30 Feline corneal sequestrum
A cat with a corneal sequestrum will be presented with a slightly uncomfortable watery eye. Thus blepharospasm will be present – the cat holds the affected eye partially closed, and the discharge is likely to be serous in nature, although sometimes it can be mucopurulent. It is not uncommon for the discharge to be dark brown in colour – owners may think that the eye has been bleeding due to the colour. The owners might also have noticed a dark discolouration on the cornea, although since this can often be central in position it is not always obvious to them against the dark background of the pupil. The position in the central or subcentral cornea is typical, and the colour can vary from mid amber to very dark brown or black. Any breed of cat can be affected, but Persians of all types and Burmese are over-represented.
The history of previous corneal ulceration is common. Thus the cat could still be under treatment for an ulcer, and on subsequent examination is found to have the typical dark corneal deposition of a sequestrum developing. Slow to heal superficial ulcers are most likely to progress to sequestrum formation. In domestic short haired cats the ulcer is likely to be associated with a traumatic event – a cat scratch for example – while in Persians and Burmese no such history is always present. It is not uncommon for the cat to have had problems with one eye in the past and for similar ulceration and sequestrum formation to recur or to develop in the second eye. Occasionally there will be no history of ulceration and the sequestrum seems to develop spontaneously. In some cats there will be a history of previous upper respiratory disease associated with feline herpes virus (FHV-1). Indeed chronic and recurrent ulcers are frequently associated with FHV-1 infection and secondary sequestrum formation is a well-recognized complication of this.
General clinical examination is usually unremarkable, unless an active FHV-1 infection is concurrent. Ophthalmic examination will reveal several abnormalities. The affected eye is usually wet, and Schirmer tear test readings will confirm this with a higher reading than in the normal eye. However, on occasion the opposite can be true – a sticky discharge might suggest an associated keratoconjunctivitis sicca so tear test readings should not be overlooked.
Blepharospasm is normally present. Conjunctival hyperaemia and some chemosis are common, as is superficial corneal vascularization. A darkly pigmented patch of cornea defines the sequestrum. This can be raised above the epithelium or subepithelium (the latter is less uncomfortable). The colour can vary from a mid brown to almost black. The ocular discharge is often also darkly stained; however, this is normal for many cats so should not be over-interpreted. Corneal ulceration is often superficial or mid-stromal and fluorescein dye should always be used for any suspected sequestrum. Variable amounts of corneal vascularization can be encountered – in some patients none is noted, while in others there is profuse superficial vascularization and even granulation tissue deposits at the edge of the sequestrum.
In more severe cases the pupil can be miotic, with rubeosis iridis and some aqueous flare – a reflex uveitis – which is most common if bacterial infection is present with the surface disease. In long-standing cases there might be some degree of secondary lateral lower lid entropion with the hairs rubbing on the cornea and exacerbating the problem – this is more frequent in older domestic short haired breeds and is rarely seen in Persians. However, in this latter breed a medial lower lid entropion can be present which may contribute to the problem. In addition, brachycephalic cats can suffer from a relative lagophthalmos such that the central cornea is not fully protected during blinking and tear film dynamics are abnormal (Figure 30.1). This is a serious problem in some of the ‘ultra-type’ Persians with the excessively flat face and bulging eyes – some of these poor cats even sleep with their eyes partially open and corneal health can be severely compromised as a result.
Figure 30.1 Bilateral corneal sequestrum in a Persian. The left eye has the typical central dark discolouration while the right has a superficial corneal ulcer and an amber discolouration to the exposed stroma.
Checking the frequency of blinks, and whether these are complete or partial – where the eyelids do not fully meet – is important. In some patients, particularly the predisposed breeds, both eyes can be affected although often not at the same time, and certainly rarely symmetrically if both are involved.
If there is any mucopurulent discharge then swabs should be taken for bacterial culture and sensitivity testing. These should be taken before the cornea is stained with fluorescein and are more accurate if the swab is gently rolled at the edge of the lesion rather than sampling the discharge on the face. Samples for FHV-1 polymerase chain reaction (PCR) should be taken if there is a suspicion of a viral component.
Most affected cats will require topical medication – sometimes for life in predisposed breeds – and owners must be taught how to apply this effectively. Some cats will need surgery, and nursing before, during and after the procedure is important for its successful outcome.