NOTES, CASES, INSTRUMENTS A GANZFELD CONTACT L E N S ELECTRODE IRWIN M. SIEGEL, P H . D .
New York, New York The brilliant flash used for clinical electroretinography (ERG) recordings produces a veil of scattered light that is unevenly dis tributed across the retina. Thus, the more peripherally located receptors receive less quanta than those located within the pro jected image of the flash. Under these circumstances, the summated electrical ac tivity is composed of local potentials differ ing in amplitude and latency. Techniques have been devised to approximate large fields of homogeneously distributed light known as ganzfeld illumination. Two approaches have achieved such stimulus conditions. One involves the use of a large-diameter, short focal length lens that gathers a collimated beam of light to a focus in the plane of the pupil, from which point it diverges to illuminate a large area of retina.1 This is the so-called Maxwellian view and to the observer it appears as if the entire imaging lens is evenly filled with light. However, a mouth bite and head rest are necessary to maintain a critical alignment of the patient's eye, making this technique unacceptable for routine clinical testing. A second technique2'8 requires that the patient's head be surrounded by a large light-diffusing hemisphere (similar to the Goldmann perim eter) that is indirectly illuminated by the stimulus flash. For this procedure the patient is seated with his head on a chin rest in front of the bowl opening. Despite the ob vious advantages of this technique over the From the Department of Ophthalmology, New York University Medical Center, New York. This study was supported by grant EY00213 from the National Eye Institute. Reprint requests to Irwin M. Siegel, Ph.D., De partment of Ophthalmology, New York University Medical Center, SSO First Ave., New York, NY 10016.
Maxwellian view stimulation, it is not ideal. For example, it cannot be used with an esthetized infants or young children, and it necessitates inserting lenses in patients while they are sitting, which in our experi ence is awkward. Ganzfeld illumination can also be achieved by putting a small bowl made of a diffusely translucent substance directly over the eye and backlight it with the stimulus flash. If only a small section of such material, a little greater than the pupil diameter, was placed close to the cornea it would still act as a large diffuser, and be as effective a ganzfeld as a large indirectly illuminated bowl some dis tance from the eye. Heck and Rendahl* used an ERG electrode covered with a thin layer of unspecified fabric called "matte glass" to diffuse the stimulus flash. More recently, Cone6 found that half of a PingPong ball placed over a rat eye effected homogeneous stimulation, a condition essen tial to his studies of the ERG b-wave latency and amplitude. I modified Cone's technique for human application by using a small sec tion of a Ping-Pong ball and integrating it into a Burian-Allen electrode. The result is an inexpensive ganzfeld that still allows the patient to be prone during the ERG record ing. METHOD
The completed electrode, an extra diffuser, and the shaping tool used in its construction is shown in Figure 1. A circular piece of Ping-Pong ball (11 mm in diameter) v/as cut to fit over the contact lens portion of the electrode. Almost any smooth surfaced PingPong ball has the necessary properties, though trivial differences may exist in thick ness and density. It is necessary to reduce the radius of curvature of the section to conform roughly to the anterior curvature of the lens. A simple two-piece ball and socket mold (Fig. 1) was fabricated for
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this purpose, although a less elaborate method will no doubt occur to others. The socket portion was made by lathe-cutting a hemispheric concavity of 12 mm diameter and a 7-mm radius of curvature into the end of a short brass rod. The other half of the tool was a brass ball with a radius of 6.3 mm brazed to the end of a narrow diameter rod 2x/2 inches long. The ball and socket were placed on a hot plate and warmed to about 150°C. The Ping-Pong ball section was placed, concave side up, in the mold and the heated brass ball was pressed firmly against it. After a few moments of contact, the material was removed and allowed to cool. The resulting section was more steeply curved than the anterior surface of the contact lens that aids in its being a "snap-fit." Notches were filed or cut along the rim of the section to ac commodate the small projections of the stainless steel retaining ring surrounding the contact lens. The diffuser is thus easily removable for adequate cleaning after use, and can be conveniently replaced when neces sary. Figure 2 shows two oscilloscope tracings of ERG recordings from a dark-adapted eye with and without the diffuser on the contact lens. The same intensity flash was used and the lamp distance was fixed for the com parison. The amplitude of the ganzfeld ERG is considerably greater (by 100 y.V) despite
Fig. 2 (Siegel). The ERG tracings with and without diffuser in response to or produced by a brief, intense flash of white light from a Grass PS22 strobe at stimulus strength SM. Calibration marker, 40 msec and 200 u-V.
