Mediastinitis is an infection involving the mediastinum. It is a surgical emergency with a high mortality rate. Mediastinitis may begin primarily from structures in the mediastinum (often as a result of iatrogenic injury) or may result from an infection extending downward from the oropharynx, in which case it is called descending necrotizing mediastinitis (DNM).
The mediastinum is a sterile space, so infection results from disruption of normal mucosal and tissue barriers. This usually occurs either via (1) direct injury (usually iatrogenic) to adjacent structures such as the esophagus, trachea, bronchi, or sternum or (2) extension of cervicopharyngeal infections.
Direct mediastinal injury is the most common cause of mediastinitis in developed nations. This may be due to trauma (eg, motor vehicle accident [MVA], swallowed fish bone), Boerhaave syndrome, neoplasm, cardiothoracic surgery, upper endoscopy, bronchoscopy, tracheal or esophageal dilatation, or even endotracheal intubation. See Esophageal Perforation, Rupture and Tears for more detail on this subject.
When infection extends from the head and neck downward into the mediastinum, the condition is referred to as descending necrotizing mediastinitis. The spread downward is facilitated by gravity, breathing, and negative intrathoracic pressure.  It is necrotizing, as the infection is often polymicrobial in etiology with gas-producing organisms. This is the most lethal form of mediastinitis.
Odontogenic infection is the most common cause of descending necrotizing mediastinitis, but pharyngeal abscess, sinusitis, and other ENT infections may also be sources.  This type is particularly prevalent in developing nations, where poor access to care leads to delayed diagnosis and treatment of ENT infections.
Because cervicomediastinal spaces contain loose areolar tissue with poor vascularization and few defensive mechanisms, pathogens may easily spread across fascial planes. For this reason, any cervical infection may involve the entire mediastinum.  The potential spaces that can allow infections from the head or neck to enter the mediastinum are discussed below.
The carotid sheath is a thick, matted, fibrous investment over the main longitudinal vessels of the neck. Lymph nodes are contained within the sheath, and infection in these nodes has the potential to spread downward into the mediastinum. The carotid sheath extends from the arch of the aorta to the base of the skull.
This space is bounded anteriorly by the prevertebral fascia, which overlies the prevertebral muscles in the neck. The prevertebral fascia extends from the base of the skull to the lower limit of the longus colli muscle, which is approximately at the level of T3 vertebra.
This potential space lies between the alar and prevertebral fasciae. It is patent from the skull base to the diaphragm. Its upper part is the retropharyngeal space, which lies between the prevertebral fascia and the buccopharyngeal fascia on the outer surface of the pharynx. Lymph nodes are present in this space. The lower part of this potential space extends behind the esophagus, through the superior mediastinum, and into the posterior mediastinum. Seventy percent of cases of descending necrotizing mediastinitis occur in this space. 
Mediastinitis may also result from direct extension from an adjacent source of infection, including osteomyelitis of the sternoclavicular junction. Pulmonary infections may also extend into the mediastinal space. Mediastinitis may also result from extension of granulomatous disease from mediastinal lymph nodes.
Mediastinitis is often a mixed infection, with facultative and strict anaerobes acting together. Obligate anaerobes usually outnumber facultative organisms by 10:1. Streptococcus species are the most common facultative organisms, while Bacteroides species are the most common strict anaerobes. Other organisms implicated include Staphylococcus, Escherichia coli, Peptostreptococcus, Fusobacterium, Haemophilus influenzae, Enterobacter cloacae, Histoplasmosis, Tuberculosis, and Pseudomonas aeruginosa.
Case reports have also identified Eikenella corrodens and species of Prevotella, Stenotrophomonas, Propionibacterium, Candida, Aspergillus, and Salmonella as responsible pathogens. [1, 5] As the incidence of iatrogenic mediastinitis rises compared with infections acquired outside the hospital, methicillin-resistant Staphylococcus aureus (MRSA) infections become a cause for great concern. 
Esophageal rupture is currently the most common cause of mediastinitis. Descending necrotizing infection is relatively rare in the era of antibiotic use.
In developing countries, mediastinitis still is a common devastating potential complication of head and neck infections.
Morbidity is significant for a mean hospital length of stay greater than 1 month, as well as a long period of outpatient recovery. 
Data suggest that mediastinitis has an overall lifetime mortality rate of 19%-47%; however, a review by Ridder et al suggests an improved mortality of 11.1%. 
Studies of descending necrotizing mediastinitis in the last decade indicate mortality rates ranging between 11.1% and 34.9%. 
Mortality likely has improved secondary to the widespread use of antibiotics and improved oral hygiene. 
In the presence of comorbid conditions, the mortality rate among patients presenting with established infections may be as high as 67%.
Age and risk factors are better predictors for the development of mediastinitis than race.
The prevalence of mediastinitis is higher among males than females, with a male-to-female ratio of 6:1.
Most persons who develop mediastinitis are in their third to fifth decades of life; however, it can occur in all age groups, with documented mediastinitis in patients as young as 2 months and as old as the eighth decade of life.
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