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Thoracic ultrasonography is one of the most readily available and widely used diagnostic techniques for the evaluation of the thoracic cavity in horses [1-8]. Its growth has been exponential since the original description of its use in 1981 [9,10]. With continued rapid changes in ultrasound technology, the image quality of diagnostic ultrasound machines available to the equine practitioner has dramatically improved and the cost has continued to decline, making ultrasonographic equipment a mainstay of most equine practices. The portability and versatility of the diagnostic ultrasound equipment, the lack of exposure to ionizing radiation and the breadth of diagnostic information obtained in horses with thoracic disease, have made diagnostic ultrasound a routine part of the examination in horses with thoracic disease.
Sonographic examination of the thorax is best performed when the hair over the sides of the thorax has been removed, either with a #40 surgical clipper blade or when the skin has been shaved. An ultrasound coupling gel should then be applied to the skin prior to beginning the sonographic examination. However, if sonographic evaluation of the thoracic cavity is being performed as a screening procedure, to determine if any areas of pulmonary or pleural pathology exist, clipping the hair off the skin may be avoided until areas of pathology that need closer scrutiny have been identified. In these horses, adequate images can often be obtained by saturating the hair and skin over the sides of the thorax with warm water or emu oil, or by spraying the sides of the thorax with isopropyl alcohol.
Before using any product other than ultrasound coupling gel or water, the equipment manufacturer should be consulted to be sure that damage to the transducer does not occur with the use of other coupling agents. If the warm water or alcohol is sprayed onto the side of the thorax under pressure, the skin and hair appear to become better saturated, facilitating the ability to obtain a good quality image. In all the above situations, the hair should be made to lie flat, along its grain. A screening ultrasonographic examination can often be performed with just alcohol or emu oil as the coupling agent without the use of an ultrasonographic coupling gel. Once an area of pathology has been identified, the affected area should be surgically clipped or the skin shaved to improve the resolution of the image of the thoracic pathology detected. This improved quality of the thoracic image enables the sonographer to better characterize the disease process. The skin should be thoroughly cleaned with surgical scrub and an ultrasound coupling gel should then be applied. Preparation of the entire thorax involves preparing the area from the 3rd to the 17th intercostal spaces, including the area overlying the triceps musculature. Thorough thoracic auscultation, including the use of a rebreathing bag, can also help in identifying areas of pulmonary and pleural pathology. A screening examination can be performed over the areas that auscult normally, with the areas that auscult abnormally being those subject to the initial skin preparation. This later technique usually identifies the majority of areas of thoracic pathology identifiable ultrasonographically, as the sounds detected with the stethoscope are usually located within approximately 8 - 10 cm of the thoracic wall.
The lung and pleural cavity should be scanned initially with a high frequency transducer in each intercostal space (3 - 17) and from the dorsal most to ventral most portion of the thorax [1,4,11]. The initial scan should be performed parallel to each rib sliding slowly in a dorsal to ventral direction (Video 1). Each area of the thorax should be evaluated through both inspiration and expiration, because areas of pulmonary pathology near the surface of the lung are only visible during exhalation . Also as the lung moves with inhalation and exhalation, the parenchyma previously hidden underneath the rib will be visible for ultrasonographic examination. If the scan is also performed with the transducer orientation perpendicular to each rib, further characterization of areas of pulmonary and pleura pathology can be obtained, however rib artifacts will be present on the cranial and caudal sides of the images. The dorsal most portion of the lung in each intercostal space should be the starting point for the ultrasound examination. The transducer should be moved ventrally in the intercostal spaces until the lung is imaged crossing the diaphragm, so that the ventral most tip of the lung and pleural cavity is evaluated. Once an area of pulmonary pathology is identified, the frequency of the transducer should be decreased as needed to completely examine all abnormal areas.
Video 1 - 0.5 Mo. Video of an ultrasound examination of the thorax beginning at the dorsal most portion of the right seventh intercostal space extending to the ventral most portion of the thorax in a horse with severe anaerobic pleuropneumonia. Notice the anechoic pleural fluid dorsally between the parietal and visceral pleura, the adhesion of the ventral lung to the parietal pleura, and the hypoechoic fibrin and hyperechoic free gas echoes in the fluid trapped between the axial side of the lung and the diaphragm. Dorsal is to the right and ventral is to the left.
