Introduction
The following points are especially pertinent when dealing with a dog that has started to lose its eyesight.
What is the signalment?
It is an axiom that many eye conditions present themselves with a particular history in certain ages and breeds of pedigree dogs. To understand veterinary ophthalmology the beginner should know which breeds develop certain eye conditions. For example, if presented with an 8-year-old Labrador with a history of night blindness and ophthalmic examination confirms the presence of bilateral retinal degeneration, then this is very suggestive of generalized progressive retinal atrophy (gPRA). A dog that presents with a painful, cloudy blind eye may well have glaucoma, but if it is a Jack Russell Terrier, the glaucoma is very likely secondary to primary lens luxation. Ophthalmic examination, including tonometry, would confirm the diagnosis and enable prompt treatment. Indeed, misdiagnosis of this case might be considered negligent, so by learning which conditions affect each breed the aspiring ophthalmologist will be half way to understanding the subject.
Is the failing eyesight acute or chronic in onset?
Unfortunately, some owners can be unobservant when it comes to noticing something wrong with their pet’s eyes, and they may delay seeking help if they do observe a problem. However, this is not always the case, and many clients may seek timely help. If the problem is associated with ocular pain, namely lacrimation and blepharospasm, this may spur owners to present their animal early in the course of a disease. However, if there is no overt pain then the first signs may be missed. An owner’s personality and attitude to seeking medical help has a strong influence on whether or not an animal is presented in the early stages of sight loss, and as part of the history taking it is important to determine how long the disease condition may have been present, asking open questions wherever possible to allow the owner time to speak.
What is the client’s primary complaint, and what other ocular features might be present?
Find out if the eye is painful; ask if any redness or discharge has been noticed, and whether the eye appears abnormal (e.g., cloudy) in any other way. Determine (both from the history and from the examination) if the condition affects one or both eyes. If blindness is the primary presenting feature then one would expect both eyes to be affected with a degree of symmetry in the lesions seen. However, it is also possible for a dog to have lost the sight in one eye due to a certain condition and for the other eye to then become affected at a later date from either the same or a new underlying pathology.
Clinical examination
Many systemic conditions can present with ophthalmic features and the alert clinician should always include the eyes during any general physical examination. Equally the ophthalmologist should be aware of the “ocular support structures” (which may be regarded as the rest of the animal!) and examine the entire animal thoroughly, particularly where bilateral ocular conditions are present. A complete physical examination should always be considered appropriate and conducted where time allows. For example, a diabetic dog not uncommonly develops secondary cataracts and therefore will require a full examination, although the animal will usually already have presented with other symptoms such as polydipsia and may indeed be receiving insulin medication by the time the cataracts develop.
Ophthalmic examination
Ophthalmic examination is the key to making a specific diagnosis. Localizing and identifying a lesion within the eye is the essence of ophthalmology. There is a great variation in eye appearance in normal animals and an essential part of understanding this subject is learning to differentiate what is normal from an acquired or congenital abnormality. A full description of how to perform an ophthalmic examination is outwith the scope of this article, and the reader is referred to other literature for more detail 1, although it is pertinent to discuss vision testing here. It is also worth noting that some knowledge of tonometry (the measurement of intraocular pressure) could be considered essential, as glaucoma not infrequently causes vision loss, and it is very useful if a clinician has access to a tonometer device to allow intraocular pressure to be measured.
The first part of the ophthalmic examination begins with observation of the pet. In general, veterinarians are very keen to get the pet into the consultation room as quickly as possible, and will usually immediately put the patient on the examination table. Do not become distracted at this point; observe the pet from a distance if you can. It is when I am greeting the owner that I first look at the patient from a distance. Watch the animal’s movements as it comes into the room and while on the floor, undisturbed by the owner. Acutely blind dogs, especially those of a nervous temperament, will show signs of anxiety on their faces. In contrast, dogs that have become gradually blind can adapt well, and may become so skilled at spatial awareness that even in an unfamiliar area like a consult room they can move around as though apparently sighted. You need to have a feeling for whether or not the dog can really see, as preconceived ideas can influence the clinician’s approach to a case.
Vision testing is generally the first part of my eye examination. It is a truism in veterinary work that a vision test is subjective; if our animal patients could talk and tell us what they can or cannot see, ophthalmology would be a very different discipline. My favorite test is visual tracking using a cotton-wool ball. This is dropped from above, within the pet’s eye line, observing for reflex movement of the eyeball or head as the pet watches the object moving downwards. A cotton-wool ball is the best object to use for this test because of the speed at which it falls; the white color also assists visibility. Each eye is tested in turn, and allowance should be made for the overlapping visual fields. I ask the owner to gently cover one eye of the animal with their hand held flat while the other eye is tested for vision. It is important to make sure there isn’t too much restraint to avoid encumbering movement of the head.
In larger dogs the test can be done with them standing on the floor, whilst medium sized dogs can be stood on the exam table and gently restrained and reassured by the owner. Small toy dogs may be held in the owner’s arms – if the dog is nervous or excitable it is essential to ensure the dog’s head is facing towards you in a comfortable manner, not tucked under the owner’s arms or towards their chest. Some dogs won’t cooperate, and in fact cats are notorious for this.
Other forms of visual testing include the following:
1. Obstacle course. If the waiting room is empty and I am unsure about the degree of vision present, I will set up an obstacle course. Be aware that this might not be appropriate to do immediately in a first opinion setting, as it takes time and needs space; it may be necessary to admit the pet to the clinic for this degree of assessment. A sufficiently large and secure room, devoid of other animals and owners, must be available; all doors need to be closed so the animal cannot escape. Objects of various sizes are placed across the floor to create a maze-like course for the pet to negotiate; I use readily available items such as chairs placed on their side, leaflet holders and waste paper bins (Figure 1). The owner should assume a position on the far side of the maze whilst I hold the pet on the near side. The owner is then asked to calmly call their dog towards them, allowing me to assess the dog’s vision. Over-enthusiastic calling must be avoided as the dog may rush through the maze and hurt itself.
Where possible it might be appropriate to attempt to assess vision in both light (photopic) and dark (scotopic) conditions, as some conditions, such as inherited retinopathies, (specifically gPRA) initially affect nighttime vision because of particular effects on rod function. The history may give a clue in this situation, so it is important to ask the owner what the pet’s vision is like when taken out at night, but the clinician should also judge if the response to the visual test seems worse in dim light, although of course this will be very much a subjective assessment.