Ultrasonography of the Pleural Cavity, Lung, and Diaphragm
Rood and Riddle Equine Hospital, Lexington, KY, USA
Thoracic Ultrasonography
Ultrasonography has largely replaced radiography in the diagnosis of pulmonary disease since it can be performed stall-side relatively quickly without the need for transport to a referral center. The pleural surfaces, the pleural space, and the surface of the lung can be rapidly evaluated for mild to severe pathological changes. In some cases of thoracic disease, the extent of involvement of the pleural space and lung may be subtle and unable to be detected by auscultation, radiography, or percussion; but these changes may be readily detected by ultrasonography. Severity of pulmonary consolidation is better assessed with ultrasonography, and consolidation can be readily identified in horses with pleural effusion. The musculature of the thoracic wall, bony and cartilaginous continuity of the ribs, and the integrity of the diaphragm can also be evaluated.
Technique
The entire thorax should be examined in a dorsal to ventral direction from the third to seventeenth intercostal space (ICS). While clipping this area provides the best image, removal of hair is best reserved for discrete areas of pathology, if at all necessary, to maintain cosmesis. Acceptable acoustic contact can be achieved by thoroughly wetting the hair with isopropyl alcohol before slicking down the hair to remove trapped air.
The probe is placed in the ICS parallel to the ribs to maximize the acoustic window, beginning at the most dorsal aspect of the visualized lung and traversing down the ICS to past the level of the diaphragm to ensure the entire lung field and pleural space are imaged. The probe is generally placed perpendicular to the thoracic wall; however, the areas shielded by the ribs can be assessed during horizontal oscillations applied to the probe to sweep the ultrasound beam along this plane. Assessment of the cranial thorax (cranial to the third ICS) presents some difficulty as the triceps musculature covers this area. The probe may be applied to the triceps muscle with depth adjusted to allow pleural and pulmonary evaluation. Alternatively, the probe can be placed under the right triceps musculature with this limb protracted cranially, and the probe angled appropriately to image the cranial mediastinum.
Normal Anatomy
The cranial thorax encompasses the heart which is deep to the triceps musculature [1]. The cardiac notch of the lung allows visualization of the heart, extending from the third rib to the fourth ICS on the right side, and the third to sixth ribs on the left side [1]. The diaphragm (Figure 21.1) extends cranially to contact the heart, with the most ventral aspect reflecting caudally. Always identify the hyperechoic diaphragmatic musculature as it reflects on to the thoracic wall to provide visual separation between the lung fields and the abdominal viscera. This will ensure the correct location is ascribed to any fluid seen, and determines whether contact is occurring between the viscera and the lungs resulting from discontinuity of the diaphragm.
Examination of the left hemithorax caudal to the heart finds the left liver lobe deep to the diaphragm. Traversing caudally, the spleen will come into view, adjacent and medial to the left lobe of the liver. The stomach may be noted in cross-section deep to this interface alongside the splenic vein. The spleen is visible between the seventh and seventeenth ICS [2].
Examination of the right hemithorax ventrally, caudal to the heart, reveals the diaphragm and right lobe of the liver extending from the ninth to sixteenth ICS, with the large colon medial to the liver or in apposition with the diaphragm [2]. Dorsally, as on the left side, the lung fields are easily visualized moving across the screen in time with inspiration and expiration during breathing. Mid-thorax, deep to the diaphragm, the liver and right dorsal colon may be seen in close apposition. Caudally, between ICS sixteen and seventeen, the cranial aspect of the right kidney is visible [2].
The apposition of the parietal and visceral pleura may be noted as a hyperechoic interface with evenly spaced reverberation artifacts indicating a highly reflective interface (Figures 21.2, 21.3, 21.4). Motion of the pulmonary parenchyma can be seen synchronously with thoracic excursions during respiration.
An anechoic space from a small amount of fluid (up to 3.5 cm reported) [1] is present in the majority of horses over the right cranioventral lung field. This fluid creates separation of the parietal and visceral pleura. This may also be present in a lesser amount on the left side in these horses. This is not an indicator of a disease state, rather this is thought necessary for lubrication of the pleural surfaces.
Pathology
Thoracic Wall
Rib fractures (Figures 21.5, 21.6, 21.7, 21.8) are more readily detected by ultrasonography than radiography in neonatal foals [3]. Changes in continuity of the hyperechoic border of the ribs, as well as size and location of rib fragments, are visualized. Associated subcutaneous and intramural hematomas can also be detected and their extent assessed.
Pleural Abscess
Abscesses may form on the pleural surface (Figures 21.4, 21.9) and image as hypo- or hyperechoic, encapsulated structures.
Pleuritis
Uniformly aerated lung adjacent to the parietal pleura creates a hyperechoic gas echo and is responsible for the smooth appearance of healthy lung sliding rhythmically past the thoracic wall. With pleuritis (Figures 21.10, 21.11, 21.12, 21.13
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