Discussion

Examination in the dark: Indirect and direct ophthalmoscopy.

The posterior segment (the internal structures of the globe behind the lens) consists of the viteous, retina and optic nerve and is examined using indirect ophthalmoscopy followed by direct ophthalmoscopy. The 2 methods are complementary rather than exclusive.
The normal appearance of the equine fundus requires considerable practice for correct interpretation, because there is much normal variation. Most pathological lesions of the fundus are identified near and below the optic nerve head and typically involve hyperpigmentation or depigmentation.

Indirect ophthalmoscopy.
This is a useful technique for screening the ocular fundus and can be performed most simply using a bright pen light or transilluminator and a condensing lens. The system produces a low magnified, reversed, inverted, virtual image, such that a large field of view is produced. Mydriasis, a bright light source and darkness are essential for detailed fundic examination. A strong lens (high plus: 30D) produces a small, bright image whereas a weaker lens (lower plus: 5.5D) produces a larger, less bright image. It takes some practice to learn the indirect method.
A condensing lens is held some 2 - 8 cm from the horse's eye and the light source is held level with the bridge of the observer's nose. The aim is that the observer's eye, the light source, the lens and the patient's pupil should all lie in the same axis. The plane of the lens must be parallel to that of the horse's iris and pupil. The light is directed into the horse's eye so that the tapetal reflection is obtained and the lens is moved to and fro until a sharp, clear image is produced. The observer-patient distance is approximately 50-75 cm.
In horses a +20 dioptre (D) condensing lens is the most versatile in use although the image is minified. A 20D lens minifies the fundic view with 0.79X and 0.84X magnification laterally and axially, respectively. The 20D lens provides a nice panoramic, screening view of the equine fundus, but it is not satisfactory for detailed, highly magnified observations. Indirect ophthalmoscopy with a 30D lens provides a magnified view of 0.51X lateral magnification and 0.35X axial magnification. Indirect ophthalmoscopy with a 14D lens provides a magnified view of 1.18X lateral magnification and 1.86X axial magnification. Indirect ophthalmoscopy with a 5.5D lens provides 3.88X lateral magnification and 20.10X axial magnification in the horse.
Binocular indirect ophthalmoscopes have an integral light source and a prism system for delivery of separate images to the observer's eyes. They are mounted on a headpiece or spectacle type frame. The working principle is the same as for monocular indirect ophthalmoscopy, but the instruments have the benefits of stereopsis and more powerful light sources. They also allow the observer one free hand.

Direct ophthalmoscopy.
The use of a standard direct ophthalmoscope produces an upright image of greater magnification than is possible with the indirect ophthalmoscope when used close to the patient's eye. However, viewing the fundus directly along a beam of light necessarily restricts the field of view. The direct ophthalmoscope provides the most magnified view of the fundus in the horse, with a lateral magnification of 7.9X and an axial magnification of 8.4X. Both distant direct ophthalmoscopy and close direct ophthalmoscopy should form part of direct ophthalmoscopic examination.
There are also controls on the head of the ophthalmoscope for spot size (use the largest diameter), and brightness (use as bright as the animal will tolerate). There may be controls for a red-free filter (do not use), and a cobalt blue filter (use to look for corneal ulcers).
Red is seen as black with a 'red free' or green filter. The 'red- free' (don't call it 'green') filter is useful for enhancing the appearance of blood vessels and haemorrhages by making blood show up black. The 'red free' filter in the ophthalmoscope is used to differentiate between retina nevi or choroidal nevi. The retinal blood supply and its retinal pigment epithelium (RPE) act like a red filter. Therefore, a nevus that is behind the retina, and located in the choroid, will not be seen when viewed with the 'red free' filter since red and green cancel each other out. On the other hand a nevus located on or in the retina will still be seen with the
'red free' filter in place.
If the scope has a cobalt blue filter this too works to detect nerve fibre drop out. Some feel the aforementioned condition is best viewed with the cobalt blue since the blue light wave length is focused more directly in the nerve fibre layer. Other than the nerve fibre layer the cobalt blue filter has little use when examining the retina.
The slit on the ophthalmoscope is designed to look at the optic disc and is too wide for detecting flare.

Distant direct ophthalmoscopy.
This technique uses the tapetal fundus as a means of retro- illuminating the structures anterior to it. The ophthalmoscope is set to 0 dioptre (no magnification) and directed to find the tapetal reflex in the pupil at an observer-patient distance of 25 - 40 cm. It is a useful way of assessing whether there are any opacities between the observer and the fundus and is usually used as a quick screening method prior to more detailed assessment. Any opacities present in the ocular media (cornea, aqueous, lens, vitreous) will appear as black forms against the fundus reflex. Assessment of comparative pupil sizes may also be made using this technique.

Close direct ophthalmoscopy.
Direct ophthalmomoscopy can be performed with direct or
Panoptic ophthalmolscopes. The Panoptic ophthalmoscope has an intermediate level of magnification between the direct and indirect ophthalmoscopes. Direct ophthalmoscopy can be used to examine all aspects of the eye and adnexa but is most commonly done to examine the retina and optic nerve head. The aperture of the instrument must be held as close as possible to the observer's eye and the subject's eye. The lens range is approximately +30D (magnifying lenses) to -30D (reducing lenses). As the examiner reduces the strength of the plus lenses, the focus of observation gradually extends posteriorly, so that magnified details of the lids, cornea, aqueous, iris, lens and vitreous are successively visualised until features of the fundus are brought into focus. For detailed examination darkness is essential and mydriasis is helpful.
For examination of the external eye and adnexa a setting of
+20 to +15D (green numbers are plus) is required. The iris may be examined with a setting of +15 to +12D. For the lens, the setting will be about +12 to +8D depending on whether the anterior or posterior parts are being examined. Intermediate settings will be required for the aqueous and vitreous. Close examination of the fundus is usually performed with the ophthalmoscope placed some 2 cm from the eye and a setting of between +2D or -2D (red numbers are minus) (usually 0) is required. The examination is often made easier and safer if the hand holding the ophthalmoscope is rested lightly against the horse's head, so that sudden movements do not damage the eyes of the horse or the examiner.