How to Use Ophthalmoscopes

1. Introduction

The posterior segment or fundus (the internal structures of the globe behind the lens) consists of the vitreous, retina, and optic nerve and is examined using low magnification indirect ophthalmoscopy followed by high magnification direct ophthalmoscopy. The two methods are complementary rather than exclusive.¹  The results of an examination of the horse fundus are always more easily achieved in a dark environment with a dilated pupil.

2. Methods

Examination in the Dark

Indirect and Direct Ophthalmoscopy

The normal appearance of the equine fundus requires considerable practice for correct interpretation, because there is much normal variation. Most pathologic 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 (Figs. 1–4). Mydriasis, a bright light source, and darkness are essential for a detailed fundic examination. 

3. Results and Discussion

How to Do Indirect

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 (Figs. 1, 3, and 4). 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 diopter (D) condensing lens is the most versatile in use, although the image is minified. A 20 D lens minifies the fundic view with 0.79x and 0.84x magnification laterally and axially, respectively. The 20 D 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 14 D lens provides a magnified view of 1.18x lateral magnification and 1.86x axial magnification. Indirect ophthalmoscopy with a 5.5 D lens provides 3.88x lateral magnification and 20.10x axial magnification in the horse.
 

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 (Fig. 5). 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 a direct ophthalmoscopic examination.
 

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