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Correlation between static automated and scanning laser entoptic perimetry in normal subjects and glaucoma patients
Correlation between Static Automated and Scanning Laser Entoptic Perimetry in Normal Subjects and Glaucoma Patients Daniel J. Plummer, PhD,1 Ann Lopez, MD,1 Stanley P. Azen, PhD,3 Laurie LaBree, MS,2 Dirk-Uwe G. Bartsch, PhD,1 Alfredo A. Sadun, MD, PhD,3 William R. Freeman, MD1 Objective: To compare the effectiveness of scanning laser entoptic perimetry with static automated perimetry as a noninvasive instrument for screening for glaucomatous damage in visually asymptomatic subjects within the central 60° (diameter) of vision. Design: A masked cross-sectional study comparing entoptic perimetry to achromatic threshold perimetry. Participants: Twenty-three subjects and controls from the Sharp Rees-Stealy Hospital and the Shiley Eye Center at the University of California, San Diego. Testing: Virtual reality– based entoptic perimetry was compared with achromatic threshold perimetry. Main Outcome Measures: For each testing session, we compared the presence of a disturbance in the entoptic perimetry stimulus with the presence of defects in visual function as measured by Humphrey automated visual field perimetry. Results: Scanning laser entoptic perimetry reasonably estimates the overall visual field loss for moderateto-severe scotomas as measured by the pattern deviation in standard visual field perimetry. Scanning laser entoptic perimetry has a sensitivity from 27% to 90% and a specificity from 50% to 100% for screening moderate-to-severe visual field defects caused by glaucoma within the central 60° diameter of vision. Conclusions: Scanning laser entoptic perimetry may be an effective and inexpensive screening test in hospitals and community clinics for diagnosing visual field loss caused by glaucoma. Ophthalmology 2000;107: 1693–1701 © 2000 by the American Academy of Ophthalmology. One of the most challenging problems in ophthalmology is the development of effective retinal screening tests for peripheral retinal disease. Remarkably, subjects experiencing peripheral visual field damage often remain visually asymptomatic. Subjects generally do not notice any disturbance of the visual field until damage occurs close to the fovea. Glaucoma is a disease of the optic nerve that produces characteristic visual field loss. Early stages are typically associated with relative loss of peripheral visual field sensitivity producing wedgelike defects in the peripheral visual field. Left untreated, it will typically progress and involve the central 10 degrees of vision. Until visual field loss is advanced and has an impact on central vision, paOriginally received: November 9, 1999. Accepted: April 17, 2000. Manuscript no. 99732. 1 Shiley Eye Center, Department of Ophthalmology, School of Medicine, University of California, San Diego, La Jolla, California. 2 Statistical Consultation and Research Center, Department of Preventive Medicine, University of Southern California, Los Angeles, California. 3 Doheny Eye Institute and Department of Ophthalmology, Keck Medical School of the University of Southern California, Los Angeles, California. Supported by NIH grant NEI EY11961 (DJP), NIH grant NEI EY07366 (WRF), Core Grant for Vision Research NEI EY-03040 (SPA, LLB), and a departmental grant from Research to Prevent Blindness (WRF). Reprint requests to Daniel J. Plummer, PhD, Shiley Eye Center, Department of Ophthalmology, School of Medicine, University of California, San Diego, La Jolla, CA, 92093-0946. © 2000 by the American Academy of Ophthalmology Published by Elsevier Science Inc.
tients generally remain asymptomatic to scotomas caused by decreasing peripheral visual field sensitivity. Repression of pathologic peripheral scotomas is related to the Troxler phenomenon. In the Troxler phenomenon1 a fixed spot of light above threshold presented to the peripheral visual field will disappear from view. This phenomenon applies primarily outside 12° from fixation. This is likely due to neural mechanisms in the brain and has several adaptive values in human vision. For example, the Troxler phenomenon allows structures in a constant position in the visual field (e.g., blood vessels) to be repressed and not interfere with visual function. Furthermore, it permits most cortical function to remain focused on a centrally placed object of regard, except when peripheral items are moving or changing in luminosity. However, scotomas caused by retinal injury or other pathologic condition are also repressed by the Troxler phenomenon and therefore are not perceived by subjects, especially if they are far from the fovea. Early detection of potentially treatable diseases such as glaucoma is essential for the prevention of severe vision loss. A procedure that can quickly and reliably measure the extent and locations of eccentric retinal scotomata would therefore be valuable, especially if it could give a relatively precise determination of the damage. Until now, there have been no highly sensitive, rapid, noninvasive screening techniques for peripheral visual dysfunction. The standard diagnostic tool has been threshold perimetry, which can efISSN 0161-6420/00/$–see front matter PII S0161-6420(00)00248-7
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Ophthalmology Volume 107, Number 9, September 2000 fectively assist the ophthalmologist in diagnosing glaucoma but is impractical for screening by the primary care provider. Entoptic, or snow-field perimetry, is a technique we have used to detect cytomegalovirus retinitis,2,3 with sensitivities and specificities greater than 95%, and has also been used by other groups.4 – 6 A computer monitor filled with random particle motion, when viewed by someone with a normal visual system, will appear as “visual noise.” Subjects with peripheral retinal lesions are able to outline their scotomas. Those areas corresponding to the damaged retina appeared to have no random motion and were “gray” or “motionless” in appearance. Those areas in which subjects reported no random particle motion corresponded to retinal lesions. Entoptic perimetry has, until now, been presented on a computer monitor. Practical considerations limit the amount of retina that can be screened by flat-panel technology. This technology suffers from several problems including (1) loss of contrast and lack of lighting control, which are critical in entoptic perimetry3; (2) the requirement for a large amount of space to present and store the equipment; (3) a distortion of the stimulus as people get close to the large screen (but outside the accommodative limit) while attempting to view the image in the peripheral retina; and (4) refractive and accommodative error correction. The limitations of flat-screen technology presentation of entoptic perimetry can be overcome by using a virtual reality device (Fig 1) in the form of the Microvision Virtual Retinal Display™ system (VRD)™ (Microvision Inc., Seattle, WA). There are several advantages to this technology over monitors. Images are projected directly into the eye, presented at virtual infinity, and can be imaged over the peripheral retina. This compensates for all but the most severe refractive errors and also eliminates peripheral image distortion, and the quality of the image allows for extremely high contrast. The scanning laser equipment is portable, easily fitting within a briefcase, allowing mobility within a clinical setting. A narrow exit pupil in our device ensured that subjects were fixated centrally, greatly reducing error rates caused by inappropriate fixation. Our group has recently demonstrated7 that entoptic perimetry in conjunction with the Microvision VRD™ platform can be used to screen subjects for damage caused by infectious retinopathies out to 60° radius from the fovea. In this study, we found that scanning laser entoptic perimetry was able to detect visual field loss caused by full-thickness retinal damage within the central 120° (diameter) of vision with a sensitivity of 92%, specificity of 95%, positive predictive value of 94%, and a negative predictive value of 94%. We present results evaluating the effectiveness of the VRD™ platform for screening for glaucomatous visual field defects.
