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AAEP FOCUS ON AMBULATORY PROCEEDINGS / 2015 Applied Ophthalmic Anatomy and How to Describe Ocular Lesions Chelsey Miller, DVM Author’s address— Iron Will Mobile Veterinary Services, P.C., 2445 Vaughn Lane, Burlington, NC 27217; e-mail: [email protected] identification of non-displaced fractures with no associated external trauma Globe movement results from the action of six extraocular muscles (medial rectus, lateral rectus, dorsal rectus, ventral rectus, and dorsal and ventral oblique). The retractor bulbi muscle is a muscular cone with multiple, large muscle bellies that attach to the posterior half of the globe. Additional ocular muscles control the movement of the eyelid, most relevant to the equine clinician being the large orbicularis oculi muscle. The orbicularis oculi muscle is the major muscle that closes the eyelids and akinesia (cessation of voluntary movement following peripheral anesthesia) of this muscle is critical when evaluating an injured eye where the integrity of the globe is compromised, performing diagnostic procedures, placing a SPL catheter, and during standing surgery of the globe. Depositing a small volume of an anesthetic solution subcutaneously using a small gauge (26- or 25-gauge) needle as the nerve passes over the zygomatic arch, caudal to the bone process of the frontal bone, will anesthetize the palpebral branch of the auriculopalpebral nerve and facilitate opening of the upper eyelid. While the auriculopalpebral nerve can be anesthetized at multiple sites as it courses from the base of the ear towards the dorsal (superior) palpebral, the location where the nerve lies perpendicular to the zygomatic arch is the easiest to palpate and horses typically do not resist if placement of the needle is swift. Using a slip-tip syringe and placing the needle prior to attaching the syringe facilitates injection. I. INTRODUCTION B oth normal and abnormal anatomy play important roles in ophthalmic diagnoses. Unlike those of many medical disciplines, however, most ophthalmic diagnoses are made on the basis of anatomical observations. In this lecture, the anatomy of the equine eye and associated structures will be reviewed with emphasis on clinically relevant elements. II. MATERIALS AND METHODS Ocular Adnexa The adnexa of the eye include the bony orbit, eyelids, extraocular muscles, conjunctivae, lacrimal glands, and nasolacrimal duct system. In various ways these structures protect the globe and provide a first line of defense to the horse’s environment. These are the ocular structures that a veterinarian first appreciates when meeting a horse, even prior to pursuing a complete ophthalmic examination. The equine orbit is made up of six bones (lacrimal, zygomatic, frontal, sphenoid, palatine, and temporal) and is completely enclosed. Specifically relevant to the ophthalmic examination is the supraorbital fossa or foramen in the frontal bone, where the supraorbital nerve emerges. This nerve provides sensation to the upper eyelid and it is often helpful to anesthetize the nerve at this location to facilitate manipulation of the eyelid or place a subpalpebral lavage (SPL) catheter in the upper eyelid. The eyelids are important not only as they serve a protective function to the globe, but also as an indicator of discomfort in the horse. The upper eyelid is more mobile than the lower eyelid, and thus injury to the upper eyelid can present a greater risk to proper function of the eyelids. Eyelashes or cilia are present in greatest number on the lateral two thirds of the upper eyelid. Vibrissae (long, tactile hairs) are located dorsonasal to the upper lid and ventral to the lower lid. The horse has a large nictitating membrane or third eyelid, situated at the medial canthus. The third eyelid moves laterally in a horizontal and slightly dorsal action across the globe. The prominent bony rim of the orbit is formed by the frontal process of the zygomatic bone, and the zygomatic processes of the frontal and temporal bones. It is important to pay particular attention to this area when evaluating a horse following trauma to the head as the dorsal orbital rim and zygomatic arch are at greatest risk of fracture. Even if the hair and dermis overlying these structures appear intact, careful palpation and sonographic examination may reveal a fracture. Subtle fractures are most easily diagnosed using ultrasonography as the anatomy and complicated structures of the skull preclude Lacrimal and Nasolacrimal System The horse’s major lacrimal gland lies beneath the dorsolateral orbital rim and is innervated by a combination of sympathetic and parasympathetic nerve fibers from the lacrimal branch of cranial nerve (CN) VII. At the base of the third eyelid, the horse 1 AAEP FOCUS ON AMBULATORY PROCEEDINGS / 2015 Posterior Segment has a serous nictitans gland, which is innervated by parasympathetic nerve fibers of CN IX. Two lacrimal puncta are located 8-9 mm from the medial canthus on the upper and lower eyelids. A canaliculus from each punctum join to form the lacrimal sac, which is the proximal dilation leading into the long nasolacrimal duct. The nasolacrimal duct terminates in the lower punctum in the ventromedial aspect of the nostril, near the mucocutaneous junction. Multiple