Right Middle Lobe Syndrome

Middle lobe syndrome (MLS) is a relatively uncommon clinical entity that is under-recognized in clinical practice. It was first identified clinically in 1948 by Graham et al. in a case series involving 12 patients with nontuberculous middle lobe atelectasis secondary to extensive compression by enlarged lymph nodes. Recurrent or fixed right middle lobe opacification is almost pathognomonic for MLS; however, lingular involvement is common due to similar anatomic and physiological burdens. Though there has been no established definition of middle lobe syndrome, it has been recognized to involve two fundamentally different pathophysiological pathways that lead to recurrent middle lobe atelectasis: obstructive and nonobstructive.