Fluctuations on the Humphrey and Octopus Perimeters

Transcription

1 May 987 Vol. 28/ Investigative Ophthalmology & Visual Science A Journal of Dosic and Clinical Research Articles Fluctuations on the and Perimeters Randall S. Drenton and William A. Argus Fluctuation of threshold determinations must be considered when evaluating visual field abnormalities. The short- and long-term threshold fluctuation were measured on 36 normal subjects using the and automated perimeters. Subjects were tested using similar threshold programs that tested 6 locations twice. Short-term fluctuation was greater on the perimeter (.6 ±.3 db) than on the perimeter (.3 ±.3 db; P <.). The heterogenous long-term fluctuation was greater on the perimeter (.6 ±.4 db) than on the perimeter (.3 ±. db; P <.). The homogenous long-term fluctuation was near for both machines. There are numerous variables that can alter the reproducibility of perimetry. Reproducibility can be estimated by short- and long-term fluctuation, but differences between the and also influence their fluctuation. Invest Ophthalmol Vis Sci 28:767-77, 987 Statistical analysis of visualfieldshas become a useful tool in interpreting visualfieldson automated perimeters. Automated perimeters have greatly decreased the variability introduced by the technician. However, there is a substantial amount of variability or fluctuation that results because of the subject. The fluctuation of test results also can occur because of measurement errors by the perimetric technique. Fluctuation is the uncertainty in quantifying differential light sensitivity. If an infinite number of repeat determinations of a threshold were performed at a given location, these observations would presumably have a Gaussian distribution around the physiologic threshold. This dispersion of measurements is called short-term fluctuation}- 2 If repeat testing is performed at another time, there isfluctuationthat occurs between sessions called long-term fluctuation. Long-term fluctuation can be divided into heterogenous and homogenous fluctuation. 2 If the sensitivity of the entire visualfieldwere to fluctuate on different testing sessions, this would be homogenous fluctuation. Thefluctuationbetween sessions that varies from one location to another and is From the Department of Ophthalmology, The University of Iowa Hospitals and Clinics, Iowa City, Iowa. Supported in part by grant EYO333O from the National Institutes of Health and in part by an unrestricted departmental grant from Research to Prevent Blindness. Submitted for publication: April 22, 986. Reprint requests: Randall S. Brenton, MD, Department of Ophthalmology, The University of Iowa Hospitals and Clinics, Iowa City, IA not accounted for by short-term fluctuation is the heterogenous long-term fluctuation. Fluctuation of threshold measurements can result from subject factors, such as patient attention, physiologic changes, retinal adaptation, and fatigue. Perimetric factors that can induce fluctuation include variations in stimulus and background luminance, threshold methods, and cartographic errors. The goal of this study was to compare fluctuation on the and perimeters in normal subjects. In this study, we attempted to eliminate subject factors by testing the same normal subjects on both perimeters using similar short programs. Subjects Materials and Methods We tested 36 normal subjects between the ages of 22 and yr (mean, 34.8 yr). Informed consent was obtained from all subjects after the procedure was explained fully. Nineteen subjects were inexperienced in visual field testing and 7 were experienced. Subjects previously tested on either perimeter with any program before this study were considered experienced. All subjects first had a screening eye examination. We excluded subjects () who were taking medications such as tranquilizers that might affect their test performance; (2) who had a history or physical signs of an ocular or systemic disease that might affect the visual fields; (3) whose best corrected visual acuity was 2/3 or worse; (4) who required more than ±7. diopters of spherical correction; () whose pupils were 767 Downloaded From: on 9//28