The cranial mediastinum should be examined in all horses with suspected pulmonary, pleural and/or cranial mediastinal pathology. Ultrasonographic examination of the cranial mediastinum can be performed 2 ways, either by placing the transducer in the horse’s axilla in the 3rd intercostal space or by scanning through the upper limb musculature overlying the 3rd intercostal space. When scanning in the axilla, the horse’s foreleg should be as far forward as possible. The ultrasound transducer should be placed in the third intercostal space with the beam parallel to the ribs (Fig. 1) [1,2,4]. The transducer should then be angled towards the point of the opposite shoulder. A mid to low frequency transducer is usually optimal for evaluating the cranial mediastinum from the axilla. When scanning the cranial mediastinum through the upper limb musculature, a lower frequency transducer is usually needed that should be oriented with the beam again parallel to the ribs (Fig. 2). The transducer should then be moved from a dorsal to ventral direction over the 3rd intercostal space. In the normal horse, the cranial mediastinum is best imaged from the right side of the thorax, because the right ventricular outflow tract and pulmonary artery are in the left 3rd intercostal space. In the horse with cranial mediastinal pathology, the heart is often caudally displaced and the cranial mediastinum can also be imaged successfully from the left axilla.
Figure 1. Position of the horse’s right foreleg and ultrasound transducer for imaging the cranial mediastinum through the axilla.
Figure 2. Position of the ultrasound transducer over the upper arm musculature of the horse for scanning the cranial mediastinum.
Transducers for Thoracic Ultrasonography
Transducer frequencies of up to 14 MHz are available that can penetrate through the intercostal musculature in adult horses and obtain excellent quality images of the lung and pleura. The initial scan should begin with the highest frequency (7.5 - 14 MHz) that is available for the equipment being used [1,11]. As thoracic pathology is identified and more penetration is needed, a 6.0 - 2.0 MHz frequency transducer should be used. In all cases, the highest frequency transducer should be selected that can penetrate to the depth needed, for optimal image quality. A high frequency microconvex transducer is ideal for performing the initial screening sonographic examination as the footprint of these transducers fits nicely in the equine intercostal space. However, all types of ultrasound transducers can be used to perform the thoracic ultrasound examination. High frequency straight linear, T shaped linear, annular array, sector scanner, matrix array and phased array transducers can all be used for the initial thoracic examination with mid frequency and lower frequency transducers usefulness limited often by the footprint of the transducer and how well it fits into the intercostal space.
The ultrasonographic appearance of normal lung is a bright, hyperechoic echo emanating from the aerated lung surface that moves in a dorsal to ventral direction with respiration (Video 2) [1,11]. The normal lung glides across the diaphragm in inspiration and expiration, the "curtain sign" (Video 3) . In most normal horses, there is no visible pleural fluid detected surrounding the lung, although up to 2 cm of anechoic pleural fluid has been reported to be within normal limits [1,4,5,9-11]. Heteroechoic gelatinous tissue representing fat can be imaged surrounding the heart and in the ventral portion of the thorax and cranial mediastinum. An echoic mediastinal septum can be imaged in normal horses in the ventral most portion of the cranial mediastinum. In the neonate and yearling the thymus is visible as a hypoechoic to echoic glandular structure in the ventral aspect of the cranial mediastinum [1,2,11].
Video 2 - 4.5 Mo. Video of a sonogram of the normal lung gliding against the thoracic wall in inspiration and expiration obtained in the mid portion of the thorax. Notice the bright hyperechoic line that represents the visceral pleural surface of the lung gliding smoothly against the parietal pleural surface of the chest wall. Dorsal is to the right and ventral is to the left. Currently not available
Video 3 - 4 Mo. Video of a sonogram of the normal lung gliding across the diaphragm in inspiration and expiration. Dorsal is to the right and ventral is to the left. Currently not available
There are characteristic abnormalities detected upon ultrasonographic examination of the thorax that help the ultrasonologist differentiate a variety of pulmonary pathologies; in particular pulmonary atelectasis, consolidation, necrosis, and pulmonary abscess [1,4,5]. The sonographic findings in horses with pulmonary edema, exercise induced pulmonary hemorrhage, chronic obstructive pulmonary disease, and horses with scarring from previous pleuropneumonia are usually very similar and other diagnostics are needed to differentiate the different conditions [1,2,4,5]. Similarly, the sonographic findings in horses with pulmonary fibrosis, granulomatous pneumonia, fungal pneumonia and metastatic neoplasia are not diagnostic for the disease process [1,2,4,5]. A lung biopsy and/or culture of the affected lung parenchyma are needed to definitively differentiate these conditions.
