Most cases of acute mediastinitis are secondary to postoperative
complications and esophageal perforation . Acute sternal
osteomyelitis and septic arthritis of the sterno-clavicular joint
may spread into the mediastinum . Risk factors for sternal
osteomyelitis include IV drug use, HIV, and other immunodeficiency
conditions . Risk factors for sterno-clavicular septic
arthritis (most commonly S. aureus or P. aeruginosa) include
diabetes, rheumatoid arthritis, and IV drug use . Despite
appropriate antibiotic therapy and surgical removal of infected
and necrotic tissue/debridement relapse occur in up to 18% of
patients with staphylococcal mediastinitis .
Mediastinitis is a relatively uncommon (incidence 0.5 to 5%), but
serious complication following median sternotomy (mortality 7-80%)
. Mortality can be decreased if the diagnosis is made early and
sternal debridment and antibiotic lavage are performed . The
most common organism for post-op mediastinitis is S. aureus . Risk factors
include diabetes, obesity, internal mammary artery harvest,
excessive cautery, and prolonged surgical duration . Symptoms
include fever, eryhtema, surgical site drainage, chest pain,
sternal instability, and sternal dehiscence .
Normal post-surgical findings after sternotomy include callus
formation (which begins after the 3rd post-op month), small gaps
or offsets, and impactions . Only 50% of sternotomies appear
completely healed after 6 months; however, sternal union should be
complete within one year of the procedure .
Dehiscence- which is defined as identifiable disruption of the
sternotomy- is frequently associated with the development of
mediastinitis [4,5]. The development of dehiscence later than the
2nd week after sternotomy is suggestive of mediastinitis .
Clinically patients present with acute chest pain, high fever,
chills, and leukocytosis . Obesity, insulin-dependent
diabetes,and the use of internal mammary artery grafts (especially
bilateral grafts) are risk factors for the development of
postoperative mediastinitis .
CXR: Radiographic abnormalities frequently precede the clinical diagnosis of sternal dehiscence by an average of 3 days . On plain film radiographs, sternal wire migration or rotation is an indication that the patient has developed a sternal dehiscence which may be secondary to mediastinitis . Occasionally, a mid-sternal vertical lucency wider than 3 mm can be seen representing the separating fragments . Gaps less than 3 mm are usually not associated with clinical instability, but progressive widening of the incisional gap is indicative of dehiscence . Other findings of mediastinitis include mediastinal widening and loss of the normal mediastinal contour . Air-fluid levels may be seen within the anterioir mediastinum on the lateral view.
Computed tomography: Following mediansternotomy strandy densities can be seen within the anterior mediastinal fat for up to 2 to 3 weeks, however, the mediastinal fat planes are generally preserved. Small collections of air may be identified normally for up to 7 days after the procedure. Focal retrosternal fluid collections or hematomas may also be identified for several days to 2 weeks- again, the mediastinal fat planes are usually preserved. The sternal defect may be slightly irregular or offset. A small amount of pericardial thickening or fluid can be expected in the early post-operative period.
Similar to CXR, a lucent separation between the sternal fragments
of mor than 3mm is suggestive of dehiscense . Findings of
mediastinitis include localized mediastinal fluid collections and
the presence of mediastinal air. Secondary findings include
edema/increased density within the adjacent mediastinal soft
tissues (with obliteration of the mediastinal fat planes ),
sternal separation, pleural effusion, and adenopathy. Early
sternal osteomyelitis can be very difficult to differentiate from
normal post-surgical sternal irregularities.The presence of
mediastinal gas bubbles and fluid collections after the 14th
post-operative day is has a sensitivity and specificity
approaching 100% for acute mediastinitis .
In the early post-operative period (to day 14 post-op), normal post-operative changes within the mediastinum (as discussed above) may mimic mediastinitis- during this period, CT scan has a specificity of as low as 33%. Significant improvement in specificity occurs after day 14 (up to 100% in one article). The sensitivity remains high (up to 100%) both in the early and late post-operative periods. Despite its low specificity, CT can still prove useful in guiding aspiration and drainage of fluid collections which can then be sent for culture. [1,2]
(1) AJR 1992; Templeton PA, Fishman EK. CT evaluation of poststernotomy complications.159; 45-50 (Review. No abstract available)
(3) AJR 2003; Li AE, Fishman EK. Evaluation of complications after sternotomy using single- and multidetector CT with three-dimensional volume rendering. 181: 1065-1070
(4) Radiographics 2009; Restrepo CS, et al. Imaging
appearances of the sternum and sternoclavicular joints. 29:
(5) Radiographics 2011; Katabathina VS, et al. Nonvascular,
nontraumatic mediastinal emergencies in adults: a comprehensive
review of imaging findings. 31: 1141-1160
(6) J Nucl Cardiol 2015; Rouzet F, et al. Diagnostic value of
99mTc-HMPAO-labeled leukocytes scinitgraphy in suspicion of
post-sternotomy mediastinitis relapse. 22: 123-129
(7) AJR 2018; Hota P, et al. Poststernotomy complications: a multimodality review of normal and abnormal post operative imaging findings. 211: 1194-1205