2 768 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / May 987 Vol. 28 Table. Hardware of and perimeters Perimeters 2 - A Measurements Bowl diameter Background luminance Stimulus size Stimulus duration meter 4asb.43 deg msec 66 cm 3. asb.43 deg 2 msec H h smaller than 2. mm; (6) who had suspicious optic discs, asymmetric cupping (.2 or greater), or a cupto-disc ratio greater than.; or (7) who had an intraocular pressure measured by Goldmann applanation tonometry of 22 mm Hg or more. Subjects were randomly assigned to start testing on one of the two perimeters at thefirstsession. Each subject was allowed min to adapt to the background illumination of the perimeter. Each subject was given a -min demonstration trial before testing on each machine. Subjects were tested on both perimeters during the same session with the right eye. Sessions were from 3-4 days apart (mean, 2.2 days). On the second session, the subjects were tested on the perimeters in the opposite order from that of thefirstsession. No subject was excluded on the basis of fixation losses, false-positive, or false-negative catch trials. Perimeters The 2 (Hitron Corporation, East Providence, RI) is a projection perimeter with an infrared video monitor for fixation monitoring. 3 It will discard Table 2. Test locations of Neuro-2 program of the perimeter Quadrant Superior Temporal Superior Nasal Inferior Temporal Inferior Nasal Coordinates of Test Locations X coordinate (deg) Y coordinate (deg) Fig.. Graphic representation of locations tested with both perimeters. The coordinates are given in Table 2. measurements when there is afixationloss. The field analyzer ( Instruments, San Leandro, CA) is also a projection perimeter. Fixation is monitored by catch trials in the blind spot, but no measurements are discarded for loss of fixation. 4 The stimulus subtends a visual angle of.43 deg on both perimeters, equivalent to Goldmann size III. See Table for bowl diameter, background luminance, and stimulus duration. Programs On both perimeters a threshold program was used to test the locations in Table 2. These locations (Fig. ) were tested twice during a single session and correspond to those on the Neuro-2 program of the perimeter. 4 This program was chosen to allow repeat threshold measurements on all locations for each machine. A short program was selected to minimize testing fatigue. Test time was 3- min for each program and was slightly longer on the. Statistical Analysis Both perimeters used a decibel scale to record differential light sensitivity (Table 3). 3>4 The decibel values are defined on the as follows: db = 24+og The decibel values on the are defined as follows: db = 2+Log Downloaded From: on 9//28

3 No. FLUCTUATIONS ON THE HUMPHREY AND OCTOPUS PERIMETERS / Drenron and Argus 769 Both of these equations are derived from values given in Table 3. Short-term fluctuation or reproducibility is calculated from repeat threshold determinations at the same location. For this study, short-term fluctuation was calculated by an independent computer using the following equation : Short-term fluctuation = N *-> i= R-l r = repetition R = total repetitions N = number of locations X ir = threshold for repetition R Xj = mean of threshold at location i Homogenous and heterogenous long-term fluctuation were calculated using the statistical model of Bebie et al. 2> The equations for the calculation of heterogenous and homogenous long-term fluctuation are as follows: (SHET) 2 ] = f-jj Zjk(y jk. - y }. ) 2 - N.-y...) 2-2(S HET 2 )-(S ST F) 2 ] S S TF = variance owing to short-term fluctuation SHOM = variance between sessions or homogenous long-term fluctuation SHET = variance between location and session or heterogenous long-term fluctuation j = session j =,2 k = location k =, y... = mean threshold for all observations for one subject yj. = mean threshold for session j for one subject y jk. = mean threshold of repetitions for one subject for session j at location k Results The average threshold for all locations and subjects was 3. ±.4 db on the perimeter and 3.6 ±.4 db on the. The mean difference of.6 db was significant (P <.). There was no significant difference between the mean threshold between the first and second sessions for either perimeter. The mean short-term fluctuation on the was.6 ± :3 db and on the was.3 ±.3 db. This difference was significant at P <.. If we consider only the second session in which all subjects were experienced, the short-term fluctuation Table 3. Decibel scales Apostilbs, 3,2, Background luminance asb Stimulus Intensity asb (.6 ±.4 db) was significantly greater than that found on the (.4 ±.4 db; P <.). The range was from.-2.4 db on the and from.8-2. db on the. Heterogenous long-term fluctuation was greater on the (.6 ±.4 db) than on the (.4 ±. db; P <.). The range was from db on the and from db on the. Homogenous long-term fluctuation was not significantly different between the machines with a mean of. db on the and.4 db on the. The value should presumably be or near for normal subjects. The highest value was 3.3 db on the and 2.4 db on the. The highest values occurred with inexperienced subjects. The difference between the first and second threshold for all locations and subjects is given in Table 4. These differences are illustrated in Figure 2 for the and Figure 3 for the. All differences between repetitions are even numbers on the, whereas the difference between repetitions on the may be either an even or odd number. On the there were locations with 6 db or more Table 4. Fluctuation: number of locations Difference Downloaded From: on 9//28