Comet tail artifacts
Comet tail artifacts are bright hyperechoic reverberating artifacts emanating from an irregularity in the aeration of the pulmonary periphery (Video 4) [1,4]. Comet tail artifacts are non specific findings and can be seen in horses with a wide variety of pulmonary pathology including pulmonary atelectasis, pneumonia (especially interstitial), acute bronchopneumonia, scarring from previous pneumonia, pulmonary edema, exercise induced pulmonary hemorrhage, chronic obstructive pulmonary disease, pulmonary fibrosis, granulomatous pneumonia, or metastatic neoplasia [1,2,4,7]. The significance of the comet tail artifacts detected in an individual horse needs to be examined in light of the animal’s history, physical examination and clinicopathological findings. Other diagnostics such as a transtracheal aspirate, bronchoalveolar lavage, thoracic radiographs, echocardiogram or lung biopsy may be indicated to further define the significance of the comet tail artifacts detected ultrasonographically.
Video 4 - 2.5 Mo. Video of a sonogram of the right lung obtained in the 12th intercostal space from a horse with comet tail artifacts. Notice the bright "comet tail" like echoes emanating from the visceral pleura surface of the lung. Small hypoechoic areas of pulmonary parenchyma are imaged occasionally at the periphery of the lung with the comet tail artifact originating deep to these small non-aerated areas of lung. Dorsal is to the right and ventral is to the left.
Atelectasis of the lung occurs when normal lung parenchyma is compressed by fluid or air surrounding it. If there is a pneumothorax, the more dorsal lung will be compressed but this portion of the lung will usually be obscured from view by the intervening air. If surrounded by fluid, the ventral most lung parenchyma will be compressed by the fluid and appear hypoechoic (Fig. 3) [1,4,7]. The lung will enlarge towards the hilus of the lung and the pulmonary vessels will be imaged as branching vessels within the collapsed compressed lung [1,4]. The bronchi will also be visible as branching tubes with more echoic walls than the pulmonary vessels but lacking hyperechoic air echoes [1,4]. Pulsed wave, color flow or power Doppler can be used in the consolidated lung parenchyma to help the ultrasonologist differentiate vessels from bronchi. The ventral compressed tip of the lung will be imaged floating in the surrounding pleural fluid (Video 5) [1,4].
Figure 3. Sonogram of the ventral tip of the right lung in a horse with compression atelectasis obtained in the 8th intercostal space. Notice the marked compression of the ventral lung due to the large pleural effusion (arrow), the enlargement of the lung dorsally towards the hilus and the lack of any hyperechoic air echoes in the ventral portion of the lung. The ventral tip of the lung is floating in the adjacent fluid. Dorsal is to the right and ventral is to the left.
Video 5 - 4 Mo. Video of a sonogram of the ventral tip of the left lung in the 7th intercostal space from a horse with compression atelectasis. The ventral tip of the lung is hypoechoic and floating in the adjacent fluid. Notice the lack of any hyperechoic air echoes in the ventral portion of the lung other than the few residual air echoes within the main bronchial structures and the enlargement of the lung dorsally towards the hilus. The pericardiodiaphragmatic ligament is visualized floating in the fluid adjacent to the diaphragm. To the left of the diaphragm in the abdomen the more hypoechoic left lobe of the liver is imaged adjacent to the more echoic spleen. A small tag of hypoechoic material consistent with fibrin is imaged on the ventral most tip of the lung. Dorsal is to the right and ventral is to the left.
Consolidated lung is not compressible because the airways and alveoli are filled with fluid and cells. Thus the ventral portion of the lung retains its triangular shape when filled with exudate (Video 6) [1,2,4]. This fluid filled lung is heavier than the normal air filled lung and thus does not float when surrounded by fluid in the pleural space (Video 6) [1,4]. The consolidated lung often begins as small hypoechoic areas of the pulmonary parenchyma, usually located in the more cranial and ventral portions of the lung (Video 7) [1,4,7].
Video 6 - 6.5 Mo. Video of a sonogram of a consolidated ventral tip of the right lung in the 6th intercostal space from a horse with severe consolidation. The abnormal pulmonary parenchyma is first detected parallel with a line level with the point of the shoulder. Notice the large triangular shaped hypoechoic ventral lung consistent with fluid and exudate filled areas of lung parenchyma. Notice the hyperechoic tubular air echoes casting dirty acoustic shadows consistent with air remaining in the larger airways, particularly during inspiration. The ventral portion of the thorax contains anechoic pleural fluid and the pericardiodiaphragmatic ligament is imaged floating in the pleural fluid. Dorsal is to the right and ventral is to the left.