Methods Subjects A total of 16 glaucoma subjects (29 eyes, because 3 were too impaired) and 7 normal subjects (12 eyes) were recruited from
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Figure 1. The virtual reality– based entoptic perimeter with headrest.
local eye clinics. A total of 41 eyes were tested using both standard Humphrey automated visual field perimetry (SAP) and scanning laser entoptic perimetry. All patients had previously undergone multiple Humphrey Field Analyzer tests for all eyes as part of their routine examination protocol using the Humphrey Field Anaylzer protocol 30-2. All glaucoma subjects were recruited during ophthalmologic visits for treatment or follow-up of ocular disease. Participation was entirely voluntary, and we received informed consent and institutional review board approval.
Procedures Standard Automated Visual Field Perimetry. SAP, an ophthalmologic clinical procedure that measures the ability of the eye to detect a near-threshold visual stimulus in the paracentral or peripheral visual fields was used to determine the extent and severity of visual field damage or whether the subject was normal. SAP was performed on all subjects on a visit before the entoptic field testing using a Humphrey Visual Field Analyzer, model 640 (Humphrey Instruments, Inc., San Leandro, CA). Standard parameters provided by the internal settings of the machine using program 24-2 and a size III (Goldmann) target were used. Scanning Laser Entoptic Perimetry. Scanning laser entoptic perimetry consisted of a monocular presentation on a VRD™ of monochromatic random particle motion. Each “pixel value” could be either on at 635 nm or off. The VRD™ consisted of a scanning retinal laser that delivered the image through a 1-mm exit pupil.
Plummer et al 䡠 Scanning Laser Entoptic Perimetry Table 1. Criteria for Determining Severity of Visual Field Loss Using the Glaucoma Hemifield Test, Corrected Pattern Standard Deviation, and Mean Defect Percentiles and dB Values GHT Normal (0) Early (1) Moderate (2) Advanced (3) MD only (4)
⬍99.5% ⬎99.5% ⬎99.5% ⬎99.5% ⬍99.5%
CPSD and or or or and
⬍95% 95%–99% ⬎99% ⬎99% ⬍95%
and with or w/o with or w/o with or w/o with
MD
MD in decibel
⬍95% ⬎95% ⬎95% ⬎95% ⬎95%
n/a ⫺6 ⬎⫺6 but ⫺15 ⬎⫺15 and ⫺25 Any value
CPSD ⫽ corrected pattern standard deviation; GHT ⫽ glaucoma henifield test; MD ⫽ mean defect
The stimulus presented to the subject through the VRD™ was also “mirrored” by virtue of a video signal splitter that displayed the identical stimulus on a computer monitor. This allowed the experimenter to view the identical stimulus as the subject and control the entoptic perimetry program without interfering with the view of the subject in the VRD™. The VRD™ we used had a capability for screening out to 30° radius when the patient was fixated centrally on a fixation crosshair. Subjects were not dilated, and, as we have previously reported,7 because of the nature of the VRD™, subjects required no refractive correction because the image is placed at virtual infinity within the eye. Subjects were seated and initially viewed a computer monitor that mirrored the stimulus inside the VRD™ and were shown an example of the entoptic stimulus. They were given instructions that they were to fixate centrally on the screen at a crosshair and maintain fixation throughout the testing session. The technician explained that a stimulus would appear on the screen consisting of monochromatic particle motion. If there were any areas in which there was a permanent disturbance or change in the random particle motions, the subject was to indicate this area through a computer pen. The participants were then given instructions on how to use the virtual pen. The entoptic program has two modes of display. The stimulus mode displays the entoptic stimulus. As the virtual pen was brought close to a touch-sensitive pad, the stimulus mode ended and the program entered the “recording” mode, in which subjects see only a blank workspace for drawing. The recording mode had several options. Placing the pen on the pad and moving it (keeping a firm, light pressure on the stylus) produced a black line against the background. Removing the pen from the pad but keeping it close to the pad (i.e., closer than 1 cm) allowed the subjects to move the cursor on the screen without drawing. Pulling the pen away from the pad further than 1 cm returned the viewer to the stimulus mode of moving random dots. Placing the pen close to the pad would again return the subject to the drawing screen, and previously drawn scotomas would remain. In this way, subjects were able to alternate between viewing the stimulus or their own drawing under their own control. All actions were monitored by the technician who viewed the computer monitor during testing. This instructional phase rarely took longer than 2 minutes. After instructions, subjects were then seated in front of the VRD™ and asked to fixate centrally and, while looking at the target, to report any permanent perceptual change. All subjects were satisfied with their initial entoptic tracing. Unlike computer monitors that can be viewed from a wide variety of angles, by virtue of the narrow exit pupil subjects had to concentrate on fixating within the VRD™ to see the entoptic stimulus. If their gaze wandered, the stimulus disappeared from view, and they saw a black field. Thus, the subject saw the stimulus only while maintaining fixation centrally within the VRD™.
Scoring Perimetry Findings Humphrey automated perimetry printouts provide a series of measures that evaluate visual function. For this study, we evaluated the effectiveness of entoptic perimetry against the Humphrey pattern deviation plot. The pattern deviation plot performs an algorithm that “corrects” for diffuse loss caused by cataracts from the total deviation, which analyzes individual visual field locations for deviations from normal. In some cases, subjects can have a large number of points outside of normal limits on the total deviation plot but appear relatively normal on the pattern deviation plot. In this study, we compared the sensitivity of entoptic perimetry against pattern deviation. Scoring was performed as follows. Using the results of Humphrey visual field STATPAC printout, each of the 52 points tested during the 24-2 threshold algorithm were scored as either normal or abnormal. (The points directly above and below the blind spot were eliminated from the analysis, reducing the number from 54 to 52.) For both pattern deviation and total deviation, each point was classified as either normal or abnormal for four different conditions, representing scotoma severity: (1) a point was scored as abnormal if its sensitivity was 95% or more below normal limits (all scotomas), (2) if sensitivity was 98% or more below normal limits (mild to severe scotomas), (3) if sensitivity was 99% or more below normal limits (moderate-to-severe scotomas), and (4) if sensitivity was 99.5% or more below normal limits (severe scotomas only). These deficit severity parameters were derived from the Humphrey STATPAC probability symbols. This provided us with a progressive method for comparing entoptic perimetry tracings against visual fields with all types of defects (all scotomas) to only those with the most severe loss of sensitivity (severe scotomas only). Normal limits were determined by the Humphrey STATPAC internal database.
