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4.3

Mediastinum and esophagus

Normal mediastinum and variants Trauma

The margins of the mediastinum are formed by the Pneumomediastinum
internal parietal pleura of the two hemithoraces. The Pneumomediastinum can occur as the result of thoracic
cranial mediastinum contains mediastinal and sternal or cervical trauma or incidentally as a sequela to jugular
lymph nodes, the thymus or thymic remnants, major phlebotomy. Mediastinal gas distributes along fascial
blood vessels, the esophagus, and a variable amount of planes and surrounds organs and tissues contained within
fat (Figure 4.3.1). The middle, hilar, and caudal regions the mediastinal space. On CT images, mediastinal gas is
of the mediastinum contain the heart, aorta, thoracic low attenuating, may have a fragmented distribution, and
duct, esophagus, and tracheobronchial lymph nodes. accentuates margins of the soft‐tissue structures, around
The normal thymus has a mildly striated, glandular which the gas collects (Figure 4.3.3). Gas can migrate into
appearance with soft‐tissue attenuation on unenhanced the cervical region in patients with large active leaks, lead-
CT images and a homogeneous appearance on MR ing to pronounced subcutaneous emphysema.
images with T1 isointensity and T2 hyperintensity as
compared to skeletal musculature. Hemomediastinum
Hemomediastinum can result from thoracic trauma or
bleeding diatheses, such as those induced by anticoagulant
Developmental disorders toxicity. Depending on the volume of blood in the medi-
astinum, uniform or nonuniform mediastinal widening
Cranial mediastinal cyst may occur. Hemorrhage will appear mildly hypo‐ to
Cranial mediastinal cysts are uncommon and most hyperattenuating compared to soft tissues on unenhanced
often arise from branchial cleft remnants that contrib- CT images and may be amorphous or organized depend-
ute to the formation of the thymus. Uncomplicated ing on the age of the bleeding episode (Figure 4.3.4).
1–3
mediastinal cysts are generally well‐delineated, thin‐
walled, fluid‐filled masses that may displace other cra- Inflammatory disorders
nial mediastinal structures when large. Mediastinal
cysts are fluid attenuating on unenhanced CT images Reactive lymphadenopathy and lymphadenitis
and do not contrast enhance, although thymic rem- Sternal, cranial mediastinal, and tracheobronchial lymph
nants adherent to the margins of the cyst may enhance nodes can enlarge as a result of reactivity from a regional
(Figure 4.3.2). Pleural effusion may also be present in or systemic inflammatory disorder or from overt infec-
some patients. tion within the nodes. Bacterial and mycotic agents are

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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