Video 7 - 12 Mo. Video of a sonogram of a ventral area of early consolidation in the left lung of a horse with pleuropneumonia. Notice that the small area in the periphery of the ventral lung that is hypoechoic and somewhat triangular in shape. There is a small amount of anechoic pleural fluid adjacent to the abnormal lung. Dorsal is to the right and ventral is to the left.
Small areas of consolidation are detected more frequently sonographically than radiographically, especially when present in the cranioventral portion of the lung [1,4,12]. As the consolidation becomes more severe and larger areas of lung parenchyma are affected, air bronchograms appear. A hypoechoic area of lung surrounding a hyperechoic tubular branching structure in the lung is consistent with an air bronchogram (Fig. 4) [1,2,4,5,7,13]. Air bronchograms are recognized because the lung parenchyma surrounding the air filled bronchus is no longer air filled but is filled with fluid and cells, giving it a hypoechoic appearance. As the consolidation of the lung becomes more severe, a fluid bronchogram may be recognized. A hypoechoic area of lung surrounding a hypoechoic or anechoic tubular branching structure that does not contain flowing blood is consistent with a fluid bronchogram (Fig. 5) [1,2,4,13].
Figure 4. Sonogram of an air bronchogram in the right lung obtained from the 10th intercostal space at a level 4 cm dorsal to a line parallel with the point of the shoulder in a horse with severe pneumonia. Notice the tubular hyperechoic air filled bronchus (arrow) surrounded by hypoechoic lung parenchyma. Dorsal is to the right and ventral is to the left.
Figure 5. Sonogram of a fluid bronchogram in the right lung obtained from the 8th intercostal space at a level 4 cm dorsal to a line parallel with the point of the shoulder in a horse with severe cranioventral pneumonia. Notice the echoic wall of the fluid filled bronchus (arrow) and the severe hepatization of the ventral lung parenchyma. Dorsal is to the right and ventral is to the left.
Fluid bronchograms are imaged when the lung appears hepatized or "liver-like" and is devoid of air in the affected area (Fig. 6) [1,2,4,5]. As the consolidation worsens the ventral lung looses its triangular shape and has a more rounded appearance (Fig. 7). This rounded or swollen appearance is consistent with severe consolidation and is usually followed by the development of areas of cavitation within the lung parenchyma consistent with necrosis. The detection of pinpoint hyperechoic echoes consistent with gas imaged in an otherwise severely hepatized pulmonary parenchyma is consistent with an anaerobic pneumonia () [1,2,4,5,14].
Figure 6. Sonogram of a hepatized area of lung parenchyma in the ventral most portion of the right lung in the right 8th intercostal space in a horse with severe fibrinous pleuropneumonia. Notice the liver-like appearance of the ventral lung with only the echoic walls of the bronchi visible containing residual amounts of air. The hepatized lung was imaged ventral to a line level with the point of the shoulder. There is a small echoic layer of fibrin overlying the visceral pleural surface of the lung. Dorsal is to the right and ventral is to the left.
Figure 7. Sonogram of a bulging ventral area of severe consolidation obtained from the ventral most portion of the right lung in the 5th intercostal space. Notice the swollen ventral tip of the lung that has lost its triangular shape (arrow). A few hyperechoic gas echoes remain in the ventral airways. Dorsal is to the right and ventral is to the left.
Video 8 - 4.5 Mo. Video of a sonogram of a ventral area of anaerobic pneumonia obtained from the right 5th intercostal space at a level 5 cm dorsal to a line parallel with the point of the shoulder from a horse with anaerobic pneumonia. Notice the large ventral area of hepatized lung with a large central fluid bronchogram. There are multiple tiny pinpoint hyperechoic echoes scattered throughout the axial portion of the ventral lung consistent with free gas echoes. Dorsal is to the right and ventral is to the left.
Necrotic lung is consolidated lung parenchyma that is beginning to lose its normal architecture due to the necrosis of the lung tissue [1,4]. The lung parenchyma is usually more anechoic than the adjacent consolidated lung parenchyma. This anechoic necrotic area appears gelatinous with respiratory movement (Video 9) [1,4]. Cavitating areas appear in the anechoic lung parenchyma and either result in the formation of pulmonary abscesses or bronchopleural fistula [1,4,7].
