How I approach… The dog with failing eyesight

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.

2. Pupillary light reflex (PLR). A bright light is shone into the eye to assess constriction of the pupil. Beware: this is not a test of vision. PLR is subcortical and is a test of the afferent and efferent arms of the autonomic nervous system, i.e., the neuroretina, the optic nerve and oculomotor nerve. I consider this test to be useful in giving clues as to the state of health of the retina, optic nerve, optic chiasma and oculomotor nerve. However, the following comments on this test are important:

• It is possible to have a good, rapid PLR even where there is complete retinal detachment or advanced retinal degeneration. Until recently the reason for this was unknown, although it was thought that perhaps the response relied only on a very small number of functioning photoreceptors. Recently it has become apparent that PLR is elicited by different colors (i.e., wavelengths) of light, so the chosen light source may have a much greater influence on this test than most clinicians realize 2.
• The PLR test will be of no help without a sufficiently powerful light source; the iris muscle will not contract rapidly. This also occurs if there is age-related iris atrophy (see later) or if the dog is particularly fearful or aggressive. A negative test does not necessarily mean there is a lesion; if necessary get a better light source or replace the battery in the flashlight!
• PLR will be absent and/or the pupil will be dilated if a mydriatic drug has been applied, so use open questions to verify this point when taking a thorough history. If a case has been seen previously by another center, identify what topical drugs have been used recently. Remember that if atropine has been applied topically then it can last several days, especially in a normal eye where no uveitis is present (or if an acute uveitis has been present but quickly controlled).
• The test is particularly useful if there is a unilateral ocular opacity. The presence of a consensual PLR (where the pupil in the opposite eye constricts) will give a clue that the retina is functional in the affected eye (Figure 2). Further tests, such as the swinging flashlight test, are recommended for clinicians with a major interest in ophthalmology 3.

3. Dazzle test. A bright light is suddenly shone onto the eye to see if there is a reflex blink; this is also a subcortical reflex and gives an indication of retinal function. Again it is a useful pointer in some cases, e.g., when investigating mature cataracts; a poor PLR in an older pet with cataracts might be due to iris atrophy, whereas a positive dazzle test may indicate the retina is healthy enough to warrant consideration of cataract extraction.

4. Menace text. The hand is suddenly moved into the visual field to determine if the animal can see – strictly speaking this should be called a menace response as it is a learned behavior. The full pathway differs from that of the PLR because it has a component that involves the cerebellum. There are good and bad techniques to performing the menace response, and there is more to it than simply waving a hand close to the animal’s face. Test each eye in turn, and be aware that there is a nasal and temporal field of vision in each eye due to the crossover in central optic nerve pathways. Do not create air currents with your hands; some authors recommend using a plastic screen to shield the air currents but in my opinion this makes performing the test too complicated!

The aim of an ophthalmic examination is to determine the anatomical location of any abnormality within the eye and then to reach a conclusion as to the possible etiology.

The remainder of the examination involves detailed assessment of the adnexa (eyelids), conjunctiva, cornea, anterior chamber, iris, lens, vitreous and retina.

Normal features of aging

Care should be taken as it is not uncommon to be presented with an animal with failing eyesight that also has a normal ocular feature of aging, and it is essential to be able to differentiate this from an acquired pathology. A normal aging feature will have no effect on vision and there may be another lesion present which needs differentiation and definitive diagnosis.

Normal features of aging include:

1. Iris atrophy. This is an age-related atrophy of the iris muscle, particularly the constrictor muscles which lie more centrally than the dilator muscles. The pupil develops a “ragged” edge and the iris tissue becomes thinner. Transillumination with a bright light source will highlight this. Iris atrophy can be a feature of aging of any animal, particularly over the age of ten, and small breeds (e.g., toy poodles) are commonly affected. It has no known effect on vision but it is relevant as it can lead to a negative or poor PLR.

2. Nuclear sclerosis. The lens, which has a structure comparable to the layers of an onion, grows throughout life. The nucleus becomes more compressed with age and can give the illusion that the central portion of the lens is cloudy when viewed under normal background lighting (Figure 3). Owners will often present old dogs with eyes that appear opaque with the presumption they have cataracts. By using distant direct ophthalmoscopy, nuclear sclerosis can be readily differentiated from a true cataract by using the technique of retroillumination (Figure 4).

