in deciding whether a particular case of this kind should be considered as an example of Group 3 agenesis or hypoplasia. The first cases to be described were all diagnosed post mortem, when it became apparent that in this condition there was also absence of the pulmonary artery on the affected side. It has been suggested, therefore, that if vessels can be demonstrated giving blood to the fleshy structure that this no longer constitutes agenesis and that such a case must therefore be classified as hypoplasia. However, as will be shown later, this is not strictly true, and there remain some cases that defy attempts at inclusion in Schneider's Group 3 or hypoplasia. More recently (Boyden, 1955) a more practicable classification has been recommended based on the degree of developmental arrest, viz. Group 1 complete absence of one or both lungs (agenesis); Group 2 suppression of all but a rudimentary bronchus (aplasia); Group 3 abortive growth (hypoplasia). In this terminology there is no need to attempt to distinguish between 'type 3 agenesis' and 'hypoplasia'.