Right middle lobe syndrome (RMLS) is a rare and clinically variable condition. It is defined as chronic or recurrent atelectasis of the right middle lobe with silhouetting of the right heart border on the frontal view of chest radiograph and a wedge-shaped density extending anteriorly and inferiorly from the hilum of the lung that can be best visualized using lateral chest radiography. It is thought that the obstructive type of RMLS results from extrinsic compression of the bronchus by lymphadenopathy or tumors which exploit the anatomic peculiarities of the right middle lobe bronchus, while the nonobstructive variety results from poor collateral ventilation which may be due to underlying pulmonary disease, such as asthma.

The initial diagnosis of MLS is often made by examining the posteroanterior and lateral X-ray chest films and determining the etiology of the patient’s airway diseases, as well as confirming the presence of a wedge-shaped density on lateral chest radiograph. If the diagnosis is obstructive, surgical lobectomy should be considered.

In the present study, we evaluated a series of 17 patients with recurrent or chronic atelectasis of the right lung with silhouetting of the right coronary border and a wedge-shaped density on lateral radiograph that extended from the hilum anteriorly and inferiorly. The etiologies of the cases were lined as allergic asthma (23%), idiopathic asthma (23%) reactive airway disease, tuberculosis and others.

In the present study, we found that a history of asthma is associated with increased incidence of infection, especially bacterial infections (MP). A recurrent atelectasis and MP infection may lead to lung parenchymal damage, ciliary clearance dysfunction, epithelial cell shedding, mucus plug and eventually blockage of the bronchial lumen.