Differential Diagnosis(Common)

1. Compensatory distention of adjacent lobe or lung secondary to atelectasis, agenesis, lobectomy or shunting procedure.
2. Contralateral increased density (chest wall hemihypertrophy, pleural effusion)
3. Emphysema, bullous or diffuse; large bulla or pneumatocele
4. Normal variant
5. Obstructive distention, air trapping (bronchial FB, neoplasm, stricture)
6. Pectoral muscle absence, congenital (Poland Syndrome) or surgical (mastectomy), or atrophy (polio)
7. Pneumothorax
8. Scoliosis
9. Technical (heel effect, lateral decub film, positioning/rotation, grid cutoff)

Differential Diagnosis(Uncommon)

1. Adenomatoid malformation
2. Bronchial atresia, pulmonary bronchogenic cyst, sequestration, lobar agenesis or aplasia
3. Lobar emphysema, infantile
4. Pulmonary artery atresia, hypoplasia, coarctation, or anomalous origin (sling)
5. Pulmonary artery compression by inflammatory process or neoplasm
6. Pulmonary artery thromboembolism
7. Swyer-James Syndrome
8. Venolobar Syndrome

Swyer-James Syndrome Discussion

In Swyer-James Syndrome, Bronchiolitis obliterans is triggered by a lower respiratory adenovirus infection in early childhood.
Diffuse small airways obliteration, air trapping, alveolar wall destruction, and emphysema follow.

Patients can be asymptomatic or complain of dyspnea and recurrent lower respiratory tract infections.
Affected lung can be normal-sized or small
Hyperlucency due to air trapping and reduced pulmonary vasculature

Perfusion lung scanning demonstrates decreased pulmonary blood flow on the affected side as well as decreased ventilation and markedly delayed washout of radiotracer.
This helps distinguish Swyer-James from primary central pulmonary artery occlusion or hypoplastic lung (which both have normal ventilation)