You were probably first introduced to an ophthalmoscope during your physical examination training. Most physician assistants are given an ophthalmoscope and told, “This is what you use to examine the back of the eye.” A brief look at the red reflex of the eye, a few vessels, and perhaps the optic disc may be the extent of your use of the ophthalmoscope. You are not alone if you come away from an eye examination feeling as if you really did not gain much useful information. After your clinical training is complete, however, you may hesitate to ask about how to expand your use of the instrument. This article aims to provide you with some tips that will make your use of the ophthalmoscope more rewarding, accurate, and practical.
THE INSTRUMENT
Invented by Hermann von Helmholtz in 1851, the instrument was called an augenspiegel (eye mirror). The term ophthalmoscope (eye observer) came into common use in 1854. Helmholtz's instrument consisted of three essential elements: a source of illumination, a method of reflecting the light into the eye, and an optical means of correcting an unsharp image of the fundus.1 The instrument used in today's clinical setting is still composed of these basic elements. Used effectively, the ophthalmoscope will enable you to make an accurate diagnosis in most clinical situations where more sophisticated eye-examination equipment is not available.
THE SETTINGS
The different light and aperture settings in the head of the ophthalmoscope change the color and shape of the light source. Each setting provides the lighting and/or view used for different diagnostic purposes2 (Figure 1).
White beam This beam can be large or small. The large white beam is the most frequently used beam of light. At a very close range, the small white beam only covers a small area of the pupil and iris. This beam can make visualization through a small pupil easier because it reflects less light toward you from the structures around the pupil and directs less light toward a reacting pupil. Either the small white beam or the half beam is used to view the retina from around the opacity of an early-stage cataract. The light just barely enters the bottom of the pupil while you look through the area at the top of the pupil.
Half beam This setting is used to examine patients with early lens opacity. The light is allowed to enter only the lower half of the pupil; the disc is visible through the upper half of the pupil, from around the opacity. This improves the blurriness and offers a clearer view of the retina.
Grid The grid pattern in the beam helps make rough measurements within the eye. For example, this setting can be used to measure the relative distance between a retinal lesion and the macula or disc.
Blue light Corneal abrasions or ulcers are revealed by examining the eye with this setting after fluorescein staining of the cornea.
Slit beam This setting is used to examine the cornea and anterior chamber of the eye. If a separate magnifying loupe is used, the slit beam is directed obliquely towards the cornea and anterior chamber. The slit beam can be thought of as a poor man's slit lamp view. The slit beam is also used to examine the contour of structures or abnormalities in the cornea, lens, or retina.
Red free This setting makes the other beams visible as a green light. Green is a contrasting color to the red of the retina; therefore, the view of the retina will appear black and white. The red free setting is used to examine the blood vessels. The vessel caliber and distribution appear more clearly and with better contrast when viewed through this setting. The red free setting is particularly useful when looking for nicking in patients with hypertension and new vessel growth in patients with diabetes. Retinal bleeding appears as black.
THE LENSES
An understanding of the lenses is helpful when examining the structures inside the eye, such as the vessels and the retina. What do the numbers in the small window of the ophthalmoscope head signify? You might want to read this section with an ophthalmoscope in hand for reference.
The red numbers are concave or minus lenses. The green numbers are convex or plus lenses. A convex lens converges or focuses beams of light to a spot on the opposite side of the lens from the light source. A concave lens diverges or directs the beams that pass through the lens apart from one another. An imaginary line drawn from where the light rays start to diverge backwards to a point where the light rays would meet will lead to a theoretical focal point (Figure 2). This point is called a virtual image because the light rays theoretically focus between the light source and the lens.
Lens power is measured in diopters. The numbers on the ophthalmoscope represent the power of the lens you are looking through.3 The focal length is the distance between you and the object you are focusing on when you look through the ophthalmoscope. A 1-diopter lens has a focal length of 1 m. To find the focal length of the lens you are using, divide 100 by the diopter number seen in the ophthalmoscope window. For instance, a 4-diopter lens focuses on an object 25 cm away (100 / 4 = 25). Likewise, a 5-diopter lens focuses on an object 20 cm away; a 10-diopter lens focuses on an object 10 cm away.
To see corneal detail up close, set the ophthalmoscope lens on +20 and view the eye at a distance of 5 cm from the cornea. You should be able to see a sharp, magnified image of the cornea. Adjust the focus by moving slightly toward or away from the patient's eye until your view is at its sharpest. At this level, the eye is magnified approximately 6 times, which should give you a clear view of any foreign bodies on the cornea or eyelid. If you view the red reflex of the eye from 25 cm away at +4, the pupil margin comes clearly into focus and the iris detail can be seen quite well. This is an excellent distance from which to examine the red reflex, as it will also let you see shadows caused by opacities in the lens. This provides much more diagnostic information than just looking at the red reflex at 0.3
All the distance settings change when looking inside the eye because you are now looking through the cornea and the lens as well. Figure 3 illustrates the approximate depth into the eye that is in focus at each diopter setting. When looking into the eye from a very close distance, changing the lens power can allow the structures at various depths inside the eye to come into focus. Start with +20 at the cornea, and decrease the diopter setting gradually (+10, +8, +6, etc.), you will be able to see structures, opacities, floaters, and even foreign bodies in the anterior chamber, lens, vitreous, and finally, arrive at a clear view of the retina.3
When examining the emmetropic eye (no refractive error, therefore, the patient does not need to wear corrective lenses), the ophthalmoscope will focus on the retina at 04 (Figure 4). This number is usually white in the ophthalmoscope window. This is, of course, assuming that the examiner also has no refractive error. The minus, or red, numbers are rarely used when both the examiner and the patient have emmetropic eyes. However, a minus setting may be used to observe the depth of the optic cup from the rim of the optic disc.
At a very close range (the ophthalmoscope is almost touching the patient's eyelashes [2 cm]) and a 0-diopter lens setting, the retina of an emmetropic eye is in focus. If there is cupping in a patient with chronic glaucoma, the center of the optic disc will be out of focus because it is at a greater depth than the retinal surface. By adding –1 or –2 to the diopter setting, the cup of the disc is now in focus and the rest of the retina will be out of focus. This is a way to evaluate the depth of the cup from the rim and thereby the severity of glaucoma damage.