Rodolfo Gialletti University of Perugia, Perugia, Italy Equine ocular ultrasonography (EOUS) provides a complete image of globe structures, regardless of opacities in the ocular media and eyelid swelling [1,2]. A feasible and easy to perform procedure that is safe and practical, EOUS can be performed in the standing horse, as sedation or local nerve block is required only in some cases [1,3]. EOUS should always be included in an ophthalmologic examination, especially when, in conditions such as corneal disease, uveitis, cataracts, or trauma, ocular opacities preclude adequate evaluation of posterior structures using ophthalmoscopy and slit-lamp biomicroscopy [1]. In human and veterinary medicine, ultrasonography (US) is crucial for investigating ocular opacities in conditions such as corneal disease, uveitis, cataracts, or trauma [4] and essential when screening candidates for cataract surgery and for diagnosing posterior segment diseases [1,5,6]. Additionally, US in veterinary medicine is a very useful diagnostic tool for many other ophthalmic conditions such as enophthalmos, buphthalmos, or exophthalmos, ocular protrusion, and suspected disparity in eyeball sizes [7,8]. In human beings, ocular US is remarkably important in emergency cases of acute vision loss and acute posterior ocular disease [4,9]. The Bedside Ocular Ultrasound is used by trained general physicians to diagnose retinal detachment with a high degree of accuracy; moreover, its features such as speed, non-invasiveness, and cost-effectiveness make it an ideal tool for busy emergency medicine clinicians [6]. US visualizes intraocular structures in case of loss of transparency and accurately distinguishes between diseases that need immediate ophthalmologic intervention (e.g., retinal detachment, vitreal hemhorrhage, etc.) and those that require only outpatient-type follow-up treatment [5,10]. Although clinical examination is the gold standard for diagnosing eye abnormalities, in case of ocular media loss of transparency, US becomes essential for diagnosis. Ocular surface and anterior segment pathologies, i.e., involving the eyelids, cornea, anterior chamber, lens, vitreous, and retina are recognized and diagnosed better by clinical examination but, in cases of ocular media loss of transparency (i.e., diffuse corneal edema or cataract), US examination is more reliable, thus becoming crucial for diagnosis [11]. The equine ocular ultrasonography technique diagnostic technique is easy to perform, non-invasive, pain-free, and usually well-tolerated. Although EOUS is usually conducted on the standing horse, the sonographer’s safety is ensured by sedating it, usually by administering α-2 agonists like xylazine hydrochloride(0.3–0.4 mg/kg IV) or detomidine hydrochloride (0.04–0.08 mg/kg IV) with or without butorphanol (0.01–0.02 mg/kg IV) in cases of trauma or severe pain[8]. Sometimes local anesthesia may suffice (oxybuprocaine hydrochloride 0.4% on the cornea) or a supra-orbital, auriculo-palpebral nerve block [12,13]. Even though eyelid hair does not usually need to be shaved, the supra-orbital area may need to be if images are required of the retro-bulbar space [14]. Corneal damage is a counter-indication to this approach [8]. A US scan of the eye and the orbital area can be performed by positioning the transducer on the eyelid (transpalpebral) or directly on the cornea (transcorneal).Introduction
Equine Ocular Ultrasonography Technique
Although transcorneal ultrasound provides best-quality images, it is not always well tolerated. Therefore, corneal anesthesia by instilling tetracaine solution 0.5% or proparacaine solution 0.5% into the eye is recommended.
An auriculo-palpebral nerve block can also facilitate the scan, particularly in cases with severe blepharospasm or ocular pain. A standoff pad must be used to assess the cornea and anterior chamber when using this technique.
Trans-palpebral ultrasound is easier to perform, and is usually well tolerated by the horse, but does, however, create more artifacts because of the eyelid-cornea interface [12,15]. A sterile ultrasound coupling gel is recommended for use with either technique. It should be applied directly to the cornea or the eyelid, to facilitate transducer contact with the eye. When the gel is applied directly to the cornea, the eye should be thoroughly washed after the scan [12], so as to prevent potential gel-related irritation. EOUS is not indicated or should only be performed with care when corneal perforation is imminent.
