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Mean length of absorption for the PRT tear test in cats is 23.0 mm ± 2.2 mm/15 seconds. The normal range in cats for the PRT tear test is 18.4 to 27.7 mm/15 seconds.
In dogs the mean length of absorption using the PRT tear test is 29.7 to 38.6 mm/15 seconds.
Tear Drainage
The excretory component of the nasolacrimal system is evaluated by the presence or absence of medial canthal tearing; passage of fluorescein instilled onto the eye; nasolacrimal flush; catheterization of the entire system, and by dacryocystorhinography. The nasolacrimal drainage apparatus consists of two puncta and canaliculi, a poorly developed nasolacrimal sac and the nasolacrimal duct. The oval puncta are situated in the upper and lower medial eyelid margins about 1 to 2 mm in the palpebral conjunctiva. A partial to complete ring of pigment may surround the puncta and facilitates their detection.
Passage of fluorescein from the eye to the external nares is a reasonable test for patency of the nasolacrimal system. A strip of fluorescein is moistened with a few drops of sterile eyewash and touched to the upper bulbar conjunctiva. The dye usually appears at the external nares in 3 to 5 minutes. Both sides should
be performed at the same time to compare passage times. Ultraviolet light enhances detection of the dye. Fluorescein passage in brachycephalic dogs and is
not reliable as the dye may exit more readily into the nasopharynx. The animal’s tongue and saliva should be examined with a UV light in these cases.
The nasolacrimal flush determines patency of the system and the treatment of many of its disorders. The upper punctum is cannulated with a 22-23 g blunt lacrimal needle or 22-24 gauge teflon catheter under topical anesthesia. Tranquilization or general anesthesia is seldom necessary for the dog but often necessary for the cat. A 2 to 3 ml plastic syringe with sterile saline is used to inject the solution through the upper punctum, canaliculus, nasolacrimal sac, lower canaliculus and out the lower punctum. Once this “arc” is established, the lower punctum is compressed digitally and the solution is forced through the nasolacrimal duct and out the external nares. If the dog’s head is positioned upward, the dog will swallow or gag on the solution. Excessive pressure should be avoided to minimize the danger of rupturing the N-L system above an obstruction.
D. External Ophthalmic Stains Fluorescein
Examination of the cornea is incomplete without utilization of topical ophthalmic stains. Fluorescein is used to demonstrate the presence or absence of corneal ulcers. For topical use, fluorescein impregnated paper strips are preferred to fluorescein solution to insure sterility.
Rose Bengal
Rose bengal is retained by the cornea and conjunctiva in keratoconjunctivitis sicca, early fungal keratitis, pigmentary keratitis, exposure keratitis, viral keratitis, and certain other corneal ulcers.
E. Intraocular Pressure Measurement (Tonometry)
Intraocular pressure (IOP) is estimated digitally, and measured by Schiotz tonometry or applanation tonometry. Subtle elevations in intraocular pressure, repeated measurements of glaucomatous eyes under medical treatment, or after surgical intervention require instrument tonometry.
Applanation tonometers (especially the Tonopen
type) are very accurate and easy to use. Applanation tonometers are becoming more common in practices. The Tonopen applanation tonometer has made it much easier to diagnose and treat the animal glaucomas.
IOP is 16.8 ± 4.0 mm Hg in dogs; 20.2 ± 5.5 in cats; and
23.2 ± 6.9 in horses.
F. Ophthalmoscopy
1. Direct Ophthalmoscopy
Direct ophthalmoscopy is used more frequently by practitioners than indirect ophthalmoscopy. However, both techniques have advantages that complement
each other when used together. The method is termed “direct” because a condensing lens is not interpositioned between the ophthalmoscope and the patient’s
eye. The examiner has a direct optical image of the patient’s eye. The fundus image is real, upright and approximately about 17 to 19 times magnified in dogs and cats. The fundus area visualized is about 10 degrees or approximately 2 disc diameters.
The direct ophthalmoscope head also offers a range of lenses to enable focusing at various depths within the eye. These lenses are calibrated in diopters. A lens with
a power of 1 diopter will focus light from an infinite source (parallel rays) at 1 meter. The higher the diopters, the
more converging power the lens possesses. Negative diopters denote diverging lenses. When an emmetropic eye (observer) looks into an emmetropic eye (patient) with an ophthalmoscope the retina of the patient should be in focus at the 0 diopter setting. Minor lens corrections are usually needed to focus on the patient’s fundus. Within the eye, a distance of 3 diopters equals 1 mm.
In performing ophthalmoscopy, the patient’s body and head are minimally restrained by an assistant. The examiner holds the muzzle and/or lids with one hand and with the other hand holds the ophthalmoscope to make the necessary diopter changes. It is preferred to view the tapetal fundus several inches from the patient and then
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