nasal punta are common findings in the horse. Often a single puntum lead to the naso lacrimal duct, while the others are blind pouches not connected to the nasolacrimal duct. This is important to keep in mind when diagnosing and/or treating a blocked nasolacrimal duct as the clinician may have to flush each punctum in order to determine which one connects to the nasolacrimal duct. The lens in the horse is very large and it is common to identify prominent lens sutures (normal finding). The lens in its entirety can only be evaluated following complete dilation. In the posterior segment of the eye, the vitreous should be transparent and uniform in appearance. Abnormalities in the vitreous include loss of uniformity with debris and fibrin membranes, which can lead to a change in the tapetal reflection to a yellow color. The tapetum is located in the dorsal choroid and is usually green to blue in color. Small dark dots throughout the tapetal fundus are end-on choroidal capillaries called stars of Winslow. The ventral fundus is the non-tapetal area and is usually dark brown. Both the tapetal and non-tapetal areas can have variable appearances depending on the iris and coat color. Globe The optic disc is oval, salmon pink, and located in the nontapetal fundus. Radiating from the periphery of the optic disc are 30-60 small arterioles and venules. The horse has a paurangiotic retina meaning that it is partially vascularized by the vessel radiating from the optic nerve disc. The remainder of the retina is avascular and obtains nourishment from the underlying choroid. Anterior Segment The cornea is the transparent anterior portion of the fibrous tunic of the globe. Due to its avascular nature, the cornea relies on the aqueous humor and tear film for nourishment and immune surveillance. The thickness of the equine cornea ranges from 770 μm1 peripherally to 793 μm2 centrally. Histologically, the cornea has three primary layers: epithelium (lipophilic), stroma (hydrophilic), and endothelium (lipophilic). The alternating lipophilicity of the corneal layers is useful in identifying corneal ulcers and has implications for drug penetration into the cornea. The conjunctiva is continuous with the corneal epithelium and consists of three primary regions: palpebral conjunctiva, bulbar conjunctiva, and fornix. The conjunctiva is highly vascular and variably pigmented. The conjunctiva functions primarily to prevent desiccation of the cornea and to provide a barrier against microorganisms and foreign bodies. III. DISCUSSION Ocular Lesions Ophthalmology is a visual science that demands a detailoriented approach. The correct diagnosis often relies entirely on abnormalities from the clinical examination. Ruling out possibilities on the clinician’s differential diagnosis list often depend solely on the clinician appreciating the differences between deep and superficial corneal vasculature, buphthalmia and exophthalmia, or corneal cellular infiltrate and fibrosis. The clinician must not only understand the difference between these abnormalities and the significance of them, but possibly more necessary is the clinician’s ability to “see” them. Critically thinking about every structure while scanning the eye and conducting the ophthalmic examination by evaluating the “layers” of the eye is a good start to avoid missing key abnormalities. Once a lesion is identified, noting the shape, surface, margins, color, distribution/location, and size will help guide the clinician towards a morphologic diagnosis. Posterior to the cornea is the anterior chamber, which is filled with aqueous humor produced by the ciliary body. The visible part of the uveal tract is the iris. Most horses have a dark brown to golden color, while blue, white, and heterochromia iridis are more common in certain breeds and coat colors. The horizontally oval pupil of the horse becomes more circular following dilation due to the great vertical pull of the dilator muscle.3 Prominent corpora nigra (CN) are located on the dorsal pupillary rim of the equine pupil and are thought to provide “shade” to the eye especially when the horse is grazing.3 Evaluating the CN and comparing within one eye and between both eyes is important when evaluating for signs of equine recurrent uveitis (ERU) in a quiet, non-painful eye as atrophied CN in the affected eye is a common finding in eyes with ERU. A useful characteristic of the equine eye is that the iridiocorneal angle (ICA) can be directly visualized medially and laterally. 3 Even though the ICA is easily evaluated in the horse, it is often overlooked during the ophthalmic examination and the clinician should take notice of any abnormalities that could provide a key to previous ocular inflammation. REFERENCES 1. 2. 3. 2 Ramsey D, Hauptman J, Petersen-Jones S. Corneal thickness, intraocular pressure, and optical corneal diameter in Rocky Mountain horses with cornea globosa or clinical normal corneas. Am J Vet Res 1999;60:1317-1321. Woerdt Avd, Gilger B, Wilkie D, et al. Effect of auricolopalpebral nerve block and intravenous administration of xylazine on intraocular pressure and corneal thickness in horses. Am J Vet Res 1995;56:155158. Samuelson D: Ophthalmic anatomy. In Gelatt K, editor: Veterinary Ophthalmology. Ames: Blackwell, 2007:37148.