Silke Hecht Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN, USA The mediastinal space is located between the right and left mediastinal pleura which is a continuation of the costal parietal pleura and marks the medial border of the left and right pleural sacs, respectively [1–3]. It is bordered dorsally by the spine and ventrally by the sternum (Figure 17.1). Cranially, the mediastinal space connects to the deep fascial planes of the neck musculature. Caudally, the ventral aspect of the mediastinal space is bordered by the diaphragm, while its dorsal aspect connects to the retroperitoneal space via the aortic hiatus. Organs contained within the mediastinal space include the heart, vessels (aorta, azygous vein, cranial and caudal vena cava [CVC], brachiocephalic trunk and left subclavian artery, main pulmonary arteries and veins, and smaller vessels), trachea and proximal mainstem bronchi, esophagus, lymphatic structures (thoracic duct, lymph nodes, thymus) as well as the vagus nerve and other smaller nerves. Additional structures within the mediastinal space include musculature (most notably the M. longus colli in the cranial thorax) and a variable amount of fat. Although the majority of the mediastinum is located roughly in the midsagittal plane of the thorax, there are exceptions. The mediastinum can be categorized into a dorsal part and a ventral part using the tracheal bifurcation or heart base as an anatomic marker (Figure 17.3A). The mediastinum can also be subdivided into cranial, middle, and caudal (Figure 17.3B). The cranial mediastinum extends from the thoracic inlet to the cranial margin of the heart. The middle mediastinum is occupied by the cardiac silhouette and is significantly widened. The caudal mediastinum extends from the heart to the diaphragm. FIGURE 17.1 Schematic transverse image of the thorax at the level of the heart. The parietal pleura, which covers the inner margin of the thoracic wall, continues into the mediastinal pleura which separates the left and right pleural cavities. The mediastinal pleural layers contain the mediastinal space. Source: Drawing courtesy of Dr Elodie Huguet. FIGURE 17.2 Dorsal multiplanar reconstructed computed tomographic images of the normal thorax in a 16‐year‐old cocker spaniel. (A) The cranial aspect of the mediastinum crosses midline in a right‐cranial to left‐caudal direction (arrows). (B) The caudal mediastinum is bordered on the right by the caudal vena cava (CVC) and forms a pocket bordered to the left by the caudal mediastinal reflection (arrowheads) to accommodate leftward extension of the accessory lung lobe (ALL). Note also widening of the middle mediastinum to accommodate the heart. FIGURE 17.3 Right lateral (A) and ventrodorsal (B) radiographs demonstrating subdivision of the mediastinum into dorsal and ventral (A) as well as cranial, middle, and caudal (B). As for any radiographic study of the thorax, the mediastinum should be evaluated on orthogonal views [2–4]. While the ventrodorsal (VD) view is generally considered superior for evaluation of the mediastinal structures, especially in the ventral aspect of the thorax, a dorsoventral (DV) view may aid in the evaluation of more dorsally located structures such as tracheobronchial lymph nodes and esophagus [5, 6]. On lateral thoracic radiographs, several mediastinal organs are visible (see later in this chapter), but most of the mediastinum itself is not seen as a distinct structure or compartment. The cranial mediastinal reflection may be apparent cranial to the heart and dorsal to the sternum as a curvilinear soft tissue opacity band which may contain a variable amount of fat (Figure 17.4A). On VD and DV radiographs, the mediastinal space is mostly located on the midline and superimposed with the spine and sternum. Similar to the lateral view, the cranial mediastinal reflection may be seen and appears as an obliquely oriented soft tissue opacity band in the right cranial thorax (Figure 17.4B), and the caudal mediastinal reflection which borders the left margin of the caudal mediastinum may be visible as a soft tissue opacity band of variable width (Figure 17.4C). Overall, the width of the cranial mediastinum should not exceed twice the width of the spine in dogs (Figure 17.5A) although a large amount of fat deposits may result in significant mediastinal widening. In cats, the cranial