4 77 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / May 987 Vol. 28 NumbRr of Locations n L Difference Between Repetitions Fig. 2. Differences between the first and second threshold at all locations for all patients on the perimeter. difference between repetitions and 29 locations on the. Subjects were analyzed on the basis of prior experience with visualfieldtesting. There was no significant difference between experienced and inexperienced subjects. Short-term fluctuation for experienced subjects was greater on the (.6 ±.4 db) than on the (.3 ±.3 db; P <.). The same relationship held true for inexperienced subjects when analyzed separately. Short-term fluctuation with inexperienced subjects on the (.6 ±.3 db) was greater than on the (.3 ±.3 db; P <.). There was a trend for the heterogenous longtermfluctuation to be greater on the perimeter when considering subjects by experience, but these trends were not statistically significant. A linear regression analysis was performed to determine the slope of the hill of vision in each quadrant for each subject. The average of the slope values for each subject was used to compare the slope between these machines by subjects. The average slope between -2 deg eccentricity for all subjects on the was -.24 (db/deg) and -.22 (db/deg) on the. This difference is not statistically significant. Number of Locations r\ U I II _ Difference Between Repetitions * One observation is off-scale with a difference of 3 db Fig. 3. Differences between the first and second threshold at all locations for all patients on the perimeter. Discussion The mean sensitivity was.6 db higher on the. The difference in the definition of decibel as given in the Materials and Methods section accounts for db difference between machines if the value of the Weber fraction is a constant. Another difference between the two machines is the method of obtaining the threshold. Both machines use a similar "double pass" threshold method. 3 On the perimeter, the average of the last seen and unseen value is taken as the threshold, 3 and on the, the last seen stimulus is taken as threshold (Personal communication: M. Patella; Instruments). Because the final thresholding steps are made by intervals of 2 db, 34 one would expect the values to be about db lower on the. The value of the threshold may be affected by the stimulus duration and state of retinal adaptation. Aulhorn and Harms 6 indicate that temporal summation occurs below. sec of stimulus duration. Using the Tubinger perimeter, they studied the effect of background luminance and various stimulus sizes on perimetry. Based on their graphs at deg temporal to the fovea, one would expect about 3 db higher sensitivity on the than on the based on background luminance and stimulus duration. 6 ' 7 The exact contribution of each factor would need to be measured by altering the stimulus duration and background luminance on these automated perimeters. Anderson et al 7 showed that the operates on the nonlinear portion of the Weber-Fechner curve. Because the Weber fraction is not constant for the background luminance differences between these two perimeters, the exact difference in threshold values cannot be determined directly. However, the comparison of the actual threshold values is usually not a clinically important question because patients are only tested on one perimeter. Hoskins and Migliazzo 8 studied nine normal eyes on the perimeter and the 2 perimeter. They found a 4 db higher mean threshold on the perimeter than the perimeter. They predicted this 4 db difference by theoretic means but did not consider the influence of the nonlinearity of the Weber-Fechner curve on the or the difference in the definition of a decibel on the two perimeters. Short-term and heterogenous long-term fluctuation were both greater on the perimeter. Fluctuation is known to increase toward the periphery of the visualfield, 9 but this should not have affected the results of this study because the same locations were tested on each perimeter. Also, longer testing times may increasefluctuationthrough fatigue, but these tests Downloaded From: on 9//28

