Introduction

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].

Equine Ocular Ultrasonography Technique

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).

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.

Cornea

Both the trans-eyelid and trans-corneal approaches visualize the cornea, though with the latter, pad use is recommended. The cornea appears as a fine, regular curvilinear hyperechoic line (corneal epithelium), an anechoic area (corneal stroma), and an unbroken, 2.33 ± 0.39 mm thick, hyperechoic line which should not vary in the entire cornea (corneal endothelium) [12,19,20].

The most common corneal abnormalities are:

• Corneal Edema
  
• Corneal Infiltrates
  
• Stromal Abcesses
  
• Corneal Ulcers

Anterior and Posterior Chambers

The anterior chamber is bordered by the cornea, iris, ciliary body, and the anterior surface of the crystalline lens. The posterior chamber, lying between the iris and the anterior lens surface, appears as a fine anechoic line which is often hard to visualize in the horse. The complex measures about 4.22 ± 1.29 mm [12,19,20]. Since it contains acqueous humor, it appears uniformly anechoic in physiological conditions, providing a good contrast for identifying and examining other ocular structures. Echoes in these chambers must be considered as abnormal as they indicate increased cellularity in the acqueous humor. During the scan, no undue pressure should be applied to the probe so as to avoid deforming the chamber. Pathological findings in the chambers include:

• Blood (ifema)
  
• Fibrin
  
• Pus (ipopion)
  
• Synechiae, adhesions between the cornea and the iris or crystalline lens, which are usually linked to chronic inflammation
  
• Changes in depth (whether more or less) due to lens dislocation or subluxation, uveitis, and/or glaucoma or trauma – related anterior chamber collapse so that the cornea overlies the crystalline lens
  
• Foreign bodies
  
• Intra-ocular mass in the anterior chamber [11].

Iris and Ciliary Bodies

In a standard EOUS scan, the iris and ciliary bodies appear as a single irregular hyperechoic structure extending from the periphery to the center, posterior to the anterior chamber and lateral to the crystalline lens. The corpora nigra are visualized as irregular hyperechoic structures on the anterior edge of the iris that protrude into the anterior chamber. Cysts, appearing as spheres with an anechoic content, may occasionally be detected on the iris and/or the ciliary bodies (Figure 24.32). Other common findings are a rounded iris, due to previous inflammation and/or post-uveitis glaucoma [8], crystalline lens dislocation or subluxation, and neoplasias (adenoma, adenocarcinoma, melanoma). Iris thickness is 2.5 ± 0.66 mm; ciliary body thickness is 3.99 ± 1.13 [19].

Crystalline

The crystalline or lens includes an anechoic nucleus and anterior and posterior surfaces that appear on a US scan as two hyperechoic curvilinear bands, one convex and the other concave. Often both bands are not visible on one scan. As the crystalline lens is roundish in shape, only areas perpendicular to the suspensory ligaments can be visualized [1], so the probe needs to be rotated during the scan for its complete visualization. Lens dimensions: antero-posterior 11.93 ± 1,10 mm; diameter about 20 mm [12,19,20]. The most common EOUS findings are:

• Variations in dimensions (due to cataract, lens resorption, trauma-related rupture)
  
• Changes in position (dislocation or subluxation)
  
• Changes in echoic status (cataract)
  
• Lens rupture [11].

The Vitreous Chamber

The vitreous chamber is bordered anteriorly by the posterior surfaces of the iris/ciliary bodies and the crystalline lens and posteriorly by the retina/choroid/sclera complex. Containing vitreous humor, it measures approximately 17.37 ± 1.98 mm [12,19,20].

Its most common pathologies are:

• Variation in dimensions (posterior crystalline lens dislocation)
  
• Extraneous material (blood due to trauma, fibrin due to infection or inflammation, vitreous degradation, asteroid hyalosis, lipid and calcium-containing spheres that are visualized as echoic granules)
  
• Retina detachment
  
• Mass or neoplasia
  
• Change in echoic status (e.g., due to uveitis)
  
• Posterior vitreous detachment

Increasing the gains is often needed to achieve better images of the vitreous chamber and visualize abnormalities like, for example, vitreous membranes, which are fibrous layers that develop after inflammation and clot formation [17].

Retina/Choroid/Sclera

In healthy eyes the retina, choroid, and sclera cannot be distinguished separately on a US scan. Altogether, they form the posterior eye segment, appearing as a hyperechoic circular concave line that borders the vitreous. Retina detachment is the most common abnormality in this segment. Choroid detachment may be found but is extremely hard to visualize.

Choroid melanoma may protrude into the vitreous.

Retrobulbar Area

Since visualization of this region requires tissue penetration to a depth of 6–8 cm, lower frequency probes are used [8]. Clearly visible on US images are the optic nerve, extraocular muscles, retrobulbar fat, and the optic disc.

The optic nerve appears as hyperechoic triangle posterior to the optic disc. The extraocular muscles, which surround the eyeball and the optic nerve, are slightly less echoic than the optic nerve. Fat appears as heterogeneous echogenicity. At further depth, bone is visualized as a smooth hyperechoic surface with a typical bone US pattern.

The most common abnormalities in this site, and the reason why it is usually scanned, are retrobulbar mass or abscess, and bone trauma to the eye cavity.

Principal Diseases of the Equine Eye and Appurtenances. US Scanning

Corneal Edema

Corneal edema, a symptom of many eye diseases, develops due to injury to the endothelium or, in some cases, the epithelium. Even though uveitis is its main cause, corneal edema may be consequent to trauma, glaucoma, surgery, keratitis, and aging. In EOUS images, the cornea appears thickened, with hypoechoic central and stromal layers. The endothelium, its deepest layer, appears as a broken line with alternating hyperechoic and anechoic areas (Figures 24.2, 24.5).