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depressed.
- Grid aperture can be used to directly measure the size of lesions or to relate the size of a lesion to that of the optic nerve.
- Green light is red-free and permits differentiation of melanin pigment from blood. With red-free light, blood looks black and pigment still looks brown.
- Blue light filter excites fluorescein dye.
INDIRECT OPHTHALMOSCOPY
• This technique can be performed using a hand-
held lens and a bright focal light source (monocular indirect ophthalmoscopy), or for those with a stronger interest in ophthalmology, an indirect headset can be purchased and will provide stereopsis (binocular indirect ophthalmoscopy).
• Indirect ophthalmoscopy is recommended to the small animal practitioner as the PREFERRED METHOD to visualize the fundus of the small animal patient by this author. The image is significantly less magnified than that seen with direct ophthalmoscopy, has a much larger field of view, and is better for routine screening of the eye.
• Indirect ophthalmoscopy provides the examiner with an inverted, reversed image.
• Monocular indirect ophthalmoscopy is inexpensive and required equipment consists only of a bright focal light source and a 28 or 20 diopter lens. It is performed as follows:
• Step 1: Begin at arms length from the patient. An assistant is required to restrain the patient and to help hold the eyelids open.
• Step 2: Darken the exam room (the darker the room, the better the exam).
• Step 3: With a focal light source such as a transilluminator or penlight held close to your lateral canthus of your dominant eye or at your cheek and standing at arms length from the patient, visualize the tapetal reflection (you will see a red reflex in animals with poorly developed or no tapetum).
• Step 4: Hold a 20- or 28-diopter condensing lens in your other hand between the thumb and index finger, and use your ring finger and pinky to elevate the upper lid. Initially, hold the lens to one side of the eye until the tapetal reflection is established, and then rotate the lens such that the eye can be observed through it. Initially, hold your lens close to the corneal surface (about 1/2 inch from the eye) and then slowly pull the lens towards you until the image of the fundus fills the lens (usually about 1-2 inches away from the animal’s eye, depending on the diopter power of the condensing lens you are using). When the image is lost (usually because you or the animal moved), immediately rotate the lens away
An Urban Experience
from the patient’s eye (the left ring finger and pinky continue to hold up the upper lid), re-establish the tapetal reflex, and again, rotate the lens back into place in front of the eye so the fundus comes into view. The fundus should appear as a virtual image in front of the lens. (Hint: some people find it helpful to look at the image which
is in front of the lens and not at the lens or the eye; this author “sees” the fundic image within the lens).
• Step 5: Maintain alignment with your light source, lens, and patient’s eye and then move yourself up, down, left, or right to examine all quadrants of the patient’s fundus. (Hint: remember that the image is inverted, so you must move in the opposite direction to the image). If the image is lost, move the lens out of the light beam and start again.
• Indirect ophthalmoscopy is relatively easy to master providing you PRACTICE OFTEN!
• Various lenses are commercially available for
in indirect ophthalmology and will provide varying magnification and field of view. The less the magnification, the greater the field of view. Although a 20 or 2.2 Diopter lens is considered “standard” for most board-certified veterinary ophthalmologists in practice, I routinely begin all of my exams using a 28 diopter lens, and then work my way up to higher magnifications by making use of the 2.2 D, 14 D, and if necessary the 5.5 MAC lens.
INTERPRETATION OF FINDINGS
When viewing the fundus, be systematic: ASSESS:
1. Reflectivity (Uniformly or focally increased? Uniformly or focally decreased?) – Note it is very helpful to learn how to “shimmy” the lens to help determine is a lesion is hyper or hypo-reflective. This will be demonstrated in lecture.
2. Vascularity (Attenuated? Engorged? Tortuous? Out of focus due to elevation or depression?)
3. Optic nerve (Color? Degree of myelination? Raised? Depressed?)
4. Pigmentation of tapetal versus nontapetal areas
Ask yourself: Normal versus Abnormal? If Abnormal à is it due to an addition (something is there that shouldn’t normally be there such as fluid or blood or a granuloma) or subtraction (the retina and possibly underlying choroid has undergone a selective or uniform degeneration such has what we see in inactive chorioretinal lesions from distemper or end stage retinal degeneration diseases)?
References available upon request
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