Video 9 - Video of a sonogram of a ventral gelatinous area of pulmonary consolidation obtained from the 8th intercostal space in a horse with severe necrotizing pleuropneumonia. The abnormal pulmonary parenchyma was first detected in the left lung 18 cm dorsal to a line level with the point of the shoulder. Notice the anechoic to hypoechoic area of the superficial portion of the ventral lung that giggles with respiration. There are some large hyperechoic echoes in the ventral anechoic lung consistent with residual air in bronchi. Notice the sharp demarcation between the normal and abnormal parenchyma and the bulging ventral tip of the lung, consistent with a severe necrotizing pneumonia. The sharp area of demarcation of the ventral lung parenchyma and the hypoechoic segmental appearance in the gelatinous area of lung is consistent with a pulmonary infarct in this region. Power and color flow Doppler revealed an absence of blood flow to this region. There is a hypoechoic pleural effusion between the gelatinous ventral lung and the chest wall. Dorsal is to the right and ventral is to the left. Currently not available
Horses with pulmonary infarction have a sharp line of demarcation between the ventral infarcted region of lung and the more dorsal consolidated lung. Segmental echoic areas of lung parenchyma have been imaged in these horses within the infarcted area of lung. Color flow and power Doppler ultrasound has been used in horses to evaluate blood flow in severely hepatized areas of lung. Parenchymal areas with reduced blood flow or areas completely lacking in blood flow have been identified in horses with a pulmonary infarct (Video 10).
Video 10 - Video of a sonogram of an area of decreased pulmonary circulation and infarction obtained from the left 8th intercostal space in a horse with severe pleuropneumonia. The abnormal pulmonary parenchyma was first imaged 17 cm dorsal to a line level with the point of the shoulder. Notice the sharp demarcation of the echoic consolidated/infarcted lung from the more normal dorsal aerated lung. The ventral lung is echoic with a segmental appearance. There is a slightly rounded appearance to the ventral lung consistent with severe consolidation. The echo from the visceral pleural surface of the lung is prominent due to the adjacent hypoechoic pleural fluid and the hypoechoic separation of the ventral lung into segments. Notice the small pulmonary arterial flow that extends only midway into the ventral most portion of the lung before it stops with no further arterial flow visible into the ventral most lung. Dorsal is to the right and ventral is to the left. Currently not available
A pulmonary abscess is an anechoic or hypoechoic cavitated area in the lung parenchyma that does not contain any pulmonary vessels or bronchial structures (Fig. 8) [1,2,4,5,13,15]. The exudate within the abscess is usually more hypoechoic than the adjacent lung parenchyma. A dorsal hyperechoic area is detected when a dorsal gas cap is present [1,2,4,7]. When a large dorsal gas cap is detected, a bronchopleural fistula should be suspected Fig. 9). In most instances in the adult horse, pulmonary abscesses are not encapsulated. In foals with Rhodococcus equi pneumonia, encapsulation of the pulmonary abscesses is more common (Fig. 10). Differentiating an abscess from consolidated parenchyma is often difficult. The detection of airways or vessels in a hypoechoic to anechoic area is indicative of consolidation, not abscessation [1,5,7]. Monitoring the sonographic appearance of the abscess during the course of treatment can often clarify the progression of an area of severe consolidation to the development of a pulmonary abscess [5,7].
Figure 8. An extended field of view sonogram of a pulmonary abscess obtained from the right side of the thorax in a horse with a pulmonary abscess. This sonogram begins in the dorsal most portion of the right lung and extends ventrally to the ventral portion of the thorax. Notice the circular hypoechoic cavitated area with a hyperechoic rim (arrow) in the mid thorax consistent with a pulmonary abscess. The hyperechoic echoes lining the circular cavitation are free gas echoes, consistent with an anaerobic abscess or, less likely, a bronchial pleural fistula. Notice the ventral anechoic pleural fluid. The hypoechoic membranous tissue immediately dorsal (to the right) to the anechoic pleural fluid is consistent with the pericardial diaphragmatic ligament (arrowhead). Dorsal is to the right and ventral is to the left.
Figure 9. Sonogram of the dorsal gas cap in a thoracic abscess in a horse with a bronchopleural fistula. This sonogram was obtained in the 5th intercostal space on the left side of the thorax. Notice the hypoechoic semicircular echo (arrow) in the center of the sonogram that represents the abscess capsule. The abscess capsule is imaged along the parietal pleural surface of the chest wall as a thin hypoechoic line, superficial to the dorsal gas cap within the abscess. The ventral portion of the abscess (not visible in this frozen image) is the ventral aspect of the thorax. The aerated lung is dorsal to the abscess capsule (to the right) and has an irregular aerated border adjacent to the fibrous abscess capsule. Dorsal is to the right and ventral is to the left.