Scoring Entoptic Perimetry Findings The absence of entoptic perimetry visual field disturbance was determined by an expert psychophysicist (DJP). If a subject drew an area using the computer interface that corresponded to a permanent, localized change in the entoptic stimulus, the eye was classified as having a visual field disturbance. Entoptic perimetry scores were derived by superimposing each subject tracing over a scaled grid that represented the 6° spacing of the Humphrey stimulus display pattern. Each of the 52 points was scored as either normal or abnormal in the following manner: if the entoptic perimetry tracing crossed a border anywhere within the 6° ⫻ 6° area, that point was considered to be abnormal; otherwise, the point was scored as normal.
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Figure 2a.
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Figure 2A. Mild and severe glaucoma. A, Standard Humphrey automated visual field perimetry (SAP) printout from a patient with a mild-to-moderate phase glaucoma on the left with the corresponding entoptic perimetry tracing on the right.
Clinical Assessment of SAP An ophthalmologist experienced in the treatment of glaucoma (AL) reviewed all Humphrey visual fields in a masked manner and assigned one of five classifications to each visual field (normal, suspect, early, moderate, severe) on the basis of experience in diagnosing glaucoma. On the basis of the ophthalmologist’s five classifications, we grouped patients into one of two groups, as either normal/suspect/early or moderate/severe.
Standardized Assessment of SAP We classified subjects into two categories, as either normal/early or moderate/severe on the basis of clinical evaluation using the Ocular Hypertension Treatment Study classification clinical evaluation of the automated Humphrey visual fields8 (Table 1).
Statistical Analysis For each study eye, we computed the sensitivity, specificity, and overall predictive value (⫽ percent correct) for scanning laser entoptic perimetry for strata defined by one of the two assessments (clinical, standardized). Because motivation and attention may be a factor in determining the reliability and validity of entoptic perimetry, we analyzed data on both a per-eye and per-subject basis. For the per-subject basis, we randomly selected one eye for each subject. For each clinical assessment, sensitivity was calculated as the ratio of the number of eyes scored positive by scanning laser entoptic perimetry to the number of eyes scored positive by Humphrey automated visual field perimetry. Specificity was calculated
as the ratio of the number of eyes scored negative by entoptic perimetry to the number of eyes scored negative by Humphrey automated visual field perimetry. Summary statistics were calculated for two different measures of threshold sensitivity: (1) percentage of the 52 points outside normal limits (pointwise analysis) and (2) percentage of four quadrants classified as abnormal, in which a minimum of three neighboring points within that quadrant have been classified as abnormal (cluster analysis). The rationale for the cluster analysis is that occasionally even normal subjects will produce a spurious point outside normal limits. The cluster analysis would in effect raise the “threshold” for detection of a true scotoma. We calculated summary statistics at four levels of significance on the basis of Humphrey STATPAC analyses: (1) at least one point (cluster per quadrant) outside of a 95% normal limit, (2) at least one point (cluster per quadrant) outside of a 98% normal limit, (3) at least one point (cluster per quadrant) outside of a 99% normal limit, and (4) at least one point (quadrant, cluster per quadrant) outside of 99.5% normal limit. The percentage of normal limits is determined by the Humphrey internal STATPAC database.
Results There were 29 glaucomatous eyes included in the study. All 29 eyes had abnormalities on SAP, and 19 of these eyes reported entoptic perimetry disturbances. None of the 12 normal eyes had abnormalities on either SAP or entoptic perimetry. An example of an entoptic tracing and an SAP printout appear in Figure 2. Average testing time for entoptic perimetry per eye was 30 seconds.
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Figure 2b.
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Figure 2b. Mild and severe glaucoma. B, Standard Humphrey automated visual field perimetry (SAP) printout and entoptic tracing of a patient with advanced glaucoma. Note that there is good correspondence between the areas of entoptic tracings and the locations of the most severe scotomas.
Predictive Measures of Entoptic Perimetry versus SAP Stratified by Category of Clinical Assessment
Predictive Measures of Entoptic Perimetry versus SAP Using the Standardized Assessment Table 3 presents for each of the two categories the per-eye and per-subject sensitivity, specificity, and percent correct classifications of entoptic perimetry for detecting glaucoma-related visual field defects. Results are similar to those given in Table 2, namely, sensitivities and specificities are relatively high for subjects classified in the moderate/severe group and increase with increasingly deeper scotomas, represented by greater levels of probability of abnormalities. In contrast, subjects classified as normal/early have a relatively high specificity but have a much wider range in sensitivities from low to high, despite the fact that the overall percentage correct remains the same. As with the moderate/severe group, predictive value also increases with the more severe scotomas. For both sets of analyses, all measures tend to have more predictive power using the by-subject analyses and the pattern cluster analyses.
Table 2 presents for each of the two categories the per-eye and per-subject sensitivity, specificity, and percent correct classifications of entoptic perimetry for detecting glaucoma-related visual field defects. In general, the sensitivity of entoptic perimetry was relatively high for subjects classified in the moderate/severe group (range, 0.71– 0.90) and increased with increasingly deeper scotomas, as reflected by greater levels of probability of abnormalities. Specificity was 1.00 for clinical assessment. In contrast, subjects classified as normal/early/suspect had low-to-moderate sensitivities (range, 0.27– 0.67). Specificity was adequate (range, 0.78 –1.00). For both sets of analyses, all measures tended to have more predictive power (as measured by percent correct) using the bysubject analyses and the pattern cluster analyses.
Table 2. Sensitivity, Specificity, and Predictive Value of Entoptic Perimetry by Clinical Classification of Severity (number of moderate/severe subjects/eyes ⫽ 12/21; number of normal/early/suspect subjects/eyes ⫽ 11/20) Sensitivity
Specificity
% Correct
Normal/ Early/ Suspect
Moderate/ Severe
Normal/ Early/ Suspect
Moderate/ Severe
Normal/ Early/ Suspect
Pointwise deviation by eye (%) ⱖ95 0.71 ⱖ98 0.75 ⱖ99 0.75 ⱖ99.5 0.75
0.27 0.50 0.40 0.50
— 1.00 1.00 1.00
1.00 1.00 0.87 0.88
71 76 76 76
45 80 75 80
Cluster deviation by eye (%) ⱖ95 0.75 ⱖ98 0.83 ⱖ99 0.83 ⱖ99.5 0.83
0.25 0.33 0.33 0.50
1.00 1.00 1.00 1.00
0.81 0.82 0.82 0.83
76 86 86 86
70 75 75 80
Pointwise deviation by subject (%) ⱖ95 0.75 ⱖ98 0.82 ⱖ99 0.82 ⱖ99.5 0.82
0.38 0.60 0.67 0.67
— 1.00 1.00 1.00
1.00 1.00 0.88 0.88
75 83 83 83
55 82 82 82
Cluster deviation by subject (%) ⱖ95 0.82 ⱖ98 0.82 ⱖ99 0.90 ⱖ99.5 0.90
0.50 0.50 0.50 1.00
1.00 1.00 1.00 1.00
0.78 0.78 0.78 0.80
83 83 83 92
73 73 73 82
SAP
Moderate/ Severe
SAP ⫽ Standard Humphrey automated visual field perimetry.
