The Thoracic Wall



The Thoracic Wall


Valerie F. Samii


The thoracic wall is composed of skin, fat, subcutaneous and intercostal musculature, parietal pleura, blood vessels, nerves, and lymphatics. The spine, ribs, costal cartilages, and sternum provide rigid support for the thoracic wall soft tissues. Thoracic wall abnormalities are often overlooked on initial appraisal of thoracic radiographs. Careful inspection of the extrathoracic soft tissue and bony structures is always warranted and can provide information critical to the appropriate diagnosis and treatment.



Normal Radiographic Appearance


The soft tissues of the thoracic wall are normally homogeneous in opacity. In particularly obese animals, curvilinear soft tissue opacities, representing extracostal musculature outlined by fat, may be seen paralleling the lateral curvature of the ribs on dorsoventral (DV) and ventrodorsal (VD) projections (Fig. 28-1). Thirteen pairs of ribs and eight sternebral segments are normal. On the lateral projection, the first few ribs are oriented vertically but become progressively more caudoventral in orientation, from rib head to costochondral junction, in the mid to caudal aspect of the thoracic spine (Fig. 28-2). On the DV or VD projection, the first few ribs are oriented perpendicular to the spine. In the mid to caudal thoracic spine, ribs curve in a caudolateral direction from their respective vertebrae to their most lateral extent, then continue caudomedially (Fig. 28-3).


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Fig. 28-1 DV thoracic radiograph of an obese cat. Note the bilateral, curvilinear, soft tissue opacities peripheral to the lateral rib margins caused by extracostal muscles interposed between fat (black arrows).

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Fig. 28-2 Right lateral thoracic radiograph of a normal adult dog. The first few ribs are oriented vertically but become progressively more caudoventral in orientation, from rib head to costochondral junction, in the mid to caudal aspect of the thoracic spine.

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Fig. 28-3 DV thoracic radiograph of a normal adult dog. The first few ribs are oriented perpendicular to the spine. In the mid to caudal aspect of the thoracic spine, ribs curve in a caudolateral direction from their respective vertebrae to their most lateral extent, then continue caudomedially.

Slight differences in thoracic wall conformation are common among various canine breeds (Fig. 28-4). Breed-associated costochondral conformation leading to a misdiagnosis of pneumothorax or pleural effusion is discussed in Chapter 31.


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Fig. 28-4 Right lateral (A) and DV (B) thoracic radiographs of an adult Boston terrier. There are multiple wedge-shaped vertebrae and hemivertebrae in the midthoracic spine. The congenital vertebral anomalies in this dog have resulted in cranioventral angulations of the ribs on the lateral radiograph and a radiating appearance to the rib cage on the DV projection. Scoliosis of the midthoracic spine is evident. Extensive mineralization of the costal cartilages is present; this is a normal finding seen commonly in young and old dogs.

Mineralization of the costal cartilages may be seen in young dogs and cats and is nearly always present in older animals (Fig. 28-5). Movement at the costochondral and costosternal joints increases as the costal cartilages stiffen from mineralization. This in turn results in osseous proliferation at the costochondral and costosternal joints. The opacities resulting from enlargement of these joints may be confused with lung nodules on DV or VD radiographs. Excessive costochondral or costosternal mineralization may also be confused with aggressive processes such as infection or neoplasia.


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Fig. 28-5 Right lateral radiograph of a middle-aged German shepherd with extensive costal cartilage mineralization and sternebral and costochondral degeneration.

Pedunculated soft tissue opacities on the thoracic wall, such as nipples, papillomas, or engorged ticks, may summate with the pulmonary parenchyma and be misdiagnosed as pulmonary nodules. Inspection and palpation of the thoracic wall surface usually clarify the significance of this finding. The inability to see the presumed pulmonary nodule in the lung on an orthogonal radiograph increases the likelihood of the suspicious opacity being caused by summation created by a superficial structure. Application of positive contrast medium, such as barium paste, on the thoracic wall nodule followed by repeat radiographs can be performed if localization is still in question (Fig. 28-6).


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Fig. 28-6 Right lateral thoracic radiographs before (A) and after (B) application of barium paste to a nipple. In A, a nodular soft tissue opacity summates with the seventh costal cartilages (black arrow). In B, barium paste was applied to a palpable nipple on the skin, confirming that the nodular opacity was created by the nipple (black arrow) and not a lung nodule.