Diagnosis

There are many diseases and conditions of the eye which are potentially injurious to vision, and there are too many to cover in any great detail within this article. However, vision loss can result either from conditions that cause opacity of the ocular media or conditions that are injurious to specific structures of the eye (such as the retina and optic nerve), and these can result from either congenital (Table 1) or acquired (Table 2) pathologies. Two case reports offer examples as to conditions that can leading to failing eyesight in the dog.

Table 1. Congenital/early developmental eye conditions associate with vision impairment.

Table 2. common acquired eye conditions associated with vision impairment*.

* Note that this is not an exhaustive list and there is a degree of crossover between acute and chronic conditions 4.

Case 1

An 8-year-old Springer Spaniel presented for assessment of blindness. Overtly, there were no other symptoms and neither eye was painful or opaque. The dog was one of eight animals that lived outside on a smallholding; the owner had noticed poor eyesight over the last few days, and was unsure whether the dog could see or not. The dog seemed well systemically and was reported to be eating and drinking normally.

Visual tests, although inconclusive, suggested that vision was poor; the dog could negotiate around a room without colliding with stationary objects but could not follow cotton-wool balls easily. Both pupils were dilated and poorly responsive to bright light. There was no other cranial nerve or neurological deficit present.

Ophthalmic examination did not reveal any ocular lesion, other than absent PLR, and each retina appeared fundoscopically normal. The referring vet was concerned about retinal bleeding but none was apparent.

A normal color variation (an orange-brown color) of the non-tapetal fundus was noted, this was perhaps mistaken for retinal hemorrhage by the referring vet.

Differential diagnoses include sudden acquired retinal degeneration (SARD), optic neuritis or a CNS lesion affecting the central pathways.

Electroretinography (Figure 5) was performed and showed a negative trace, which supported a possible diagnosis of SARD (Figure 6) and a more complete work-up, including MRI imaging to investigate for a central lesion, was therefore deemed unnecessary.

No treatment is possible for this condition but the owner can be counseled on their pet’s welfare and how to help the animal adapt to vision loss.

Case 2

A 10-year-old Cairn Terrier presented because of concerns about failing eyesight. The dog had started to become lost on walks and collide with certain objects around the house. On questioning the owner did not feel their pet was showing any signs of ocular pain but did report that the eyes had become cloudy some months before and were now looking even more “odd”, in that they seemed to be bulging and red.

The dog failed to respond to sight tests and both pupils were dilated and non-responsive. Ophthalmic examination on this occasion revealed a number of ocular changes. Each globe was buphthalmic (enlarged) and there was episcleral congestion and dark brown pigmentation of the scleral and peripheral cornea (Figure 7).

Diffuse corneal edema was also present, making the intraocular examination more challenging. With indirect ophthalmoscopy, bilateral retinal degeneration was visible with cupping of the optic nerve. Tonometry revealed a high intraocular pressure reading in each eye (45 mmHg).

This case was diagnosed as pigmentary glaucoma, a condition which has been reported in the Cairn Terrier. The disease course is chronic and insidious but medical management with anti-glaucoma medication can slow the inevitable deterioration and give the dog and the owner time to adapt to the blindness.

Conclusion

Having seen thousands of eye cases over the last 25 years, I take a relatively simple view when making a diagnosis: if it looks like the last case I saw with a certain condition, then it has probably got that one. This method of achieving a diagnosis is called “pattern recognition” and for a clinician who has had lots of case experience it can work well. However, the beginner to ophthalmology might need to adopt what is called a “problem-oriented” approach, and even experienced veterinarians should use this method when presented with a rare or unusual case. At its most basic level, determine which part of the eye is affected, list all the salient features, consider the differential diagnoses, reach a provisional diagnosis and conduct confirmatory tests. Never forget that history, signalment and full clinical examination are also key to a successful diagnosis!

Finally, it must be stressed that in some situations a quick diagnosis is essential – for example, a cloudy, painful eye with episcleral congestion and impaired vision, together with a sluggish dilated pupil, can be the cardinal signs of glaucoma. Tonometry will generally confirm this diagnosis, and gonioscopy of the other eye might help determine if it is a primary or secondary glaucoma. If the clinician waits until the globe is grossly enlarged before confirming the diagnosis then it is too late – so if ever in doubt, consider referral to a specialist ophthalmologist!