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Retinal Fixation Point Location in the Foveal Avascular Zone

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Retinal Fixation Point Location in the Foveal Avascular Zone Investigative Ophthalmology & Visual Science, Vol. 31, No. 10, October 1990 Copyright © Association for Research in Vision and Ophthalmology Retinal Fixation Point Location in the Foveal Avascular Zone Bernard 5. Zeffren,* Raymond A. Applegare,* Arthur Bradley,f and W. A. J. van Heuven" The site of normal fixation is often assumed to be centered in the foveal avascular zone (FAZ). This assumed anatomic relationship is used during photocoagulation therapy as an objective guide to avoid damaging critical retinal structures on or near fixation. With laser therapy being directed closer and closer to the center of the FAZ, the accuracy with which the center of the FAZ locates the retinal point of fixation becomes an important therapeutic issue. Using an optimized technique for visualizing the retinal vasculature entoptically, the authors determined the location of the retinal point of fixation with respect to the foveal area vasculature in 26 eyes of 14 healthy subjects. In 23 eyes (12 subjects), a traditional FAZ was observed, the other three eyes (two subjects) had capillaries near or crossing the center of fixation. Of the 23 eyes with a traditional FAZ, 20 had centers of fixation located eccentric to the center but in the FAZ, (average deviation from the center of the FAZ, 66.5 ± 49.5 /xm) with the direction of deviation from the FAZ center appearing random. Consequently, when following protocols that advocate photocoagulation treatment with spot centers closer to the FAZ center than 300 nm, the center of the FAZ is a poor locator of a subject's retinal point of fixation. When using the FAZ as a reference, the resulting uncertainty in the location of the subject's retinal point of fixation increases the probability of significant damage to the actual point of fixation by up to 20%. Invest Ophthalmol Vis Sci 31:2099-2105, 1990 The posterior pole of the eye contains the anatomic In a normal observer, visual performance is maxi- fovea and foveola. These structures are highly special- mized at the subject's point of fixation. That is, when ized to maximize visual acuity. In the center of the asked to fixate a point in space, normal observers anatomic fovea, the inner retinal layers down to the move their eyes such that the point of interest is outer nuclear layer are displaced, forming a pit, the imaged on the retinal region providing the highest foveola, which contains the highest density of cones resolution.3-4 The connection between fixation and in the retina (147,000 cones per mm2).1 The retinal optimal resolution, together with the histologic spe- capillaries in the area of the fovea typically form con- cialization of the retina, suggests that a normal indi- centrically arranged channels ending in a capillary vidual will move the eye to place the image of the loop approximately 0.5 mm across which outlines a fixation point on the foveola. Thus the retinal point capillary-free zone known as the "foveal avascular of fixation and the foveola are assumed to be coinci- zone" (FAZ).1 The lateral displacement of inner reti- dent and reasonably centered in the FAZ. nal structures, including the retinal vasculature, pre- Given current therapeutic trends to photocoagu- sumably exists to leave an unobstructed light path to late closer and closer to the point of fixation using the the site of phototransduction, thereby enhancing center of the FAZ as a guiding landmark,5"9 the accu- image quality.2 racy with which the center of the FAZ locates the point of fixation becomes critical. That is, when using the FAZ as an anatomic landmark for locating the From the *Department of Ophthalmology, University of Texas retinal point of fixation, significant deviation of the Health Science Center at San Antonio, San Antonio, Texas, and point of fixation from the center of the FAZ could the fDepartment of Visual Science, School of Optometry, Indiana lead to placement of laser burns closer to, or further University, Bloomington, Indiana. Supported by NIH grant EYO8OO5 to RAA, NIH grant EY07863 from, the fovea than anticipated. As a result, burns to AB, and an unrestricted research grant to the Department of may unintentionally lead to inappropriate laser-in- Ophthalmology, University of Texas Health Science Center at San duced damage or incomplete coverage or nontreat- Antonio from the Research to Prevent Blindness, Inc. New York. ment of a (sub)retinal neovascular lesion due to its Arthur Bradley is also supported by the Indiana Institute for the assumed (as opposed to known) proximity to the reti- Study of Human Capabilities grant AFOSA #870089. nal point of fixation. Reprint requests: Raymond A. Applegate, OD, PhD, Depart- 10 