Can you see what I see? The ophthalmic examination by Sally Turner

How often do you look at something without really seeing what's there? It's often the case that we don't take careful note of what we look at in everyday life, but when it comes to ophthalmology it's critical to pay attention to detail in what is a very visual subject!

Careful examination of the patient’s eyes can often enable a diagnosis without having to perform lots of diagnostic testing. Often the condition is readily visible to those who really examine the eyes meticulously. It is also vital to record all observations, so this article will address the principles of the ophthalmic examination.

Initial examination

The initial part of the ophthalmic examination involves just observing the patient. As with many veterinary disciplines, the breed and age of the animal often give clues as to the possible problem. Is it one of the breeds that are predisposed to various eye complaints?

For example, the five-year-old Jack Russell terrier with a sudden onset, painful, cloudy eye, might be suffering from a lens luxation; the three-year-old Persian cat with a sore eye, for several weeks, might have a corneal sequestrum; the nine-year-old Cocker spaniel with a gradual deterioration in its eyesight might have progressive retinal atrophy. The list goes on – but, of course, we must not jump to conclusions.

It is important to study the patient carefully, observe how it enters and negotiates the consulting room. Cats should be encouraged out of their baskets to explore the new environment. This ‘hands off’ approach enables an initial assessment of how well the animal can see. It is important to observe such things as the posture, a high-stepping gait, obviously bumping into objects, how confident the patient seems and so on. Look for signs of discomfort, including:

• blepharospasm (squinting)

• discharge (nature and amount)

• photophobia (light sensitivity)

• tear staining, both on the face and the paw (which is suggestive of rubbing)

• redness and swelling in the periorbital area

• symmetry between the eyes

• colour change to the eye itself.

Obtain a history from the owner, in which the following information is critical:

• what the owner is worried about

• onset and progression of signs

• whether the owner has treated the eye(s); and if so,

• how effective was the treatment?

Hands on

It is important to know how to hold the animal properly. A table with adjustable height is essential. Dogs of Labrador size and below are best examined on the table, but bigger dogs are easier on the floor. In general, minimal restraint should be used – one hand behind the back of the head to stop the patient from shying away from any bright lights used to examine the eyes, and the other hand to steady the muzzle (Figure 1).

Figure 1: How to hold a dog's head for an ophthalmic examination

For cats, gently cuddling them while holding their elbows to stop them trying to claw the vet’s face is usually sufficient! The ophthalmologist will probably hold the cat’s head.

The first part of the ‘hands on’ exam is conducted in the light. The eyes are briefly checked again for symmetry, discharge and discomfort, while the conjunctiva is inspected for redness, the position of the eyelids and the corneal reflex. The neurological part of the ophthalmic examination is performed next (Table 1).

Schirmer tear tests should also be undertaken at this early stage of the examination, and any samples for bacteriological culture and sensitivity testing taken.

Detailed examination

The detailed ophthalmic examination is undertaken in the dark. Pupillary light reflexes are tested again before using good illumination and magnification to examine the adnexa (eyelids, third eyelid, periorbital region), the conjunctiva, the cornea and the anterior segment. An initial examination of the lens is also included here.

The ideal instrument for examining this anterior part of the eye is the slit lamp biomicroscope (Figure 2). In general practice, a good light source and magnification, for example with surgical loupes, (Figure 3) or a direct ophthalmoscope on the correct magnification setting (Table 2) will suffice.

Figure 2: Use of the slit-lamp biomicroscope. Note the way the nurse holds the cat gently, preventing it from putting a paw up, while the ophthalmologist holds the cat's head to move it where she wants to achieve a complete examination

Figure 3: Magnifying loupes and a good light source (Finhoff transilluminator on the base of the direct ophthalmoscope) can be used for examination of the adnexa and anterior parts of the eye

To fully evaluate the lens, and examine the posterior segment and fundus (i.e. vitreous, retina, choroid and optic nerve) the pupils should be dilated. Normally 1% tropicamide (Mydriacyl -Alcon) is used and it takes 20 to 30 minutes for full dilation to occur.

