Overview

The feline thorax is presented in this separate chapter because cats are not small dogs, and have their own set of idiosyncratic “rules of the road.” All the rules and guidelines established for the dog do not fit for the cat and thereby normal anatomy, variants, and common thoracic abnormalities are presented in this chapter even though feline examples have been presented in the previous chapters.

Feline Thorax Interpretation Paradigm

The same interpretation paradigm as set forth in the previous chapters will be used. The paradigm will still work from the outside in, providing the clinician and student with a framework to work through the extrathoracic structures, the pleural space, the pulmonary parenchyma and vasculature, and the mediastinum.

The overall shape of the feline thorax is narrower ventrodorsally and more elongated (in a craniocaudal direction) compared with the dog, giving the feline thorax a more triangular appearance (Figure 19.1). The cardiac silhouette has a more acute angle relative to the sternum on lateral projections compared with the dog. Radiographs should be made on peak inspiration as noted for the dog (Figure 19.2).

Extrathoracic Structures

Cranially, the thorax extends to the level of the thoracic inlet at the level of the manubrium ventrally, first ribs laterally, and cervicothoracic spinal junction dorsally. The diaphragm marks the caudal extension of the thorax. Ventrally, the diaphragmatic cupula extends cranially to the level of T8–T10. Dorsally, the diaphragmatic crura extend to the level of T13–L1 on peak inspiration and attach to the ventral border of L3 and L4. On right versus left lateral radiographs, minimal separation of the right and left crura is noted, unlike that seen in the dog.

The 13 pairs of ribs protect the normal thoracic wall [1–3]. The first pair of ribs is attached to the manubrium whereas ribs 2 through 9 attach to the intersternebral disc spaces of the sternum. The costal portions of the ribs will mineralize in cats as in dogs; however, the mineralization is often multipartite and thin lined rather than solid and complete. Congenital anomalies of the sternebrae are common, often with fusion or decrease in number compared to the normal eight sternebrae (the manubrium is the first, xiphoid process is the eighth).

Pectus excavatum is an uncommon congenital sternal anomaly resulting in dorsal deviation of the caudal sternum and ventrodorsal narrowing of the thorax noted on lateral radiographs [4, 5] (Figure 19.3).

Pleural Space

The pleural space is a potential space located between the parietal and visceral pleura, and is not visualized on normal thoracic radiographs. The pleural space is closed cranially and caudally, but there are mediastinal fenestrations connecting the right and left pleural spaces. The parietal pleura is a mesothelial lining located along the thoracic wall, diaphragm, and mediastinum; the visceral pleura is a mesothelial lining around the pulmonary parenchyma. A small amount of lubricating fluid allows the visceral and parietal pleura to slide against each other during respiration.

The pleural space extends between lung lobes, creating pleural fissure lines between the right cranial and middle lung lobes, the right middle and caudal lung lobes, and the left cranial and caudal lung lobes (Figure 19.4). Occasionally, pleural fissure lines will be seen as linear opacities due to pleural thickening from fibrosis. The longus coli and hypaxial muscles are located ventral to the thoracic spine, creating a soft tissue opacity between the caudodorsal lungs and spine. This can be mistaken for pleural effusion on peak inspiration on lateral images (Figure 19.5).

Pulmonary Parenchyma and Anatomy

There are six lung lobes present in the cat, as in the dog (Figure 19.4). The right lung is divided into right cranial, middle, caudal and accessory lung lobes. The left lung is divided into left cranial (cranial and caudal subsegments) and caudal lung lobes. The trachea terminates into the right and left principal bronchi. The right cranial lung lobe bronchus is the first branch off the right principal bronchus and extends cranially. The right middle lung lobe bronchus courses ventrally over the cardiac silhouette, best seen on the left lateral radiograph. The right caudal lobe bronchus continues caudal and dorsal. The accessory lung lobe bronchus originates along the ventromedial aspect of the right caudal lung lobe bronchus. The left cranial lung lobe bronchus extends ventral and cranial away from the left principal bronchus. Within several millimeters of its origin, it bifurcates into cranial and caudal subsegment bronchi. The cranial subsegment bronchus extends cranially; the caudal subsegment bronchus extends ventrally. The left caudal bronchus extends caudal and dorsal into the left caudal lung lobe.