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Ophthalmology Volume 107, Number 9, September 2000 Table 3. Sensitivity, Specificity, and Negative Predictive Value of Entoptic Perimetry By Standardized Classification of Severity (number of moderate/severe subjects/eyes ⫽ 13/23; number of normal/early subject/eyes ⫽ 10/18) Sensitivity
% Correct
Normal/ Early
Moderate/ Severe
Normal/ Early
Moderate/ Severe
Normal/ Early
Pointwise deviation by eye (%) ⱖ95 0.65 ⱖ98 0.68 ⱖ99 0.67 ⱖ99.5 0.67
0.31 0.67 0.75 1.00
— 1.00 0.50 0.50
1.00 1.00 0.93 0.93
65 70 65 65
50 89 89 94
Cluster deviation by eye (%) ⱖ95 0.67 ⱖ98 0.74 ⱖ99 0.74 ⱖ99.5 0.78
0.67 1.00 1.00 1.00
0.50 0.75 0.75 0.75
0.87 0.88 0.88 0.88
65 74 74 78
64 91 100 100
Pointwise deviation by subject (%) ⱖ95 0.69 ⱖ98 0.75 ⱖ99 0.73 ⱖ99.5 0.73
0.43 0.75 1.00 1.00
— 1.00 0.50 0.50
1.00 1.00 1.00 1.00
69 77 69 69
60 90 100 100
Cluster deviation by subject (%) ⱖ95 0.73 ⱖ98 0.80 ⱖ99 0.80 ⱖ99.5 0.89
1.00 1.00 1.00 1.00
0.50 0.67 0.67 0.75
0.88 0.88 0.88 0.88
69 77 77 85
90 90 90 90
SAP
Moderate/ Severe
Specificity
SAP ⫽ Standard Humphrey Automated Visual Field Perimetry
Discussion Our previous studies with entoptic perimetry have suggested that entoptic perimetry would be useful in screening subjects who have absolute or near-absolute scotomas caused by retinal damage from a variety of pathologic conditions (e.g., cytomegalovirus retinitis, ocular melanoma, age-related macular degeneration). Although several studies examine how well entoptic perimetry can detect scotomas caused by glaucoma (e.g.,4), to our knowledge no study to date has implemented testing using a scanning laser– based system that can eliminate optical problems caused by refractive error, cataracts, or poor contrast conditions. These results demonstrate that entoptic perimetry reasonably estimates the overall visual field loss for moderate-tosevere scotomas as measured by the pattern deviation in standard Humphrey automated visual field perimetry. This version of scanning laser entoptic perimetry was less sensitive and did not have as strong a predictive power for detecting all visual field scotomas, including the shallow (relative) ones that may be indicative of early visual field damage. Detection of the moderate-to-severe visual field loss and rapid referral to a qualified ophthalmologist is a critical public health issue, particularly before subjects become symptomatic when vision loss has an impact on central vision and at stages that might be more amenable to treatment. These results also suggest that using a “cluster” analysis, in which several adjacent points must be outside normal limits for the quadrant to be classified as abnormal, may be an advantageous method of determining abnormality.
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We performed two subanalyses on our subject population. Subjects were placed into one of two severity groups on the basis of classifications used by a glaucoma specialist and in a separate analysis using a standardized system based on threshold perimetry analyses. We found that for both subanalyses, the sensitivity and predictive values of entoptic perimetry for detecting moderate-to-severe visual field loss increased in subjects with more severe visual field loss but that both the clinician’s and standardized rating system produced similar results. This again suggests that scanning laser entoptic perimetry would be a useful screening test for moderate-to-advanced glaucoma. There are several advantages for this current version of entoptic perimetry. As we have previously reported,7 the VRD™ places the stimulus at virtual infinity, eliminating the need for refractive correction except in the most extreme cases. The brightness and contrast can be varied along a large dynamic range in presenting the stimulus to the retina even in cases of opacification caused by vitritis, cataracts, or high refractive error. This eliminates many of the confounds that can affect the interpretation of standard threshold perimetry of newer devices such as Frequency Doubling Technology (Welch-Allyn, Skaneateles Falls, NY). VRD-based entoptic perimetry also has the advantage of requiring very little time of the subject. We have reported that subjects can see the scotomas within a matter of seconds, typically less than 30. The Frequency Doubling Technology takes a minimum of 1 minute and up to 5 minutes for a full-threshold test. The Humphrey Field Analyzer II SITA program requires a minimum of 4 and up to 10 minutes for each eye. The singular advantage of allowing the subject to actually
Plummer et al 䡠 Scanning Laser Entoptic Perimetry see the extent of his or her area of scotoma is that this may lead to increased compliance. Several major differences exist between glaucoma and other retinal diseases (e.g., ocular melanoma, cytomegalovirus retinitis). Retinal damage caused by these latter diseases often causes full-thickness destruction of the retina in the affected areas, manifesting functionally as absolute visual field scotomata. Glaucoma-related visual field changes initially begin as shallow, relative scotomas that progress and deepen over a long period of time, depending on treatment and early detection. In the final stages of the disease, glaucoma will also cause absolute visual scotomas similar to those we have previously reported with cytomegalovirus retinitis and other diseases.7 We predict that the sensitivity and specificity would be similar to those rates previously reported for full-thickness scotomas1,3,7 for subjects with advanced glaucoma. The VRD™ may be an ideal candidate for a communitybased screening test for glaucoma and retinal diseases. The incidence and prevalence of glaucoma is much higher in the general population than diseases such as cytomegalovirus retinitis or ocular melanoma. Entoptic perimetry is rapid and inexpensive, making it an ideal candidate for a visual function screening test. Scanning laser– based entoptic perimetry may also be useful for screening populations not only in the primary care clinic but also for public health applications such as the Department of Motor Vehicles, where persons may in fact pass a standard vision test but have severely limited peripheral vision caused by glaucoma. In these and other general screening examinations, the screening test must be rapid, user-friendly, and noninvasive To create a validated virtual-reality– based entoptic screening test, we need to create an algorithm that incorporates measures of fixation loss, false-negative results, and false-
positive results. Although this will potentially increase duration of testing, measures of validity will allow physicians to assess entoptic perimetry results, particularly in the case of patients who are suspect for glaucoma but report no entoptic perimetry stimulus disturbance.
