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 25-28 September, 2018 | Singapore
move to 1 to 2 inches from the patient’s eye when the optimum focus is achieved and the animal has adapted to the restraint. The diopter setting is usually started at “0” and adjusted to between +3 to -3 diopters to provide the sharpest image possible. By using more positive lenses the lens can be seen at +8 to +12 diopters and the cornea at +20 diopters.
Direct ophthalmoscopy has certain limitations. Penetration of cloudy or partially crystallized media is limited. Because of magnification, there is a small field of view. Examination of the peripheral fundus is difficult. There may be difficulty in compensating for refractive errors and eye movements. Stereopsis is absent, and depth of focus is limited. The small working distance between examiner and patient may be hazardous to certain species of animals.
The PanOptic ophthalmoscope is available and provides an intermediate level of magnification to the direct and indirect techniques.
2. Indirect ophthalmoscopy
Indirect ophthalmoscopy complements direct ophthalmoscopy. To perform indirect ophthalmoscopy
a fairly bright light source is directed into the eye. A condensing lens is interposed between the light source and the eye. Incident light is condensed to illuminate
the fundus. The reflected light then is condensed by the same lens to form a virtual, inverted, and reversed image between the lens and the light source.
The advantages of binocular indirect ophthalmoscopy are penetration of cloudy media, large field of view (hence an excellent survey instrument), examination
of the peripheral fundus, ease of compensation of refractive errors and eye movements, stereopsis, greater distance between examiner and patient, two
to three simultaneous observers and the ability to readily examine the more intractable patients with less hazard to the examiner. The disadvantages include less magnification for studying particular areas, and the need for drug-induced mydriasis.
Indirect ophthalmoscopy can be employed with only
a light source and a lens. Several commercial indirect ophthalmoscopes are available. Regardless of the
light source used, the power and type of lens used determines the ease and accuracy with which the fundus exam will be conducted.
The indirect ophthalmoscope is adjusted so the light
is slightly off center of the examiner’s visual field (to reduce glare). The patient’s muzzle is held gently and the lens is positioned three to five cm from the cornea and the upper eyelid retracted. The lens is usually held close to the cornea initially to permit observation of the ocular fundus and then moved away from the eye until the image is maximum size. When the hand lens is
interposed between the light source and the eye, the fundus is visualized. Image magnification (2X to 4X) is dependent on the dioptric power of the hand lens. The +20 lens is the most versatile. Occasionally, an annoying light reflection occurs and is remedied by slightly tilting the hand lens.
Image magnification is dependent on the dioptric power of the hand lens. The +20 D lens is the most versatile.
H. Ultrasonography
Ultrasonography has become increasingly useful in the diagnosis of intraocular disease. High frequency sound waves are directed through the eye. A portion of these sound waves “echo” off tissue interfaces. These echoes are amplified and projected onto an oscilloscope. Echoes from the corneal surfaces, the anterior and posterior lens surfaces, the retina, and any abnormal intraocular material will project an image which aids intraocular diagnosis. This is especially useful when dense corneal opacity or mature cataract obscures the view of the fundus.

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