mediastinum rarely exceeds the width of the spine (Figure 17.5B). Of the organs and structures located in the mediastinum, only a few are visible on radiographs in normal animals as the others are either too small to be discernible (e.g., normal lymph nodes) or have the same (soft tissue) opacity as adjacent mediastinal structures, resulting in silhouetting (e.g., cranial mediastinal vessels). The following mediastinal structures are seen (Figures 17.6–17.9). This chapter will discuss mediastinal shift, mediastinal fluid accumulation, masses, lymphadenopathy, pneumomediastinum, and esophageal and tracheal disorders. The heart is covered in a separate chapter. FIGURE 17.4 The cranial mediastinal reflection appears as a curvilinear soft tissue opacity band on lateral (A) and ventrodorsal (B) radiographs (arrows). The caudal mediastinal reflection borders the left margin of the caudal mediastinum and is visible as a soft tissue opacity band of variable width on ventrodorsal views (arrows; C). FIGURE 17.5 Ventrodorsal radiographs of the cranial thorax in a normal dog (A) and cat (B). While the cranial mediastinum (arrows) in normal dogs may reach twice the width of the vertebral column it usually does not extend significantly beyond the spinal margins in cats. FIGURE 17.6 Lateral (A) and ventrodorsal (B) radiographs indicating mediastinal structures visible in normal animals: cardiac silhouette (CS), trachea (arrows), aorta (arrowheads), caudal vena cava (dotted outline). Note also mediastinal fat deposits delineating the ventral margin of the heart on the lateral view. FIGURE 17.7 Lateral view of the thorax in a 9‐year‐old mixed‐breed dog demonstrating a small amount of gas within the thoracic esophagus (arrows) which is an incidental finding. FIGURE 17.8 Ventrodorsal thoracic radiograph (A) and transverse CT image cranial to the heart (B) demonstrating accumulation of a large amount of fat in the cranial mediastinum in a 13‐year‐old Pekingese resulting in cranial mediastinal widening (arrows). While mediastinal shift is not a disorder of the mediastinum per se, it is a very common finding in animals. A mediastinal shift is recognized on VD/DV views based on displacement of mediastinal structures (most notably the heart) into the left or right hemithorax [4, 7]. It can occur secondary to pull or push forces. The most common example of pull forces is pulmonary volume loss (atelectasis) due to lateral recumbency. This is a very common incidental finding and is frequently noted when obtaining radiographs under sedation or anesthesia (Figure 17.10). Other examples of pull forces include volume loss of a lung lobe secondary to bronchial obstruction (Figure 17.11) and decreased lung volume due to prior lung lobectomy (Figure 17.12). Any kind of space‐occupying lesion within the thorax can result in a push force and cause mediastinal shift. Examples include severe lateralized pneumothorax or pleural effusion, abnormal contents in the thoracic cavity due to diaphragmatic hernia (Figure 17.13), thoracic masses (Figure 17.14), and deformities of the thoracic wall and/or sternum (Figure 17.15). FIGURE 17.9 Lateral (A) and ventrodorsal (B) radiographs of the thorax in a puppy. The thymus is seen as a triangular soft tissue opacity structure in the cranioventral mediastinum (arrows), resulting in a classic “sail sign” on the VD view (B). Source: Courtesy of Dr Janina Bartels, University of Tennessee. FIGURE 17.10 Right lateral (A), left lateral (B), and VD (C) views of the thorax in a normal 2‐year‐old Bouvier de Flanders. The dog had been sedated and positioned on its left side for several minutes prior to obtaining the VD view radiograph. (A,B) The lateral views are unremarkable. (C) On the VD view the cardiac silhouette is shifted into the left hemithorax. Note also diffusely increased opacity of the atelectatic caudal subsegment of the left cranial lung lobe. The large mineral opaque structure within the stomach is consistent with ingestion of a rock which was incidental. FIGURE 17.11 Right lateral (A), left lateral (B), VD (C), and DV (D) views of the thorax in an 8‐year‐old mixed‐breed dog presented with lower airway disease. (A) There is faint increased opacity in the plane of the cranial thorax following the expected margins of the left cranial lung lobe. This lobe appears decreased in volume. (B) The radiographic