Veterinary ophthalmology: what is feline corneal sequestrum?
Feline corneal sequestrum is an eye disease that begins with the appearance of brown or black plaques on the corneal epithelium and stroma (the most superficial layers of the cornea). The lesion is exclusive to cats and horses.
What is corneal sequestrum?
Corneal sequestrum involves the degeneration of pigmented collagen and mainly occurs in adult cats, affecting one or both eyes. These plaques may darken as the disease progresses and affect deeper into and perforate the cornea, possibly resulting in the loss of the eye.
What causes it?
Although it seems to have a genetic component, since it mainly affects Persians, Siamese or Sphinx, there are other triggering factors, such as:
- Herpesvirus infection, which commonly causes virus-related conjunctivitis or ulcers.
- Inadequate lubrication of the eye.
- Factors causing corneal ulcers or irritation: for example, cats living in aggressive climatic conditions or who suffer from entropion (the edge of the eyelid rolls inwards towards the cornea, causing the eyelashes to be in continuous contact with the ocular surface).
- Trauma.
Corneal sequestrum arises whenever the cornea suffers disruption due to any of the above reasons and the effected area develops necrosis rather than healing.
Clinical signs
We should normally begin to suspect corneal sequestrum if a corneal ulcer does not heal or changes colour, but the main indication is pain; cats keep the eye closed or half-closed (blepharospasm), which can make examination very challenging. Other signs are photophobia, protrusion of the third eyelid, excessive tearing and mucopurulent discharge. It can be mistaken for a foreign body or another type of eye disease.
Diagnostic tests
The eye must be examined with a torch with white light, which reveals a dark spot on a bed of neovascularisation. Schirmer’s test can be used to assess the volume of tears produced by the affected eye. After application of anaesthetic eye drops or even mild sedation, the intraocular pressure can be measured with an applanation tonometer, and the eye examined in detail with a slit lamp. The most reliable method for aetiological diagnosis of sequestrum involves taking a conjunctival sample for PCR confirmation of herpesvirus infection.
Treatment
The first step is to treat the cause. We cannot eliminate any genetic factors, but we can surgically correct entropion or improve the eye’s lubrication if found to be insufficient. Once the factor predisposing ulceration has been corrected, corneal sequestrum can be approached in two ways, medical or surgical.
Medical treatment consists of dealing with the ulcer, using eye drops, antibiotics, lenses or ophthalmic ointments. Associated antiviral treatment may also help.
Surgical treatment aims to remove the sequestrum. Remember that sequestrum may penetrate deeper into the cornea, so perforation is a real possibility if it is left untreated or only remedied with superficial keratectomy, so all scenarios should be closely assessed and the eye, or eyes, undergo a thorough slit lamp examination. Of course, the sooner treatment starts, the more favourable the prognosis.
Why are the necrotic plaques brown or black?
A study conducted on corneal samples from 12 cats with corneal sequestrum employed different bioanalytical techniques and observed differences between control eyes and the study sample in terms of the type of lipids found in the tears, but the observation of melanin in optical microscopy was finally deemed to be the cause of the colour.
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