Welcome to the all-new Vetlearn

  • Exciting News Coming to Vetlearn in July 2014!
    Coming soon you'll be able to access...
  • The latest issues of Compendium and
    Veterinary Technician
  • Thousands of industry Conference Proceedings
  • All-new articles (CE and other topics) for the
    entire healthcare team
  • Over 500 hours of interactive CE Videos
  • An engaging new community for asking
    questions, making connections and more!

To access Vetlearn, you must first sign in or register.

registernow

  • Registration for new subscribers will open in early July 2014!
  • Watch for additional exciting news coming soon!
Become a Member

Veterinarian Technician December 2008 (Vol 29, No 12)

Managing Feline Corneal Sequestrum

by Kris Pratt, BS, LVT

    CETEST This course is approved for 0.5 CE credits

    Start Test

    Corneal sequestrum is a condition characterized by necrosis of the corneal stroma. The cause is unknown, but previous corneal ulceration, eyelid malformation, or feline herpesvirus type 1 (FHV-1) infection may be predisposing factors. Early recognition of clinical signs and prompt treatment may slow the progression of tissue damage and minimize the need for surgical intervention.

    Corneal sequestrum is predominately observed in cats. Although all cats are susceptible, the Burmese, Persian, and Himalayan are the most common breeds reported with corneal sequestrum.1 These cats may be more vulnerable because of their facial conformation and protruding globes.2

    A sequestrum is usually located in or near the center of the cornea and most often occurs unilaterally. Clinically, the condition presents with a darkened lesion overlying layers of ulcerated or nonulcerated cornea. In mild cases, the corneal epithelium remains intact and the lesion may be light amber. In more advanced cases, the cornea can become black and the epithelium may slough off. The blackened area is necrotic tissue and features properties consistent with melanin granules.3 Corneal mummification, corneal necrosis, and corneal nigrum are other names describing this condition. The necrotic tissue can extend from the corneal stroma to Descemet's membrane.1

    Clinical Findings

    Corneal irritation, FHV-1 infection, abnormal eyelid formation, or keratoconjunctivitis sicca (KCS) may lead the veterinarian's differentials list for feline corneal sequestrum, but the definitive cause remains unknown.4 It has been reported that approximately 55% of corneal sequestrum cases test positive for FHV-1 when a sample is submitted for polymerase chain reaction (PCR) testing.4 In addition to a complete history, thorough physical examination, and ophthalmic evaluation, identification of clinical signs is essential for the veterinarian to determine the diagnostic differentials.

    Cats with corneal sequestrum can present with epiphora, blepharospasm, ocular discharge, elevation of the nictitating membrane, corneal neovascularization, chemosis, and hyperemia. Corneal edema also can appear around the lesion, and inflammation (flare) can be appreciated inside the anterior chamber. All these signs suggest an uncomfortable eye.5

    Depending on the stage of the sequestrum, the pain level can range from mild to severe. The pigmented defect can extend to various depths of the corneal stroma, ranging from superficial to very deep. If the necrotic crust separates from the underlying tissue, the examiner can determine the depth of the corneal ulcer. If the darkened area remains intact, assessment can be difficult.4

    Complete Eye Examination

    The eye examination begins with visual function testing, which involves eliciting the menace response. To elicit this response, the examiner simply places a hand over one of the cat's eye and gestures toward the opposite eye with the other hand, being careful not to push air into the cat's face. It is important not to stimulate the cornea or touch any whiskers while the menace response is being elicited.

    The tracking response can be measured by tossing a cotton ball over the cat's head and observing whether the cat tracks the ball. However, because the cat may not react if the environment is stressful, the menace response should be used in conjunction with this test.

    An additional vision test is the visual placement response, which consists of holding the cat under one arm while moving it toward the edge of a table and watching for appropriate placement and extension of the forelimbs on or toward the table.

    Next, the pupillary light reflex (PLR) needs to be assessed by shining an illuminator into one eye for a direct PLR and then observing the opposite eye for the indirect or consensual PLR. The examiner should then palpate the head and ocular adnexa (surrounding eyelids and tissue) for any signs of trauma or malformation. The eyelids, sclera, conjunctiva, and nictitating membranes should be examined to detect any abnormality, such as chemosis, hyperemia, or significant masses. Next, the cornea, anterior chamber, iris, and lens should be scanned for any defects or signs of inflammation. A slit lamp is ideal for performing this examination, but a transilluminator also can be used.

    Diagnostic testing begins with the Schirmer tear test, which should be conducted before the eyes are cleansed with eyewash or any drops are instilled into the eye because either can interfere with the results. Although this test is not particularly accurate in cats, an extremely low reading may change treatment protocols.1

    An important tool in diagnosing feline corneal sequestrum is the fluorescein stain test, which is done by placing one drop of stain into each eye and flushing the eyes with eyewash. The stain adheres to any corneal lesion that is devoid of epithelium, thereby exposing the stroma. In a darkened room, a cobalt blue light is shined into the cat's eyes; stain retention causes any ulcerated area on the cornea to luminesce. If no fluorescein is observed, rose bengal stain can be applied. This pink dye binds with devitalized tissue, making these areas more obvious to the examiner. A dendritic ulceration pattern, which looks like it is branching out, is indicative of a cat infected with FHV-1.2 All observations should be clearly documented in the medical record.

    Treatment

    The veterinarian bases therapeutic choices on the severity of the sequestrum, discomfort level of the cat, and patient's anesthetic risk. Financial constraints of the owner also may be an issue. Numerous treatment options have been reported, but surgical treatment is often the most successful method for treating a corneal sequestrum and preventing recurrence.5 Because corneal surgery requires the use of an operating microscope, delicate ophthalmic instruments, and fine suture, patients are usually referred to a veterinary ophthalmologist.