the fact that the Ping-Pong ball material had an optical density of 0.7 (20% transmissivity). Although the b wave was larger, its implicit latency was the same in both conditions (about 40 msec), indicating a greater stimulus effectivity produced by the diffuser. The latency value agrees with rec ordings obtained with a similar strength stimulus projected into a large diffusing bowl.8 Patients did not appear to object to their vision being occluded by the diffuser dur ing the recording, and no problems were en countered even with bilateral stimulation. Large-bowl ganzfelds may be advantageous when the experimenter wishes to vary flash and background illumination independently, but for most clinical situations, the contact lens ganzfeld provides a large field of evenly diffused light with no additional equipment at a negligible cost. SUMMARY
Fig. 1 (Siegel). From left to right, A BurianAllen contact lens electrode with Ping-Pong ball diffuser snapped into place; diffuser section before insertion; and a two-piece ball and socket mold used in diffuser fabrication.
A small piece of Ping-Pong ball material fitted to a Burian-Allen contact lens electrode provided a large field of evenly distributed light on the retina that resulted in larger
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electroretinographic amplitudes and more accurate measurement of b-wave latencies. REFERENCES
1. Dodt, E., Copenhaver, R. M., and Gunkel, R. D.: Electroretinographic measurement of the spectral sensitivity in albinos, Caucasians, and Negroes. Arch. Ophthalmol. 62:795, 1959. 2. Gouras, P.: Clinical electro-oculography. In Straatsma, B. R., Hall, M. O., Allen, R. A., and Criscitelli, F. (eds): The Retina. Berkeley, University of California Press, 1969, pp. 565-581. 3. Rabin, A. B., and Berson, E. L.: A full field system for clinical electroretinography. Arch. Ophthalmol. 92:59, 1974. 4. Heck, J., and Rendahl, I.: Components of the human electroretinogram. Acta Physiol. Scand. 39: 167, 1957. 5. Cone, R. A.: The rat electroretinogram. 1. Contrasting effects of adaptation on the amplitude and latency of the b wave. J. Gen. Physiol. 47: 1089, 1964.
A DUAL V I E W I N G A T T A C H M E N T W I T H T H E HAND-HELD OPHTHALMOSCOPE A.
P. W. J.
MAKEPEACE, B . S C ,
C. DEAN HART,
AND
B.SC.
Bristol, England AND
H. H.
HOPKINS,
S. J.
F.R.S.,
DOBSON,
AND
M.Sc.
Reading, England To ensure that undergraduate medical students become conversant with the use of the hand-held ophthalmoscope as early in their clinical course as possible, preliminary teaching on the techniques of ophthalmoscopy is valuable. Since teacher and student are not able to From the Departments of Medicine (Mr. Make peace) and Ophthalmology (Mr. Hart), University of Bristol, Bristol, and the Department of Physics (Dr. Hopkins and Mr. Dobson), University of Reading, Reading, England. This study was sup ported in part by grant 33-R/E1050/31 (STB5) from the Department of Health and Social Se curity. Reprint requests to Anthony Makepeace, B.Sc, Department of Medicine, Royal Infirmary, Bristol BS2 8HW England.
AUGUST, 1975
see the same image simultaneously by using a direct hand-held ophthalmoscope, we devised a dual viewing attachment that is consid ered useful as an ophthalmoscopic training aid. Initially, we explored the possibility of us ing commercially available dual viewing fiber optic endoscopic devices, but image resolution in such systems depends to a large extent on the number of fibers in the optic bundles. To obtain a quality of resolution suitable for ophthalmoscopic examination, the thickness of the fiber optic cable so increased the size of the image-splitter assembly that the un modified image field became unacceptably re duced. Additionally the image seen by the ob server was focused on the proximal end of the fiber optic bundle and the focus was fixed, usually at infinity. If the examiner is not emmetropic or is accommodating to some de gree the two images seen will not be parfocal. For these reasons we employed an attach ment utilizing conventional optical systems that eliminated these problems. MATERIAL
The attachment (Fig. 1) consists of a head, 20 mm thick, containing a beamsplitting prism and an observer's eyepiece, linked by four light tubes. The connecting joints between each section contain pairs of right angle reflecting prisms, permitting complete freedom of flexion and extension. Each tube also has a coupling that
Fig. 1 (Makepeace and associates). The direction of joint movements is indicated "by arrows.