Equine/Bovine Sonographic Evaluation of the Orbital Space and Eye
Ruffin’s Ruminations: Equine/Bovine Sonographic Evaluation of the Orbital Space and Eye
Ultrasound is an excellent, if under-utilized modality for imaging the equine or bovine eye. While you may not see a daily caseload of trauma to the periorbit, adnexa, or globe, when you do, a convex or micro convex probe can be used to provide key information relating to the structures involved, the severity of the injury, and prognosis. Cases that may benefit from this include chronic swelling of the eyelid or adnexal structures, foreign bodies, retrobulbar abscesses, lens luxation, retinal detachment, and uveitis.
For any procedures involving an eye, especially one that is painful and/or non-visual, sedation is important to getting a good quality image. In some painful cases, local anesthesia may also be necessary. These issues are better discussed in other publications, but I find the use of Dormosedan (detomedine HCl) and a palpebral nerve block adequate for most needs. A head stand can also make the process much easier, as it will steady the patient’s head and minimize risk to a client, technician, or student that may otherwise be placed in danger while holding the head.
One of the most important things to keep in mind is that alcohol can be irritating to the adnexal structures and toxic to the cornea, so in these patients, try to limit imaging media to ultrasound gel. Once you have applied lubricant to the upper eyelid, apply your probe to the lid. Due to the close proximity of all of the structures involved, the C20 probe is best suited here, but the C60 or Convex Rectal probes can provide serviceable images. Higher frequencies will provide the ideal image quality in this scenario, and due to changes in tissue densities, you’ll likely need to adjust your IP, Gain, and Far-field Gain settings to compensate for attenuation. Typically, you’ll want to set your depth in the neighborhood of 8 cm for deeper structures (retrobulbar foreign bodies, abscesses, retinal detachment, etc) and as close as possible (3-5 cm) for evaluating the adnexa, lens, anterior chamber, or zygomatic arch.
Once imaging the orbit and its associated structures, you’ll be visualizing a top-down cross-section of the space. I usually start perpendicular to the orbit and aim to hit where the optic nerve should sit. For consistency, the notch on the probe is placed rostrally. From there, rolling vertically and panning left to right provides a complete image of the orbit, globe, and adnexa. If you’re using the C60 or Convex Rectal probes, you may notice some bone shadowing at the margins of your screen. You can move your probe rostrally and caudally to image underneath the bony structures if this is the case.
To image the lens and anterior chamber, a 5 mm silicon pad standoff can be useful, as can decreasing focal depth and imaging depth. With a standoff, you’ll first notice the skin and eyelid, which will be dense and grey. A more closed-textured and more echogenic line will appear next. This is the cornea. Behind that, the anterior chamber will be an anechoic area, and then two moderately echogenic and intimately associated structures (the iris and lens). Without a standoff, typically you’ll lose most of the eyelid and associated adnexa and will see the anterior chamber. In the abnormal anterior chamber, you may see dense, gravity-dependent material (suppurative material), flocculant strands that resemble seaweed in a current (fibrin), and the lens if it is luxated. In the posterior chamber, you’ll normally see an anechoic structure usually 2-3 cm in diameter. There may be the occasional echogenic spot even in normal eyes. These are typically just artefacts due to the density changes between the vitreous and the aqueous humors. Again, you may see fibrin or suppurative material. One of the more important structures is the retina, should it be detached. In this case, you’ll see it most clearly with a micro convex probe like the C20. It will appear as an irregular relatively hyperechoic line in the medial or interior aspect of the globe. The surrounding adnexa should appear to have a fairly close texture but may become less homogenous and less echogenic with swelling and edema. Immediately medial to the globe will be a 3-5 mm structure that appears hyperechoic and has a linear texture. This is the optic nerve and sheath. The amount of detail in this structure is highly dependent on focal depth, frequency, and how cooperative your patient is. -Ruffin Hutchison, DVM