changes are similar but less pronounced than on the right lateral view. (C, D) The left cranial lobar bronchus is not visible due to obstruction with a mucous plug. The secondary volume loss of this lung lobe results in leftward mediastinal shift and diffuse soft tissue opacity of the left cranial lung lobe. Fluid accumulation within the mediastinum is mostly seen in conjunction with pleural effusion [8] and may in those cases not always be recognized due to overlying fluid opacity within the pleural space. Similarly, if mediastinal effusion is present along with a mediastinal mass (see below), it may not be recognized as a separate entity. Hemorrhagic effusion within the mediastinum may develop, e.g., secondary to trauma, invasive neoplasia, coagulopathy and vascular pathology, or may be idiopathic [9–14]. Other possible causes of mediastinal fluid accumulation include but are not limited to inflammation (mediastinitis, perforating esophageal foreign body, pyothorax, others), chylous effusion, and transudate [8, 15]. Radiographically, depending on the underlying cause and nature of effusion, the mediastinum appears generally, multifocally or focally of soft tissue opacity and widened on the VD view (Figure 17.16; see also Figures 17.26 and 17.72). Tracheal narrowing may be observed secondary to compression by surrounding fluid (Figure 17.17). FIGURE 17.12 Right lateral (A), left lateral (B), and VD (C) views of the thorax in a 7‐year‐old Weimaraner following prior right cranial lung lobectomy for treatment of pulmonary carcinoma. (A,B) The lateral views are unremarkable although on close inspection only one pair of pulmonary vessels is seen supplying the right cranial and middle lung field on the left lateral view. (C) On the VD view there is rightward mediastinal shift and increased soft tissue opacity to the right cranial thorax. FIGURE 17.13 Right lateral (A), left lateral (B), and VD (C) views of the thorax in a 4‐year‐old DSH cat who was found outside and suspected to have been hit by car. (A,B) On the lateral views abdominal viscera are present in the thoracic cavity and the diaphragm is discontinuous. In addition to the diaphragmatic hernia, mild pneumothorax and pulmonary contusions are also evident. (C) On the VD view the cardiac silhouette is pushed into the left hemithorax. FIGURE 17.14 Right lateral (A), left lateral (B). and VD (C) views of the thorax in an 11‐year‐old cocker spaniel with a large pulmonary mass (adenocarcinoma) of the left caudal lung lobe which pushes the cardiac silhouette toward the right on the VD view (C). Note also obliteration of the left caudal lobar bronchus on this view. Additional smaller pulmonary nodules are consistent with metastatic disease in this patient. FIGURE 17.15 Right lateral (A), left lateral (B), and VD (C) views of the thorax in a 3‐month‐old Pekingese. (A,B) On the lateral views there is displacement of caudal sternebral segments dorsally beyond the costochondral junctions, consistent with pectus excavatum. (C) On the VD view the cardiac silhouette is completely displaced into the left hemithorax. FIGURE 17.16 Right lateral (A) and ventrodorsal (B) views of the thorax in a 7‐month‐old DSH cat with mediastinal effusion due to a systemic inflammatory disease process, the underlying etiology of which was not elucidated. (A) On the lateral view there is diffuse increased opacity throughout the entire thorax with poor delineation of the cardiac silhouette. Additionally, mild pleural effusion recognized by scalloping and retraction of lung margins from the sternum and spine and diffuse patchy increased opacity to the lung fields is evident. (B) The VD view reveals diffuse generalized widening of the mediastinum which is of soft tissue opacity and silhouettes with cranial and caudal margins of the cardiac silhouette as well as the diaphragm (arrows). There is also more focal widening of the cranial mediastinum from the thoracic inlet to the third intercostal space, consistent with cranial mediastinal mass or lymphadenopathy. Pleural effusion (indicated by retraction of the right lung from the thoracic wall and right‐sided pleural fissure lines) as well as multifocal unstructured interstitial to alveolar pulmonary patterns are again noted. Mediastinal masses and enlarged lymph nodes within the mediastinum manifest as variably sized and shaped mass lesions which