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Mediastinal Disease
Martha M. Larson

Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, USA

Mediastinal disease is common in cats. Imaging of some is difficult and additional imaging, such as ultrasound or
type (radiology, ultrasound, computed tomography [CT], CT, is necessary for better localization of the origin of the
or magnetic resonance imaging [MRI]) is critical for evalu- mass. Masses located in the cranial mediastinum are the
ation of this area, as other diagnostic methods (palpation, most common. If large enough, these masses can result in
blood tests, auscultation) are ineffective and nonspecific. dorsal displacement of the trachea, and may silhouette with
The most common mediastinal diseases include mediasti- the cranial margin of the heart (Figures 17.4 and 17.5).
nal shift, mediastinal mass, and pneumomediastinum. With larger cranial mediastinal masses, this border efface-
ment can resemble severe cardiomegaly. Large cranial
mediastinal masses will cause caudal displacement of the
17.1 Mediastinal Shift heart and carina (beyond the sixth intercostal space). This
abnormal location of the carina indicates displacement of
A mediastinal shift occurs with a change in size of one or the heart, not cardiomegaly. On VD/DV projections, cra-
more lung lobes in one hemithorax. A loss of volume due nial mediastinal masses result in widening of the cranial
to atelectasis or lobe removal will result in a mediastinal mediastinum, with caudal and lateral displacement of the
shift toward the affected side (Figures 17.1 and 17.2). The cranial lung lobes. Pleural effusion often accompanies
heart is the largest organ in the mediastinum, and a devia- these masses, making identification on radiographs alone
tion of the heart to one side (on a properly positioned ven- more difficult.
trodorsal [VD] or dorsoventral [DV] view) is indicative of a The most common neoplastic cranial mediastinal
mediastinal shift. Alternatively, the mediastinum will shift masses are thymoma and mediastinal lymphoma
toward the normal hemithorax when the opposite side has (Figures 17.4–17.6) [1,2]. Other less common cranial
increased in volume, secondary to unilateral pleural effu- mediastinal masses in the cat include ectopic thyroid
sion or pneumothorax, unilateral pulmonary overinflation, tumor (Figure 17.7), rarely sarcomas, and metastatic neo-
or large pulmonary mass (Figure 17.3). The mediastinal plasia [3]. Thymomas are most commonly seen in older
shift is an important radiographic sign, as it helps to dif- cats (mean 10 years), and can be benign or malignant [3].
ferentiate consolidated lung (maintained volume, no medi- This assessment appears to be based on invasiveness and
astinal shift) from increased lung opacity with decreased resectability rather than histopathology [3]. Malignancy is
volume (atelectasis) with the associated mediastinal shift. associated with invasiveness, vascular infiltration, and
close or distant metastasis. Many thymomas are cystic,
filled with serous or serosanguinous fluid. Radiographically,
17.2 Mediastinal Masses these masses are noted as a soft tissue opacity in the
cranioventral or occasionally craniolateral aspect of the
Masses may originate from any portion of the mediasti- thorax (within the mediastinum). Tracheal deviation or
num (cranial, middle, caudal), and must be differentiated compression, pleural effusion, or in some cases megae-
from masses of pulmonary origin. Mediastinal masses are sophagus and aspiration pneumonia can be seen [2–6].
midline in location, and may result in deviation of medias- Thymoma has been associated with acquired myasthenia
tinal structures. In some cases, radiographic differentiation gravis in cats (Figure 17.8) [7,8].

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.