Pulmonary hyperplasia refers to abnormal overgrowth of lung parenchyma, reflected by increased number and enlargement of airspaces. It is always a secondary phenomenon and is thought to be a response to airway obstruction during development. Pulmonary hyperplasia may affect both lungs equally, as in fetal laryngeal atresia, but more commonly affects a single lung or portion of one lung. Laryngeal atresia may be an isolated anomaly or syndromic, as in Fraser syndrome (cryptophthalmos, ear anomalies, syndactyly, renal anomalies, and cryptorchidism). The bilateral pulmonary hyperplasia in this setting is related to obstructed outflow of fluid from both lungs in utero. The lungs may weigh 150% to 300% of normal weight and are characterized histologically by airspaces that are increased in number, enlarged, and poorly subdivided.

In its most dramatic form, pulmonary hyperplasia has been described as the solid (or “adenomatoid”) form of cystic adenomatoid malformation (Stocker type III), a rare lesion recognized as massive enlargement of one or more lobes in fetuses and stillborn infants. By ultrasound, these are large, brightly echogenic masses, which may enlarge or regress in utero. Grossly, the lung parenchyma appears uniform and solid, except for scattered tiny cystic spaces. The histologic features are similar to those described in the setting of fetal laryngeal atresia, with markedly increased numbers of alveolar structures that are mildly enlarged and poorly subdivided. The airways are widely separated by the increased lobular parenchyma but are otherwise normal. Interlobular septa are infrequent to absent. The parenchyma is often also described as having an immature appearance, although this is explained by the immaturity of the fetuses and premature stillborns in whom these lesions are identified. Although not clearly documented to date, airway obstruction during development likely accounts for the pathogenesis of these rare lesions.