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Corneal Ulceration Diagnosis Andtreatmentof Promote Healing Veterinary NURSING EDUCATION Diagnosis and Treatment of Corneal Ulceration Sara Grabiel-Duncan, BS, LVT, VTS (Ophthalmology) Anatomy gland, which contributes 35-50% of LEARNING OBJECTIVE: The cornea is composed of 5 distinct layers: tears and the gland of the nictitating Readers should gain a basic membrane which contributes 35-65% understanding of the anatomy 1. Pre-corneal tear film—the most of the aqueous component of tears. and physiology of the cornea superficial (outermost) layer of the cornea This layer is responsible for providing and diagnosing corneal ulcers. is a fluid layer which functions to hydrate, nutrition and oxygen to the cornea and They will also be able to identify protect and lubricate the cornea, provide conjunctiva and also functions to keep potential management options nutrition to corneal cells and tissues the epithelium hydrated. available. of the avascular cornea, and maintain optical clarity. The pre-corneal tear film is • Mucoid layer—the innermost Diagnosis and Treatment composed of at least three distinct layers: layer of the pre-corneal tear film is lceration of the cornea is one of the composed of mucin and is produced Umost prevalent clinical abnormalities • Oil layer—the superficial (outermost) primarily by conjunctival goblet cells. affecting the canine cornea. Diagnosis layer consists of oils produced by the This layer increases the surface area and treatment of corneal ulceration is not meibomian glands located within and binds the middle aqueous layer to identical for all types of corneal ulcers. the eyelids. The oil layer permits the the corneal surface. (Blinking partially An inaccurate diagnosis and improper eyelids to move in a gliding action inter-mixes the three component treatment of corneal ulcers may have and prevents evaporation of the second layers of the tear film) profound effects on corneal sepsis, healing, (aqueous) layer of the tear film. corneal clarity, and ultimately may cause 2. Corneal epithelium and its basement impaired sight, blindness, or even loss of the • Aqueous layer—the aqueous layer membrane—the corneal epithelium affected eye. Understanding basic anatomy constitutes the largest volume of the serves as a barrier to prevent an and physiology of the cornea, its response pre-corneal tear film (95%) and is excessive volume of tear film fluids from to injury, recognition of normal corneal located between the superficial oil entering into the corneal stroma, which wound healing as well as different medical layer and the underlying mucus layer maintains relative dehydration of the and surgical treatment options is essential to of the pre-corneal tear film. This layer corneal stroma. The epithelial layer has promote healing. is produced by the orbital lacrimal remarkable regenerative capabilities This program was reviewed and approved by the AAVSB RACE program for 1 hour of continuing education in jurisdictions which recognize AAVSB RACE approval. Please contact the AAVSB RACE program if you have any comments/concerns regarding this program’s validity or relevancy to the veterinary profession. Photo by iStockphoto.com / Photo by iStockphoto.com 10 | THE NAVTA JOURNAL | NAVTA.net Veterinary NURSING EDUCATION Canine Corneal Ulcers Superficial Deep Young Middle age to older Uncomplicated Complicated Trauma Free epithelial edges Topical tobramycin, Descemetocele atropine, protective collar Yes No Yes No Yes No Normal Normal Surgical Medical Topical SCCED anatomy anatomy repair (graft) treatment tobramycin, gabapentin, protective collar Yes No Yes No Corneal debridement, Example: diamond topical ofloxacin burr, topical qid, and atropine Foreign Determine and treat ofloxacin, Treat sid, oral doxycline body underlying cause i.e. atropine, underlying bid, NSAID sid-bid, entropion, ectopic cilia, oral NSAID, cause i.e. and gabapentin distichiasis, etc. gabapentin, entropion, bid-tid, protective protective distichiasis, collar collar Yes No etc. Other ocular Remove foreign abnormalities body, topical i.e. Dry Eye ofloxacin, (KCS), corneal atropine, calcium gabapentin, degeneration, protective collar etc. Yes No Treat underlying Treat, topical cause, topical tobramycin, tobramycin, gabapentin, gabapentin, protective collar protective collar VETERINARY NURSING IN ACTION | June/July 2019 | 11 Veterinary NURSING EDUCATION and re-epithelialization occurs rapidly “dehydrated” to maintain optimal optical the corneal stroma. Depending on the under normal circumstances. If the clarity for refraction of light entering the amount of epithelial cellular loss, fluorescein entire epithelial layer is removed, it eye. The cornea is highly innervated by the may still enter the corneal stroma. Full- takes approximately 11-14 days to re- ophthalmic