Introductory Philosophy

The phrase "more is missed through not seeing than not knowing" is particularly true for management of small animal eye diseases. Although historical data may provide essential clues to the diagnosis, ready visualization of almost all parts of the eye means nothing can replace a complete examination. Fortunately, a thorough and revealing ophthalmic examination is readily performed using easily learned skills and equipment that is almost certainly already in your clinic. During this session, we will "diagnose and treat" a number of cases designed to exemplify many clinically relevant facets of veterinary ophthalmology. The notes cover some tips on how to perform an excellent ophthalmic exam plus the use of corneal colours to aid the diagnosis of ocular disease.

The Ophthalmic Exam

There are 4 essential requirements for a thorough ophthalmic examination:

1.  The patient and doctor must be at eye level with each other

2.  The exam must be performed in dim ambient light

3.  A bright, focal light source and a means of magnification are essential

4.  Always perform an orderly and complete examination

Mastering just 4 procedures will provide all of the essential information from the anterior segment:

1.  Retroillumination

2.  Focal illumination or transillumination

3.  Tonometry (measurement of intraocular pressure)

4.  Assessment of aqueous flare

Retroillumination is a simple but extremely useful technique for assessment of pupil symmetry and all parts of the transparent ocular media (tear film, cornea, aqueous, lens, and vitreous). A focal light source held close to the examiner's eye and directed over the patient's nose from at least arm's length is used to elicit the fundic reflection. Each eye is illuminated equally and the fundic reflex is used to assess and compare pupil size, shape, and equality. Additionally, opacities in the ocular media will obstruct the fundic reflection and are noted for more detailed examination using transillumination (or retroillumination after pupil dilation). Retroillumination is particularly useful for differentiating nuclear sclerosis from cataract.

Focal illumination or transillumination (with magnification) utilizes a bright, focal light source directed from an angle that differs from the viewing angle. Varying the viewing and lighting angles relative to each other permits utilization of parallax, reflections, perspective, and shadows to gain valuable 3D information. This technique is used to examine the anterior segment in a sequential manner. An obvious method is to progress from the front to the back of the eye and use a mental checklist - eyelids (periocular skin, eyelid margin, cilia), conjunctiva (nasolacrimal puncta, third eyelid, bulbar and palpebral conjunctival surfaces), sclera, cornea (tear film and limbus), anterior chamber, iris, and lens. Anterior segment examination should be initiated prior to dilation so that the iris face is easily examined; however examination of the lens requires full dilation.

Tonometry is essential for differentiation of the two major, vision-threatening conditions in which red-eye is the hallmark feature - uveitis and glaucoma. The availability of easily used and reasonably priced applanation tonometers such as the Tonopen® (www.danscottandassociates.com/) makes measurement of intraocular pressure (IOP) easy in all species, particularly cats. Across large populations, normal canine and feline IOP is reported as 10–25 mm Hg. However, some variation occurs. Comparison of IOP between right and left eyes permits application of a reasonable rule of thumb that IOP should not vary between eyes of the same patient by more than ~20%. The obvious application for the Tonopen is the diagnosis of glaucoma where IOP is elevated. However, tonometry is also used to diagnose uveitis, in which IOP is lowered. Perhaps the most important role for tonometry is the monitoring of progress of these diseases and the titration of medications needed.

Aqueous flare is a pathognomonic sign of uveitis due to breakdown of the blood-ocular barrier with subsequent leakage of proteins into the anterior chamber. It is best detected using a very focal, intense light source (the small circular aperture on the direct ophthalmoscope works well) in a totally darkened room. The passage taken by the beam of light is viewed from an angle. In the normal eye, a focal reflection is seen where the light strikes the cornea. The beam is then invisible as it traverses the almost protein- and cell-free aqueous humor in the anterior chamber but becomes visible again as a focal reflection on the anterior lens capsule and then as a diffuse beam through the body of the normal lens. If uveitis has allowed leakage of serum proteins into the aqueous humor, then these cause a scattering of the light as it passes through the anterior chamber. Aqueous flare is therefore detected when the beam of light is visible traversing the anterior chamber.

Other Ophthalmic Diagnostic Techniques

Unlike assessment of other less accessible organs, visual examination of the eyes frequently provides all the clues necessary to reach a clinical diagnosis. Some of the more commonly used tests are briefly covered here.

The Schirmer tear test (STT) records basal and reflex tearing as millimetres of wetting of a strip of absorbent paper placed in the lateral, ventral conjunctival fornix for one minute. The lateral fornix is used to ensure that the strip lightly touches the cornea and produces reflex tearing. If the strip is placed more medially, it may be prevented from touching the cornea by the third eyelid. The STT should be performed prior to application of any topical solutions. General anaesthesia and sedation also cause a temporary (at least 48 hours) depression of normal tearing. Normal STT values for dogs are > 15 mm in 60 seconds. However STT values in normal cats range widely (3–32 mm; mean = 17 mm in 60 seconds) and are more difficult to interpret than in dogs.