Figure 10. Sonogram of a pulmonary abscess obtained from the right lung in the 8th intercostal space from a foal with Rhodococcus equi pneumonia. Notice the cavitated hypoechoic area lacking in pulmonary vessels or bronchi with a slightly more echoic capsule in the periphery of the right lung (arrow). The hypoechoic appearance of the fluid within the abscess is consistent with purulent fluid within the abscess. The abscess is located 4 cm dorsal to a line level with the point of the shoulder. Dorsal is to the right and ventral is to the left.
Marked dilation of the pulmonary artery in the absence of left sided congestive heart failure or severe chronic lower airway disease has been detected in several horses with pulmonary thromboembolism. Dilation of the pulmonary artery is commonly detected in horses with pulmonary hypertension  in horses with acute dilatation of the pulmonary artery and severe acute respiratory distress and detectable pulmonary pathology; pulmonary thromboembolism should be seriously considered.
Pulmonary fibrosis, diffuse granulomatous disease, fungal pneumonia and metastatic neoplasia. Small multifocal hypoechoic soft tissue masses are typically imaged throughout both lung fields in horses with fibrosing interstitial pneumonia (Fig. 11), diffuse granulomatous disease, fungal pneumonia and metastatic neoplasia [1,4,7]. Occasionally horses with a diffuse suppurative bronchopneumonia will present with this sonographic appearance of the lung. Many of these hypoechoic masses are best visualized during exhalation, as they are close to the visceral pleural surface but do not involve the most superficial alveoli. Hemangiosarcoma has occasionally been imaged ultrasonographically and usually appears as very small, miliary to multifocal anechoic to hypoechoic fluid filled masses in the periphery of the lung . Similar masses should be looked for on the parietal pleural surface as hemangiosarcoma typically seeds this surface as well. Mesotheliomas may also be detected ultrasonographically as small to moderate sized soft tissue density masses involving the parietal and visceral pleura [1,3,7].
Figure 11. Sonogram of an irregular soft tissue mass in the periphery of the lung from a horse with pulmonary fibrosis. This sonogram was obtained in the 10th intercostal space at 10 cm dorsal to a line level with point of the shoulder. Notice the irregular hypoechoic area of pulmonary parenchyma lacking visible bronchi or vessels (arrow). Multiple similar areas in the pulmonary periphery were imaged throughout both lung fields. Ultrasound guided lung biopsy confirmed the diagnosis. Dorsal is to the right and ventral is to the left.
High-resolution ultrasound transducers must be used in most horses with suspected hemangiosarcoma or mesothelioma due to the small size of many of these masses. However, one horse with hemangiosarcoma had a large anechoic loculated neoplastic mass involving the ventral portion of the lung. Pulmonary hydatidosis has also been reported in a horse with a large anechoic loculated mass in the lung parenchyma . A large encapsulated mass with a hypoechoic center was detected ultrasonographically in one horse with a fungal granuloma associated with aspergillosis . Cytological evaluation of the transtracheal aspirate and pleural fluid (if present), bronchioalveolar lavage fluid, or a lung biopsy is needed to definitively diagnose the underlying pulmonary pathology.
Horses with metastatic neoplasia may have large pleural effusions [1,2,5,6,13,15]. Cytological examination of the pleural fluid should be performed and may yield a diagnosis. Swirling echoic fluid consistent with hemothorax is often detected in horses with hemangiosarcoma involving the lung and pleura. A diaphragmatic hernia should also be considered in horses with hemothorax. A large anechoic pleural effusion is also usually present in horses with mesothelioma.
Primary Pulmonary Neoplasia
Primary pulmonary neoplasia is rare in horses but should be considered in horses with large heterogeneous areas in the lung lacking recognizable pulmonary anatomical structures (airways or vessels) [1,19,20]. Not all primary pulmonary tumors are visible ultrasonographically but those that involve the periphery of the lung usually appear as large multilobulated soft tissue masses of varying echogenicities (Fig. 12). Color flow Doppler or power Doppler ultrasonographic examination of these areas should demonstrate an increase in blood flow to the mass that may be visible if there is adjacent parenchymal consolidation or a concurrent pleural effusion.
