F. ter Woort
Ability to recognize and interpret the abnormal images found on thoracic ultrasound evaluation of the horse.
A probe with a frequency between 6–14 MHz should be selected. In most adults and foals the pleural surface can be adequately imaged with a linear probe. The large curvilinear probe (2–6 MHz) is sometimes needed to penetrate deeper if extensive lung pathology is present. For the cranial mediastinum, the large curvilinear probe is needed. Horses with a sport clip frequently don’t require additional clipping, but horses with thick hair will need clipping. The hair and skin are saturated with alcohol and then ultrasound gel is applied.
Evaluation of the normal lung is limited to the pleural surface, since the ultrasound beam is unable to penetrate the air within the lung. It’s important to realize that minimal peripheral air is required to obscure underlying pathology. As such a “normal lung ultrasound” really means “normal pleural surface” and doesn’t exclude deeper pathology. The cranial mediastinum can be imaged from the right side in the horse, ether angling the probe cranial under the triceps muscle from the 3rd intercostal space (this requires the horse to be cooperative and stand with the right leg forward), or through the triceps muscle over the level of the 3rd intercostal space. In the normal mediastinum, an echoic mediastinal septum can be seen and in some case heteroechoic pericardial fat can also be present.1
Scan for free pleural fluid (>3.5 cm of fluid is considered abnormal) note the depth and high on the chest for monitoring, and check for a lack of normal gliding motion between the parietal and visceral pleura. Although pneumothorax is uncommon in adult horses, ensure the air is contained within the lung and not the pleural space. Pleural irregularities will cast an acoustic shadow (“comet tail”), and this is a very non-specific finding which can be associated with a pleural scarring, viral or bacterial infection.
Scan for anything that is not air, this includes hypoechoic lung tissue, anechoic fluid with or without echoic gas bubbles characteristic of pulmo-abscesses.
In addition to the abnormalities described above, foals are more prone to rib fractures and pneumothorax.
Ultrasound evaluation is more sensitive then radiographs at detecting rib fractures.2 For this, a high frequency linear probe is used to evaluate the surface of each rib, scanning for a lack of continuity in the surface.
Pneumothorax (often as a result of a rib fracture), can be imaged in the dorsal aspect of the thorax, although if the foal is lying down this may be altered. The air contained within the normal pleura moves rhythmically with respiration, so scanning carefully from dorsal to ventral, a transition can be seen between dorsal still air and ventral moving air within the lung. In addition, if pleural irregularities are present, these will cast small acoustic shadows, which are useful to identify air that is within the pleura.
Pulmonary abscessation is an important component of Rhodococcus equi pneumonia. Ultrasonography has been used as a screening tool for early detection of infection and treatment with antimicrobials has been instituted in foals with pulmonary abscesses. However, not all foals with pulmonary abscesses require antibiotics. A first study showed that foals for which the sum of the diameter of the abscesses was 10 cm or less, recovered without treatment.3 A second study showed that foals that developed clinical signs and required treatment had diameter sums of 20 cm or more.4
In conclusion, thoracic ultrasound evaluation provides valuable information that affects the patient’s treatment plan. However, it’s important to interpret these findings with solid knowledge of the limitations of thoracic ultrasonography.
1. Reef VB, Whittier M, Allam LG. Thoracic ultrasonography. Clin Tech Equine Pract. 2004;3(3):284–293.
2. Jean D, Picandet V, Macieira S, Beauregard G, D’Anjou MA, Beauchamp G. Detection of rib trauma in newborn foals in an equine critical care unit: a comparison of ultrasonography, radiography and physical examination. Equine Vet J. 2007;39(2):158–163.
3. Venner M, Rödiger A, Laemmer M, Giguère S. Failure of antimicrobial therapy to accelerate spontaneous healing of subclinical pulmonary abscesses on a farm with endemic infections caused by Rhodococcus equi. Vet J. 2012;192(3):293–298.
4. Chaffin MK, Cohen ND, Blodgett GP, Syndergaard M. Evaluation of ultrasonographic screening methods for early detection of Rhodococcus equi pneumonia in foals. J Equine Vet Sci. 2012;32(10):S20–21.