Introduction to mediastinitis

Mediastinitis is defined as infection and inflammation of the mediastinum. The mediastinum is the central part of the chest cavity containing the heart, the main blood vessels, the oesophagus (gullet), the trachea (windpipe) and the nerves and structures of the lymphatic system in the chest; basically, everything in the chest cavity that is not the lungs and their lining (pleura). 

The prognosis, or outcome, of mediastinitis is dependent on its cause. There are several types of mediastinitis each with differing causes. In terms of outcome, they are best considered separately as they have differing prognoses and respond to treatment strategies specific to each type.

Types of mediastinitis

Post-surgical mediastinitis

This form of mediastinitis usually occurs following surgery to open the chest cavity to gain access to the heart in open heart surgery. A median sternotomy is the name given to the incision through the breastbone (the sternum) performed in procedures where surgical access to the mediastinal cavity is required. Infection may be introduced by the bacteria on the surface of the patient’s skin or from the surgical team. The term deep sternal wound infection (DSWI) is used to denote infection that has spread from the wound into the mediastinum causing mediastinitis. Rates of DSWI following median sternotomy range from 0.25%-5%,¹ although an infection rate greater than 2% in a particular cardiac surgery centre is generally considered a marker of poor performance.²

Mediastinitis following esophageal perforation

A perforation of the oesophagus (gullet) is also often classified as post-surgical because 50% of all perforations occur following procedures to visualise the oesophagus and stomach (endoscopies) in which an instrument containing a camera (endoscope) or a device to directly visualise the oesophagus (direct oesophagoscope) is inserted into the oesophagus.³ Rates of perforation following routine endoscopy are around 0.5% of all procedures.⁴ Other causes are spontaneous rupture frequently associated with alcohol abuse (Boerhaave’s syndrome) accounting for 15%, the ingestion of a foreign body which tears the oesophagus, external trauma to the chest or neck, surgery to the oesophagus and cancers of the oesophagus.³ Following perforation, gastric juices and bacteria leak through the defect into the mediastinum which leads to rapid infection of the mediastinum.

Descending necrotising mediastinitis

This form of mediastinitis arises from infections in the deep spaces of the neck, usually from either a dental or oropharyngeal (mouth and throat) source. The infection spreads downwards, under the influence of gravity along the thick planes of fibrous tissue known as fascia that separate the deep spaces of the neck into the mediastinum. Whilst traditionally these infections were associated with a dental cause (Ludwig’s angina), improvements in dental care have seen a shift to oropharyngeal sources such as a peritonsillar abscess (quinsy), tonsillitis or occasionally throat infections (pharyngitis). This form of mediastinitis is particularly dangerous and has a high mortality rate (10-40%), depending on the extent of the infection.⁵

Chronic fibrosing mediastinitis

This is the most rare form of mediastinitis. As opposed to all of the previously mentioned forms which are all acute forms of infection and inflammation, chronic fibrosing mediastinitis has a long and indolent course. Chronic fibrosing mediastinitis can be further divided into two forms. The most common, granulomatous type, is associated mostly with infection with the fungus histoplasma capsulatum, found in soils rich in bird droppings. Occurring in the USA, parts of Africa and South America, it is extremely rare in Europe. Very rarely, tuberculosis can also precipitate chronic fibrosing mediastinitis. The formulation of a granuloma (a ball of inflammatory tissue) in the mediastinum is thought to occur as a result of a disordered immune response to proteins found on the surface of histoplasma capsulatum, which then leads to perpetual inflammation. There is currently no known cure.^6,7 A less common, non-granulomatous form of chronic fibrosing caused by an auto-immune response has also been identified. This form of the disease may show some response to corticosteroids.⁷

General factors affecting the prognosis (outcome) of mediastinitis

Whilst the prognosis of each type of mediastinitis will be considered separately in this article, there are some general factors which affect all of the acute forms of mediastinitis adversely. These factors also increase the risk of developing mediastinitis in the first place, particularly in post-surgical patients. The following conditions are associated with negative outcomes in mediastinitis:⁸

• Diabetes
• Obesity
• Chronic obstructive pulmonary disease
• Smoking
• Heart failure
• Kidney failure
• Dialysis
• Gender (more common in people assigned female at birth)
• Mechanical ventilation / prolonged stay in intensive care after surgery
• Circulatory problems (peripheral vascular disease)

Additionally, in all forms of mediastinitis prompt diagnosis and treatment are associated with better outcomes. Delays in diagnosis have a significant effect on mortality rates from the disease.

The prognosis (outcome) of post-surgical mediastinitis

Historically deep sternal wound infections (DSWI) following cardiac surgery have a particularly poor outcome. Studies from the 1980s report overall mortality rates of between 20-45%.² However, this high mortality rate has been reduced by new advances in surgical techniques employed in DSWI. More recent studies of mortality from DWSI place the rate between 1-14%.⁹ Once mediastinitis due to DSWI has been established, intravenous antibiotics are commenced. The next stage is the re-opening of the wound and surgical debridement (cutting away) of infection. The prognosis is dependent on the extent of the infection, in particular, the viability of the remaining sternum (breastbone). 