may partially obscure and displace normal mediastinal structures on the lateral view and will result in focal or multifocal mediastinal widening on the VD/DV view. Concurrent thoracic abnormalities such as pleural effusion may be encountered and may make identification of a mediastinal mass difficult. Even though enlarged mediastinal lymph nodes may resemble mediastinal masses of different origin, differentiation between an enlarged mediastinal lymph node and other types of mediastinal mass is crucial to establish an appropriate list of differential diagnoses and appropriately plan the additional diagnostic workup of a patient. A decision tree is shown in Figure 17.18 to help with the interpretation of focal or multifocal mass lesions. Although normal mediastinal lymph nodes are not visible radiographically, one has to be familiar with their location to recognize thoracic lymphadenopathy. Additionally, it is important to know their tributary areas as lymph node enlargement may be related to primary disease processes in their draining region. Three groups of lymph nodes are located within the mediastinum [1] (Figure 17.19). Enlargement of a single lymph node or group of lymph nodes is commonly related to neoplasia and less likely an inflammatory process in the tributary area. Occasionally, and especially for the tracheobronchial lymph nodes, enlargement of a single lymph node or group of lymph nodes can be associated with a systemic disease process such as round cell neoplasia (e.g., lymphoma) or fungal disease [16–18]. Radiographically, there will be a well‐ or ill‐defined increased soft tissue opacity in the expected location of the respective lymph node group (Figures 17.20–17.23). In case of enlargement of two or all three groups of thoracic lymph nodes (Figure 17.24), disseminated neoplasia (lymphoma or other round cell neoplasia) and fungal disease are the most likely differential diagnoses [16, 17]. FIGURE 17.17 Right lateral (A,C) and ventrodorsal (B,D) radiographs of the thorax in a 12‐year‐old miniature poodle with dyspnea, thrombocytopenia, and disseminated intravascular coagulopathy (DIC). Initial radiographs (A,B) were obtained by the referring veterinarian at the initial onset of clinical signs 2 days prior to recheck radiographs obtained at the referral institution (C,D). (A,B) Initial radiographs show increased opacity in the perihilar region with a J‐shaped course of the caudal trachea, consistent with tracheobronchial lymphadenopathy. No significant pleural, pulmonary or cranial mediastinal abnormalities are observed at this point. On recheck radiographs (C,D) there are additional findings of pleural effusion, multifocal unstructured interstitial to alveolar pulmonary infiltrates, and increased opacity to the cranial mediastinum with loss of visualization of the cranial cardiac margin and diffuse circumferential tracheal narrowing indicative of mediastinal effusion. Postmortem examination revealed necrotizing lymphadenitis and interstitial pneumonia with multiorgan hemorrhage including mediastinal bleeding. In many cases, mediastinal masses can be categorized based on their location. However, it is important to realize that exceptions to these general rules exist and that mediastinal malignancies as well as benign mass lesions (e.g., abscesses, granulomas, or hematomas) may be encountered in various locations. These masses are the most common type within the mediastinum. They are located dorsal to the sternum and cranial to the cardiac silhouette. Dependent on their size and location, they may be silhouetting with and/or displacing the heart and cranial mediastinal vessels, and/or they may be displacing and compressing the trachea [2, 4, 7]. On the VD view, these masses are associated with a variable degree of mediastinal widening. On lateral views, they may lead to confusion with sternal lymphadenopathy; however, they typically can be distinguished due to their more acute angle with the sternum rather than broad‐based contact, their larger size, and a differing location compared to the sternal lymph nodes. Concurrent pleural effusion may be present due to impairment of venous or chylous return to the heart or vascular invasion by the mass [8, 14, 19]. The most common etiologies of a cranial mediastinal mass are thymoma (Figure 17.25) and (thymic) lymphoma (Figure 17.26) [16,20–22]. Other