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Ultrasonography in Equine Respiratory Diseases 2

Pleural Pathology

Pleural effusion is the most common pleural abnormality found in horses with thoracic disease [1]. The pleural fluid displaces the lung parenchyma dorsally and the pericardial diaphragmatic ligament becomes visible. The pericardial diaphragmatic ligament is a normal pleural reflection that extends from the pericardium to the diaphragm and it is imaged floating in the pleural fluid (Fig. 13) [1,4,5]. The pericardial diaphragmatic ligament is thicker than fibrin and should not be mistaken for fibrin in the pleural space, because it is imaged in every horse with a pleural effusion [1,3,4]. The echogenicity of the pleural fluid varies with the cellularity of the pleural fluid, the type of cells present within the pleural space and the amount of protein and free gas present within the pleural space [1,7,14]. Anechoic pleural fluid is usually a transudate or a modified transudate and can be found in the ventral thorax in horses with congestive heart failure, thoracic neoplasia or pleuropneumonia. Cranial mediastinal lymphosarcoma or mesotheliomas are most likely to result in the production of large quantities of anechoic fluid [1,2]. Hypoechoic pleural fluid is usually seen in horses with pleuropneumonia or thoracic neoplasia (Fig. 13).


A hemothorax is recognizable by the detection of echoic fluid within the thoracic cavity (Fig. 14). In real time the more echoic cells are imaged swirling in a more hypoechoic fluid [1]. This swirling is most likely associated with the formation of red blood cells rouleaux in the vasculature of the thoracic cavity (Note from the Editor: Rouleaux is a condition wherein the blood cells clump together forming what looks like stacks of coins). Thoracic trauma, such as fractured ribs or a ruptured diaphragm and resultant diaphragmatic hernia should be considered in horses with hemothorax. Most foals with fractured ribs do not have a significant hemothorax, however. Hemangiosarcoma should also be on the initial differential diagnosis list for a horse with hemothorax.

Fibrinous Pleuritis

Fibrin in the thorax appears as hypoechoic filmy strands floating in the fluid, often creating loculations or as a hypoechoic shaggy layer on the parietal and/or visceral pleural surfaces (Fig. 15) [1-5,12]. Fibrin usually appears more hypoechoic than the pericardial diaphragmatic ligament and more filamentous. Strands are often imaged crises-crossing the thoracic cavity creating numerous loculations and a spider web appearance.


Fibrin between the parietal and visceral pleural surfaces can result in the lung being adhered to the chest wall, diaphragm or pericardium. An echoic thick organized band of tissue crossing from the lung to the parietal pleural surface of the chest wall or diaphragm is consistent with an adhesion [1,3-5]. Often the adhesion distorts the normal lung shape or restricts the movement of the lung during respiration [1,2,4,5].

Polymicrobullous Fluid

A hypoechoic fluid containing hyperechoic pinpoint echoes within the fluid consistent with gas is a polymicrobullous or composite fluid. The detection of free gas echoes mixing within the pleural fluid is most consistent with an anaerobic infection within the pleural cavity [1,2,4,5,14]. In horses with a fibrinous pleuropneumonia, the hyperechoic pinpoint echoes of gas often are first detected stuck on fibrin coating the pleura or on the fibrinous loculations that have formed (Fig. 16) [1,2,4].

Pleural Abscess

In horses with a fibrinous pleuropneumonia a pleural abscess may develop in the ventral portion of the thorax. The fibrin lining the pleural surfaces organizes and becomes a thick echoic abscess capsule (Fig. 17). Usually the fluid within the abscess is heterogeneous and strands of fibrin are recognizable initially and may remain persistently visible. Free gas echoes are usually imaged adhered to the fibrinous inner lining in horses with anaerobic pleural abscesses (Fig. 17). A Dorsal gas cap is usually present in horses with a bronchial pleural fistula. Homogeneous echoic fluid within a pleural abscess is less common and most consistent with infection with Streptococcus sp. (Fig. 18).

Pleural Fluid Volume

The volume of pleural fluid can be estimated from the level of the pleural fluid line detected ultrasonographically in the thorax [1-4,12]. Consideration should be made for the amount of parenchymal consolidation or the size and number of pulmonary abscesses present. A pleural fluid line at the level of the point of the shoulder corresponded to recovery of 1 - 5 liters of pleural fluid per side, while a pleural fluid line above the point of the shoulder to the middle of the thorax corresponded to a recovery of 4 - 10 liters of pleural fluid per side [1,4,12]. When the pleural fluid line extends to the dorsal most portion of the thorax, 15 - 30 liters of pleural fluid can be recovered per side on thoracocentesis.


