Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

FELINE OPHTHALMOLOGY

ACQUIRED DISEASES OF THE FELINE CORNEA

Epithelial erosion

The cat appears to have a chronic type of refractory superficial erosion, which is similar to canine epithelial basement membrane dystrophy in presentation (see Section 3, pp 114–16, for description of typical clinical appearance) (Figure 4.17). In addition to the usual diagnostic work up (including tear production and adequacy of blink) it is worth checking for feline herpes virus (the commonest confirmed aetiological agent) and also the cat’s FeLV and FIV status. In some cases there is also a history of topical or systemic corticosteroid use.

Figure 4.17 Epithelial erosion of the cornea in an adult Domestic Shorthair. The erosions have been stained with fluorescein and demonstrated using a cobalt blue light source, note that superficial vascularisation is also present.

Treatment

•If no predisposing cause can be found, the ulcer should be treated by removing redundant epithelium at the rim of the ulcer with saline-soaked cotton wool wound round the tips of Halstead’s mosquito forceps

•The procedure can be performed under topical local anaesthesia and can be repeated after 10 days only if there has been no improvement, if the erosion persists despite treatment, consider referral

•A therapeutic soft contact lens can be used to provide protection and pain relief during healing (but avoid if tear production is abnormally low)

•If there is any suggestion of abnormal tear production, then topical tear replacement therapy should be given until tear production returns to normal, and for life if it does not

Herpetic keratitis

Aetiology and clinical signs

•Caused by feline herpes virus-1

•Epithelial keratitis occurs commonly during primary infection in young cats, but

resolves spontaneously in most cases (Figure 4.18)

• Stromal keratitis (usually unilateral) in adult cats often represents reactivation of latent virus (see below); the prognosis is poor in these cases, because corneal complications are likely (Figure 4.19)

Figure 4.18 Acute herpetic keratitis in a Domestic Shorthair. The pathognomonic superficial dendritic lesions have been stained with fluorescein.

Figure 4.19 Chronic herpetic keratitis in a Domestic Shorthair. The lesions are predominantly stromal in the chronic phase. This cat was also FIV positive.

Clinical findings

•Mild blepharospasm and lacrimation, or a serous ocular discharge, are common presentations in the initial stages

•The epithelial keratitis consists of discrete superficial punctate opacities in the early stages; later the pathognomonic linear branching (dendritic) ulcers may form and these can progress to produce an irregular and superficial geographical ulcer (Figure 4.18). These changes are the direct result of viral cytopathic effect on the corneal epithelium

•Chronic cases, with stromal involvement, are a consequence of a cell-mediated immune response

•Stromal keratitis is usually characterised by stromal oedema, superficial and deep vascularisation and cellular infiltration (Figure 4.19)

•A number of affected cats are FIV or FeLV positive, indicating that herpetic keratitis in these cases is an example of an essentially opportunist infection in an immunosuppressed host. The prognosis is poor in immunosuppressed animals

Diagnosis

Diagnosis is based on the clinical findings, which can be so diverse as to be unhelpful in chronic cases, and the detection of FHV-1 by viral isolation (insensitive in chronic cases) or FHV-1 DNA using PCR (more sensitive) from corneal samples.

FELINE OPHTHALMOLOGY

FELINE OPHTHALMOLOGY

Management

There is no simple and effective means of managing chronic disease. In practice, many cases relapse because of persistent conjunctival infection, or reactivation of a latent viral infection (in trigeminal ganglia). Reactivation of latent virus is triggered by many forms of stress (both local ocular and systemic), intercurrent disease and systemic corticosteroid administration. In view of the known association with FIV and FeLV, it is sensible to screen affected animals for the presence of these viruses.

Antiviral agents

Herpetic keratitis responds unpredictably to topical antiviral agents (e.g. idoxuridine, trifluorothymidine, acyclovir). Ideally, treatment should be given every 2–4 hours for at least six days, and then less frequently until the eye appears quiet. Acyclovir may also be given systemically (200mg tid) but can cause bone marrow depression. All these agents, together with newer agents, such as ganciclovir (used to treat acute herpetic keratitis in humans), require evaluation in clinical trials to assess their efficacy against FHV-1.

Mechanical removal

Mechanical removal of affected corneal epithelium may assist in the treatment of epithelial keratitis, whereas lamellar keratectomy or even penetrating keratoplasty

(corneal graft) may be of value in the treatment of stromal keratitis.

Immunotherapy

The use of one drop of intranasal vaccine to each eye, or oral human interferon alpha-2 are being evaluated at present, as is dietary therapy with L-lysine (200mg mixed with food on a daily basis, maximum 400mg bid).

