ABSTRACT: Working as a veterinary nurse, it is expected that you will come across visually impaired and blind cats, along with their concerned owners. The VN plays a crucial role in helping the owner to understand why his or her cat has eye problems, and what the different treatment options involve for both them and their pet. It can be a very emotional subject for owners, who imagine themselves being blind and the implications that would have on their lives. They worry that they or their pet will not cope, and therefore need thoughtful advice and support.

You can guide them through changes they can make to their home to help the cat to continue to enjoy a great quality of life, and support them when they need to make difficult decisions, such as enucleation of one or both eyes. Indeed you may give advice to a prospective owner wanting to re-home a cat with vision problems, and help him or her to create an enriching home life.

Part 1 of this article deals with signs and causes of blindness, with potential treatment options. Part 2 in June will deal with the emotional side of things, and how to adapt the home to make a secure and happy environment for the visually impaired cat.

Signs of blindness

The signs which lead the owner to suspect that the cat has vision problems can be subtle or more obvious, depending on the rate at which blindness develops.

Sudden blindness causes more dramatic changes, which the owner is more likely to notice. These include a sudden loss of confidence, manifested as reluctance to jump, bumping into things, and walking with a low crouching gait around the edges of a room rather than through the centre. The eyes may change depending on the cause – they may be red with hyphaema (blood in the anterior chamber) or cloudy with cataracts. There may be a bright green glow from the tapetum at the back of the eye, as the shine of this reflective mirror is more intense if the overlying retina is thin or absent, as in the case of retinal degeneration (Figure 1).

Figure 1: Blue tapetal hyper-reflectivy can be seen in this one-year-old cat with retinal degeneration

Not all blind eyes look obviously abnormal. Cats with a slow onset of blindness adapt so well that the owner may not appreciate any change at all until the layout of the furniture is changed or doors which are normally open are closed, which will catch a blind cat out.

Reasons for blindness

The most common causes of blindness in cats are summarised as follows.

Retinal detachment may occur as a result of high blood pressure (hypertension). There may be hyphaema (blood in the anterior chamber) (Figure 2). Underlying causes include renal disease, hypertrophic cardiomyopathy and hyperthyroidism, although some cases are idiopathic. All older cats presenting with sudden blindness should have blood pressure measurement.

Figure 2: Hyphaema in the right eye and a retinal detachment was seen in the left eye, caused by hypertension

Cataracts are opacity of the lens or the lens capsule (Figure 3). Total/mature cataracts cause blindness. They are less common in cats than in dogs, and are usually secondary to inflammation (uveitis), trauma or glaucoma.

Figure 3: Bilateral cataracts in a six- year-old cat

Trauma can precipitate blindness by causing globe rupture, cataracts or retinal detachments. Head trauma may cause traumatic brain injury which can result in blindness.

Glaucoma occurs as a result of sustained raised pressure within the eye, caused by impairment of aqueous humour drainage from the eye. It is a painful and blinding disease, and in the cat is most often a result of uveitis.

Uveitis is inflammation of the uveal tract (the iris, ciliary body and / or choroid). Causes include infectious disease, such as feline immunodeficiency virus, feline leukaemia virus, feline infectious peritonitis and feline herpesvirus (FHV). Other possible causes are trauma, corneal ulcers and neoplasia, although some cases are idiopathic and are predominated with the inflammatory cells lymphocytes and plasma cells.

Birth defects may occur sporadically, with one or more kittens in a litter affected. The eyes may be very small and non-functioning (Figure 4).

Figure 4: Evie was born with no eyes and tiny eyelid openings

Corneal ulceration usually heals with the appropriate treatment but it can lead to blindness if the ulcers deepen and become perforated, if intraocular structures were also damaged, or if there is significant scarring during healing. 

Tumours can arise from within the eye or in the immediate peri-orbital area. They can cause blindness by giving rise to glaucoma and uveitis, or may cause the vet to remove the eye. Both eyes may be affected by lymphoma.