ment of Ophthalmology, University of Texas Health Science We used optimization techniques for visualizing Center at San Antonio, San Antonio, TX 78284-7779. the smallest foveal capillaries entoptically to deter- 2099 2100 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / Ocrober 1990 Vol. 31 mine whether or not the retinal point used for fixa- Source S, a 1-mm pinhole, back-illuminated by tion actually lies in the center of the FAZ, and if not, light from a fiber optic passing through a blue filter whether or not the deviations are significant enough (3M, Visual Products Division, 3 M Center, St. Paul, to warrant clinical consideration. MN, color filter part #47 with peak transmittance at 470 nm), was set to rotate at a speed of 3.5 hertz in a Materials and Methods circular path 2 mm from, and concentric with, the optical axis of the instrument. Light from S was first A number of techniques have been used to exam- collimated by lens L, and then imaged into the plane ine the FAZ and its component capillaries in living of the subject's pupil with unit magnification by lens, humans, including fluorescein angiography,""14 and L . An iris diaphragm (aperture A), imaged at optical two noninvasive subjective psychophysical tech- 2 1516 infinity by L , served as the field stop for the eye-ap- niques: the "blue-field entoptic effect" and the 2 paratus system. entoptic visualization of the retinal vasculature (commonly known as the Purkinje image)."1718 The A second channel provided the subject a view of blue-field entoptic effect provides the viewer an two dim point sources (PSi and PS2) optically conju- image of the white corpuscles as they flow through gate with aperture A through a beam splitter, Bi. the retinal capillaries. The capillaries remain unseen, Point source, PS,, was attached to a motorized X-Y and the actual form of the vascular tree must be con- positioning plate and served as a guidelight for mark- structed in the mind of the viewer. An optimized ing positions of interest in the field of view. Point Purkinje image allows simultaneous evaluation of a source, PS2, served as a stationary fixation point cen- persistent, easily visible, high-contrast image of the tered on the optical axis of the apparatus. The subject foveal capillaries and the point of fixation.10 controlled the position of PSi with a joystick and could thereby locate, with respect to the point of fix- ation, any point on the capillary loop defining the Instrumentation FAZ. A variable voltage signal reflecting the location Stimulus optimization for the entoptic visualiza- of moveable point source, PSl5 was sent from the X-Y tion of the macular area retinal vasculature and FAZ position plate to an X-Y plotter for hard-copy docu- was achieved by modifying a standard Maxwellian- mentation of the border of the FAZ. view system.19 We call the system, schematically il- Alignment of the subject's pupil to the optical axis lustrated in Figure 1, the Vascular Entoptoscope. of the apparatus and stabilization of the subject's Monitor Subject Fig. 1. Schematic repre- sentation of Maxwellian view system. S = source, a rotating 1-mm pinhole, peak wavelength 470 nm; L, = collimating lens; A = variable iris diaphragm; B! = beam splitter; L2 = Maxwellian view lens; B2 = beam splitter; VC = video X-Y camera; FSM = front sur- Joystick face mirror; AR = align- ment ring; PSi, PS2 = point X-Y Plotter sources. A toB^Bj toPSj FSM to B2 = B2 to Subject S to L j = A to L2 = L2to Subject = 22cm PS is mobile PS 2 is stationary No. 10 RETINAL FIXATION POINT LOCATION / Zeffren er ol 2101 head was obtained with a dental bite-bar mounted on near their center of fixation and subsequently con- a compound vice mobile in three planes. To insure nect the dots free hand again, adding as much detail the subject was properly aligned to the apparatus, a as possible to form their final tracing. third channel provided a closed-circuit video view of the pupil entry location of the rotating beam and the Results corneal reflection of alignment ring AR (a circle of infrared light-emitting diodes concentric with the op- All of the subjects easily saw the Purkinje image of tical axis of the apparatus). The video view of the their retinal capillaries. Ten of the 14 subjects rotating beam was obtained by deflecting some of the graphed details of the shadow of their FAZ in both beam from source S at beam splitter B2 onto a front eyes and two (due to personal time constraints), in surface mirror (FSM; optically conjugate with the only one eye. Another subject observed a traditional subject's entrance pupil) and back through beam FAZ in one eye but saw capillaries running through splitter B2 into the video camera. The video view of what should have been the FAZ in the other, and one alignment ring, AR, was obtained by reflection off the subject saw.
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