Application of fluorescein dye to the cornea is routinely used to diagnose ulcers but also provides information regarding nasolacrimal duct patency (Jones test) and corneal perforation (Seidel's test). For a Seidel's test excessive dye is applied and not rinsed from the cornea. The site of potential corneal rupture is then examined with a cobalt blue light and magnification. Leakage of aqueous humor will cause small rivulets to form in the fluorescein dye pooled on the corneal surface.

Basic neuro-ophthalmic testing includes assessment of cranial nerves involved in ocular function (CN II–VII), the central visual pathways, and the visual cortex. This can be performed reasonably completely with relatively few simple tests:

 Menace response (CN II & VII, visual cortex, and cerebellum)

 Behavioural vision testing (CN II, visual cortex, and all areas involved in motor function)

 Direct and consensual pupillary light responses (CN II & III, and central visual pathways excluding the visual cortex)

 Palpebral and corneal reflexes (CN V & VII)

 Dazzle reflex (CN II & VII, and subcortical visual pathways)

 Doll's eye reflex (CN III, IV, & VI)

Retropulsion of the globe is a simple but useful method for investigating orbital disease. This is performed by applying gentle digital pressure to both globes through closed lids. The resistance to retropulsion and the resilience with which the globes "spring" back against the retropulsive force are subjectively assessed. Retropulsion of the globe in a variety of directions may further localize orbital masses or outline smaller masses that would be missed by direct caudal retropulsion only.

Corneal Colours As A Diagnostic Aid

Any decrease in corneal clarity is indicative of one of a number of pathologic processes, each of which is associated with a defining colour change. Learning to recognize and interpret these colour changes and the mechanisms responsible for them provides a simple and logical approach to diagnosis of all corneal and some intraocular diseases.

1.  Corneal vascularization causes a red discoloration of the cornea. It is a non-specific indicator of chronic irritation; however, distribution of corneal blood vessels provides valuable information regarding location and depth of the inciting cause. Therefore, differentiation of deep from superficial corneal is critical (Figure 1). Superficial corneal vessels arise from the conjunctiva at the limbus. They tend to be fine, branch to form "tree-shaped"patterns on the cornea, and often are seen crossing the limbus. Superficial vessels reflect ocular disease due to inadequate protection or excessive frictional irritation of the corneal surface.
Deep corneal vessels arise from perilimbal ciliary and scleral vessels and tend to be darker, shorter, straighter, and not to branch. They cannot be seen crossing the limbus, but instead arise from under the scleral shelf to form a "hedge-shaped" pattern. They are characteristic of serious, vision-threatening disease such as deep keratitis, uveitis, or glaucoma.

Figure 1

2.  Corneal (stromal) oedema is usually evident as a bluish "fluffy" discoloration of the cornea. Corneal oedema represents dysfunction of one or both of the cell layers responsible for corneal deturgescence (the epithelium or endothelium). Application of fluorescein stain and careful attention to the severity and area of oedema will assist in differentiating which cell layer is dysfunctional (Figure 2). Epithelial loss (corneal ulceration) produces more focal and milder oedema in close spatial association with a currently or recently fluorescein-positive area of cornea. Endothelial decompensation tends to produce more severe and diffuse corneal oedema because endothelium has the more important role in maintaining deturgescence. Endothelial dysfunction may occur as a primary event, especially in dogs or be secondary to 3 intraocular diseases - uveitis, glaucoma, or lens luxation.

Figure 2

3.  Corneal scars are due to derangement of the usual highly regular array of stromal collagen. The scattering of light this induces causes grey, wispy discolorations in an uninflamed cornea. These lesions do not retain fluorescein stain. Scars are by definition inactive and require no further treatment.

4.  Lipid and/or mineral accumulation appears as crystalline or creamy silver-white areas in the cornea. Varying combinations of lipids and minerals are possible. It occurs as a primary inherited, but not necessarily congenital condition in many dog breeds (corneal lipid dystrophy) but rarely in cats, or secondary to corneal inflammation or injury (corneal degeneration) in dogs (and sometimes cats) (Figure 3).

Figure 3

5.  Corneal pigmentation causes an obvious black discoloration of the corneal surface. In dogs, the pigment is melanin and tends to encroach insidiously from the limbus. Black discoloration of the feline cornea is rarely due to melanin. Instead, the feline corneal pigment is soluble and appears to originate from or be spread in the tear film and is often associated with a corneal sequestrum. Regardless of pigment type (or species affected), corneal pigmentation is a sign of chronic irritation. It is often seen in conjunction with superficial corneal vascularization, which shares the same mechanism (Figure 1).

6.  Fibrin and WBCs appear as a tannish-grey corneal discoloration. These appear in two common locations with characteristic appearances. Keratic precipitates (KPs) are multiple, clumped accumulations of inflammatory cells and debris on the inner corneal (endothelial) surface. They have a "greasy" appearance, and tend to be deposited in vertically aligned, linear arrays along the ventral cornea. KPs are a pathognomonic sign of uveitis. Thorough investigation for causes of uveitis should be undertaken. A staphyloma describes herniation of uveal tissue through the corneo-scleral tunic.

7.  Inflammatory cell infiltration of the corneal stroma appears as yellowish-green discoloration. This is most often due to an infectious cause or an intracorneal foreign body, but can sometimes be non-septic. Corneal cytology along with culture and sensitivity testing should be performed.