branch of the trigeminal nerve, thickness loss of corneal stoma exposes epithelialize under normal circumstances. thus its exceptional sensitivity. Sensory Descemet’s membrane which does not retain receptors are concentrated in the superficial fluorescein dye. 3. Corneal stroma—the third layer of (anterior) third of the corneal stroma and Corneal ulcers are categorized by their the cornea is the corneal stroma which in the corneal epithelium. The epithelium topographic location, depth and etiology. is located just underneath the basement contains the greatest density of sensory Location of an ulcer is important because membrane of the epithelial layer. The nerves per square millimeter than any other it often provides clues to determine the stroma accounts for approximately tissue in the body. It is for this reason that underlying cause. When an ulcer is located 90% of the corneal thickness and is superficial ulcerations or abrasions are more in the superior portion of the central cornea, composed mostly of uniformly arranged painful than deep ulcerations. detailed examination of the palpebral parallel bundles of collagen fibers. The surface of the upper eyelid may reveal transparency of the cornea is achieved by Corneal Erosions and Ulcers ectopic cilia or embedded foreign body. A this precise arrangement of collagen and Corneal erosion is defined as loss of corneal horizontally-oriented ulcer located in the prevents scattering or refraction of 99% of epithelial cells without the loss of the central cornea is suggestive of exposure from light entering the cornea. The stroma is epithelial basement membrane. Corneal incomplete blinking (lagophthalmos). Most extremely hydrophilic (water loving), but ulceration is defined as a loss of the corneal traumatic ulcers also occur in the central absorption of fluids by the stroma is limited epithelium and its basement membrane. or paracentral region. If an ulcer is located by the hydrophobic (water repelling) Ulceration can also include loss of part in the inferior-nasal region, examination corneal epithelium (preventing absorption or the entire corneal stroma. Solubility behind the nictitating membrane may reveal of tear film) and the corneal endothelium properties of the epithelium and stroma an embedded or retained foreign body. (preventing absorption of aqueous humor). help differentiate between ulcerations and Ulcers located in the paralimbal region Dysfunction or loss of either the corneal erosions. The epithelium acts as a barrier are suggestive of an eyelid conformational epithelium or endothelium may permit to water-soluble solutions, whereas the abnormality or an eyelash abnormality. over-hydration of the corneal stoma and hydrophilic stroma acts as a “sponge” Limbal-based ulceration is suggestive of an result in corneal edema. to water-soluble solutions. Fluorescein immune-mediated etiology. sodium dye is water-soluble—therefore, if The depth of an ulcer is much more 4. Descemet’s membrane—The fourth an ulcer is present the stroma will retain important than the geographic size of an layer of the cornea is Descemet’s the dye. If a corneal erosion is present, the ulcer when determining the severity of membrane, which is the basement fluorescein stain will be partially repelled corneal ulceration. A superficial ulcer is an membrane of the corneal endothelium. by the remaining epithelial cell layers. ulcer only involving the epithelium and Descemet’s membrane is composed of The basement membrane of the corneal its basement membrane. When an ulcer collagen, is highly elastic, and continues to epithelium is a weak barrier to prevent involves less than one third of the stroma, thicken with advancing age. water-soluble compounds from entering it is classified as a superficial stromal ulcer, and if it involves more than one third of 5. Endothelium—The innermost layer of Corneal erosion is the stroma, it is classified as a mid to deep the cornea is the corneal endothelium stromal ulcer. Full-thickness loss of corneal which lines the inner surface of defined as loss of corneal stoma exposes Descemet’s membrane and is Descemet’s membrane and is in direct epithelial cells without called a descemetocele. A perforated corneal contact with aqueous humor. This layer ulcer occurs when Descemet’s membrane consists of a monolayer of cells that have the loss of the epithelial ruptures, resulting in leakage of aqueous a poor regenerative capacity. Endothelial basement membrane. humor from the anterior chamber. cells are the predominant barrier to Indisputably, there is always an the movement of aqueous humor into Corneal ulceration is underlying cause of corneal ulceration— the corneal stroma and are essential to defined as a loss of the determining the primary origin may maintain the transparency of the cornea.
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