Ophthalmic examination and diagnostics (Proceedings)

A thorough and systematic examination is important in ophthalmology as in other areas of veterinary medicine. Generally, an exam should start with the outer periocular area and then proceed to an intraocular exam is recommended. Initial observation of the symmetry of gaze and visual capacity can be performed while talking with the owners. An obstacle course using objects in the room, such as trash cans and chairs, may also be performed under both light and dark room conditions if the owners report visual abnormalities.

Once the patient is on the exam table symmetry of the eyes should again be evaluated from above, from the sides, and from a forward aspect. Deviations in gaze may additionally be evaluated by slowly rotating the head from side to side and observing the symmetry of the globes during rotation. Defects may occur secondary to space occupying lesions or due to neurological deficits. Next the blink reflex, the menace response and the dazzle reflex should be evaluated. Pupillary light reflexes are the last part of the neurophthalmic portion of the exam.

Discharge should be noted, including type, consistency and amount. The eyelids should be examined for masses, defects, additional eyelashes (distichia), ingrown eyelashes (ectopic cilia), and size of the eyelids in relation to the globes. The meibomian gland openings should appear non-inflamed with a small amount of meibum present at each opening. Inflammation, increased amounts of meibum accumulation, or the formation of a chalazion are some of the possible abnormalities that may occur. After examining the eyelids, the third eyelid and then the conjunctiva should be evaluated next. Redness, swelling, excoriations, masses, and thickening of the conjunctiva are abnormal.

After the conjunctiva the cornea should be evaluated next. Anything that disrupts the clarity of the cornea is considered abnormal and includes vascularization, pigment, scarring, edema, cellular infiltrates, and masses. The clarity and thickness of the cornea, as well as the presence of ulceration are all important in obtaining a diagnosis. Proceeding into the eye the aqueous humor in the anterior chamber (AC) is evaluated next. Any turbidity of the aqueous humor is abnormal and could include fat, blood, cells, proteins, or masses. Cells may also accumulate on the corneal endothelium to form keratic precipitates (KP's). The amount of turbidity or flare may be graded on a 1-4 basis; the same applies for the amount of cell. Any material that accumulates in the AC may obstruct the iridocorneal angle (ICA) and play a role in secondary glaucoma. Additionally, accumulation of material against the corneal endothelium may affect its function.

The iris should be evaluated for a normal, symmetrical pupil and normal pupillary movement. Thickening of the iris, infiltrates into the iris, congestion of blood vessels in the iris or changes in iris pigmentation are all abnormal and important in making a diagnosis. Masses may also be present associated with the iris. Position of the iris in relation to both the cornea and the lens is important as changes in position may lead to adhesions and play a role in glaucoma. The ICA at the periphery of the iris is routinely evaluated by veterinary ophthalmologists, but cannot be visualized without additional equipment. The ciliary body is adjacent to the posterior iris, but cannot be visualized on routine ophthalmic exam. The iris and the pupil act as a curtain over the front of the lens so mydriasis should be induced for complete evaluation of the lens. Tropicamide is a short-acting drug that is effective in approximately 20minutes. The lens should be completely transparent. Lenticular sclerosis or normal aging changes may lead to increased density and haziness of the central lens, however once dilated, the fundic reflex should be apparent even through the central lens. Any opacity in the lens is technically a cataract. However, not all cataracts are vision impairing so lenticular opacities are graded as far as progression, area of the lens affected, and pattern of opacity.

The vitreous body comprises the majority of the eye when considering volume and is usually transparent. Cysts, consolidations, masses, and liquefaction are all abnormalities to note on examination. Cloquet's Canal, the remnant of the path of the hyaloid artery is often present and is not considered abnormal. Lastly, the retina is evaluated. Many variations of normal retinas exist so experience and familiarization with the different range of normal will aid in interpretation. Most domestic animals have a non-tapetum and a tapetum, however the subalbinotic fundus is a variation of normal. Sometimes it is difficult to differentiate hemorrhage from a subalbinotic variation without some experience so routine dilation is recommended to become familiar with fundic variation.

In general practice a direct ophthalmoscope, a headloupe, a transilluminator, and a 20D lens should allow you to perform a complete ophthalmic examination. A transilluminator or even the otoscope head will allow you to evaluate PLR's and then examine the periocular structures in conjunction with a headloupe. Many different magnifications of headloupes are available in art supply stores and most are modestly priced. Alternatively, they may purchased through a medical supplier. Direct ophthalmoscopes give a highly magnified view of the structure under examination and work well for corneal examination. They are monocular so it is more difficult to use the slit beam to evaluate corneal depth, however with experience deep ulcers vs. shallow ulcers should be identifiable. By adjusting the focus wheel structures of different depths in the eye may be assessed.

Direct ophthalmoscopes are excellent for evaluating the optic nerve head due to their high magnification. It is harder to evaluate the whole retina so the use of a 20D lens with the transilluminator allows a less magnified and more comprehensive view of the retina. The examiner should be at arm's length from the patient's eye. The light source should be held up by the ear and a fundic reflex visualized. Then the lens should be moved into position as close to the patient's eye as possible. By moving the lens both up and down and side to side as well as by angling the lens the whole retina may be examined.

After thorough evaluation testing is performed. The first test to be run should always be a Schirmer Tear Test (STT). Once drops are instilled into the eye STT results are not valid since the drops will be absorbed onto the paper, not tears. The STT evaluates the aqueous portion of the tear film and is a quantitative test. The test strips should be handled as little as possible and be held by their far ends since the oils on our hands will affect the absorption capacity of the test strip. Each strip should be inserted under the lateral lower lid to the notch in the strip and remain in place for 60 seconds. Over 15 mm wetting/minute is normal and 10-15 mm wetting/minute is suspicious for Keratoconjunctivitis sicca.

If a culture and sensitivity needs to be submitted it is optimal to obtain it before instilling anesthetic and stains. The ventral fornix of the conjunctiva should be swabbed in conjunctivitis; otherwise the abnormal area should be sampled. Cytology is also best obtained prior to instillation of drops, however in many cases the pain associated with the disorder dictates that anesthetic be used prior to sampling. Anesthetic and fluorescein staining usually are performed next. Sodium fluorescein binds to the corneal stroma; it does not bind to healthy epithelium or to Descemet's membrane. Sodium fluorescein may also be used to evaluate the quality of the tear film. The stain should coat the cornea evenly and adhere to the cornea for at least 20 seconds. Additionally, passage of fluorescein through the nasolacrimal ducts may assist in evaluating their patency. Blotchiness in the distribution of sodium fluorescein is consistent with a defect in the quality of the tear film. Rose Bengal stain may also be used to evaluate the health of the corneal epithelium. Rose Bengal adheres to "sick" or degenerating epithelium cells of both the cornea and conjunctiva. The classic use of Rose Bengal stain is in the diagnosis of dendritic corneal ulcers in feline herpes virus.

Tonometry is usually the next test performed. Currently, applanation tonometry (Tonopen) and rebound tonometry (Tonovet) are the most commonly used instruments. Both these instruments are accurate and comparable in results. The above diagnostics are the most accessible for the general practitioner. Other testing available for in room diagnostics include pachymetry (measurement of corneal thickness), aesthesiometry (measurement of corneal sensitivity), ocular ultrasound, and electroretinogram.

References

1. Veterinary Ophthalmology, 4th ed., Gelatt KN, editor, Blackwell Publishing, Ames IA 2007.

2. Ophthalmic Disease in Veterinary Medicine, Martin CL, Manson Publishing, 2005.