The pulmonary parenchyma consists of alveoli, interstitial tissue, bronchial walls, and pulmonary vasculature. Background opacity of the normal lung is primarily due to pulmonary arteries and veins, with the bronchi and interstitium contributing to a lesser extent. The small airways will not be visualized in the lung periphery although larger airways will be seen centrally. The fourth‐ and fifth‐generation vessels (pulmonary arteries and veins) can be seen in the peripheral portions of the lungs. The descending thoracic aorta and the caudal vena cava are well visualized due to surrounding lung. The plica vena cava of the accessory lung lobe wraps around the caudal vena cava, which enters the right side of the diaphragm at the caval foramen. The descending thoracic aorta enters the abdomen at the level of the aortic hiatus in a central position between the right and left diaphragmatic crura (Figure 19.6).

The pulmonary arteries lie dorsal to the cranial bronchi on the lateral view, with the corresponding pulmonary veins located ventral to the bronchus (Figure 19.6). They should be approximately equal in size, and smaller than the diameter of the proximal third of the fourth rib [6]. On ventrodorsal or dorsoventral radiographs, the pulmonary artery is located lateral to the caudal bronchus with the corresponding pulmonary vein located medial. The pulmonary artery and vein are typically equal in size and the vessels are smaller than the right and left ninth ribs where they intersect on ventrodorsal/dorsoventral projections. Caudal lobar vessels are best seen on a dorsoventral projection when surrounded by more fully expanded caudal lung lobes and are magnified.

Mediastinum

The mediastinum is a true open space between the right and left pleural cavities. The cranial mediastinum connects to the deep fascia of the neck, while the caudal mediastinum communicates with the retroperitoneum via the aortic hiatus. The mediastinum is incomplete in cats and dogs such that a transudate and modified transudate may pass through mediastinal fenestrations, and will present as a bilateral pleural effusion. In the case of exudative effusion, the pleural fluid may not cross the mediastinum easily, and may be unilateral.

The mediastinum can be divided into cranial, middle, and caudal portions. In the dorsal aspect of the cranial mediastinum are a collection of vessels, esophagus, nerves, and cranial mediastinal lymph nodes. The sternal lymph nodes and internal thoracic vessels are in the cranioventral mediastinal reflection between the ventral aspects of the left and right cranial lung lobes (Figure 19.7). The sternal lymph nodes (not normally visible) are located dorsal to the cranial aspect of the third sternebra, and drain the pericardium, cranial aspect of the diaphragm and the abdominal cavity [7, 8]. In young cats, the thymus is located in the ventral aspect of the cranial mediastinum. The thymus will enlarge until the age of maturity, then involutes and will be no longer visualized (Figure 19.8) [8, 9].

The trachea extends through the cervical region into the thoracic cavity at the thoracic inlet. The trachea does not deviate as dramatically from the thoracic spine as in dogs, but does angle ventrally to the base of the cardiac silhouette. The trachea terminates at the carina typically located at the fifth or sixth intercostal space. The normal thoracic tracheal diameter in the cat has been reported as 5.5 mm or 18% of the thoracic inlet (20% in normal Persian cats) [10]. The esophagus, vagus, and phrenic nerves, other cranial mediastinal vasculature (cranial vena cava, left subclavian, aortic arch, brachiocephalic trunk), and lymph nodes (cranial mediastinal dorsally and sternal ventrally) are not routinely visualized.

The descending thoracic aorta, esophagus, and tracheobronchial lymph nodes are located in the dorsal aspect of the middle mediastinum. The esophagus and tracheobronchial lymph nodes are not seen on normal thoracic radiographs. There are three primary tracheobronchial lymph nodes or lymphocenters. The right tracheobronchial lymph node is found medial to the right cranial lung lobe bronchus and lateral to the trachea. The left tracheobronchial lymph node is found medial to the cranial subsegment of the left cranial lung lobe bronchus and lateral to the trachea. The central tracheobronchial lymph node is found between the two caudal lobar bronchi just caudal to the bifurcation of the carina. These nodes are located slightly more cranial than those in the dog, and receive afferent lymphatics from the lungs [9].

The cardiac silhouette is located in the ventral middle mediastinum, and in the cat measures between 2 and 2.5 intercostal spaces on the lateral radiograph. It has an elongated, “almond” shape when compared with the dog. The vertebral heart scale can be used to assess cardiac size, and in the normal cat measures 7.5 ± 0.3 on the lateral image and 8.1v ± 0.45 on DV/VD projections (Figure 19.9) [11–13]. The apex of the cardiac silhouette is typically separated from the cupula on both lateral and ventrodorsal projections. On the ventrodorsal radiograph, the apex of the feline cardiac silhouette is typically to the left of the midline. In geriatric cats, the cardiac silhouette may align more horizontally along the sternum on the lateral projections. In addition, the aortic arch may elongate and become tortuous, or “redundant” (Figure 19.10). This creates an aortic “knob” on the ventrodorsal image just to the left of the midline. The cause of these changes has not been determined. However, thickening of the tunica intima and media has been shown histologically in aged cats. It is hypothesized that these changes in the aortic arch and descending aorta will shift the base of the cardiac silhouette into a more cranioventral position [14]. In obese cats, fat deposition around the pericardium creates the appearance of cardiomegaly, especially on ventrodorsal or dorsoventral images (Figure 19.11).