A dorsal pneumothorax in a horse with pleuropneumonia is most indicative of the presence of a bronchopleural fistula [1,14]. A dorsal pneumothorax is most easily recognized in patients with a concurrent pleural effusion. In these horses, a dorsal hyperechoic gas echo moves up and down like a curtain with the ventral pleural fluid (Fig. 28) [1]. The underlying lung parenchyma is imaged deep to the gas-fluid interface and moves independently from the pneumothorax. Thoracic trauma should be considered in horses with a pneumothorax without a concurrent pleural effusion. Diagnosing a pneumothorax in a horse without a concurrent pleural effusion is more challenging. In the absence of pleural effusion the dorsal pneumothorax can be identified by visualizing a bright hyperechoic echo dorsally that is static [4-6]. At the junction of the dorsal pneumothorax and the adjacent compression atelectasis in the lung, the dorsal hyperechoic air echo moves up and down with respiration over the adjacent underlying lung [1]. There is a small hypoechoic area of lung parenchyma usually detectable at the ventral most extent of the pneumothorax associated with the dorsal compression atelectasis of the lung [1,4].

Cranial Mediastinal Pathology

Fluid imaged in the cranial mediastinum is common in horses with a wise variety of thoracic pathology. Fibrin and loculations are commonly imaged in the cranial mediastinum in horses with fibrinous pleuropneumonia. These hypoechoic lacy loculations can be detected unilaterally in horses with a complete mediastinum (Fig. 19) or may be imaged in both sides of the thorax. As the infection progresses and the fibrin becomes more organized, it may begin to wall off an area within the cranial mediastinum, forming a cranial mediastinal abscess.

Masses in the cranial mediastinum in horses are usually associated with either a cranial mediastinal abscess or cranial mediastinal neoplasia, most frequently lymphosarcoma [1,2,4,23,24]. Differentiating the abscess from the neoplasm can be done ultrasonographically as they have very different sonographic characteristics. The cranial mediastinal abscess is usually composed of a heteroechoic fluid, although a homogeneous hypoechoic fluid may occasionally be detected, most commonly with Streptococcal sp. infections. A more echoic capsule is usually imaged surrounding the central fluid area, lining the parietal pleural surface of the chest wall and the ventral visceral pleural surface of the lung [1,2,4,24]. Hyperechoic free gas echoes may be imaged in the dorsal portion of the abscess if there is a concurrent anaerobic infection or a bronchopleural fistula is present [1,2,4,24]. The abscess, if large, may indent the right ventricular outflow tract and cause obstruction of venous return to the right side of the heart. Some of the horses will develop a systolic murmur over the pulmonic valve area, consistent with a murmur of pulmonic stenosis. A well-encapsulated mediastinal mass with a hypoechoic center compressing both the right and left atria has been described in a horse with an Aspergillus spp. granuloma [17].
The cranial mediastinal mass in a horse with cranial mediastinal lymphosarcoma is a soft tissue mass that usually occupies the entire cranial mediastinum (Fig. 20). The mass may appear homogeneous or heterogeneous and is often 20 cm or greater in diameter, by the time the horse is presented with clinical signs [1,2,4,24]. Large anechoic pleural effusions are most frequently detected in horses with cranial mediastinal lymphosarcoma [1,2,4,24]. These pleural fluid lines can extend more than 20 cm dorsal to the point of the shoulder. In some horses, small homogeneous hypoechoic or heteroechoic masses are detected in the mediastinal septum or floating freely within the pleural fluid in the cranial mediastinum [1,4,24]. The transducer should be angled dorsally as much as possible in all horses in which cranial mediastinal lymphosarcoma is suspected because occasionally, only the ventral most tip of the mediastinal mass is visualized just ventral to the right apical lung, but is not present in the ventral most portion of the cranial mediastinum. In these horses the ventral cervical lymph nodes may be enlarged and infiltrated and easily scanned ultrasonographically from the thoracic inlet region. The transducer can also be angled ventrally from the thoracic inlet to look for enlarged lymphoid tissue extending ventrally into the cranial mediastinum. Metastatic melanoma has also been described in horses with a cranial mediastinal mass and pleural effusion and should be considered in the differential diagnosis of grey horses with a large cranial mediastinal mass [25,26].

Diaphragmatic Hernia

Diaphragmatic hernias can be diagnosed ultrasonographically by the detection of abdominal viscera in the thoracic cavity (Fig. 21). Small intestine, large colon, spleen, liver and stomach have all been imaged within the thoracic cavity in horses with a diaphragmatic hernia. A portion of the defect in the diaphragm can often be imaged due to its location in the periphery of the diaphragm (Fig. 18). An estimate of the size of the diaphragmatic hernia can be made by determining the number of intercostal spaces on each side of the thorax where the defect in the diaphragm can be visualized. The sonologist must recognize, however, that a diaphragmatic hernia in the mid portion of the diaphragm could be missed if abdominal viscera were not adjacent to the thoracic wall.


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15. Chaffin MK, Ruoff WW, Schmitz DG, et al. Regression of hypertrophic osteopathy in a filly following successful management of an intrathoracic abscess. Equine Vet J 1990; 22:62-65.
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Click on the author's name to view a list of his/her publications: V. B. Reef

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