Congenital and Developmental Abnormalities


Anomalies of the ribs and sternum are fairly common. Rudimentary ribs are sometimes present on the seventh cervical vertebra (Fig. 28-7) or the thirteenth thoracic vertebra (Fig. 28-8). Ribs may be hypoplastic or absent on the thirteenth thoracic vertebral segment (Fig. 28-9). These anomalies may be unilateral or bilateral (Fig. 28-10). The clinical significance of rib asymmetry at the thoracolumbar junction with regard to their use as a surgical landmark was discussed in the “Incidental Factors” section of Chapter 7.


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Fig. 28-7 Right lateral radiograph of a Pekingese dog with a left-sided rudimentary rib on the seventh cervical vertebra, forming a pseudoarthrosis with the first thoracic rib (black arrows). T1, First thoracic vertebra.

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Fig. 28-8 VD radiograph of a dog with a transitional thoracolumbar vertebral segment. Note the broad-based, thick appearance to the proximal extent of the left thirteenth rib (black arrows) compared to the hypoplastic right. The left thirteenth rib is taking on the characteristics of a transverse vertebral process.

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Fig. 28-9 VD radiograph of a dog with a transitional thoracolumbar vertebral segment. The left thirteenth rib is hypoplastic (black arrows) and only partially mineralized.

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Fig. 28-10 Right lateral (A) and DV (B) thoracic radiographs of a young cat with congenital deformation of the rib cage. In A, the distal extent of right ribs 5 to 12 curve caudally, the severity of which is most pronounced in ribs 9 to 11. The xiphoid is displaced dorsally (black arrow). In B, the asymmetry between the right and left thoracic wall is evident. The right caudolateral rib cage has a compressed appearance, and the right diaphragmatic crus is displaced cranially.

Congenital sternal deformities, such as reduction in number of segments, fusion of neighboring segments (Fig. 28-11), pectus carinatum (ventral displacement of the sternum), and pectus excavatum (funnel chest or dorsal displacement of the sternum) may be incidental findings, but some sternal anomalies have also been reported in animals with peritoneopericardial diaphragmatic hernia (Fig. 28-12).1,2 Peritoneopericardial diaphragmatic hernias are discussed in Chapter 29.


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Fig. 28-11 Right lateral radiograph of a young dog. There is fusion of the fourth and fifth sternebral segments, likely because of congenital malformation. The dorsal concave margin of the fused segments is smooth, and no evidence of degenerative disease is present to suggest trauma.

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Fig. 28-12 Left lateral thoracic radiograph of a dog with a peritoneopericardial diaphragmatic hernia. Note the reduction in number of sternal segments. The cardiac silhouette is enlarged and oval in shape, causing dorsal deviation of the trachea. Multiple loops of small intestine are present within the thorax summating with the cardiac silhouette. The caudal margin of the cardiac silhouette is confluent with the cranioventral margin of the diaphragm. A segment of large intestine containing granular material is identified crossing the peritoneopericardial junction (black arrows).

Pectus excavatum (Fig. 28-13) results in dorsal to ventral narrowing of the thorax and is often associated with respiratory and cardiovascular anomalies. Although pectus excavatum may be congenital or acquired in human beings, all reports in animals describe congenital malformations.3 The etiology is unknown; however, a hereditary component has been suggested.4


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Fig. 28-13 Left lateral (A) and VD (B) thoracic radiographs of a 4-year-old pug with pectus excavatum that causes severe dorsal to ventral narrowing of the thorax. The cardiac silhouette and caudal aspect of the thoracic trachea are displaced dorsally, and the heart is also deviated into the left hemithorax. The thoracic trachea is focally, dorsally deviated in the cranial aspect of the thorax, likely an artifact of head and neck positioning. Multiple congenital vertebral anomalies are present to include shortened, wedge-shaped, and butterfly vertebral segments from T3 to T11.

Most congenital thoracic wall defects are not repaired surgically unless they are associated with life-threatening complications.



Thoracic Wall Trauma


Thoracic wall trauma is common in small animals but may go unnoticed. Direct soft tissue injury may produce soft tissue swelling or subcutaneous emphysema (Fig. 28-14), or air can accumulate because of leakage from the airway (Fig. 28-15). Tears of the intercostal musculature can result in separation of ribs and are a common sequela to bite wounds, resulting in uneven spacing between ribs (Fig 28-16).5,6


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Fig. 28-14 DV thoracic radiograph of a cat with subcutaneous emphysema along the right thoracic body wall caused by bite wound trauma (white arrows).

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Fig. 28-15

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May 27, 2016 | Posted by in ANIMAL RADIOLOGY | Comments Off on The Thoracic Wall
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