References 1. Plummer DJ, Are´valo JF, Fram N, et al. Effectiveness of entoptic perimetry for locating peripheral scotomas caused by cytomegalovirus retinitis. Arch Ophthalmol 1996;114:828 – 31. 2. Plummer DJ, Sample PA, Are´valo JF, et al. Visual field loss in HIV-positive patients without infectious retinopathy. Am J Ophthalmol 1996;122:542–9. 3. Plummer DJ, Banker A, Taskintuna I, et al. The utility of entoptic perimetry as a screening test for cytomegalovirus retinitis. Arch Ophthalmol 1999;117:202–7. 4. Aulhorn E, Ko¨st G. Rauschfeldkampimetrie. Eine neuartige perimetrische Untersuchungsweise. Klin Monatsbl Augenheilkd 1988;192:284 – 8. 5. Aulhorn E, Ko¨st G. Noise-field campimetry. A new perimetric method (snow campimetry). In: Heijl A, ed. Perimetry Update 1988/1989. Proc VIIIth Int’l Perimetric Society Meeting, 1988. Amsterdam: Kugler & Ghendini, 1988. 6. Schiefer U, Gisolf AC, Kirsch J, et al. Rauschfeld-Screening. Ergebnisse einer Fernsehfeldstudie zur Detektion von Gesichtsfelddefekten. Ophthalmologe 1996;93:604 – 616. 7. Plummer DJ, Azen SP, Freeman WR. Scanning laser entoptic perimetry for the screening of macular and peripheral retinal disease. Arch Ophthalmol; in press. 8. Gordon MO, Kass MA. Ocular Hypertension Treatment Study (OHTS) Manual of Procedures v. 2.0. Bethesda, MD: National Institutes of Health, 1995.
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tients generally remain asymptomatic to scotomas caused by decreasing peripheral visual field sensitivity. Repression of pathologic peripheral scotomas is related to the Troxler phenomenon. In the Troxler phenomenon1 a fixed spot of light above threshold presented to the peripheral visual field will disappear from view. This phenomenon applies primarily outside 12° from fixation. This is likely due to neural mechanisms in the brain and has several adaptive values in human vision. For example, the Troxler phenomenon allows structures in a constant position in the visual field (e.g., blood vessels) to be repressed and not interfere with visual function. Furthermore, it permits most cortical function to remain focused on a centrally placed object of regard, except when peripheral items are moving or changing in luminosity. However, scotomas caused by retinal injury or other pathologic condition are also repressed by the Troxler phenomenon and therefore are not perceived by subjects, especially if they are far from the fovea. Early detection of potentially treatable diseases such as glaucoma is essential for the prevention of severe vision loss. A procedure that can quickly and reliably measure the extent and locations of eccentric retinal scotomata would therefore be valuable, especially if it could give a relatively precise determination of the damage. Until now, there have been no highly sensitive, rapid, noninvasive screening techniques for peripheral visual dysfunction. The standard diagnostic tool has been threshold perimetry, which can efISSN 0161-6420/00/$–see front matter PII S0161-6420(00)00248-7
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Ophthalmology Volume 107, Number 9, September 2000 fectively assist the ophthalmologist in diagnosing glaucoma but is impractical for screening by the primary care provider. Entoptic, or snow-field perimetry, is a technique we have used to detect cytomegalovirus retinitis,2,3 with sensitivities and specificities greater than 95%, and has also been used by other groups.4 – 6 A computer monitor filled with random particle motion, when viewed by someone with a normal visual system, will appear as “visual noise.” Subjects with peripheral retinal lesions are able to outline their scotomas. Those areas corresponding to the damaged retina appeared to have no random motion and were “gray” or “motionless” in appearance. Those areas in which subjects reported no random particle motion corresponded to retinal lesions. Entoptic perimetry has, until now, been presented on a computer monitor. Practical considerations limit the amount of retina that can be screened by flat-panel technology. This technology suffers from several problems including (1) loss of contrast and lack of lighting control, which are critical in entoptic perimetry3; (2) the requirement for a large amount of space to present and store the equipment; (3) a distortion of the stimulus as people get close to the large screen (but outside the accommodative limit) while attempting to view the image in the peripheral retina; and (4) refractive and accommodative error correction. The limitations of flat-screen technology presentation of entoptic perimetry can be overcome by using a virtual reality device (Fig 1) in the form of the Microvision Virtual Retinal Display™ system (VRD)™ (Microvision Inc., Seattle, WA). There are several advantages to this technology over monitors. Images are projected directly into the eye, presented at virtual infinity, and can be imaged over the peripheral retina. This compensates for all but the most severe refractive errors and also eliminates peripheral image distortion, and the quality of the image allows for extremely high contrast. The scanning laser equipment is portable, easily fitting within a briefcase, allowing mobility within a clinical setting. A narrow exit pupil in our device ensured that subjects were fixated centrally, greatly reducing error rates caused by inappropriate fixation. Our group has recently demonstrated7 that entoptic perimetry in conjunction with the Microvision VRD™ platform can be used to screen subjects for damage caused by infectious retinopathies out to 60° radius from the fovea. In this study, we found that scanning laser entoptic perimetry was able to detect visual field loss caused by full-thickness retinal damage within the central 120° (diameter) of vision with a sensitivity of 92%, specificity of 95%, positive predictive value of 94%, and a negative predictive value of 94%. We present results evaluating the effectiveness of the VRD™ platform for screening for glaucomatous visual field defects.
Methods Subjects A total of 16 glaucoma subjects (29 eyes, because 3 were too impaired) and 7 normal subjects (12 eyes) were recruited from
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Figure 1. The virtual reality– based entoptic perimeter with headrest.
local eye clinics. A total of 41 eyes were tested using both standard Humphrey automated visual field perimetry (SAP) and scanning laser entoptic perimetry. All patients had previously undergone multiple Humphrey Field Analyzer tests for all eyes as part of their routine examination protocol using the Humphrey Field Anaylzer protocol 30-2. All glaucoma subjects were recruited during ophthalmologic visits for treatment or follow-up of ocular disease. Participation was entirely voluntary, and we received informed consent and institutional review board approval.
Procedures Standard Automated Visual Field Perimetry. SAP, an ophthalmologic clinical procedure that measures the ability of the eye to detect a near-threshold visual stimulus in the paracentral or peripheral visual fields was used to determine the extent and severity of visual field damage or whether the subject was normal. SAP was performed on all subjects on a visit before the entoptic field testing using a Humphrey Visual Field Analyzer, model 640 (Humphrey Instruments, Inc., San Leandro, CA). Standard parameters provided by the internal settings of the machine using program 24-2 and a size III (Goldmann) target were used. Scanning Laser Entoptic Perimetry. Scanning laser entoptic perimetry consisted of a monocular presentation on a VRD™ of monochromatic random particle motion. Each “pixel value” could be either on at 635 nm or off. The VRD™ consisted of a scanning retinal laser that delivered the image through a 1-mm exit pupil.