The descending thoracic aorta is seen in the dorsal aspect of the caudal mediastinum. The azygous vein and thoracic lymphatic ducts run adjacent to the aorta, but are too small to be seen. The esophagus enters the abdomen just to the left of the central diaphragm at the level of the esophageal hiatus. Ventrally, a mediastinal reflection between the accessory lung lobe and the left caudal lung lobe is present, extending from the apex of the heart to the left side of the diaphragm. This is best visualized on the ventrodorsal image but is less commonly seen in the cat (Figure 19.12).

Extrathoracic Abnormalities

Rib fractures (most commonly due to trauma) are a common cause of extrathoracic abnormalities. The appearance will be variable [15]. A flail thorax is a condition where multiple contiguous segmental rib fractures are present (potentially along with sternal or thoracic vertebral anomalies, pleural effusion, pneumothorax, or pulmonary contusions). Paradoxical motion of the flail segment results in outward motion on expiration and inward motion on inspiration. Nontraumatic rib fractures can occur with chronic pleural, pulmonary or mediastinal diseases [15, 16]. This condition is called a thoracic bellows effect (Figure 19.13).

Rib tumors are rare in the cat [1517–19]. Expansile, proliferative, and lytic lesions often result in an extrapleural sign and/or a pleural effusion. The most common rib tumor in the cat is osteosarcoma. Metastatic lesions to the ribs have also been reported with pulmonary carcinomas and plasma cell tumors (multiple myeloma) (Figure 19.14).

The most common congenital diaphragmatic abnormality in the cat is a peritoneo‐pericardial diaphragmatic hernia. In this condition, the central portion of the diaphragm (septum transversum) fails to fuse with the pleuroperitoneal and pleuropericardial folds, resulting in a cranioventral opening in the diaphragm [20–23]. The condition results in variable abdominal contents herniating cranially into the pericardial sac. Commonly this is an incidental finding. On lateral radiographs, a short curvilinear opacity between the caudal pericardium and the cranial diaphragm, called the dorsal pericardial‐peritoneal mesothelial remnant, can be seen (Figure 19.15) [21]. The most common acquired diaphragmatic abnormality is diaphragmatic rupture secondary to trauma (Figure 19.16) [24–27]. Depending on the abdominal contents cranially displaced and size of diaphragm tear, the degree of contralateral mediastinal shift and pleural space changes will vary. Pleural effusion may be present with both acute and chronic hernias.

Pleural Space Abnormalities

Pleural space abnormalities include pleural effusion and pneumothorax. Fluid in the pleural space results in a widened, radiopaque pleural space, retraction of the lung lobes from the thoracic wall (resulting in a scalloped appearance), and the presence of multiple pleural fissure lines. A mild pleural effusion will result in border effacement of the ventral aspect of the cardiac silhouette and cranioventral aspect of the diaphragm (Figure 19.17). With more severe effusion, there is an increase in the retraction of the lung lobes with fluid separation of parietal and visceral pleural surfaces on both lateral and VD/DV views. There is widening of the pleural fissure lines adjacent to the thoracic wall with a convex curvilinear soft tissue opacity extending toward the pulmonary hilum. These fissure lines occur in the specific locations as noted previously.

The lungs take on a leaf‐like or triangular shape as they become atelectatic. This will also result in an overall increased pulmonary opacity due to an unstructured interstitial pattern. In cats, as pleural fluid progresses, it is very common for lung lobes to collapse with only a single air bronchogram extending into the completely atelectatic lung lobe (Figure 19.18). The borders of the atelectatic lung lobe will not be visualized due to border effacement with the pleural fluid. Severe pleural effusion results in the cardiac silhouette and inflated lung lobes appearing to “float in the fluid” on the lateral projections. The carina will still be located at the level of the fifth or sixth intercostal space, but the trachea may be elevated [28].

