Descending necrotizing mediastinitis (DNM) is an acute, polymicrobial infection of the mediastinum that spreads from oropharyngeal, cervical, and odontogenic infections. It is a serious condition, with reported mortality rates ranging from 30% to 50%. DNM is also a rare condition and was first reported by Pearse in 1938. Continuity of the fascial planes between the neck and mediastinum may allow infection to spread from the oral cavity and neck into the mediastinum, which leads to acute mediastinitis. There are three potential pathways for the spread of infection from the neck to the mediastinum: the pretracheal route to the anterior mediastinum, the lateral pharyngeal route to the middle mediastinum, and the retropharyngeal-retrovisceral route to the posterior mediastinum. Of these, the most common route is by way of the retropharyngeal-retrovisceral space.

Rarely, acute mediastinitis may result from hematogenous spread of an extrathoracic infection such as septic arthritis. This type of acute mediastinitis occurs in those who abuse intravenous drugs, are immunocompromised, and who have chronic debilitating disease. At CT, mediastinal air and multiple fluid collections commonly are seen in the anterior and posterior mediastinum. Fluid collections may extend into the soft tissues of the chest wall. There are no definitive imaging findings that pinpoint extrathoracic infection as the cause of mediastinitis; however, histologic analysis of microbial organisms obtained from mediastinal and extramediastinal sites may help establish a diagnosis.