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Diagnostic Ophthalmoscopy for the Veterinarian

Animal Eye Care is pleased to provide referring veterinarians with this informational handout on the techniques of Ophthalmoscopy in domestic animals.

Basic Concepts:

Diopter - the reciprocal of the focal length. One diopter of lens power deviates a ray of light 1 cm at a distance of one meter. Therefore, as the power (number of diopters) of a lens increases, its focal length decreases.

Focal length - the distance in meters from the lens to the focal point

Focal point - the point at which parallel rays of light are brought together by the lens

Lens Types:

  1. convex - positive lenses (black numbers on the direct ophthalmoscope dial) which are convergent. Used to examine the anterior segment and vitreous in direct ophthalmoscopy. In indirect ophthalmoscopy, used to evaluate the posterior segment.
  2. concave - negative lenses (red numbers on the direct ophthalmoscope dial) which are divergent. Used to examine the posterior segment and posterior vitreous in direct ophthalmoscopy.

Refraction and Refractive Errors:

  1. emmetropia - image of object falls on the retina.
  2. ametropia - image of object falls anterior or posterior to retina. Several causes.
  3. myopia - image of object falls in front of retina [near-sighted].
  4. hypermetropia - image of object falls behind the retina [far-sighted].
  5. astigmatism - variation in corneal curvature in one or more meridians

Most domestic animals are emmetropic, with a tendency towards mild myopia in German Shepherds, Miniature Schnauzers, and Rottweilers. Geriatric dogs are usually myopic. It is thought that the nearest point at which dogs can focus is about 30 cm.

Accommodation - Ocular reflex used to keep image of object on the retina, controlled by the ciliary body muscles.

Ophthalmoscopy

The clinician must recognize the normal fundus in all of its variant forms, in order to recognize an abnormal fundic appearance.  There are many, many variations of normal in dogs and cats.

DIRECT OPHTHALMOSCOPY - (termed because the fundus is viewed directly) Used to examine both the anterior and posterior segments of the eye, but primarily the fundus. The ophthalmoscope consists of a light source, mirror, and view hole through which a circular series of convex and concave lenses (numbered in diopters) can be used to examine different parts of the eye. The device also contains a circular dial to change the illumination to one of the following settings:

  1. green filter - helps differentiate hemorrhage from black pigmentation (hemorrhage appears red).
  2. graticule - grid to assist in estimation of lesion size, using the optic disc diameter as a reference (dog and cat, 1-2 mm diameter; cattle 3-4 mm diameter; horse 5-7 mm horizontal, 3.5-5 mm vertical).
  3. slit - slit of white light assists the detection of an elevation or depression of tissue
  4. different sizes of light apertures - fit size of circle of light to pupil size

Most devices contain a rheostat to control the amount of light delivered.

A new type of direct ophthalmoscope is available, which is easier to use than the conventional models.  It is called the PanOptic™ Ophthalmoscope, and is made by Welch Allyn.  The PanOptic™ Ophthalmoscope unit fits onto a conventional Welch Allyn power source hand piece.

Image and Magnification - The image produced is real and upright. Direct ophthalmoscopes use the refractive ability of both the patient's and the examiner's eyes to magnify the image. Therefore the refractive error of the patient and of the examiner affects the amount of dialed diopters needed to focus on the retina. In an emmetropic eye, the image is magnified 15x and a 5 mm diameter circular area of the fundus is visualized. Hyperopic eye-- more seen but less magnified myopic eye-- less seen but more magnified

At the posterior pole of the eye, axial magnification varies according to the species:
One diopter = 0.2 mm in the cat, 0.3mm in the dog, 0.7 mm in the ox, and about 1.3 mm in the horse. Example: in evaluating the depth of a cupped optic disc in a dog, the bottom of the depression is in focus at (-)5 diopters and the retina is in focus at (-)2 diopters, the disc is cupped 0.9 mm {3 x 0.3}. The change in diopters between focusing on the retina and on the lesion also helps determine if the lesion is elevated or depressed. Example: if a retina is in focus at (-)2 diopters and the lesion is in focus at (-)5 diopters (more negative) it is depressed, whereas if the lesion is in focus at 0 diopters (more positive) it is elevated. Other advantages: easy to use, can keep one hand free during examination, and relatively inexpensive.

Advantages - Direct ophthalmoscopy can be used for close evaluation of fundic lesions, both because of the high magnification afforded and because the device can be used to estimate the degree of depth or elevation of a lesion.

Disadvantages

  • In viewing the fundus, only small areas can be seen at any one time. Analogous to seeing one piece of a jigsaw puzzle. Complicated by ocular movement during examination-- our patients can be restrained physically, but not their eyeball movements.
  • To view the eye, must bring instrument and your face very close (2 - 3 cm) to patient's eye. In temperamental animals, this may pose problems.
  • It is harder to visualize the fundus through cloudy ocular media.
  • It is more difficult to examine the peripheral fundus.
  • Stereopsis is absent.


INDIRECT OPHTHALMOSCOPY - (fundus is viewed indirectly). A convex lens is placed between the patient's and the examiner's eyes, and a real, inverted image (upside-down and backward) is formed in space, between the lens and the examiner's eye.

Magnification - The magnification depends on the focal length of the lens.
20 D lens: 4-5x magnification but less field of view
30 D lens: 2-3x magnification but greater field of view

An important point to consider is that the axial magnification of the image varies with the species examined. For this reason, a very mild elevation of the optic disc seen ophthalmoscopically in a horse should be given much more significance than a disc of similar appearance in a dog. In addition, the fundic image in the horse will appear flattened.

Advantages

  • panoramic view of the fundus
  • lens is held 4-6 cm away from patient's eye, and patient is 0.5 -0.75 meters away from examiner (less risk of being bitten/scratched)
  • easier to view fundus through opacities in ocular media
  • stereoscopic view, if using a binocular head unit.

Disadvantages

  • binocular and monocular units expensive
  • both hands are occupied, if holding the lens in one hand and a transilluminator in the other (the "no frills" method of indirect ophthalmoscopy)
  • less fundic detail than with direct ophthalmoscopy
  • device only used to view fundus and posterior vitreous
  • less magnification image upside-down and backward.
  • image upside-down and backward

Indirect and direct ophthalmoscopy should both be used, as they complement each other!

An inexpensive 20 D condensing lens for Indirect Ophthalmoscopy can be ordered from Welch Allyn.

 

 		 
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