Plummer et al 䡠 Scanning Laser Entoptic Perimetry Table 1. Criteria for Determining Severity of Visual Field Loss Using the Glaucoma Hemifield Test, Corrected Pattern Standard Deviation, and Mean Defect Percentiles and dB Values GHT Normal (0) Early (1) Moderate (2) Advanced (3) MD only (4)
⬍99.5% ⬎99.5% ⬎99.5% ⬎99.5% ⬍99.5%
CPSD and or or or and
⬍95% 95%–99% ⬎99% ⬎99% ⬍95%
and with or w/o with or w/o with or w/o with
MD
MD in decibel
⬍95% ⬎95% ⬎95% ⬎95% ⬎95%
n/a ⫺6 ⬎⫺6 but ⫺15 ⬎⫺15 and ⫺25 Any value
CPSD ⫽ corrected pattern standard deviation; GHT ⫽ glaucoma henifield test; MD ⫽ mean defect
The stimulus presented to the subject through the VRD™ was also “mirrored” by virtue of a video signal splitter that displayed the identical stimulus on a computer monitor. This allowed the experimenter to view the identical stimulus as the subject and control the entoptic perimetry program without interfering with the view of the subject in the VRD™. The VRD™ we used had a capability for screening out to 30° radius when the patient was fixated centrally on a fixation crosshair. Subjects were not dilated, and, as we have previously reported,7 because of the nature of the VRD™, subjects required no refractive correction because the image is placed at virtual infinity within the eye. Subjects were seated and initially viewed a computer monitor that mirrored the stimulus inside the VRD™ and were shown an example of the entoptic stimulus. They were given instructions that they were to fixate centrally on the screen at a crosshair and maintain fixation throughout the testing session. The technician explained that a stimulus would appear on the screen consisting of monochromatic particle motion. If there were any areas in which there was a permanent disturbance or change in the random particle motions, the subject was to indicate this area through a computer pen. The participants were then given instructions on how to use the virtual pen. The entoptic program has two modes of display. The stimulus mode displays the entoptic stimulus. As the virtual pen was brought close to a touch-sensitive pad, the stimulus mode ended and the program entered the “recording” mode, in which subjects see only a blank workspace for drawing. The recording mode had several options. Placing the pen on the pad and moving it (keeping a firm, light pressure on the stylus) produced a black line against the background. Removing the pen from the pad but keeping it close to the pad (i.e., closer than 1 cm) allowed the subjects to move the cursor on the screen without drawing. Pulling the pen away from the pad further than 1 cm returned the viewer to the stimulus mode of moving random dots. Placing the pen close to the pad would again return the subject to the drawing screen, and previously drawn scotomas would remain. In this way, subjects were able to alternate between viewing the stimulus or their own drawing under their own control. All actions were monitored by the technician who viewed the computer monitor during testing. This instructional phase rarely took longer than 2 minutes. After instructions, subjects were then seated in front of the VRD™ and asked to fixate centrally and, while looking at the target, to report any permanent perceptual change. All subjects were satisfied with their initial entoptic tracing. Unlike computer monitors that can be viewed from a wide variety of angles, by virtue of the narrow exit pupil subjects had to concentrate on fixating within the VRD™ to see the entoptic stimulus. If their gaze wandered, the stimulus disappeared from view, and they saw a black field. Thus, the subject saw the stimulus only while maintaining fixation centrally within the VRD™.
Scoring Perimetry Findings Humphrey automated perimetry printouts provide a series of measures that evaluate visual function. For this study, we evaluated the effectiveness of entoptic perimetry against the Humphrey pattern deviation plot. The pattern deviation plot performs an algorithm that “corrects” for diffuse loss caused by cataracts from the total deviation, which analyzes individual visual field locations for deviations from normal. In some cases, subjects can have a large number of points outside of normal limits on the total deviation plot but appear relatively normal on the pattern deviation plot. In this study, we compared the sensitivity of entoptic perimetry against pattern deviation. Scoring was performed as follows. Using the results of Humphrey visual field STATPAC printout, each of the 52 points tested during the 24-2 threshold algorithm were scored as either normal or abnormal. (The points directly above and below the blind spot were eliminated from the analysis, reducing the number from 54 to 52.) For both pattern deviation and total deviation, each point was classified as either normal or abnormal for four different conditions, representing scotoma severity: (1) a point was scored as abnormal if its sensitivity was 95% or more below normal limits (all scotomas), (2) if sensitivity was 98% or more below normal limits (mild to severe scotomas), (3) if sensitivity was 99% or more below normal limits (moderate-to-severe scotomas), and (4) if sensitivity was 99.5% or more below normal limits (severe scotomas only). These deficit severity parameters were derived from the Humphrey STATPAC probability symbols. This provided us with a progressive method for comparing entoptic perimetry tracings against visual fields with all types of defects (all scotomas) to only those with the most severe loss of sensitivity (severe scotomas only). Normal limits were determined by the Humphrey STATPAC internal database.
Scoring Entoptic Perimetry Findings The absence of entoptic perimetry visual field disturbance was determined by an expert psychophysicist (DJP). If a subject drew an area using the computer interface that corresponded to a permanent, localized change in the entoptic stimulus, the eye was classified as having a visual field disturbance. Entoptic perimetry scores were derived by superimposing each subject tracing over a scaled grid that represented the 6° spacing of the Humphrey stimulus display pattern. Each of the 52 points was scored as either normal or abnormal in the following manner: if the entoptic perimetry tracing crossed a border anywhere within the 6° ⫻ 6° area, that point was considered to be abnormal; otherwise, the point was scored as normal.
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Figure 2a.
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Figure 2A. Mild and severe glaucoma. A, Standard Humphrey automated visual field perimetry (SAP) printout from a patient with a mild-to-moderate phase glaucoma on the left with the corresponding entoptic perimetry tracing on the right.
Clinical Assessment of SAP An ophthalmologist experienced in the treatment of glaucoma (AL) reviewed all Humphrey visual fields in a masked manner and assigned one of five classifications to each visual field (normal, suspect, early, moderate, severe) on the basis of experience in diagnosing glaucoma. On the basis of the ophthalmologist’s five classifications, we grouped patients into one of two groups, as either normal/suspect/early or moderate/severe.
Standardized Assessment of SAP We classified subjects into two categories, as either normal/early or moderate/severe on the basis of clinical evaluation using the Ocular Hypertension Treatment Study classification clinical evaluation of the automated Humphrey visual fields8 (Table 1).