tumor types (e.g., ectopic thyroid carcinoma, hemangiosarcoma or histiocytic sarcoma; Figure 17.27) as well as benign masses (abscess, granuloma) are also possible [14, 23, 24]. FIGURE 17.18 Decision tree for soft tissue opacity mass or masses within the mediastinum including pitfalls. FIGURE 17.19 (A) Schematic lateral view of the thorax showing location of sternal (1), cranial mediastinal (2) and tracheobronchial (3) lymph nodes. (B) Dorsal schematic image of the lungs showing arrangement of tracheobronchial lymph nodes around the tracheal bifurcation. Source: Drawings courtesy of Dr Elodie Huguet. FIGURE 17.20 Sternal lymphadenopathy secondary to neoplastic disease in the tributary area (peritoneal cavity) in a 6‐year‐old Labrador retriever. On the lateral view (A), a focal well‐demarcated broad‐based soft tissue opacity mass is identified dorsal to the second sternebra (arrows). On the VD view (B), the cranial mediastinum is mildly focally widened (arrows). Note loss of serosal margin detail in the abdomen consistent with peritoneal effusion. Based on laboratory findings (severe leukocytosis with predominant lymphocyte population and many lymphoblasts both in the peripheral circulation and the abdominal effusion), a diagnosis of acute lymphoid leukemia was made. Even though a definitive diagnosis based on radiographic findings is not possible, concurrent imaging or laboratory findings may allow prioritization of the list of differential diagnoses. For example, FeLV‐positive status in a young cat with a cranial mediastinal mass is most consistent with lymphoma [25]. Several paraneoplastic syndromes (myasthenia gravis, polymyositis, hypercalcemia, dermatitis) have been described in association with thymomas, and the concurrent observation of a cranial mediastinal mass with megaesophagus is highly suggestive of this condition [20, 21]. While not apparent on radiographs, ultrasonographic findings of a heterogeneous mass or internal cysts are also most consistent with thymoma [26–28]. Based on the author’s experience, mild thymic enlargement in an adult dog with maintenance of the overall normal thymic shape is occasionally seen in patients with lymphoma (Figure 17.28). Cranial mediastinal cysts are an occasional finding in cats and are typically found incidentally without clinical signs which would be expected with malignant lesions (Figure 17.29) [29, 30]. Deposition of a large amount of fat within the mediastinum will result in mediastinal widening on the VD view (see Figure 17.8) which may lead to an erroneous diagnosis of a cranial mediastinal mass. However, a distinction is usually possible as the cranial margin of the heart will contrast with fat and remain clearly visible on lateral views while it will be obscured in cases of cranial mediastinal masses (Figure 17.30). FIGURE 17.21 Sternal lymphadenopathy secondary to peritoneal neoplasia (carcinomatosis) in a 10‐year‐old domestic shorthair cat. On the right lateral view (A), an ovoid soft tissue opacity mass is present dorsal to the third sternebra (arrows) which results in mild focal cranial mediastinal widening on the VD view (B; arrows). Note granular lack of serosal margin detail in the visible abdomen consistent with effusion and carcinomatosis. FIGURE 17.22 Cranial mediastinal lymphadenopathy due to metastatic disease from a tracheal mast cell tumor. (A) The initial left lateral view at presentation shows a lobulated tracheal mass with near complete obliteration of the lumen (arrows). (B) Recheck radiograph 5 months following surgical resection of the mass and adjuvant chemotherapy reveals a focal soft tissue mass resulting in dorsal displacement of the trachea cranial to the heart base, consistent with an enlarged cranial mediastinal lymph node. FIGURE 17.23 Tracheobronchial lymphadenopathy in an 11‐year‐old mixed‐breed dog with fungal disease (blastomycosis). Note increased opacity in the perihilar region with J‐shaped course of the caudal trachea on the right lateral view ((A) arrows) and lateral deviation of the main stem bronchi (indicated by dotted lines) with increased soft tissue opacity between these bronchi on the VD view resembling a “double opacity” or “bow‐legged cowboy” sign (B). FIGURE 17.24 Generalized thoracic lymphadenopathy in an 11‐year‐old mixed‐breed dog diagnosed with lymphoma. Right lateral (A), left lateral (B), and ventrodorsal (C) radiographs of the thorax show focal areas of increased soft tissue opacity attributed to the sternal, cranial mediastinal, and tracheobronchial lymph nodes. FIGURE 17.25 Large cranial mediastinal mass in an 8‐year‐old German shepherd dog. Right lateral (A) and left lateral (B) views show a lobulated soft tissue opacity mass occupying the cranial ventral thorax and displacing the cardiac silhouette caudally. On the ventrodorsal view (C), the mass is located on the midline, is more extensive and abutting the thoracic wall on the right side, and silhouettes with the cranial margin of the cardiac silhouette. Ultrasound‐guided fine needle aspiration and surgical biopsy yielded a diagnosis of thymoma. FIGURE 17.26 Cranial mediastinal mass and pleural effusion in a 2‐year‐old FeLV‐positive domestic longhair cat. On the lateral views (A,B) there is a large, homogeneous, soft tissue opacity mass within the cranioventral thorax, the cranial margin of the cardiac silhouette is obscured, and the trachea is displaced dorsally and compressed. Additional findings include scalloped fluid opacity to the ventral thorax, retraction of the lung lobes and pleural fissure lines consistent with pleural effusion. On the VD view (C), there is marked widening of the cranial mediastinum and silhouetting of the cranial thoracic mass with the cranial margin of the cardiac silhouette. Retraction of the lung from the thoracic wall (especially right‐sided) and pleural fissure lines are again identified. Widening of the caudal mediastinum is noted which suggests concurrent mediastinal effusion. Cytology of pleural effusion yielded a diagnosis of lymphoma. FIGURE 17.27 Left lateral (A) and ventrodorsal (B) radiographs of the thorax in an 8‐year‐old Shetland sheepdog showing a cranial mediastinal mass. The cytologic diagnosis was histiocytic sarcoma. There is overlap in the differential diagnoses for midcranial and cranial dorsal thoracic masses. Severe focal esophageal enlargement (e.g., focal megaesophagus such as seen secondary to vascular ring anomaly or an esophageal mass) may be visible dorsally as well as ventrally to the trachea on lateral views and will appear as a cranial mediastinal mass lesion of variable size and echogenicity on VD/DV views (Figure 17.31; see also “Esophagus” section below) [31, 32]. A heart base mass (e.g., chemodectoma) is typically located immediately cranioventral to the tracheal bifurcation and results in dorsal displacement and J‐shaped course of the intrathoracic trachea on lateral views and rightward displacement of the trachea on VD/DV views (Figures 17.32 and 17.33) [4]. It is important to note that survey radiographs are fairly specific but not very sensitive in the detection of heart base masses in dogs [33]. Mass lesions in the dorsal mediastinum most commonly originate from the esophagus (e.g., neoplasia or foreign body; see below), the paraesophageal tissues (e.g., abscess), or the spinal and paraspinal tissues [32, 34] (Figure 17.34). Aortic lesions are typically not expected to result in radiographic changes. However, aneurysms and dissecting hematomas have been described in small animals [35–37] and may result in dorsal mediastinal abnormalities (see Figure 17.60). In addition to the options listed above, hiatal hernia is another differential diagnosis for a caudodorsal mediastinal mass lesion [38, 39]. A hiatal hernia is defined by a protrusion of any abdominal structure other than the esophagus into the thoracic cavity through the esophageal hiatus [40]. These may be encountered in dogs and cats and are of variable clinical significance [38,41–45]. Sliding hiatal hernias (type I hiatal hernias) where the gastroesophageal junction migrates cranially are most common, and manifest as soft tissue and/or gas opacity structures in the caudal dorsal mediastinum immediately cranial to the diaphragm. Unlike true caudodorsal mediastinal masses, they may only intermittently be visible on serial radiographs (Figure 17.35). In paraesophageal hernias, abdominal contents herniate through the esophageal hiatus next to the gastroesophageal junction which remains in normal position (Figure 17.36). These hernias can be subdivided into type II (herniation of portion of the fundus), type III (combination of types I and II), and type IV (presence of a structure other than the stomach within the hernia