Middle lobe syndrome: a review of clinicopathological features, diagnosis and treatment.

Middle lobe syndrome (MLS) is a rare but important clinical entity that has been poorly defined in the literature. It is characterized by recurrent or chronic collapse of the middle lobe of the right lung but can also involve the lingula of the left lung. Pathophysiologically, there are two forms of MLS, namely obstructive and nonobstructive. Obstructive MLS is usually caused by endobronchial lesions or extrinsic compression of the middle lobe bronchus such as from hilar lymphadenopathy or tumors of neoplastic origin, resulting in postobstructive atelectasis and pneumonitis. In the nonobstructive type, no obstruction of the middle lobe bronchus is evident during bronchoscopy or with computerized tomography of the chest. The etiology of the nonobstructive form is not completely understood. Inefficient collateral ventilation, infection and inflammation in the middle lobe or lingula are thought to play a role, and bronchiectasis is the most common histological finding. Patients with proven endobronchial lesions or malignancy are usually offered surgical resection directly. This contrasts with nonobstructive MLS, where most patients respond to medical treatment consisting of bronchodilators, mucolytics and broad-spectrum antibiotics. However, some patients do not respond to conservative treatment and may suffer irreversible damage of the middle lobe or lingula, in addition to having recurrent symptoms of infection or inflammation. These selected patients can be offered surgical resection of the middle lobe or lingula, which is associated with a low mortality rate and favorable outcome.