Figure 12. Sonogram of the right ventral lung obtained from a horse with primary pulmonary neoplasia. Notice the heteroechoic soft tissue mass (arrows) distorting the normal triangular shape of the lung and the absence of normal pulmonary vessels or bronchi. This sonogram was obtained from the 11th intercostal space at the level of the ventral most portion of the right lung. Dorsal is to the right and ventral is to the left.
Pleural effusion is the most common pleural abnormality found in horses with thoracic disease . 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 (Video 1, Fig. 3, Video 5, Video 9, and Fig. 13).
Figure 13. Sonogram of the pericardial diaphragmatic ligament in the ventral portion of the left 7th intercostal space in a horse with pleuropneumonia. Notice the thick undulating membrane (arrow) overlying the diaphragm in a horse with an anechoic pleural effusion. Dorsal is to the right and ventral is to the left.
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 . 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.
Figure 14. Sonogram of the left side of the thorax in the 11th intercostal space obtained from a horse with hemothorax. Notice the echoic fluid in the ventral portion of the thorax consistent with blood. The hemothorax extended dorsally to 19 cm dorsal to a line level with the point of the shoulder. Notice the ventral hypoechoic lung (arrow) that is slightly compressed and completely lacking in any air echoes, consistent with compression atelectasis associated with the large hemothorax. Dorsal is to the right and ventral is to the left.
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 (Video 11).
Figure 15. Sonogram of fibrin on the visceral pleural surface of the right lung obtained in the 7th intercostal space from a horse with fibrinous pleuropneumonia. Notice the thick hypoechoic layer of fibrin coating the superficial surface of the lung. The arrows point to the visceral pleural surface of the lung that is difficult to discern due to the thick hypoechoic fibrin layer overlying the lung surface. Close to the visceral pleural surface of the lung is a large air bronchogram. There is a hypoechoic pleural effusion visible between the thick fibrin layer overlying the visceral pleural surface and the parietal pleura of the chest wall. The axial surface of the lung appears adhered to the pericardial diaphragmatic ligament because there is no visible fluid surrounding the axial side of the lung, confirmed during the real time examination. Dorsal is to the right and ventral is to the left.
Video 11 - 5.5 Mo. Video of a sonogram of fibrinous loculations obtained from a horse with severe fibrinous pleuropneumonia. The sonogram was obtained in the left 11th intercostal space 5 cm dorsal to a line level with the point of the shoulder. Notice the hypoechoic fluid and the slightly more echoic lacy network of fibrin creating numerous loculations in the fluid. The fibrin network extends from the parietal pleural surface of the thoracic wall to the diaphragm in the ventral portion of the thorax. There is no lung visible in this video because the lung is displaced dorsally. Dorsal is to the right and ventral is to the left.
Fibrin between the parietal and visceral pleural surfaces can result in the lung being adhered to the chest wall (Video 12), 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].
Video 12 - 4.5 Mo. Video of an adhesion between the ventral tip of the right lung and the parietal pleura in a horse with an anaerobic fibrinous pleuropneumonia. Notice how the lung is pulled is inspiration due to the hypoechoic fibrinous adhesion between the tip of the lung and the parietal pleura. Ventrally, there is a fibrinous loculated pleural effusion. There is free gas dorsally adhered to the fibrin along the parietal pleural surface of the thoracic wall. The ventral lung tip is collapsed and hypoechoic consistent with compression atelectasis. Dorsal is to the right and ventral is to the left.
A hypoechoic fluid containing hyperechoic pinpoint echoes within the fluid consistent with gas is a polymicrobullous or composite fluid (Video 13). 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].
Video 13 - Video of a sonogram of a polymicrobullous effusion and a dorsal pneumothorax obtained from a horse with severe anaerobic pleuropneumonia and a bronchial pleural fistula. Notice the small hyperechoic echoes (gas or air echoes) swirling in the hypoechoic pleural fluid. There is a dorsal gas cap that moves up and down like a curtain with respiratory movement and that is first imaged deep to a hyperechoic linear echo adjacent to the parietal pleura that represents gas adhered to the parietal pleura at this level. Small hyperechoic echoes adjacent to the parietal pleura are also imaged more ventrally and also represent free gas adhered to fibrin along the parietal pleura. Dorsal is to the right and ventral is to the left. Currently not available
Figure 16. Sonogram of horse with gas adhered to the fibrin lining the parietal pleural surface of the thoracic wall and diaphragm and to a large fibrin strand within the pleural cavity (arrows). The hyperechoic echo deeper in the image (arrowhead) represents the right lung that appears nearly normally aerated but is adhered to the diaphragm. This sonogram was obtained in the right 14th intercostal space at 10 cm dorsal to a line level with the point of the shoulder. Dorsal is to the right and ventral is to the left.