Retinal degeneration results in blindness and may be caused by taurine deficiency, enrofloxacin (Baytril, Bayer) toxicity, hereditary progressive retinal atrophy (PRA) and after inflammatory retinopathy. Taurine deficiency is uncommon nowadays as all commercial cat foods contain the amino acid in sufficient quantities, but cats fed on home-made diets may be at risk. Cats which receive Baytril (usually above the recommended dose rate) can become suddenly and irreversibly blind, and thus it is imperative to adhere strictly to the data-sheet recommendations or use an alternative antibiotic for cats.

Hereditary PRA occurs in the Abyssinian cat and has occurred in the Siamese and Persian. Affected kittens usually have apparently normal vision but progress to gradual blindness by the time they are young adults.

FHV is common and can cause symblepharon (conjunctiva adhering to the cornea resulting in opacity and visual impairment), corneal ulcers which may become deep or uveitis.

Following enucleation of one eye –

because the optic nerve of the cat is very short, if excessive traction is placed on the eye being removed, the force is transmitted easily to the optic chiasm where it might damage the optic nerve of the other eye. Thus careful atraumatic surgery is essential to avoid this devastating and avoidable complication.

Following general anaesthesia –

hypoxia and hypercapnia may occur during or following general anaesthesia, which can cause damage to the visual cortex resulting in vision loss.

Treatment Options

Treatment varies greatly according to the cause of blindness.

In certain cases, no treatment is required, for example in
cases of retinal degeneration. Eye drops are required to treat glaucoma and uveitis. Hypertension is most often treated with oral amlodipine (Istin, Pfizer) tablets once daily. Herpesvirus may be treated by minimizing stress, topical eye drops (anti-virals, interferon or antibiotics), oral anti-viral tablets famcyclovir (Famvir, Novartis) or oral amino acid L-lysine dietary supplements to reduce replication of the herpesvirus.

Cataracts may be surgically removed using phacoemulsification, offering a chance to restore vision. Not every eye is suitable for cataract surgery, and cats with cataracts may be referred to a veterinary ophthalmologist for assessment. Glaucoma management involves control of the underlying cause (usually uveitis) and topical pressure-reducing drops, such as brinzolamide (Azopt, Alcon). Treatment, however, is not always successful at reducing the intraocular pressure. Glaucoma is a very painful condition, although cats may fail to display signs of overt pain. Usually they are less interactive, sleep more and hide away. In cases of uncontrolled glaucoma, or when a tumour is suspected or confirmed, enucleation is the treatment of choice.

Figures 5 and 6: An eye with a cataract before, and the morning after, cataract surgery using phacoemulsification with placement of a synthetic lens

Enucleation

This is the surgical removal of the entire globe, which is the required treatment for some eye conditions.

After removing the globe, the edges of the eyelids are removed and sutured together permanently.

There are few risks with this procedure, but they include anaesthetic concerns, bleeding, future cyst formation and blindness of the remaining contralateral eye if excessive traction is used as outlined above.

The benefits are that the cat is free of pain and will not require on-going medication. The eye is available for histopathology which can provide an accurate diagnosis so that specific treatment may be prescribed and an informed prognosis may be given.

In some cases, an ocular prosthesis may be placed. This can take the form of an intraorbital prosthesis, when a spherical silicone prosthesis is placed within the orbit prior to closure of the eyelids, thus preventing a skull depression which the owners may object to.

In certain cases, the prosthesis may actually be placed inside the globe after it is opened up behind the limbus, allowing the ocular contents (iris, ciliary body, choroid, retina and lens) to be removed. The surgical wound is then closed, leaving a formed eye structure.

This method may be considered where the owner feels strongly that they would rather euthanase their pet than have the eye removed. However, retaining the eye is of little benefit to the cat, and the material removed has little value for histology.

Enucleation is an emotive topic for owners, and this will be discussed further in Part 2 in the June issue of the VNJ.

Author

Natasha Mitchell mvb Certvophthal MRCVS

Natasha graduated from University College, Dublin in 1998. She developed a keen interest in ophthalmology and obtained a RCVS Certificate in Veterinary Ophthalmology in 2004. She now runs her own referral ophthalmology service in Limerick, Ireland – e-mail, natasha@eyevet.ie or visit www.eyevet.ie to find fact sheets, newsletters and helpful tips for both veterinary personnel and cat owners.

• VOL 25 • No5 • May 2010 • Veterinary Nursing Journal