Corticosteroids

Topical corticosteroids may reduce post-herpetic scarring, but they should only be used in conjunction with antiviral agents in chronic cases as they will exacerbate active viral infection.

Corneal sequestrum

Aetiology

This is a condition of unknown cause, which has many descriptive names. It is unique to the cat and there is both a breed disposition (e.g. Colourpoint, Persian, Siamese, Birman, Himalayan) and a tendency for the condition to appear after previous corneal insult (e.g. trauma, herpetic keratitis), irrespective of breed (Figures 4.6 and 4.20).

Clinical findings

The condition is usually unilateral, although the other eye may be affected at a later date, and, both eyes may be involved at the same time in susceptible breeds. It is sensible to conduct a comprehensive examination, to exclude complicating factors, such as entropion, tear film abnormalities and infection, for the appearance of the lesion is often so striking that the necessity for complete examination is forgotten.

The lesion is of somewhat variable appearance, ranging from an ill-defined brown staining of the corneal stroma to a clearly-demarcated black plaque (sequestrum) which

is raised above the level of the corneal epithelium (Figure 4.6). It is likely that the different appearances relate to the many causes and to different stages in the evolution of the opacity and that epithelial damage allows the pigment to deposit in the corneal stroma. The pigmented material contains melanin and it may derive from the preocular tear film and, in some cats, pigmented material also accumulates on and near the eyelid margins.

•A discrete zone of oedema sometimes surrounds the sequestrum and, apparently as a feature of chronicity, there may be obvious neovascularisation

•The sequestrum may extend as deeply as Descemet’s membrane

•In most cases the corneal sequestrum is eventually sloughed, but this is a process that may take many months and, if ulceration or other problems are present, there may be long-term discomfort for the patient and risk of additional complications, such as corneal perforation

Management

This will depend on the extent and progression of the condition and the amount of discomfort that is present. In time, many of these lesions slough without complication and the cat may be treated with topical tear-replacement preparations or antibiotic ointment during this period if any discomfort is present.

A number of cases, however, will remain uncomfortable on conservative medical treatment and, in such cases, a therapeutic soft contact lens, or keratectomy combined with a conjunctival or free pedicle graft, gives good results. Superficial keratectomy greatly reduces the time course of the disease and potential surgical cases may be best referred.

After surgery, the patient is usually given a short course of topical antibiotic. Recurrence is unusual with careful assessment and treatment. In many respects, the management approach is similar to that adopted for ulcerative keratitis.

FELINE OPHTHALMOLOGY

FELINE OPHTHALMOLOGY

sity of that seen in dogs, as chronic granulomatous types of uveitis account for a high proportion of cases.

Clinical signs of sub-acute/chronic feline uveitis

•No pain to mild discomfort, no inflammatory hyperaemia in most cases

•Usually no effect on vision, occasionally complications such as retinal detachment result in blindness

•Some or all of keratic precipitates, mutton fat deposits, hypopyon, fibrin and haemorrhage*

•Some or all of swollen iris, iris nodules, iris neovascularisation, pre-iridal fibrovascular membranes*

•Pupil responds normally to light unless synechiae are present*

•Synechiae distort pupil and result in an irregular shape*

•Intraocular pressure normal or low

•Snowflake opacities and retrolental snow banking may be seen when there is peripheral retinal vasculitis (pars planitis is the term used to describe the clinical appearance, but is actually an inaccurate description of the underlying pathology)

•Some or all of retinal vasculitis, perivascular exudates, chorioretinitis, retinal haemorrhage and retinal detachment

•Optic neuritis

Uveitis associated with infection

Systemic viral disease

Clinical signs

•It is often impossible to differentiate these conditions on clinical grounds when uveitis is the main presenting clinical sign.

•All the viruses may present with a low-grade anterior uveitis. In such cases there is a loss of iris detail and iris neovascularisation. Keratic precipitates may be obvious on the posterior cornea, especially ventrally. Frank hypopyon or cellular infiltration may be present in the ventral anterior chamber. Fibrin and haemorrhage may be present anterior to the lens and iris and may cause mechanical restriction of the pupil. Pupil mobility may also be affected by synechiae formation, so it is very common to find that the pupil shape is abnormal.

•The posterior segment should also be examined, as vasculitis and optic neuritis are possible presentations, occurring in conjunction with, or distinct from, anterior uveitis.