Statistical Analysis For each study eye, we computed the sensitivity, specificity, and overall predictive value (⫽ percent correct) for scanning laser entoptic perimetry for strata defined by one of the two assessments (clinical, standardized). Because motivation and attention may be a factor in determining the reliability and validity of entoptic perimetry, we analyzed data on both a per-eye and per-subject basis. For the per-subject basis, we randomly selected one eye for each subject. For each clinical assessment, sensitivity was calculated as the ratio of the number of eyes scored positive by scanning laser entoptic perimetry to the number of eyes scored positive by Humphrey automated visual field perimetry. Specificity was calculated
as the ratio of the number of eyes scored negative by entoptic perimetry to the number of eyes scored negative by Humphrey automated visual field perimetry. Summary statistics were calculated for two different measures of threshold sensitivity: (1) percentage of the 52 points outside normal limits (pointwise analysis) and (2) percentage of four quadrants classified as abnormal, in which a minimum of three neighboring points within that quadrant have been classified as abnormal (cluster analysis). The rationale for the cluster analysis is that occasionally even normal subjects will produce a spurious point outside normal limits. The cluster analysis would in effect raise the “threshold” for detection of a true scotoma. We calculated summary statistics at four levels of significance on the basis of Humphrey STATPAC analyses: (1) at least one point (cluster per quadrant) outside of a 95% normal limit, (2) at least one point (cluster per quadrant) outside of a 98% normal limit, (3) at least one point (cluster per quadrant) outside of a 99% normal limit, and (4) at least one point (quadrant, cluster per quadrant) outside of 99.5% normal limit. The percentage of normal limits is determined by the Humphrey internal STATPAC database.
Results There were 29 glaucomatous eyes included in the study. All 29 eyes had abnormalities on SAP, and 19 of these eyes reported entoptic perimetry disturbances. None of the 12 normal eyes had abnormalities on either SAP or entoptic perimetry. An example of an entoptic tracing and an SAP printout appear in Figure 2. Average testing time for entoptic perimetry per eye was 30 seconds.
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Figure 2b.
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Figure 2b. Mild and severe glaucoma. B, Standard Humphrey automated visual field perimetry (SAP) printout and entoptic tracing of a patient with advanced glaucoma. Note that there is good correspondence between the areas of entoptic tracings and the locations of the most severe scotomas.
Predictive Measures of Entoptic Perimetry versus SAP Stratified by Category of Clinical Assessment
Predictive Measures of Entoptic Perimetry versus SAP Using the Standardized Assessment Table 3 presents for each of the two categories the per-eye and per-subject sensitivity, specificity, and percent correct classifications of entoptic perimetry for detecting glaucoma-related visual field defects. Results are similar to those given in Table 2, namely, sensitivities and specificities are relatively high for subjects classified in the moderate/severe group and increase with increasingly deeper scotomas, represented by greater levels of probability of abnormalities. In contrast, subjects classified as normal/early have a relatively high specificity but have a much wider range in sensitivities from low to high, despite the fact that the overall percentage correct remains the same. As with the moderate/severe group, predictive value also increases with the more severe scotomas. For both sets of analyses, all measures tend to have more predictive power using the by-subject analyses and the pattern cluster analyses.
Table 2 presents for each of the two categories the per-eye and per-subject sensitivity, specificity, and percent correct classifications of entoptic perimetry for detecting glaucoma-related visual field defects. In general, the sensitivity of entoptic perimetry was relatively high for subjects classified in the moderate/severe group (range, 0.71– 0.90) and increased with increasingly deeper scotomas, as reflected by greater levels of probability of abnormalities. Specificity was 1.00 for clinical assessment. In contrast, subjects classified as normal/early/suspect had low-to-moderate sensitivities (range, 0.27– 0.67). Specificity was adequate (range, 0.78 –1.00). For both sets of analyses, all measures tended to have more predictive power (as measured by percent correct) using the bysubject analyses and the pattern cluster analyses.
Table 2. Sensitivity, Specificity, and Predictive Value of Entoptic Perimetry by Clinical Classification of Severity (number of moderate/severe subjects/eyes ⫽ 12/21; number of normal/early/suspect subjects/eyes ⫽ 11/20) Sensitivity
Specificity
% Correct
Normal/ Early/ Suspect
Moderate/ Severe
Normal/ Early/ Suspect
Moderate/ Severe
Normal/ Early/ Suspect
Pointwise deviation by eye (%) ⱖ95 0.71 ⱖ98 0.75 ⱖ99 0.75 ⱖ99.5 0.75
0.27 0.50 0.40 0.50
— 1.00 1.00 1.00
1.00 1.00 0.87 0.88
71 76 76 76
45 80 75 80
Cluster deviation by eye (%) ⱖ95 0.75 ⱖ98 0.83 ⱖ99 0.83 ⱖ99.5 0.83
0.25 0.33 0.33 0.50
1.00 1.00 1.00 1.00
0.81 0.82 0.82 0.83
76 86 86 86
70 75 75 80
Pointwise deviation by subject (%) ⱖ95 0.75 ⱖ98 0.82 ⱖ99 0.82 ⱖ99.5 0.82
0.38 0.60 0.67 0.67
— 1.00 1.00 1.00
1.00 1.00 0.88 0.88
75 83 83 83
55 82 82 82
Cluster deviation by subject (%) ⱖ95 0.82 ⱖ98 0.82 ⱖ99 0.90 ⱖ99.5 0.90
0.50 0.50 0.50 1.00
1.00 1.00 1.00 1.00
0.78 0.78 0.78 0.80
83 83 83 92
73 73 73 82
SAP
Moderate/ Severe
SAP ⫽ Standard Humphrey automated visual field perimetry.