With the cat in dorsal recumbency (ventrodorsal view), the cardiac silhouette can be visualized as pleural fluid tends to accumulate dorsally in the paravertebral regions. When the cat is in ventral recumbency (dorsoventral view), the cardiac silhouette cannot be visualized due to border effacement with the ventrally distributed fluid. On lateral projections, a radiolucent line can sometimes be seen ventral to the cardiac silhouette representing subpericardial fat.

Unilateral pleural effusions are more common in cats than in dogs (Figure 19.19). The most common cause of a unilateral pleural effusion is pyothorax, most likely the result of a ventral pneumonia and transpleural localization of bacteria into the pleural space [29–31]. Other causes of exudative effusions include chylothorax, feline infectious peritonitis, hemorrhage or neoplastic effusions [28]. Chronic effusion can result in thickening of the visceral pleura surrounding lung lobes (restrictive or fibrosing pleuritis) [32–35]. This results in rounding of lung lobes and inability to fully inflate even after removal of pleural effusion. A pneumothorax may occur to fill in the empty space between incompletely inflated lung lobes and chest wall (Figure 19.20).

Pneumothorax results in radiolucent air between the visceral and parietal pleural surfaces so that the lung lobe appears radiopaque with surrounding radiolucent pleural space. Retraction and partial collapse of lung lobes and separation of the cardiac silhouette from the sternum can be seen. The more severe the pneumothorax, the more severe the lung lobe retraction will become (Figure 19.21), resulting in progressive atelectasis of the affected lobes. Pneumothorax is routinely bilateral. In the case of a tension pneumothorax, a progressive increase in the amount of pleural gas accumulates (one‐way valve effect) as the cat breathes, resulting in a severe unilateral pneumothorax, contralateral mediastinal shift, flattening of the diaphragm on the affected side, and an increase in the diameter of the hemithorax involved (Figure 19.22). This condition is life‐threatening and pleurocentesis/thoracic tube placement is required.

Pulmonary Parenchymal Abnormalities

The pulmonary interpretation pattern has been previously reviewed in Chapter 16. A review of all the radiographic projections, and a determination of normal versus abnormal is still the first question. If abnormal, are the abnormalities radiolucent or radiopaque? Is the abnormality focal, multifocal, or generalized? If focal or multifocal, what lung lobes are involved and where in the lobe is the abnormality? The distribution of the abnormal opacity is essential in the differential diagnosis. A review of common diseases that cause or result in pulmonary changes will be described and illustrated.

A true generalized decrease in lung opacity in the cat is secondary to either hypovolemia or hyperinflation (Figure 19.23). Hyperinflation is usually associated with small airway disease such as feline asthma, where bronchoconstriction or thickening of the small airways restricts flow out of the alveoli and distal bronchioles. This results in progressive inflation of the lungs (up to a certain point) [36]. The diaphragmatic crura are displaced caudally to the level of L1–2 or beyond. On the ventrodorsal radiograph, diaphragmatic “tenting” (small diaphragmatic projections caused by pulling of the diaphragm against the costal attachment) may be visible (Figure 19.24).

Focal or multifocal areas of decreased opacity can be circular (oval) or lobar in appearance. Circular radiolucent focal or multifocal areas are commonly caused by bullae (secondary to trauma), neoplasia, or parasitic granulomas (Paragonimus kellicotti; Figure 19.25). Lobar decreased opacity is usually secondary to pulmonary thromboembolism, although this is a rare finding in cats.

Atelectasis results in loss of normal lung volume and, depending on the degree of volume loss, can result in an unstructured interstitial (mild to moderate) or alveolar (severe) pulmonary pattern. Atelectasis may be secondary to pleural effusion/pneumothorax, prolonged recumbency, or bronchial obstruction. In the cat, the right middle lung lobe may collapse secondary to bronchial obstruction by a mucous plug, and is associated with asthma/bronchitis (Figure 19.26) [37, 38]. When totally collapsed, the right middle lobe appears as a curvilinear soft tissue opacity superimposed over the cardiac silhouette on the left lateral radiograph. On the ventrodorsal radiograph, the collapsed right middle lung lobe appears as a small soft tissue opaque triangle that border effaces the right side of the cardiac silhouette.

Solitary pulmonary masses are a common radiographic presentation for primary lung tumors in cats, (Figure 19.27) [39–45]. Common primary lung lobe tumors include bronchogenic carcinoma, squamous cell carcinoma, and bronchoalveolar carcinoma [46]. Pulmonary masses can be cavitated secondary to central necrosis or invasion of a bronchus with replacement of the necrotic fluid with air. Ill‐defined areas of multifocal mineralization may also be seen (Figure 19.27

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