Feline infectious peritonitis (FIP) (Figures 4.22 and 4.23(a,b))

•This is a problem that is commoner in younger cats than older cats and is most common in pedigree cats kept in multicat households

•There is no specific diagnostic test, and it is known that enteric types of coronaviruses (FECV) may mutate to a more pathogenic type (FIPV). Interpretation of FIP serology is fraught with difficulty, as the tests detect antibody to any coronavirus, and while a high coronavirus antibody titre (>160) and hypergammaglobu-

linaemia may be suggestive of FIP disease, an intermediate, low or even negative titre does not rule out infection

•Cats with FIP often demonstrate subtle neurological involvement and are often small for their age

•FIP infection is usually confirmed by histological examination, usually of postmortem material

Figure 4.22 Anterior uveitis associated with feline infectious peritonitis. FIP is associated with typical perivascular pyogranulomatous inflammation and in this cat the aqueous is red-cell and fibrin rich and there is marked neovascularisation of the iris.

Figure 4.23(a,b) Posterior uveitis associated with feline infectious peritonitis in a Domestic Shorthair. White light (a) and blue light (b) photographs of the same area demonstrate the intense vasculitis and perivascular oedema. Both eyes were affected.

Feline leukaemia–lymphoma complex (FeLLC) (Figures 4.24 and 4.25)

•Cats of all ages may be infected by feline leukaemia virus (FeLV), although infection is commoner in young cats and is rare in cats over ten years of age

•The ophthalmic signs are usually a consequence of neoplastic involvement ± anaemia and thrombocytopenia

•Laboratory tests can provide confirmation of FeLV infection

FELINE OPHTHALMOLOGY

FELINE OPHTHALMOLOGY

Figure 4.24 Hyphaema, originating from the major arterial circle, associated with the feline leukaemia–lymphoma complex in a Domestic Shorthair.

Figure 4.25 Neoplastic infiltration of the optic nerve (infiltrative optic neuropathy) and neovascularisation associated with the feline leukaemia– lymphoma complex in a Domestic Shorthair.

Feline immunodeficiency virus (FIV) (Figure 4.26(a,b))

•FIV occurs most often in adult, free ranging, non-pedigree cats and is more common in males than females

•A positive antibody test is diagnostic of FIV However, a significant proportion of FIV-infected cats have no detectable antibody, and therefore a negative antibody result does not preclude FIV infection

Treatment

Treatment of the uveitis associated with FeLV, FIV and FIP tends to be symptomatic and supportive, although there are chemotherapy protocols available to treat FeLV infection. Some cats with FIV and FeLV can be given a good quality of life for some years after the diagnosis has been made, but this is not usually the case for FIP, where any treatment tends to be palliative only.

Figure 4.26(a,b) Chronic anterior uveitis in an FIV-positive Domestic Shorthair. Bilateral uveitis. Lens luxation has occurred in the right eye (a) and the lens has become secondarily cataractous. In the left eye (b), dark keratic precipitates (KPs) adherent to the ventral cornea indicate that the uveitis has been present for some time (KPs darken as they age). Lighter, fawn-coloured KPs indicate that the uveitis is still active. Both iridal surfaces show considerable new blood vessel formation. The anisocoria (inequality of pupil size) was due to the position of the luxated lens: the dorsal aspect was anterior and the ventral aspect occupied the pupillary aperture. Intraocular pressure (Mackay Marg) was 9 mm Hg in both eyes.

Bacterial infections

•Generalised bacterial diseases associated with uveitis are rare. Typical and atypical mycobacterial infections are rarely encountered, and ocular involvement is usually in the form of a granulomatous posterior uveitis. Combination treatment (e.g. oral clarithromycin at a dose rate of 10mg/kg and rifampicin at a dose rate of 20mg/kg) over a period of months may bring about resolution. Animals must be monitored carefully during treatment.

•Local injury, usually as a result of a bite or scratch, may result in direct intraocular inoculation of bacteria (e.g. Pasteurella multocida). This is a relatively common cause of uveitis which is amenable to symptomatic treatment for the uveitis and topical (e.g. chloramphenicol drops) and systemic administration of antibiotic (e.g. newer-generation penicillins).

Toxoplasmosis (Figures 4.27(a,b) and 4.28)

•Toxoplasma gondii is an intracellular coccidian parasite, and the domestic cat and other felidae are the only definitive hosts

•Clinical problems are most likely to be seen in debilitated or immunocompromised animals, and ocular features consist of retinitis, posterior uveitis, intermediate uveitis, anterior uveitis or panuveitis

•The zoonotic implications should be borne in mind, and it is important to discuss them with clients

FELINE OPHTHALMOLOGY