The European Association for Cardio-Thoracic Surgery expert consensus statement on mediastinitis (2017) divides the severity of sternal viability and stability into four groups, ranging from group 1- a stable sternum with a viable bone to group 4- a completely unstable sternum with necrotic (dead) bone.⁸ The sternum must be stabilised after surgery to revise the wound or there is a risk of the bone edges tearing the heart. A variety of surgical techniques are used to revise the wound. The most simple is to re-close the wound after surgery to clean the wound and to insert drains into the mediastinum which allows the mediastinum to be washed out with antimicrobial agents. In other cases, the wound is left open and packed with dressings, with a plan to close the wound at a later date. If the wound is large or a large amount of sternum is removed, a flap of muscle or omentum (the membrane surrounding the abdominal organs) is taken, along with its own blood supply to close the defect.⁸ Prognosis is very much dependent on the degree of remaining sternum and the particular surgical technique employed.

The long-term outcomes for patients with DSWI are also adversely affected. In addition to long stays in hospital and often having to undergo multiple surgical and reconstructive procedures, overall mortality from all causes is almost doubled following a DSWI. Long-term problems with recurrent infection at the wound site are also common.²

The Prognosis (outcome) of mediastinitis due to oesophageal perforation

The outcome of mediastinitis due to oesophageal perforation is largely dependent on the size and site of the perforation and the time taken to diagnose and treat the perforation. Leakage of gastric juices and oesophageal contents into the mediastinum rapidly leads to septicaemia which can progress to failure of the body's major organs. Mediastinitis can result from perforations both of the oesophagus in the region of the neck and the chest cavity, although better outcomes are seen in perforations in the region of the neck. Mortality rates for perforations in the region of the neck (cervical) are 6%, compared with 11% for those in the chest cavity.¹⁰ The cause of the perforation is also associated with differing outcomes. Mortality rates by cause are as follows:¹⁰

• Spontaneous perforation (Boerhaave’s syndrome) - 15%
• Surgical / following endoscopy - 13%
• Perforation by an ingested foreign body- 2%

Treatment in the first 24 hours of an oesophageal perforation is considered crucial to improving survival rates. Overall mortality for perforations diagnosed and treated within 24 hours in one study was 7.4%, compared with 20.4% for those discovered after a 24-hour period.¹⁰

The prognosis (outcome) of descending necrotising mediastinitis (DNM)

Traditionally, this type of mediastinitis is associated with high mortality rates of between 10-40%.^5,11 Treatment is with intravenous antibiotics and surgical drainage and debridement of the affected mediastinum. The size and extent of the infection determines to a great extent the rate of mortality and the surgical approach to treatment. Infections that are confined to the level above the division of the windpipe into the two bronchi can be accessed surgically through the neck, whereas, those below this level may require opening of the chest cavity.⁵ Interestingly, better outcomes and the need for less repeated surgical procedures are associated with surgical access through a sternotomy procedure to open the chest cavity.¹² Factors which predispose to deep neck space infections, such as diabetes, alcoholism, malnutrition etc., are associated with poorer overall prognosis.⁵

The prognosis (outcome) of chronic fibrosing mediastinitis

This rare form of mediastinitis follows a slow indolent course often over many years, with death usually occurring within six years of the onset of symptoms.⁷ There is no cure for chronic fibrosing mediastinitis and despite its cause, antifungal drugs and steroids are ineffective in the granulomatous type. Steroids may only be of limited benefit in the non-granulomatous sub-type. Eventually, inflammatory tissue causes complications by compressing the structures in the mediastinum - particularly the windpipe (trachea), oesophagus and the main veins in the chest (superior vena cava). Surgery may be considered to relieve this compression in selected cases; in other cases, the use of stents may relieve obstruction temporarily, however, all these measures are considered palliative and eventually death occurs due to complications of compression.^7,13

Complications of mediastinitis

Similar to prognosis, the complications of mediastinitis are related to the type or cause of mediastinitis. Generalised complications of mediastinitis include the following:⁸

• Septicaemia - particularly in oesophageal perforation and DNM generalised sepsis may occur, this is frequently the cause of death from mediastinitis
• Airway obstruction - compression of the trachea (windpipe) may occur, this may be a particular issue in DNM
• Obstruction of the main vein leading to the heart (Superior Vena Cava syndrome)
• Recurrent pneumonia
• Bleeding into the chest cavity
• Inflammation of the sac around the heart (constrictive pericarditis)
• Heart failure

Summary

Mediastinitis is a rare condition characterised by infection and/or inflammation of the central portion of the chest cavity containing the heart, major blood vessels, windpipe and gullet. The vast majority of cases occur secondary to surgery to open the chest cavity in open heart surgery, or via perforations of the oesophagus (gullet), many of which are also caused by medical procedures. Rarer causes include the spread of infection from the deep spaces of the neck from dental or throat infections. All acute forms of the disease carry high mortality rates but the prognosis differs depending on the cause. Prognosis is dependent on a variety of surgical approaches to draining the infection and surgically removing diseased tissue. Complications of mediastinitis are also related to the primary cause and can be fatal in nature.