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).
Figure 17. Sonogram of an anaerobic pleural abscess obtained from a horse with anaerobic pleuropneumonia. Notice the thin hypoechoic abscess capsule (arrows) that is forming along the parietal pleural surfaces of the thoracic wall and diaphragm encapsulating the pleural fluid and gas. There are hyperechoic echoes consistent with free gas that are stuck on the fibrinous lining of the pleural abscess, consistent with an anaerobic infection. The dorsal portion of this forming abscess is the ventral aspect of the left lung. This sonogram was obtained from the left 7th intercostal space in the ventral portion of the thorax. Dorsal is to the right and ventral is to the left.
Figure 18. Sonogram of a Streptococcal pleural abscess in the caudal mediastinal region of a horse. This sonogram was obtained from the left side of the thorax in the 8th intercostal space. The abscess extended from 25 cm dorsal to a line level with the point of the shoulder to the ventral most portion of the thorax. Notice the more anechoic fluid in the dorsal most portion of the abscess and the more echoic cellular material in the ventral portion of the abscess. The arrow points to the line between the more dorsal fluid and ventral cellular components of the abscess.
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) . 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 . 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 wide 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.
Figure 19. Sonogram of a horse with a mediastinitis in the right side of the cranial mediastinum. Notice the hypoechoic lacy loculations filling the right side of the cranial mediastinum with mostly anechoic fluid and a small amount of hypoechoic fibrin imaged in the left side of the thorax. The mediastinal septum divides the more normal left side of the cranial mediastinum from the right side. This sonogram was obtained from the left side of the thorax in the third intercostal space. This horse had loculated fluid imaged up to 30 cm dorsal to a line level with the point of his shoulder. Dorsal is to the right and ventral is to the left.
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 .
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].
Figure 20. Sonogram of the cranial mediastinal mass in a horse with cranial mediastinal lymphosarcoma. Notice the heteroechoic mass filling the entire cranial mediastinum. This sonogram was obtained in the right 3rd intercostal space. The thin hyperechoic line along the upper right side of the sonogram adjacent to the parietal pleura is the ventral tip of the right apical lung that is normally aerated. Dorsal is to the right and ventral is to the left.
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.
Figure 21. Sonogram of a horse with a diaphragmatic hernia obtained from the left 12th intercostal space. The echoic organ dorsal to the diaphragm is the spleen (arrow) located in the ventral portion of the left thoracic cavity. The hypoechoic curvilinear wall with the hyperechoic inner echo adjacent to the spleen is the stomach. The diaphragm (arrowhead) is much thicker than normal (2 cm) because it is contracted adjacent to the tear. Dorsal is to the right and ventral is to the left.
Ultrasonography can provide the clinician with diagnostic information about the thoracic cavity in horses with pleural and pulmonary disease that complements that obtained radiographically. Ultrasonographic examination of the thorax can be readily done in the field with the portable ultrasound equipment available to equine practitioners. The amount and character of a pleural effusion can be determined along with the presence and severity of the associated pulmonary disease in most cases. The sonographic findings in horses with pleuropneumonia can be used to formulate a prognosis before the results of the cytology and culture and sensitivity test of the transtracheal aspirate, bronchioalveolar lavage fluid and/or pleural fluid are available. The survival of horses with pleuropneumonia is adversely affected by the detection of fibrin, loculations, free gas or parenchymal necrosis on the initial ultrasonographic examination . Horses with parenchymal necrosis and/or anaerobic pleuropneumonia are also more likely to have longer treatment duration and are less likely to return successfully to their previous level of performance . The site for thoracocentesis can be determined and the response to treatment monitored ultrasonographically. Masses can be identified in the lung or mediastinum and an ultrasound guided biopsy obtained. However, in horses with suspected pulmonary edema, exercise induced pulmonary hemorrhage, chronic obstructive pulmonary disease, ARDS, fibrosing interstitial pneumonia, granulomatous pneumonia, fungal pneumonia, and interstitial pneumonia, thoracic radiographs should also be obtained to fully characterize the extent of the underlying pulmonary pathology.
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Affiliation of the authors at the time of publication
Large Animal Cardiology and Ultrasonography, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA, USA.