The retrobulbar space is scanned by positioning the US transducer on the eyelid or eye for in-depth eyeball visualization or by placing it over the supraorbital fossa to visualize features behind the eye. In some horses with masses in the retrobulbar space, scanning through the supraorbital fossa yields better images, which can be used to perform ultrasound-guided aspiration or biopsies of the retrobulbar masses [12,15].
The type of transducer or probe and the frequency are selected according to the features to be scanned. Frequency is inversely proportional to depth. Very deep tissue penetration is not usually required in EOUS, but high resolution images need to be obtained [16]. In clinical practice, the 10 MHZ frequency is usually selected as it penetrates about 3–4 cm of the anterior chamber. Lower frequencies (5–7.5 MHz) which penetrate to a depth of 6–10 cm are suitable for visualizing the posterior chamber and the retro-bulbar space [8,16,17]. Probes with greater frequencies are not used on horses. Convex or linear probes may be used as long as they are equipped with a small head [16,18]. To achieve optimal images, gains should be adjusted in accordance with the anatomical area to be scanned. Lowest gains should be used to visualize lesions and the highest for the vitreous and posterior chamber [12].
Color Doppler is used to assess eye and orbital vascularization and to determine whether the intra-ocular mass and retro-bulbar space are vascularized or not [17].
Whatever technique is selected, the approach must always be systematic. The eyeball should be fully scanned by placing the probe horizontally and moving it in a dorsal-ventral direction with a marker at 3 o’clock for the right eye and at 9 o’clock for the left (transverse scanning plane) and then placing the probe vertically for movement in the medial-lateral direction with a marker at 12 o’clock (sagittal scanning plane). Conventionally, markers are on the right of images [13]. Oblique scanning planes are used for in-depth exploration in particular cases [19]. Remember that exerting undue pressure while scanning will deform the eyeball. Both eyes ahould be scanned so as to compare whatever measurements are taken and to assess any sub-clinical abnormalities in the apparently healthy eye [8].
When patient and equipment are ready for the scan, the eye should be methodically scanned in real time, usually moving from one anatomical landmark to the next in this order: cornea, anterior chamber, iris and ciliary body, crystalline lens, vitreous, retina, choroid, sclera, and extra-ocular areas [13].
US scans may be performed in (Figure 24.1) A-mode or, more commonly in clinical practice, in B-mode.
EOUS in A-Mode
Although EOUS in A-mode is rarely used, it provides precise measurements and is useful for biometric US studies [18].
Images obtained with EOUS in A-mode show four vertical peaks, with the height of each representing echo intensity. The horizontal axis shows the distance between the anatomic feature and the probe [17]. Peak morphology produces typical images of the tissues the ultrasound passes through and the morphology changes in the presence of pathogens.
Converting the space between two peaks into distance provides measurements of the anterior-posterior eyeball axis, the depth of the two chambers, and the axial crystalline thickness [16].
EOUS in B-Mode
The B-mode two-dimension (2-D) modality is the simplest, fastest, and most used technique, as it provides 2-D images in real time. As in all US images of body parts, the ocular US distinguishes images as anechoic, hypoechoic, and hyperechoic [17].
Overall, a healthy eye appears as an oval image with four clear hyperechoic structures. Moving from the anterior to the posterior they are the cornea, iris, posterior crystalline capsule, and the retina/choroid/sclera complex, which physiologically cannot be detected separately on US images. These structures border the eye chambers and the crystalline nucleus which are hypoechoic. The anterior and vitreous chambers are visualized but the posterior chamber is usually invisible [15,17]. Other, less echoic structures include the iris, the corpora nigra, the optic nerve, peri-orbital fat, and extrinsic eye muscles.
In a trans-eyelid scan, the eyelids appear as a hyperechoic layer adhering to the probe.
Measurement taken during a US scan can be compared with reference values and the healthy eye values.The eyeball should have a diameter of 39.4 ± 2.3 mm, and a slightly larger vertical diameter (42.5 ± 3.6 mm) [19,20].