    After the patient has been anesthetized, it may be intubated with a wire-reinforced endotracheal tube because of the head and neck manipulation required during positioning. The patient may become paralyzed with the administration of atracurium, which makes the globe roll up to a central, stable position. A paralytic agent should always be used to ensure a central, stable globe position. The patient must be ventilated during the procedure and reversed with neostigmine after surgery.

    Depending on the severity of the sequestrum, a keratectomy may be performed alone or, in more advanced cases, in conjunction with a conjunctival pedicle graft5 (Figure 1). Keratectomy is a dissection of the necrotic corneal tissue that extends 1 mm beyond the affected area. The average depth of excision is 0.3 mm (full corneal thickness is 0.6 mm).5 The sample tissue is placed in formalin and submitted for histopathology. If warranted, a pedicle graft is constructed from the surrounding bulbar conjunctival tissue and sutured onto the cornea, covering the dissected area. This tissue forms a bridge from the connected portion to the lesion. The conjunctival tissue delivers antibiotics and blood supply to the avascular cornea, as well as supports the cornea as it heals.6

    After the ulceration bed has healed and filled in, the bulbar conjunctival pedicle flap can be severed from the connected portion, cutting off the blood supply to the graft. Over time the graft will lose its pink appearance, but a permanent scar will remain.

    The ideal conjunctival graft is therapeutic and very thin, thereby preserving vision after the residual tissue becomes transparent. Postoperative treatment consists of topical antibiotics, hyperviscous tear replacement, and use of an Elizabethan collar to prevent postsurgical injury.

    Nonsurgical treatment methods may be warranted when the ulceration bed is superficial, the animal is not a good surgical candidate, or the owner cannot afford surgery. Although rare, superficial necrotic epithelium can slough off, thereby exposing the underlying cornea and allowing topical medications to more easily penetrate the corneal tissue layers. Ophthalmic antibiotic and an artificial tear lubricant, along with use of an Elizabethan collar, are prescribed as treatment for a superficial corneal sequestrum. A bandage contact lens that acts as a shield protecting the cornea from constant rubbing of the eyelids onto the lesion also can provide temporary comfort for the cat while the ulcer is healing.

    Histopathology

    Microscopic observation of feline corneal sequestrum typically demonstrates a region of necrotic corneal stroma and variable mineralization. This section is usually pigmented, presumably with melanin deposition, and overlies the ulcerated cornea (Figure 2). This dark-colored portion is surrounded by stromal vascularization and inflammatory cell infiltrates, which can consist of macrophages, plasma cells, and lymphocytes but predominately neutrophils.2 The sequestrum acts as a foreign body and the surrounding tissues react accordingly.

    The Technician's Role

    Although most general practices do not have the expensive ophthalmic equipment and expertise in this field as specialists at universities do, this should not deter technicians from learning how an eye examination is performed. Ophthalmology technicians perform several every day as well as instruct veterinary students each step of the way, and any LVT, CVT, or RVT can perform these examinations from start to finish. Of course, the ophthalmologist reviews the findings and makes the diagnosis and treatment plan. Completing eye exams is a major role of technicians in a specialty ophthalmic practice, which makes them a valuable part of the team.

    Closing Remarks

    Corneal sequestrum, most commonly observed in cats, is a treatable condition. Corneal abrasion, constant exposure, keratitis, and decreased tear production associated with FHV-1 infection typically precede the progression of deep ulceration and can eventually cause stromal necrosis of affected tissue. Early recognition of clinical signs and a prompt diagnosis can reduce the need for invasive treatment.

    Based on the severity of the case, medical therapy may be elected, but in more advanced cases, surgical intervention is recommended to alleviate discomfort and remove necrotic tissue. Surgical excision in addition to a bulbar conjunctival pedicle graft may minimize the chance of recurrence.

    1. Gelatt KN. Essentials of Veterinary Ophthalmology. Philadelphia: Lippincott Williams & Wilkins; 2000.

    2. Gelatt KN. Feline ophthalmology. In: Gelatt KN (ed). Veterinary Ophthalmology. Ames, Iowa: Blackwell; 2007.

    3. Featherstone HJ, Franklin VJ, Sansom J. Feline corneal sequestrum: laboratory analysis of ocular samples from 12 cats. Vet Ophthalmol 2004;7:229-238.

    4. Barnett KC. Cornea. In: Barnett KC, Crispin SM (eds). Feline Ophthalmology: An Atlas & Text. Edinburgh: Saunders; 1998.

    5. Andrew SE, Tou S, Brooks DE. Corneoconjunctival transposition for the treatment of feline corneal sequestra: a retrospective study of 17 cases (1990-1998). Vet Ophthalmol 2001;4:107-111.

    6. Petersen-Jones SM, Crispin SM. British Small Animal Veterinary Association Manual of Small Animal Ophthalmology. Cheltenham, Gloucestershire: British Small Animal Veterinary Association; 1993.

    References »

    NEXT: Season of Suppers helps seniors and their hungry pets

    CETEST This course is approved for 0.5 CE credits

    Start Test

    didyouknow

    Did you know... In horses, topical corticosteroids and NSAIDs should be avoided if corneal ulceration is present.Read More

    These Care Guides are written to help your clients understand common conditions. They are formatted to print and give to your clients for their information.

    Stay on top of all our latest content — sign up for the Vetlearn newsletters.
    • More
    Subscribe