Abstract:
A method for assessing a patient&#39;s retinal function includes selecting a test site on a retina of the patient and stimulating the test site. In a healthy retina, this stimulation results in the generation of an entoptic signal, which is then detected. The method thus provides a simple test for detecting damage to retinal ganglion cells in glaucoma.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of the Feb. 9, 2001 priority date of U.S. Provisional Application 60/267,985, the contents of which are herein incorporated by reference. 
    
    
     FIELD OF INVENTION 
     This invention relates to ophthalmologic methods and systems, and in particular, to methods and systems for detecting damage to the retina, such as that caused by glaucoma. 
     BACKGROUND 
     The retina is a highly organized but complex neurosensory tissue that processes visual information and transmits it to the brain along the optic nerve. The optic nerve, which connects the eye to the brain, is predominantly a bundle of axons, or fiber-like projections of neuronal cells called retinal ganglion cells. These retinal ganglion cell axons fan out along the superficial aspect of the retina in arcuate bundles. When light stimulates photoreceptors, which are located under these arcuate bundles, a signal is transmitted, via a complicated array of intervening neuronal cells, to the retinal ganglion cell body. The retinal ganglion cell axons then relay this signal through an exit site, referred to as the “optic nerve head”, in the posterior wall of the eye. The optic nerve head corresponds functionally to the blind spot in vision because, at this location, there are no underlying photoreceptor cells. 
     Glaucoma is a disease of the retinal ganglion cell bodies and their axons. In a patient having glaucoma, the retinal ganglion cells slowly lose their ability to transmit nerve impulses. As a result, vision diminishes, often so slowly that a patient afflicted with this disease does not notice the degradation in vision until significant damage has occurred. It is in this insidious manner that glaucoma robs the patient of sight. When detected early enough, glaucoma can be managed. However, because glaucoma has few overt symptoms, it is difficult to detect early. 
     One approach to testing for glaucoma is to use a tonometer to measure intra-ocular pressure. This test is based on the notion that high intra-ocular pressure can damage the retinal ganglion layer. However, in practice, intra-ocular pressure has not proven to be a reliable indicator for glaucoma. 
     A more reliable test for glaucoma is a visual field test in which light is directed to various portions of the retina. By asking the patient whether he sees the light, one can map the sensitivity of the retina. Because the field vision test measures optic nerve function more directly, it is a more accurate indicator of glaucoma than the tonometric test. However, the visual field test is a time-consuming test that requires expensive equipment operated by trained personnel. Furthermore, the visual field test assesses all components of the visual system, from the tear film to the occipital cortex in the brain. It is not testing the function of retinal ganglion cells specifically. As a result, it can be difficult to distinguish pathology of retinal ganglion cells from pathologies of other components of the visual system. 
     SUMMARY 
     The invention is based on the recognition that under certain circumstances, a patient having normal retinal function will perceive an entoptic signal. This entoptic signal, which most commonly appears to the patient as a blue arc, is believed to originate in the retinal ganglion cell axons. A patient&#39;s inability to perceive this entoptic signal is correlated with the likelihood that the patient&#39;s retina has experienced damage, perhaps from early stages of glaucoma. This correlation is exploited in a simple and inexpensive test for evaluating the likelihood that a patient has glaucoma. 
     A method according to the invention includes selecting a test site on a retina of the patient and stimulating that test site to cause the generation of an entoptic signal. The entoptic signal need not be generated at the test site but can be generated sewhere in the eye. The entoptic signal generated as a result of the stimulus is then detected. In some practices of the invention, the entoptic signal is detected by accepting an input from the patient that indicates whether or not the patient has perceived any visual manifestation of the entoptic signal. In other practices of the invention, entoptic signal is detected by obtaining an objective measurement with a device for measurement of electromagnetic fields or waves. 
     In some practices of the invention, the test site is selected to be temporal, or peripheral to the patient&#39;s optic nerve head. In other practices of the invention, selecting a test site includes providing a target upon which the patient is to fixate. In those cases, stimulating the test site includes displaying, to the patient, a test figure peripheral to the target. 
     The test site can be stimulated by illuminating it with a test figure. Alternatively, the test site can be stimulated by displaying a test figure to the patient on, for example, a computer monitor or a specialized display device. In either case, the test figure can periodically be made to flash on and off. The frequency and duty cycle with which the test figure flashes is selected to enhance the patient&#39;s visual perception of the entoptic signal. 
     These and other features of the invention will be apparent from the following figures, in which: 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a time-line of a glaucoma diagnosis method; 
     FIG. 2 shows a target figure; 
     FIG. 3 shows a test figure next to the target figure of FIG. 2; 
     FIG. 4 shows the location of blue arcs perceived by the patient&#39;s right eye; 
     FIG. 5 shows the test figure on the other side of the target figure; and 
     FIG. 6 shows the location of blue arcs perceived by the patient&#39;s left eye. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, one practice of the glaucoma diagnosis method  10  begins with a first quieting interval  12  in which the patient is seated before a computer monitor, ambient lighting is turned off, and the patient closes both eyes. This allows any electrical activity within the patient&#39;s retinas to become as quiescent as possible. This first quieting interval typically lasts on the order of one or two minutes. 
