By Martin Coster, DVM, MS, DACVO
Corneal sequestration is thought to occur under conditions of chronic irritation, or dryness of the corneal surface. The sequestrum is devitalized, acellular corneal stroma (Figure 1) that can range in color from very light brunescence (Figure 2) to dark black (Figure 3). The color has been ascribed to high iron content, melanin pigment, or porphyrin pigments from the tearfilm, and no single hypothesis has been definitively proven nor disproven.2-4
Figure 1: Histopathology of a feline corneal sequestrum sample. Normal deep corneal stroma, depicted at the bottom of the image, is composed of pale to medium pink-staining collagen with evenly dispersed stromal cells and artifactual white clefts. In contrast, the necrotic anterior stroma forming the sequestrum at the top of the image is deeply purple to almost brown-tinged, devoid of cells, and more compact. The epithelium which normally forms the anterior surface of the cornea is absent.
Figure 2: Light brown corneal sequestrum in a cat. The sequestrum has been fluorescein stained, with the superior aspect highlighted in green.
Figure 3: Dark corneal sequestrum in a cat. Note the tips of the vertical pupil visible behind the dark corneal lesion; the author has sometimes seen sequestrum cases mistaken and referred for a mis-shapen pupil on a cursory examination. There is an epithelial lip around the edge of the sequestrum; to the right of the lesion is an incidental hair on the cornea.
The inciting cause of sequestrum can be irritation from entropion5 (congenital, developmental, traumatic, or secondary to aging orbital fat loss) (Figure 4), corneal ulceration (most commonly chronic herpesvirus infection), or breed-related exposure keratitis (Persians, Burmese, Himalayans, or other brachycephalic cats with lagophthalmia; Figure 5).6
Figure 4: Corneal sequestrum secondary to inferior entropion in a cat.
Figure 5: Corneal sequestrum in a brachycephalic cat.
When a sequestrum is noted, careful evaluation of the globe and orbit should be made to ensure that the eyelids are able to blink to cover the cornea – ruling out orbital space-occupying lesions (abscess, neoplasia, or feline restrictive myofibrosarcoma [orbital pseudotumor]),7,8 buphthalmia from glaucoma, or eyelid abnormalities such as agenesis. Sequestra can also form after grid keratotomy procedures in the cat, and as such grids are generally contraindicated for non-complicated superficial indolent corneal ulceration.9 Tear film insufficiency (“dry eye”) may10 or may not11 play a role.
Figure 6 – Large corneal sequestrum in a cat, sitting on the surface of the cornea like a “scab”. Although superficial sequestra can spontaneously slough, it is impossible to determine the depth of the lesion with an eye exam; high resolution ultrasound can help.
Corneal sequestra are typically extremely painful due to epithelial erosion. The corneal epithelium is generally unable to heal across the sequestrum, presumably due to an inability to form tight junctions over the devitalized tissue, and this leads to loose epithelial edges that are a constant source of pain as each blink disrupts the tissue. Rarely, epithelialization can occur over the sequestrum, leading to a more comfortable state, although further ulceration can later occur. Sometimes, the sequestrum can slough from the cornea, allowing for spontaneous resolution. Figure 6 shows a superficial corneal sequestrum that might slough. Unfortunately, the timeframe for sloughing can be weeks, months, or years – and with chronicity, the risks increase for infection or thickening of the sequestrum to full-thickness involvement, with resultant corneal perforation. This means that referral for microsurgical management is the general treatment of choice for any corneal sequestrum.
In those cases where medical management is elected with informed consent (for reasons of finances, unacceptable anesthetic risk, etc.), the principles of treatment encompass ocular lubrication, broad spectrum antibiosis, pain control, and management of any underlying conditions. Erythromycin ointment is a good first choice topical medication, avoiding the potential for anaphylaxis with triple antibiotic ointment, and is recommended q 2-6h depending on the amount of corneal coverage available from the eyelids. Suspected or presumed herpesvirus involvement or secondary bacterial keratitis require specific treatment. Prophylactic therapy of the contralateral eye should be considered in predisposed breeds or bilateral conditions, with an artificial tears ointment q 8-12h.
Surgical management of corneal sequestra will depend on the depth of the lesion, which can be determined with high-resolution ultrasound12 and even spectral domain optical coherence tomography,13 but is more often an intraoperative decision as the layers of sequestered tissue are excised. Generally, superficial sequestra respond to superficial keratectomy with or without contact lens placement and a third eyelid flap, and this is the author’s preferred approach.14-16 For mid-stromal lesions, cyanoacrylate adhesive (tissue glue) with contact lens placement may be used to aid post-healing following keratectomy.17
For deeper stromal lesions, corneoconjunctival transposition (Figure 7) can be performed (up to 2/3 stromal depth),18-20 but conjunctival grafting (Figure 8) will likely be performed for very deep, perforated, or infected lesions. Conjunctival grafting may be considered for any lesion, to provide better tectonic support and vascularization to the affected area, but will result in the least satisfactory visual outcome. Other less-commonly used materials available for providing tectonic support to a cornea following excision of a sequestrum include porcine urinary bladder, bovine pericardium, amniotic membrane, corneal transplantation, and synthetic ACell Vet™.21-25
Figure 7 – One-day post-operative corneo-conjunctival transposition graft following keratectomy for corneal sequestrum, in a cat. In this procedure, the axial sequestrum is excised by keratectomy, and normal clear cornea is transposed by sliding into the central defect, with conjunctiva following. Once healed, the central cornea regains almost complete clarity.
Figure 8 – Conjunctival graft for corneal reconstruction following keratectomy for an axial deep corneal sequestrum.
If general anesthesia is contraindicated or not possible, the author has had moderate success with corneal debridement procedures such as diamond burring and thermal cautery (a.k.a. thermokeratoplasty Figure 9), and even grid keratotomy, performed over the surface of light sequestra. These procedures are considered contraindicated with superficial ulcers due to the formation of sequestrum, but once there is a sequestrum present there is little to lose except potentially hastening the time to surgery. In highly compliant patients or with deep sedation, sequestra that are already starting to slough may be partially excised by an ophthalmologist using a Beaver scalpel blade and/or corneal scissors.
Figure 9 – Post-operative thermal cautery (a.k.a. thermokeratoplasty) for light brunescent corneal sequestrum (visible in the axial cornea with faint green fluorescein stain present). Note the opaque spots where thermal cautery was applied, and the epithelial ridge surrounding the lesion following cotton swab debridement. This cornea epithelialized and regained long-term comfort.
The author wishes to thank Dr. Pamela Mouser, DACVP for providing the histopathology image.
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