Once the pupil is dilated, the lens is examined in more detail – mainly checking its position and transparency. Dislocation or luxation of the lens is common in terrier breeds as a primary problem, and also can occur secondarily to other ocular disease, such as glaucoma in dogs, or chronic uveitis in cats.

A cataract is an opacity in the lens (including the lens capsule) and inherited cataracts are very common in some dog breeds, for example, Labrador and Golden retrievers, Staffordshire Bull terriers, Boston terriers, Bichon Frise and many more. DNA tests are available for some breeds (Table 3).

Examination of the back of the eye (the fundus) is achieved with either indirect (Figure 4) or direct ophthalmoscopy (Figure 5). Most veterinary ophthalmologists prefer the former technique – which provides a large field of view and a 3D image. It does not take long to overcome the initial confusion, in that the image seen is both upside down and back to front!

Figure 4: Indirect ophthalmoscopy

Figure 5: Direct ophthalmoscopy. Note that the ophthalmologist has a finger resting on the cat's head, so that she can detect any tiny movement that might mean the patient is about to struggle

Direct ophthalmoscopes are most common in general practice. They do provide very good magnification, but the field of view is small and to examine the fundus thoroughly it takes longer, especially in a moving patient!

There is a wide variation in the normal appearance of the fundus – the colour of the tapetum (the shiny layer which improves nocturnal vision), the size and shape of the optic disc, the amount of pigment in the non-tapetal fundus and the distribution of the retinal blood vessels. It is worth looking at as many eyes as possible so that the differences in normality can be distinguished from pathological changes (Figures 6a and 6b).

Figure 6a: Normal cat fundus

Figure 6b: Normal dog fundus

Ophthalmic dyes

Ophthalmic dyes, especially fluorescein, are frequently necessary. They are used mainly to check for corneal ulceration. The orange impregnated strips turn green when moistened in the tear film, and any areas of corneal ulceration show up bright green where the dye adheres to the damaged area. Excess dye should be thoroughly flushed before examining the cornea and use of a blue light will enhance the fluorescence seen.

A second use of fluorescein is to check the patency of the nasolacrimal ducts. A drop in the eyes should result in the appearance of the green colour at the nostrils within a few minutes. The mouth should also be checked, because in some animals (especially brachycephalics), the nasolacrimal ducts open into the oropharynx. Many false negatives occur – where the dye is not visible even though there is no blockage – so, if a negative dye test occurs, it is usually followed by nasolacrimal flushing.

Tonometry

Tonometry is the measurement of intraocular pressure and is essential for the accurate diagnosis and monitoring of glaucoma (Figure 7). It is also used to check for uveitis. With glaucoma the pressure is elevated; with uveitis is it lowered. The normal pressure range is 10 – 25mmHg in dogs and cats.

Figure 7: Tonometry – measurement of intraocular pressure – using a Tonopen

Tonopens are most commonly used in specialty practice. The far cheaper (but more cumbersome to use) Shiotz tonometers are accurate if used correctly and can often be found in general practice. However, they are frequently dumped at the back of a cupboard and they are not used often enough!

Gonioscopy

Gonioscopy is the examination of the iridocorneal or drainage angle. This is the area of the eye thorough which aqueous drains (Figure 8). It can be directly visualised in cats and horses, but a special plastic contact lens, or goniolens, is required in order to see this part of the eye in dogs.

Figure 8: The iridocorneal angle as viewed through a goniolens

Examining the drainage angle allows the ophthalmologist to evaluate the area for abnormalities that could predispose the dog to glaucoma. Goniodysgenesis, a congenital malformation of the drainage angle, is inherited in several breeds of dog (Cocker spaniel, English Springer spaniel, Flat-coated retriever, Bassett hound) and affected dogs have a higher incidence of glaucoma in early to mid-adulthood.

In conclusion

It can be seen that by careful examination and accurate recording – using an eye examination form, with diagrams together with photos, if appropriate – a huge amount of information can be gained from the ophthalmic examination. Obviously some patients will require further diagnostic testing, such as ocular ultrasonography, or electroretinography (to assess retinal function) the discussion of which is beyond the scope of this article.

Next time you have a quiet half hour, pick up an ophthalmoscope and have a look into the eyes of some dogs and cats. You will be amazed at the beauty of the anatomy of the eye!

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