Equine Ocular Ultrasonography

Equine Ocular Ultrasonography

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].

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.

Figure 24.1 Normal eye. (A) This image is a transpalpebral longitudinal scan of a normal eye. The cornea (1) appears uniformly echogenic. The anterior chamber (yellow arrow) appears uniformly anechoic. The iris leaflets (2) appear as echoic linear bands continuous with the ciliary body (4) immediately posterior. The corpora nigra (3) are highly variable in structure and sonographic appearance, but are often seen protruding into the anterior chamber on longitudinal images. Between the iris/ciliary body and the posterior wall of the eye, the only echogenic structures are the portion of the anterior (7) and posterior lens capsule (5) that fall within the primary ultrasound beam. The vitreous is uniformly anechoic. The retina, choroid, and sclera (9) appear in combination as a hyperechoic band defining the posterior aspect of the globe. Reverberation artifacts (6), reduplication, or multiple-signal echoes are commonly seen. These most commonly occur due to lack of ultrasound gel resulting in air becoming trapped between the hairs. 1: Cornea; 2: Iris; 3: Corpora nigra; 4: Ciliary body; 5: Posterior lens capsule; 6: Reverberation artifacts in the anterior chamber; 7: Anterior lens capsule; 8: Eyelid; 9: Retina/choroid; 10: Optic nerve; extraocular muscles (arrowheads); retrobulbar fat (asterick). (B) This image is ultrasound examination of a normal eye with a standoff pad. Ultrasound transducer placement for transpalpebral examination of the equine eye in the horizontal plane with the use of an ultrasound standoff pad (11). The use of the standoff device allows us to make a better evaluation of the most superficial structures such as the eyelids, cornea, and anterior chamber, as can be compared from the previous pictures without a standoff pad. 1: Cornea; 2: Iris; 5: Posterior lens capsule; 7: Anterior lens capsule; 8: Eyelid.

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].


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].


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].


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


Keratitis is an inflammation of the cornea. It may be dry, or eosinophilic and ulcerative. It is caused by bacteria, viruses, fungi, trauma, or other external agents. Corneal layers are hard to distinguish separately on a standard EOUS scan with the usual frequency.

Stromal Abscess and Corneal Infiltrates

Corneal abscesses are often due to ulcers or small corneal lesions that do not heal, become infected with foreign bodies or bacteria, and persist in the stroma. Abscesses may also be secondary to uveitis. In cases of stromal abscesses or corneal infiltrates, US images show a thickened, widely hypoechoic cornea. Use of a high frequency probe (over 10 mHz) aids imaging (Figures 24.3, 24.4, 24.8, 24.9).

Corneal Ulcer

This lesion to one or more corneal layers is caused by environmental agents (foreign bodies, etc.), lack of lacrimal film (dry eye, kerato-conjunctivitis, etc.), or continual contact with eyelashes and eyelid hair (entropion).

An EOUS scan is not indicated in cases of corneal ulcer. Although it may be helpful if severe eyelid edema prevents direct eye examination, it should be performed with great care so as not to cause any further injury. A pad should be used. Images show a thickened, irregular, pitted corneal surface (Figure 24.6).

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).

Figure 24.2 Corneal edema. This image is ultrasound scans of both eyes of the same horse. (A) The right eye has corneal edema and (B) the left eye has normal anatomical structures. Corneal edema is visualized in the ultrasound image as well as the picture. The ultrasound shows the increased corneal thickness (A), also in relation to the healthy eye (B). The edema does not allow visualilzation of the rear portions of the eye, but with ultrasound, we can evaluate all the anatomical structures of the eye as well as the structures of the posterior district. Miosis is also observed in the diseased eye (A) compared to the healthy eye (B). 1: Cornea; 2: Iris; 3: Posterior lens capsule; 4: Ciliary body; 5: Corpora nigra; 6: Anterior lens capsule (yellow arrows).

Figure 24.3 Keratitis and stromal abscess. This image is from an 8-year-old quarter horse with keratitis and a stromal abscess. The ultrasound examination allows us to evaluate the structures beyond the cornea which is very thickened and hinders any visual evaluation. The chronicity of the lesion is perceived with the formation of adhesions between iris and cornea (5) and between iris and lens (6) with the pupillary block in miosis. These are important indications for choosing therapy and assessing prognosis. 1: Cornea; 2: Iris; 3: Corpora nigra; 4: Posterior lens capsule; 5: Irido-cornea adhesion; 6: Irido-anterior lens capsule adhesion.

Figure 24.4 Stromal abscess and anterior uvetitis. These images are from a 3-year-old Thoroughbred with a stromal abscess and anterior uveitis. (A, B, and C) The stromal abscess is visible within the yellow circles and exhibits the typical appearance of non-uniform hypoechoic thickening with hyperechoic areas. (D and E) The normal eye (D) and diseased eye (E) are shown together for comparison. The thickening of the cornea (1) is evident in (E). We have the ultrasound of the diseased eye with the ultrasound comparison between the eye with an abscess (E) and the healthy eye (D). This condition is typically accompanied by significant anterior uveitis, which is evidenced here by miosis (A, B, and E) and fibrin within the anterior chamber (yellow arrowheads). In the first three ultrasounds, the effect of cycloplegic therapy with miosis in A) and B) and mydriasis in C) can be evaluated. 1: Cornea; 2: Iris; 3: Corpora nigra; 4: Posterior lens capsule; 5: Fibrin in the anterior chamber; 6:= Ciliary body.

Figure 24.5

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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Equine Ocular Ultrasonography
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