Table 1: Normal structures present in the different parts and segments of the mediastinum. The (*) indicates those structures
that are normally NOT seen on thoracic radiographs.
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Mediastinal abnormalities, including cardiac disease, are common causes of clinical signs related to the thorax. By definition,
the mediastinum is the midline potential space formed between the two pleural cavities and includes the medial portions of
the right and left parietal pleura (also called the mediastinal pleural) and the space formed between these serosal membranes.
The mediastinum is incomplete and fenestrated in the dog and cat so that transudates and modified transudates are typically
bilateral effusions, whereas exudates are unilateral effusions. The mediastinum can be divided into thirds in a cranial to
caudal direction with each part then being divided into dorsal and ventral section. Table 1 documents the specific structures
that are normally present in each of these spaces within the mediastinum.
A systematic approach to evaluating the mediastinum is critical to establishing normality for a given small animal. One can
think of roentgen abnormalities of the mediastinum as either primary (an abnormality of the mediastinum itself) or secondary
(an abnormality caused by a mediastinal structure or organ). Examples of primary mediastinal abnormalities would include abnormal
fluid or gas collections within the mediastinum. Examples of secondary mediastinal abnormalities would include lymphomegaly,
cardiac abnormalities, esophageal disorders, tracheal disorders and abnormal hemorrhage of tumors of the thymus.
Objectives of the Presentation
1. Provide practitioners with a basic interpretation paradigm for the evaluation of the mediastinal structures of the small
animal thorax.
2. Provide a summary schema for evaluating mediastinal abnormalities.
3. Think in terms of next step and how to get a cytologic diagnosis in as non-invasive a fashion as possible.
Key Etiologic and Pathophysiologic Points
1. Technical factors including technique, phase of respiration and the positioning of the patient have to be taken into account
when interpreting thoracic radiographs. High quality, well positioned thoracic radiographs are the most critical first step
to evaluating patients with intrathoracic disease (and possibly mediastinal abnormalities).
2. The basic pathophysiology related to diseases of the mediastinal structures should be understood for completing the
exercise in the formulation of differentials for the described mediastinal abnormalities.
3. The description of the mediastinal abnormalities is NOT the end point but should be considered the initiation point
for formulating a reasonable list of differential diagnoses for the described roentgen signs.
Table 1: Pulmonary Patterns
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Key Clinical Diagnostic Points
1. One should try to compartmentalize radiographic abnormalities into extrathoracic, pleural, pulmonary and mediastinal (including
cardiac), recognizing that any disease can be multicompartmental in nature.
2. One should try to determine the anatomic location of pathology within the lung first and foremost and then worry about
the pulmonary pattern. Even though there may be several pulmonary patterns, one must identify the dominant pattern in order
to evaluate for differentials.
3. Echocardiography can not diagnosis when a patient is in left sided heart failure with pulmonary edema.
Key Therapeutic Points
1. Only start to treat for a specific disease once that disease has been confirmed and is based on a solid physical examination
and diagnostic radiographs.
2. DO NOT treat a dog for pulmonary edema if you have only taken a right lateral projection that is on expiration as this
most likely is artifact and not truly edema.
Key Prognostic Points
1. The anatomic localization of the disease process to the mediastinum and the subsequent differentials will then determine
the prognosis.
