Volume loss during chest imaging is frequent and can be caused by a variety of factors, including an endobronchial blockage from mucous impaction or centrally obstructive neoplasms, past surgical resection, and visceral pleural limitation from earlier pleural inflammation or intervention. Typically, these conditions can be distinguished from one another by carefully examining the chest radiographic appearance, comparing it to prior radiographs, and correlating it with the appropriate clinical history; thoracic computed tomography may be required to clarify chest radiographic findings on rare occasions. Once the more frequent causes of unilateral thoracic volume loss have been ruled out, unusual causes of unilateral decreased lung volume, particularly congenital and developmental abnormalities, should be investigated. 

Although pulmonary agenesis and aplasia can be diagnosed in childhood, they seldom go undetected into maturity. These two disorders are distinguished by severe unilateral volume loss and a shift of the cardiomediastinal structures toward the affected side, as well as a lack of visible lung tissue on the affected side. The main difference between pulmonary agenesis and hypoplasia is the existence of a blind-ending rudimentary bronchus in pulmonary aplasia vs no bronchus in pulmonary agenesis. Pulmonary hypoplasia, which is defined by a decrease in the number and size of alveoli, can begin in utero or develop as a result of juvenile respiratory illnesses. Pulmonary hypoplasia appears on chest imaging as a unilateral volume reduction accompanied by a tiny pulmonary artery on the affected side. 

Patients with pulmonary hypoplasia, unlike those with pulmonary agenesis and aplasia, have at least some lung tissue present and may appear with a unilateral hyperlucent lung rather than total hemithorax opacification, and may therefore be recognized from those with pulmonary agenesis and aplasia.

Reference:journals.lww.com/clinpulm/Abstract/2018/01000/Unusual_Cause_of_Left_Chest_Volume_Loss.6.aspx