Principles and Advantages

The standard technique for examining the retina is binocular indirect ophthalmoscopy. With this method, reflected light from the patient’s fundus is focused by a lens into an intermediate image, which is then viewed by the observer (Fig. 3-1). The image that is created is upside down and inverted (Fig. 3-2). The headpiece of the binocular indirect ophthalmoscope is constructed so that the light from either a high-intensity incandescent or light-emitting diode bulb is focused onto a mirror that reflects the light into the patient’s eye and onto the retina (Fig. 3-3). Because the mirror is mounted above the view box, the beam of light entering the patient’s eye (illumination beam) is separated from the light rays that are viewed by the examiner (observation beams). This arrangement prevents the corneal light reflex of the illumination beam from interfering with viewing. The view box contains prisms that provide stereoscopic vision. These prisms optically “narrow” the examiner’s pupillary distance; otherwise, the light rays exiting from the patient’s pupil could not reach both of the examiner’s pupils (Fig. 3-4).

The indirect ophthalmoscope offers several advantages over other techniques for examining the fundus and is indispensable in the evaluation of retinal detachment and other peripheral vitreoretinal pathology. ^{1 ,​ 2 ,​ 3 ,​ 4 ,​ 5 ,​ 6} The strong illumination provided by the head lamp plus the light-gathering capability of the handheld lens enables the examiner to see through hazy media. When combined with scleral depression, indirect ophthalmoscopy provides the best view of the peripheral retina. It is a dynamic technique that allows for a broad area of the retina to be visualized at one time, thereby helping to ensure that no abnormalities will be missed.

Technique

Several aspheric condensing lenses are available for use with indirect ophthalmoscopy. The 14-diopter lens has the highest magnification of the commonly used lenses (3.6x), but it is difficult to use because of its long focal length (7 cm). The 30-diopter lens (f = 3.3 cm) is often preferred by beginners because it is easy to use, especially in patients with small pupils, but it does not provide adequate magnification (1.5x) for finding small breaks or other subtle retinal lesions. Most clinicians prefer the 20-diopter lens (f = 5 cm), which is relatively easy to use and provides a reasonable magnification of 2.3x. Any lens should be held with the more convex surface toward the examiner (Fig. 3-1). It is slightly tilted to move the two light reflexes (one from each surface of the lens) away from the examiner’s viewing axis. To minimize the corneal light reflex, the mirror should be adjusted so that the illumination beam is in the top of the field of view of the eyepieces. For small pupils, the light must be directed still higher (Fig. 3-5).

The patient should be reclining comfortably and have a widely dilated pupil. Initially, the transformer rheostat should be set at a low voltage. Higher light intensities can be tried later if the patient becomes less light-sensitive. Bilateral cycloplegia combined with topical anesthesia reduces photophobia and enhances cooperation. The superior periphery should be examined first because photophobia is minimized in upgaze and because the periphery is less sensitive to light than the posterior pole. Some patients are intolerant of bright light and attempt to close their eyes, inducing Bell’s phenomenon and making examination of the inferior retina difficult, if not impossible. To counteract this tendency, the ophthalmologist must constantly encourage the patient to keep both eyes open. Sometimes, a fixation target such as the patient’s thumb or a mark on the ceiling is helpful.

If the pupil is large, the examiner can increase magnification by moving closer to the eye. However, if the pupil is small, it is much easier to obtain a clear fundus image if the examiner’s arm is extended. Beginners frequently make the error of standing too close to the patient (Fig. 3-6 and Fig. 3-7).

The examiner’s head should be held to look directly into the quadrant being examined. For example, during examination of the nasal periphery, the examiner should stand on the same side as the eye being examined (Fig. 3-8 and Fig. 3-9), and for the temporal periphery, on the opposite side (Fig. 3-6 and Fig. 3-10). The hand lens is shifted from the right to left hand as necessary to avoid clumsy maneuvering, especially during scleral depression. The nose becomes less of an obstacle to viewing the temporal periphery when the patient’s head is rolled toward the examiner while the patient looks temporally.

The pupillary aperture appears elliptical when the examiner looks at the peripheral retina. This makes stereoscopic viewing more difficult and decreases the amount of light that can enter the eye. The voltage of the light source must be increased and the examiner’s head must be tilted slightly so that part of the illumination beam can enter the eye and one of the observation beams can exit (Fig. 3-11). This achieves only a monocular view, a necessary compromise to achieve an adequate image.

When a patient has a small pupil, it is easier to focus on the retina if the examiner first looks at the pupil through the lens while holding the lens away from the eye at a distance greater than its focal length. The examiner then slowly moves the lens toward the pupil. Newer ophthalmoscopes come with smaller illumination beams and adjustable prisms to enhance viewing through small pupils. Furthermore, higher-diopter condensing lenses (e.g., 25, 28, or 30 diopters) also facilitate obtaining a good view in this setting.

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