Imaging findings in mediastinal abscess
On contrast-enhanced CT scans, a mediastinal abscess is reliably visualized as a loculated fluid collection, often containing gas bubbles within it. The pathological process is prone to invasive growth and extension into adjacent anatomical regions.
In clinical practice, cases are encountered where the infection is localized in the anterior mediastinum in patients with a burdened history (e.g., in the setting of polydrug addiction). These patients complain of severe shoulder pain and redness (erythema) of the skin over the affected area.
The purulent process can extend to the anterior chest wall, specifically above the level of the manubrium sterni (manubrium). The left pectoralis major muscle (m. pectoralis major) is frequently involved in the inflammation. On CT, this manifests as its marked thickening and the appearance of gas bubbles within the muscle fibers, indicating the development of myositis and the formation of a secondary chest wall abscess.
Fibrosing mediastinitis: MRI semiotics
Magnetic resonance imaging is an important tool in the diagnosis of fibrosing mediastinitis. The appearance depends on the imaging sequence used and the stage of inflammation:
- On T1-weighted images (T1WI): The pathology appears as a heterogeneous infiltrative mass with intermediate signal intensity.
- On T2-weighted images (T2WI): The semiotics are extremely variable. Areas of high and markedly decreased signal may be simultaneously detected within the same lesion. Regions of decreased signal typically correspond to dense fibrous tissue or calcifications. Areas of increased signal indicate a more active inflammatory process.
Differential diagnosis. The presence of extensive low-signal areas on T2WI is a key criterion for distinguishing fibrosing mediastinitis from other infiltrative processes (such as lymphoma or metastatic disease), which typically show predominantly high T2 signal. Following the administration of gadolinium-based contrast agents, heterogeneous contrast enhancement is observed.
A significant limitation of MRI is its low sensitivity for detecting calcifications compared to CT. This is important to consider, as the presence of calcifications is a crucial feature that helps rule out malignant neoplasms.
Hemomediastinum (mediastinal hemorrhage)
Hemomediastinum occurs when blood extravasates into the fascial spaces of the mediastinum. The most frequent cause is trauma to major veins and arteries. Other etiological factors include aortic dissection, aneurysm rupture, and iatrogenic complications during central venous catheter placement.
Radiological signs:
- Widening of the mediastinal contour, which reaches its maximum at the level of the bleeding source.
- Formation of the "apical cap" (apical cap). Blood spreads through the mediastinal spaces and frequently arches over the apex of the left lung, creating a smooth, well-defined shadow on the radiograph.
- In massive hemorrhage, blood may rupture into the pleural cavity. Extension into the lung parenchyma via peribronchovascular pathways is also possible, radiographically mimicking the pattern of interstitial pulmonary edema.
On unenhanced CT, acute hemorrhage is visualized as an area of relatively high x-ray attenuation (high attenuation). The MRI appearance of a hematoma strictly depends on the age of the hemorrhage and exhibits significant variability.
Pneumomediastinum
Pneumomediastinum is a pathological condition in which free gas accumulates around mediastinal structures. A variety of causes can lead to the presence of air in this region:
- Penetrating or blunt chest trauma.
- Esophageal wall perforation.
- Recent invasive medical procedures involving the esophagus or tracheobronchial tree structures.
- Severe pulmonary infections and gas-forming infectious processes of the mediastinum itself (e.g., abscesses).
- Extension of air from the pleural cavity in pneumothorax.
- Specific toxic factors, such as inhalation of cocaine.
Briefly: answers to topical questions
How to differentiate pneumomediastinum from pneumopericardium on a chest radiograph?
The differential diagnosis of pneumomediastinum and pneumopericardium on a plain radiograph is based on the localization of free air and associated symptoms.
| Sign | Pneumomediastinum | Pneumopericardium |
|---|---|---|
| Air localization | Outlines the contours of the thymus, may extend into the soft tissues of the neck | Air surrounds the heart |
| Specific symptoms | "Continuous Diaphragm Sign" (Continuous Diaphragm Sign), "double bronchial wall sign" (double bronchial wall sign) | Not described in literature |
Through which anatomical pathways does the purulent process spread in posterior mediastinal abscess?
The spread of the purulent process into the posterior mediastinum occurs via several communicating fascial spaces of the neck and head.
- Retrovisceral space (spatium retroviscerale) — pus tracks downward behind the esophagus along the loose connective tissue between the 4th and 5th fascial layers.
- Prevertebral space — the infection spreads along the spine via the 5th fascial layer, which extends down to the III–IV thoracic vertebrae.
- Parapharyngeal space — its posterior compartment communicates directly with the posterior mediastinum.
What life-threatening complications are characteristic of tension pneumomediastinum?
Tension pneumomediastinum leads to life-threatening impairments of the patient's vital functions due to marked displacement of thoracic organs.
Characteristic complications of this condition include:
- Mediastinal shift — displacement of mediastinal structures causing critical impairment of vital functions.
- Reflex reaction — a pathological response mediated by the recurrent laryngeal nerves.
Class and exam questions
How to reliably identify a mediastinal abscess on CT?
On contrast-enhanced CT, the abscess appears as a fluid collection, often containing gas bubbles, with signs of inflammatory involvement of adjacent structures, such as thickening of the pectoral muscles.
What is the main difference between fibrosing mediastinitis and lymphoma on MRI?
Fibrosing mediastinitis is characterized by areas of markedly decreased signal on T2WI (fibrosis, calcifications), whereas lymphoma and metastases typically exhibit high T2 signal.
What is the "apical cap" sign?
This is a radiologic sign of hemomediastinum in which extravasated blood arches over the apex of the left lung, forming a smooth and well-defined shadow.
What else to study on this topic
- Features of fluid spread along peribronchovascular spaces in hemomediastinum.
- Evolution of the MRI signal depending on the age of the mediastinal hematoma.
- CT protocols for detecting aortic dissection as a cause of hemomediastinum.
- Pathogenesis of gas bubble formation in mediastinal infections.
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