General Considerations

• All types of atelectasis involve loss of volume in some or all of a lung with resultant increased density of the involved lung
• The atelectasis referred to here is that caused by bronchial obstruction, usually a tumor (i.e. a bronchogenic carcinoma), a foreign body or a mucus plug
• In asthmatics, chronic inflammation along with thicker and more viscous mucous leads to plugging and impaction of the bronchi 

Signs of atelectasis -- general 

• Increase in density of the affected lung
• Displacement of the fissures or the mediastinum towards the atelectasis
• Crowding of the vessels and bronchial tree in the area of volume loss
• Elevation of the hemidiaphragm
• Overaeration of the opposite lung 

Right Middle Lobe Atelectasis

• Right middle lobe is more prone to atelectasis because its bronchus has a narrow diameter, acute angle of take-off and a relatively long course
• Causes of right middle lobe atelectasis include
• Asthma
• Cystic fibrosis
• Bronchopulmonary dysplasia
• Foreign body aspiration in the airway
• Tumors
• Primary endobronchial (i.e. bronchogenic carcinoma)
• Extrinsic compression from adenopathy
• Granulation tissue, as in TB
• Bronchopneumonia
• Imaging findings
• Right middle lobe atelectasis is usually easier to recognize on the lateral view than the frontal view, where it may produce very subtle findings
• Silhouetting of the right heart border on the frontal view by the adjacent un-aerated medial segment of the middle lobe
• If the atelectatic middle lobe swings upward and anteriorly, it may produce a wedge-shaped density on the frontal view with its base at the heart
• Depression of the minor fissure and elevation of the major fissure, especially well seen on the lateral view
• On the lateral view, the atelectatic lobe forms a triangular density with its apex at the hilum and its base more peripheral in the lung
• If there is a nodular density seen at the apex of the triangle on the lateral view, suspect a mass in the hilum producing the atelectasis
• Elevation of the right hemidiaphragm may occur
• Treatment
• Chest physical therapy
• Bronchoscopy 

Right Middle Lobe Syndrome 

• Frequently occurs in children, with asthma a relatively common etiology
• Mainly based on chronic or recurrent right middle lobe atelectasis, rather than acute etiology
• Some authors believe the loss of volume is a primary inflammatory condition of the middle lobe aided by its isolation from the upper and lower lobe rather than caused by obstruction

Fraser and Pare