TWENTY-FIVE: Ultrasonography of the Eye and Orbit

CHAPTER TWENTY-FIVE
Ultrasonography of the Eye and Orbit

Caryn E. Plummer¹ and David J. Reese²

¹University of Florida, Gainesville, FL, USA

²College of Veterinary Medicine, Murdoch University, Murdoch, WA, Australia

Ultrasonography as an examination and diagnostic tool in human and veterinary ophthalmology has been in use since the 1960s [1,2,3]. With development of portable ultrasound units, ultrasonography has grown significantly in many aspects of equine practice [2]. Strikingly, despite ease of use and availability, ultrasonography has not routinely been employed for examination of the eye and orbital structures in horses. As a non-invasive imaging technique that can be performed in the standing horse, ultrasound can provide invaluable qualitative and quantitative information about intraocular and orbital structures [4,5,6,7,8,9]. Ocular ultrasonography is indicated in cases of corneal edema, cataracts, or intraocular hemorrhage, because the normally clear ocular media has become opaque and visualization of the interior structures of the globe is restricted. It is indicated in ocular trauma and when eyelid swelling prevents access to the globe. Ultrasonography may also provide biometric data, such as absolute and relative size and positions of ocular and orbital structures [8,9,10,11]. Measurement of the fetal eye size has even been used as a tool for predicting parturition in horses [12]. Ultrasound measurements of preoperative axial globe length is an important criterion for the calculation of diopter strength of artificial lens implants when performing lens replacement surgery [13,14,15].

Technological advancements have introduced various methods of ultrasonography to the field of ophthalmology. The original form, known as amplitude modulation (A-mode), displays echoes received by the transducer in one dimension and relative to time as vertical amplitudes from a baseline [3,5]. The height of each spike represents the intensity of the echo and the spacing of the spikes represents the spatial distribution of the structures being examined. This technique is the method of choice for in vivo measurements of intraocular distances such as depth of anterior and vitreal chambers, lens thickness, and axial globe length. Vector A-scan can be combined with brightness mode (B-mode) ultrasound to provide information on both the topography of a lesion (B-mode) and objective information on the lesion size and character (A-mode) [4]. The most commonly employed method of ultrasonography in clinical ophthalmology, however, is B-mode using a 10 MHz transducer with a 3–4 cm focal range and a small scan head [4,5]. Lower-frequency probes, such as 5 MHz and 7.5 MHz, may also provide adequate images and will image deeper tissues for examination of retrobulbar lesions. Color Doppler technique is used to visualize blood vessels and their flow characteristics within the eye and orbit. Normal reference values for blood velocity and vascular resistance patterns have not yet been determined in the equine eye, so comparing these parameters between seemingly normal and diseased eyes is not possible. This tool is helpful, however, in determining if a structure, such as an intraocular or orbital mass lesion, is vascular or avascular.

The latest form of ultrasonography to be employed for diagnostic purposes in the veterinary patient is high-frequency ultrasound biomicroscopy [16,17]. This method is similar to traditional B-mode ultrasonography but uses frequencies between 50 and 100 MHz. With this technique, tissue resolution is dramatically improved, permitting visualization and discrimination of structures as small as 50 µm, nearly on par with microscopic resolution [16,17]. However, tissue penetration is limited to depths of only 4–5 mm and the horse must be heavily sedated or placed under general anesthesia to avoid any movement and potential artifact. In the horse, this typically permits examination of the cornea, sclera, iridocorneal angle, and part of the anterior chamber. The clinical value of the technique in the horse eye is in determining depth and extent of lesions (e.g. corneolimbal squamous cell carcinoma) in the cornea and rostral sclera, especially prior to surgical intervention [4,16,17]. Transducers with frequencies between 25 and 50 MHz are available and allow examination of the anterior chamber. As with traditional B-mode transducers, these lower-operating-frequency transducers have a lower resolution compared to the higher-frequency transducers. However, these transducers are less prone to artifacts and may be used in standing, sedated animals.

Ultrasonographic Technique in the Normal Eye

Ocular and orbital ultrasonography is usually easily performed in the standing horse following sedation, an auriculopalpebral nerve block, and the application of topical anesthetic (e.g. proparacaine, tetracaine) to the ocular surface. The examination can be performed by placing the transducer directly on the cornea or by scanning through the closed eyelids [1,2,3,4,5,6,7]. The best possible image of the globe and orbit is achieved by placing the transducer directly on the cornea with coupling gel as a standoff medium. Scanning through closed eyelids or with a standoff device may produce sound attenuation and, therefore, may require an increase in gain setting. However, it facilitates examination of the anterior segment and is recommended in cases of corneal injury, ocular trauma, or following intraocular surgery to avoid further damage to the cornea. Standard ultrasound coupling gels should be avoided during examination of the eye and orbital structures since they can cause ocular surface irritation. Sterile methocellulose gel, such as K-Y jelly, is preferred and should be irrigated from the eye upon completion of the examination.

The eye is routinely examined in both vertical and horizontal planes through the visual axis by initially using the lens as a landmark for orientation. Once the examiner is oriented, the transducer is carefully and slowly directed off of the central axis, either rostrolateral or dorsoventral. The meridian between 12 o’clock and 6 o’clock is examined in the vertical axis, while the meridian between 9 o’clock and 3 o’clock is examined in the horizontal axis. In each plane, the transducer should be slightly tilted both dorsally and ventrally to examine the most peripheral portions of the spherical globe. Oblique positioning can sometimes add useful additional information. When orbital lesions are a concern, the transducer can also be positioned dorsal to the zygomatic arch and over the supraorbital fossa [4]. If the fellow eye in a patient is normal, it can serve as control for comparison purposes.

As with other structures of the body, ultrasonographic images of the eye are described as anechoic, hypoechoic, or hyperechoic (Figure 25.1). Four main ocular acoustic echoes are generated within the normal eye. These hyperechoic structures are the anterior cornea, anterior lens capsule, posterior lens capsule, and retina/choroid/sclera (as a single entity since these structures cannot normally be differentiated from one another with standard ultrasonography) [1,2,3,4,5,6,9]. Additionally, more hypoechoic, echogenicities can be generated by the iris, corpora nigra, ciliary body, optic nerve, orbital fat, and extraocular muscles [1,2,3,4,5,6,9]. The corpora nigrum typically appears as a well defined hyperechoic structure protruding into the anterior chamber at the pupillary margin. The anterior, posterior, and vitreal chambers, as well as the lens cortex and nucleus, are normally anechoic. The optic nerve head is typically hyperechoic, while the remainder of the nerve as it courses posterior is a relatively hypoechoic structure [1,2,3,4,5,6,9]. The orbital muscle cone, as it extends posterior from the equator to converge at the orbital apex, is a hyperechoic structure surrounding the optic nerve [1,2,3,4,5,6,9].