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% Correct
Normal/ Early
Moderate/ Severe
Normal/ Early
Moderate/ Severe
Normal/ Early
Pointwise deviation by eye (%) ⱖ95 0.65 ⱖ98 0.68 ⱖ99 0.67 ⱖ99.5 0.67
0.31 0.67 0.75 1.00
— 1.00 0.50 0.50
1.00 1.00 0.93 0.93
65 70 65 65
50 89 89 94
Cluster deviation by eye (%) ⱖ95 0.67 ⱖ98 0.74 ⱖ99 0.74 ⱖ99.5 0.78
0.67 1.00 1.00 1.00
0.50 0.75 0.75 0.75
0.87 0.88 0.88 0.88
65 74 74 78
64 91 100 100
Pointwise deviation by subject (%) ⱖ95 0.69 ⱖ98 0.75 ⱖ99 0.73 ⱖ99.5 0.73
0.43 0.75 1.00 1.00
— 1.00 0.50 0.50
1.00 1.00 1.00 1.00
69 77 69 69
60 90 100 100
Cluster deviation by subject (%) ⱖ95 0.73 ⱖ98 0.80 ⱖ99 0.80 ⱖ99.5 0.89
1.00 1.00 1.00 1.00
0.50 0.67 0.67 0.75
0.88 0.88 0.88 0.88
69 77 77 85
90 90 90 90
SAP
Moderate/ Severe
Specificity
SAP ⫽ Standard Humphrey Automated Visual Field Perimetry
Discussion Our previous studies with entoptic perimetry have suggested that entoptic perimetry would be useful in screening subjects who have absolute or near-absolute scotomas caused by retinal damage from a variety of pathologic conditions (e.g., cytomegalovirus retinitis, ocular melanoma, age-related macular degeneration). Although several studies examine how well entoptic perimetry can detect scotomas caused by glaucoma (e.g.,4), to our knowledge no study to date has implemented testing using a scanning laser– based system that can eliminate optical problems caused by refractive error, cataracts, or poor contrast conditions. These results demonstrate that entoptic perimetry reasonably estimates the overall visual field loss for moderate-tosevere scotomas as measured by the pattern deviation in standard Humphrey automated visual field perimetry. This version of scanning laser entoptic perimetry was less sensitive and did not have as strong a predictive power for detecting all visual field scotomas, including the shallow (relative) ones that may be indicative of early visual field damage. Detection of the moderate-to-severe visual field loss and rapid referral to a qualified ophthalmologist is a critical public health issue, particularly before subjects become symptomatic when vision loss has an impact on central vision and at stages that might be more amenable to treatment. These results also suggest that using a “cluster” analysis, in which several adjacent points must be outside normal limits for the quadrant to be classified as abnormal, may be an advantageous method of determining abnormality.
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We performed two subanalyses on our subject population. Subjects were placed into one of two severity groups on the basis of classifications used by a glaucoma specialist and in a separate analysis using a standardized system based on threshold perimetry analyses. We found that for both subanalyses, the sensitivity and predictive values of entoptic perimetry for detecting moderate-to-severe visual field loss increased in subjects with more severe visual field loss but that both the clinician’s and standardized rating system produced similar results. This again suggests that scanning laser entoptic perimetry would be a useful screening test for moderate-to-advanced glaucoma. There are several advantages for this current version of entoptic perimetry. As we have previously reported,7 the VRD™ places the stimulus at virtual infinity, eliminating the need for refractive correction except in the most extreme cases. The brightness and contrast can be varied along a large dynamic range in presenting the stimulus to the retina even in cases of opacification caused by vitritis, cataracts, or high refractive error. This eliminates many of the confounds that can affect the interpretation of standard threshold perimetry of newer devices such as Frequency Doubling Technology (Welch-Allyn, Skaneateles Falls, NY). VRD-based entoptic perimetry also has the advantage of requiring very little time of the subject. We have reported that subjects can see the scotomas within a matter of seconds, typically less than 30. The Frequency Doubling Technology takes a minimum of 1 minute and up to 5 minutes for a full-threshold test. The Humphrey Field Analyzer II SITA program requires a minimum of 4 and up to 10 minutes for each eye. The singular advantage of allowing the subject to actually
Plummer et al 䡠 Scanning Laser Entoptic Perimetry see the extent of his or her area of scotoma is that this may lead to increased compliance. Several major differences exist between glaucoma and other retinal diseases (e.g., ocular melanoma, cytomegalovirus retinitis). Retinal damage caused by these latter diseases often causes full-thickness destruction of the retina in the affected areas, manifesting functionally as absolute visual field scotomata. Glaucoma-related visual field changes initially begin as shallow, relative scotomas that progress and deepen over a long period of time, depending on treatment and early detection. In the final stages of the disease, glaucoma will also cause absolute visual scotomas similar to those we have previously reported with cytomegalovirus retinitis and other diseases.7 We predict that the sensitivity and specificity would be similar to those rates previously reported for full-thickness scotomas1,3,7 for subjects with advanced glaucoma. The VRD™ may be an ideal candidate for a communitybased screening test for glaucoma and retinal diseases. The incidence and prevalence of glaucoma is much higher in the general population than diseases such as cytomegalovirus retinitis or ocular melanoma. Entoptic perimetry is rapid and inexpensive, making it an ideal candidate for a visual function screening test. Scanning laser– based entoptic perimetry may also be useful for screening populations not only in the primary care clinic but also for public health applications such as the Department of Motor Vehicles, where persons may in fact pass a standard vision test but have severely limited peripheral vision caused by glaucoma. In these and other general screening examinations, the screening test must be rapid, user-friendly, and noninvasive To create a validated virtual-reality– based entoptic screening test, we need to create an algorithm that incorporates measures of fixation loss, false-negative results, and false-
positive results. Although this will potentially increase duration of testing, measures of validity will allow physicians to assess entoptic perimetry results, particularly in the case of patients who are suspect for glaucoma but report no entoptic perimetry stimulus disturbance.
References 1. Plummer DJ, Are´valo JF, Fram N, et al. Effectiveness of entoptic perimetry for locating peripheral scotomas caused by cytomegalovirus retinitis. Arch Ophthalmol 1996;114:828 – 31. 2. Plummer DJ, Sample PA, Are´valo JF, et al. Visual field loss in HIV-positive patients without infectious retinopathy. Am J Ophthalmol 1996;122:542–9. 3. Plummer DJ, Banker A, Taskintuna I, et al. The utility of entoptic perimetry as a screening test for cytomegalovirus retinitis. Arch Ophthalmol 1999;117:202–7. 4. Aulhorn E, Ko¨st G. Rauschfeldkampimetrie. Eine neuartige perimetrische Untersuchungsweise. Klin Monatsbl Augenheilkd 1988;192:284 – 8. 5. Aulhorn E, Ko¨st G. Noise-field campimetry. A new perimetric method (snow campimetry). In: Heijl A, ed. Perimetry Update 1988/1989. Proc VIIIth Int’l Perimetric Society Meeting, 1988. Amsterdam: Kugler & Ghendini, 1988. 6. Schiefer U, Gisolf AC, Kirsch J, et al. Rauschfeld-Screening. Ergebnisse einer Fernsehfeldstudie zur Detektion von Gesichtsfelddefekten. Ophthalmologe 1996;93:604 – 616. 7. Plummer DJ, Azen SP, Freeman WR. Scanning laser entoptic perimetry for the screening of macular and peripheral retinal disease. Arch Ophthalmol; in press. 8. Gordon MO, Kass MA. Ocular Hypertension Treatment Study (OHTS) Manual of Procedures v. 2.0. Bethesda, MD: National Institutes of Health, 1995.
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