An ocular examination is not difficult, but it does require knowledge of the normal eye, basic instrumentation and lots of practice. In order to be able to appreciate ocular abnormalities, it is essential to know the basic anatomy of the normal eye, and to understand how the various parts of the eye work together to provide their important function – vision.

Functional anatomy

The eye is a sense and sight organ, which functions to transmit light and focus it onto the rods and cones of the retina, in order that the process of vision can be initiated. To achieve this, the path that the light will take must be clear. In other words, the cornea, aqueous humour (fluid in the anterior chamber), lens and vitreous gel must be clear and free of opacities (Figure 1).

Figure 1: Visible anatomical features of the feline eye

The eyelids and tear film are very important for maintaining the health of the eye. The eyelids must be in a normal position and blink adequately, spreading the tear film evenly across the eye and protecting it from environmental challenges and threats. The lubricating tear film provides nutrition to the cornea, flushes foreign material and bacteria from the eye.

It also supplies antibacterial substances and protease inhibitors, which defend against invading pathogens and protect the cornea from destructive degrading enzymes released by bacteria and inflammatory cells. The cornea should be smooth and free of any opacities, such as blood vessels, oedema and pigment.

Finally, the cornea is vital because it provides the main refractive power of the eye, which is the bending of the light wave as it enters the eye in order to focus it. For this reason, people with refractive errors have corrective surgery on their corneas rather than on their lens.

The lens itself should be in a normal position and be clear, in other words free of cataract, so that it can focus passing light onto the retina. The retina is transparent in order that the light can pass through it to the inner photoreceptors (rods and cones), which initiate the process of vision.

The feline eye is particularly well adapted to nocturnal vision, with its large cornea, deep anterior chamber, posteriorly located lens, rod-dominated retina and the presence of the tapetum. The tapetum is a reflective mirror allowing light rays that pass through the retina to be reflected back, which affords a second chance of the light striking the rods and cones.

The cat iris is shaped as a vertical slit, allowing for very tight constriction (miosis) in bright (photopic) conditions (Figure 2), yet it becomes round when dilated to admit the maximum amount of light when in dark (scotopic) conditions.

Figure 2: A pupil constricted to a vertical slit protecting the retina from harmful bright light

Common signs of ocular conditions

Some ocular conditions are very painful and constitute welfare concerns for the cat. The most common painful condition is corneal ulceration, as the cornea is the most densely innervated structure in the body.

Other conditions, such as eyelid trauma, conjunctivitis, keratitis, uveitis and glaucoma are also painful and, therefore, require a rapid diagnosis so that the most appropriate treatment can be instigated (Figures 3 and 4).

Figure 3: Conjunctivitis and purulent ocular discharge caused by Chlamydophila infection

Figure 4: : Mucopurulent ocular discharge and conjunctivitis resulting from feline herpes virus infection

The most common signs of ocular abnormalities noticed by owners are:

•   blepharospasm (squinting and blinking)

•   ocular discharge

•   third eyelid protrusion

•   colour changes, such as redness or cloudiness

•   over-grooming of the eye area, or

•   known trauma, such as a cat-fight.

Gradual loss of vision may be difficult to notice, as cats have a tremendous ability to adapt to the situation. They can retain an awareness of the normal layout of their familiar environment, only getting caught out when it is changed. Signs of poor vision are more easily picked up when the onset is sudden, such as the cat starting to bump into things, walking around the edges of rooms in a crouched position and no longer jumping down from a height.

Essential skills

It is essential for you to examine the eyes of normal patients, in order to be able to recognise variations from the normal.

The role of the veterinary nurse is also in preparing the room, equipment and patient for examination, taking a history, performing ocular tests and providing a general clinical examination, which should not be overlooked.

The information may then be passed on to the veterinary surgeon, who may continue the examination. As a minimum, the mucous membrane colour should be assessed; the presence or absence of vision should be noticed; and the cornea, anterior chamber and lens should be checked for opacities. The fundus should be examined by the vet.

Equipment and disposables

Equipment required to carry out the ocular examination is listed in Table 1. It is essential to keep a spare set of batteries for all of the equipment, or appropriate re-chargeable handles.

Disposables, which should be available, include:

•   cotton wool

•   saline – to clean the eye and flush away fluorescein dye

•   mydriatic to dilate the pupil for lens and fundus examination. (The most appropriate drop is tropicamide 0.5% or 1% – available as Mydriacyl, Alcon or in Minims vials, Smith & Nephew. NB. Atropine is not a suitable mydriatic for examination as it has a slow onset of action and a very long duration (days in some cases)

•   fluorescein dye, which is available as sterile strips (Chauvin) or in Minims vials – (sodium fluorescein 1%,
Smith & Nephew)

•   topical anaesthetic – proxymetacaine 0.5% Minims vials are less irritating than amethocaine 1% and tetracaine 1%. (They need to be stored in the fridge and do not contain a preservative, therefore they must be discarded after a single use)

•   Schirmer tear test paper strips (Schering-Plough)

•   swabs for bacterial culture and virus isolation

•   lacrimal cannulae (plastic preferable).

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.

Veterinary Nursing Journal • VOL 25 • Nol • January 2010 •