Ophthalmoscopy

42 I: Principles

Table 3–1. Features of Direct vs. Binocular Indirect Ophthalmoscopy

A

F'

F N'

B

Condensing

lens

Figure 3–1. (A) Optical principles of direct method of ophthalmoscopy. (B) Optics of indirect ophthalmoscopy.

together two points can be and remain distinguished as separate when viewed through an optical system. The visualization of detail that an optical system permits is a function of its resolving power and not its magnification. Resolution is a function of the light available at the points to be resolved and of the quality of the optical components of the system. Magnification plays a role only if the resolution of the optical system exceeds the resolution of the observing human eye at a given level of magnification. Too much magnification of a poorly resolved image results in a loss of detail, such as if one were to examine a halftone newspaper photograph under a microscope.

With the direct method, the greater the degree of myopia, the higher the magnification of the fundus image and the smaller the field of view. In very high myopes, the field of view with the direct instrument becomes very limited.

High cylindric errors strongly and adversely affect the image of direct ophthalmoscopy because the high magnification of the system also magnifies the effects of refractive errors on the image. With the indirect method, the lower magnification minimizes this effect. Furthermore, the condensing lens can be tilted slightly to overcome astigmatic aberrations. Examination of the retinal periphery entails the travel of light obliquely through the cornea and lens, introducing cylindrical aberrations that are likewise problematic with the direct ophthalmoscope and easily overcome with the indirect.

Due to high illumination, binocular viewing, and high-quality optics, a good indirect ophthalmoscope and aspheric lens provide good resolution in spite of low magnification. Substantial advantages gained include a very wide field of view, stereoscopy, large depth of focus, and dynamic examination capability. During the early stages of learning indirect ophthalmoscopy, it is essential to accept that one has to work with a smaller image size; after a time, one ceases to be troubled by it. After enough experience with indirect ophthalmoscopy, one is rarely aided in an evaluation of detail by increased magnification. However, if higher magnification is needed for examining a specific lesion, this can be achieved in several ways:

Magnification is increased by moving the examiner’s head closer to the patient’s eye (rather than examining at nearly arm’s length as is usual). However, this is difficult if the pupil is not well dilated. Using a lower power condensing lens, such as a 14 diopter lens, also provides more magnification, but is also more difficult with a poorly dilated pupil (Table 3–2). The most common way to increase magnification while maintaining the advantages of indirect viewing is to use a slit lamp biomicroscope with a 60to 90-diopter lens (Figure 3–5). Higher magnification is also achieved by using a slit lamp with a contact lens with or without reflecting mirrors.

Field of view

Figure 3–2A shows what an observer sees with a direct ophthalmoscope in an emmetropic eye, in comparison with Figure 3–2B, which shows the field of view with indirect ophthalmoscopy. This is a 20-fold difference in field size, and it makes a huge difference in the facility of the instrument, especially for the diagnosis and treatment of retinal detachment. The topography of the detached retina may be complex with multiple folds, and the view with the direct ophthalmoscope

Table 3–2. Comparison of Indirect Lenses

1 Static (instantaneous) field of view varies depending on the size of the lens. 2 BIO = binocular indirect ophthalmoscope.

3 Magnification is increased as a function of the magnifying power of the slit lamp.

4 Using a slit lamp, the portion of the potential static field of view that is illuminated and visualized at one time is limited by the magnification system.

44 I: Principles

A B

Figure 3–2. (A) Shows single field of view in emmetropic eye with direct ophthalmoscope. This field is approximately 10° in diameter. (B) Shows single field of view in same eye seen with indirect ophthalmoscope. This field is approximately 37° in diameter. Since the ratio of areas of these fields is proportional to the square of radii of the fields, it follows that the field in Figure B covers an area 14 times as great as that in Figure A. The indirect method permits examination of the disc, macula, and perimacular retinal vessels at one time. Contrast between these two fields would be even more striking if the eye being examined has 20D of myopia. In such case, Figure A would show the optic disc occupying the entire field of view, while Figure B would be unchanged. Ratio of areas seen would therefore be much greater than 14-to-1.

is difficult to interpret. The field of view is so small that one sees only a small part of the convoluted retina in any one field, and it is difficult to relate each separate view to adjacent areas.

When examining for retinal detachment, perhaps even more important than the field of view is the viewable field. Direct ophthalmoscopy permits the study of approximately 60% to 70% of the total fundus area in a well-dilated, emmetropic eye (Figure 3–3). In aphakia it may be possible to visualize more than this area, whereas in myopia less than 60% of the fundus can be seen. Thus, peripheral examination is very difficult, and as explained above, even when the periphery can be seen with the direct ophthalmoscope, the image is very blurry. Piecing together what one sees is even harder, so in practical terms, the direct ophthalmoscope is rarely used to examine beyond the posterior pole. Since 30% of the retina lies anterior to the equator, failure to study this region will result in overlooking serious pathology in many, if not most, cases. Diseases such as senile retinoschisis, peripheral uveitis, and most retinal tears and detachments defy evaluation by any other technique.

Illumination

Image brightness of a direct is low due to limited power output. Direct ophthalmoscopes operated by batteries provide about one-half watt of illumination. Instruments operated through transformers deliver several times this amount, but never more than several watts. Indirect ophthalmoscopes can deliver up to 18