sac) [40]. FIGURE 17.28 Right lateral (A), left lateral (B), ventrodorsal (C) radiographs and CT image (D) of the thorax in a 6‐year‐old MC mixed‐breed dog. On the lateral views (A,B), there is faint increased opacity to the cranial mediastinum. On the VD view (C) and the CT image (D), there is enlargement and rounding of the thymic remnant (arrows), concerning for neoplastic infiltration. Ultrasound‐guided tissue sampling yielded a diagnosis of lymphoma. Some congenitally predisposed diaphragmatic hernias, including peritoneal‐pericardial diaphragmatic hernias and “true” (peritoneal‐pleural) diaphragmatic hernias, may result in a caudal ventral mediastinal mass due to protrusion of abdominal viscera into the caudal ventral thorax. Although these types of hernias may have associated clinical signs if herniated organs become incarcerated or otherwise compromised, most commonly they seem to be diagnosed as incidental findings. In peritoneal‐pericardial diaphragmatic hernias, there is congenital connection between the peritoneal and pericardial cavities [46, 47]. Radiographic abnormalities include enlargement and abnormal shape of the cardiac silhouette, abdominal organs within the pericardial sac, an indistinguishable border of the ventral diaphragm, and abnormal mostly soft tissue opacity structures between the caudal ventral margin of the cardiac silhouette and the diaphragm (Figure 17.37) [39]. In “true” (peritoneal‐pleural) diaphragmatic hernias, there is incomplete formation of the diaphragm, allowing protrusion of abdominal viscera through the resultant defect (Figure 17.38) [26, 39, 48]. The exact nature of a congenital diaphragmatic hernia may on occasion remain elusive (Figure 17.39). True caudal ventral mediastinal masses of either neoplastic or inflammatory etiology are rare. However, pyogranulomas due to actinomycosis or nocardiosis appear to have a predilection for this location (Figure 17.40) [49]. The ALL is located between the right and left caudal lung lobes and is bordered cranially by the heart and caudally by the diaphragm. Due to its location, abnormalities of this lobe (e.g., mass) may mimic an actual caudoventral mediastinal mass or even a diaphragmatic hernia (Figure 17.41) [50]. FIGURE 17.29 Incidental cranial mediastinal cyst in a 9‐year‐old cat presented with chronic nasal discharge. On the lateral views (A,B), there is faint increased opacity to the cranial mediastinum. On the VD view (C), there is mild cranial mediastinal widening (arrows). Ultrasound examination (D) confirms the cystic nature of the cranial mediastinal lesion which appears anechoic and well circumscribed. Gas accumulation within the mediastinum most commonly occurs secondary to some sort of traumatic event. Blunt thoracic trauma may lead to leakage of air from the alveoli into the pulmonary interstitium and subsequent diffusion into the mediastinum [51]. Tracheal injury is another fairly common cause of pneumomediastinum and may be secondary to perforating injury or iatrogenic (e.g., due to overinflated endotracheal cuff, following endoscopy, or following removal of a tracheal foreign body) [52, 53]. Gas within the soft tissues of the neck (e.g., secondary to bite wounds) can dissect along fascial planes and cause pneumomediastinum [54]. Pneumomediastinum secondary to other pulmonary diseases, esophageal rupture or mediastinal infection with gas‐producing bacteria, and spontaneous pneumomediastinum are also reported [55–61]. FIGURE 17.30 Thoracic radiographs in a 13‐year‐old West Highland white terrier presented for coughing. On the VD view (B), there is widening of the cranial mediastinum (arrows) which could be mistaken for a soft tissue mass. On the lateral view (A), the cranial margin of the cardiac silhouette is clearly visible and there is no evidence of a cranial mediastinal mass. A fusiform soft tissue opacity in the plane of the dorsal trachea is consistent with a redundant tracheal membrane. A diffuse unstructured interstitial and bronchial pattern and mild hepatomegaly are also identified.
CHAPTER 17
Mediastinum
Normal Anatomy
Normal Radiographic Anatomy
Mediastinal Abnormalities and Disorders
Mediastinal Shift
Mediastinal Fluid Accumulation
Mediastinal Masses and Lymphadenopathy
Cranioventral
Midcranial
Dorsal
Caudodorsal
Caudoventral
Pneumomediastinum
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