     Following the first quieting interval  12  is a first focusing interval  14  during which a computer monitor  16  displays a target figure 18, as shown in FIG.  2 . The target figure is centered on a black background on the computer monitor&#39;s display area  20 . With his left eye covered, the patient fixates on the target figure 18 with his right eye. 
     Immediately after the first quieting interval  12 , a first testing interval  22 , lasting approximately twenty-five seconds, begins. During this first testing interval  22 , a test figure 24 is periodically displayed adjacent to the target figure 18, as shown in FIG.  3 . The actual size, shape, color, luminance of the test figure 24, and its location relative to the target figure 18, can be varied in order to enhance the patient&#39;s perception of entoptic light. For example, in FIG. 3, the test figure 24 is positioned on the side of the target figure 18 that is toward the patient&#39;s nose e.g. the test figure 24 is located in the patient&#39;s nasal visual field and projects to a retinal location temporal to the fovea). Hence, the test figure 24 shown in FIG. 3 is as it would appear for testing the patient&#39;s right eye. 
     To the extent that the patient fixates on the target figure 18, the portion of the retina illuminated by the test figure 24 will lie temporal to the patient&#39;s fovea. The blind spot will lie on the side of the fovea opposite to the test figure, or nasal to the patient&#39;s fovea. 
     During the first testing interval  22 , the test figure 24 is flashed on and off. As it does so, the patient is asked whether he perceives a pair of blue arcs  26 A-B. In most cases, the blue arcs  26 A-B will appear to extend away from the test figure 24 as shown in FIG. 4, arcing around the target figure and converging toward an invisible point on the opposite side of the display. These blue arcs  26 A-B are not actually present on the display surface  20 . Instead, the patient perceives these blue arcs  26 A-B in response to the retinal stimuli provided during the first testing interval  22 . A patient who fails to perceive the blue arcs  26 A-B is considered to be more at risk of having glaucoma or other optic nerve disease than a patient who does perceive the blue arcs  26 A-B. 
     In practice, the perceived shapes of the blue arcs  26 A-B and their perceived color can vary from patient to patient. In addition, certain patterns of damage to the optic nerve, whether from glaucoma or other optic nerve diseases, can cause the patient to perceive only one blue arc. 
     The first testing interval  22  is followed by a second quieting interval  28 . This is followed by a second focusing interval  30  in which the patient covers his right eye and fixates on the target figure 18 with his left eye. During a second testing interval  32  that follows, the test figure 24 is displayed on the opposite side of the target figure 18 so that it stimulates an area of the retina temporal to the fovea of the left eye, as shown in FIG.  5 . 
     During the second testing interval  32 , the test figure 24 is again flashed on and off. As it does so, the patient is again asked whether he perceives a pair of blue arcs  26 A-B extending away from the test figure 24, arcing around the target figure  18  and converging toward an invisible point on the opposite side of the display, as shown in FIG.  6 . 
     In one practice of the invention, the test figure  24  and the target figure 18 are displayed on a conventional computer display, such as a cathode ray tube or a flat panel display. Such displays are advantageous because of their widespread availability. However, conventional computer displays offer only limited luminance. Hence, in another practice of the invention, the target figure  18  and the test figure 24 can be displayed on a specialized high-luminance display. 
     In an alternative practice of the invention, the test figure 24 can be projected directly onto the retina using known optical projection systems. This practice is advantageous because it no longer relies on the patient&#39;s ability to remain fixated on the target figure 18 during the testing interval. 
     The target figure 18 can be an “x” or a small cross rendered in gray or white. Typically, the target figure 18 is small enough so that to avoid generating unnecessary electrical activity within the retinal ganglion cell axons but large enough to capture the patient&#39;s gaze. The appropriate angular extent can be achieved by controlling the image size on the display or by seating the patient at different distances from the display. 
     The test figure 24 can be any shape or color. However, it has been found experimentally that a red vertical bar enhances the patient&#39;s perception of the blue arc. The vertical extent of the test figure 24 is selected such that the vertical image on the retina is slightly longer than the macula. In particular, with the patient&#39;s eyes located 40 centimeters from the display, the test figure 24 subtends 5 degrees of arc above and below a horizontal line defined by the target figure  18 . Along its minor axis, the test figure 24 subtends 0.86 degrees of arc. The test figure 24 is disposed approximately 2.3 degrees of arc nasal to the target figure  18 . 
     It is also not necessary that the test figure 24 flash during the first and second testing intervals. However, it has been found that flashing the test figure 24 enhances the patient&#39;s perception of the blue arc. A preferred flashing sequence includes repeatedly displaying the test figure 24 for half a second and hiding it for two seconds. 
     A disadvantage of the foregoing practice lies in its reliance on a patient&#39;s subjective perception of the blue arcs  26 A-B. To circumvent this, one practice of the invention includes a detector for detecting any electromagnetic fields or waves that cause the patient to perceive the blue arc. One example of such a detector is a photomultiplier, configured to detect any photons emitted by the retinal ganglion cell axon. 
     One implementation of the glaucoma test is in the form of a POWERPOINT (™) presentation. In this embodiment, the individual presentation slides can be sequentially displayed at a controlled rate. However, any software that can produce an animated display can be used to implement the glaucoma test of the invention.