5 No. FLUCTUATIONS ON THE HUMPHREY AND OCTOPUS PERIMETERS / Drenron ond Argus 77 were relatively short, and the difference in test times between the perimeters was less than min. Experienced subjects would be expected to have less fluctuation. 6 The subjects categorized as experienced had variable amounts of prior experience. One would expect that the experienced subjects would have a higher mean threshold than inexperienced subjects. 6 This was not true for either perimeter, which would suggest that subjects classified as experienced were probably relatively untrained subjects. However, differences between the machines in terms of short-term fluctuation are still apparent even after separating subjects on the basis of experience or looking at the second session with all subjects having prior experience. One can speculate as to other factors that may account for the differences in fluctuation between perimeters such as background luminance, stimulus duration, and variations in the luminance of the stimulus or background during testing. Another factor that could affect fluctuation is the threshold method. With the perimeter all threshold steps are made by 4 or 2 db (Personal communication: M. Patella; Instruments). The differences between the first and second threshold at each location was always an even number (Fig. 2). However, on the, the first and second threshold could differ by an odd or an even number (Fig. 3). Therefore, the threshold method might influence the estimate of reproducibility. There were twice as many repetition differences of 6 db or more on the perimeter. This finding suggests that there may be other factors besides the threshold method that would produce different fluctuation values on these perimeters. If the starting point is poorly selected, fluctuation might be greater. The perimeter starts at a level predicted by the normal slope of the hill of vision, 4 but the normal values used by the perimeter have not been published. The perimeter starts at a level of age-matched normal values that were obtained from 24 normal subjects. Because we tested normal subjects, the initial starting point was likely to be very close to the final threshold. Because these subjects were normal, this difference is unlikely to have affected the results. Fluctuation is the uncertainty in making a threshold measurement. If this uncertainty can be reduced, we can increase the certainty of detecting abnormalities. More research needs to be done to determine which of the various parameters of visual field testing affect fluctuation to help determine the optimal testing parameters. Each perimeter needs to have standard fluctuation values defined for normal subjects for a given program. The reasons for the differences in fluctuations between the machines is not entirely clear. The obvious importance of these findings is the need for standardization of the fluctuation values for each perimeter design before using this data for assessing threshold reproducibility. This study was performed on only 6 locations, fewer than the longer programs commonly used for clinical testing. A short program was used to reduce fatigue and minimize software differences between the and perimeters. Although fluctuation may vary with the testing program, we believe that this comparison would hold true if other similar programs were compared. Key words: automated perimetry, short-term fluctuation, long-term fluctuation, normal visual field Acknowledgments This article is dedicated to Charles D. Phelps, whose assistance and guidance made this study possible. The author also thanks Patricio Rojas and Dr. Robert Woolson for their statistical consultation. References. Hirsch J: Statistical analysis of the quantitative visual field. In Automated Perimetry, Whalen W and Spaeth GB, editors. Thorofare, New Jersey, Slack Publishing, 98, pp Bebie H, Fankhauser F, and Spahr J: Static perimetry: Accuracy and fluctuations. Acta Ophthalmol 4:339, Interzeag AG: Operator Manual, Section 4.3. Hitron Corporation, 978, pp Field Analyzer, Model 6, Owner's Manual. Instruments, San Leandro, CA, 983, p. 3.. Graybill FA: Introduction to Linear Statistical Models, Vol. New York, McGraw-Hill, 96, pp Aulhorn E and Harms H: Visual perimetry. In Handbook of Sensory Physiology, Vol 7, Chapter, Jameson D and Hurvich LM, editors. New York, Springer-Verlag, 972, pp Heuer DK, Gressel MG, Anderson DR, Knighton RW, and Fantes FE: Does the perimeter obey Weber's law? ARVO Abstracts. Invest Ophthalmol Vis Sci 26(Suppl):4, Hoskins HD and Migliazzo C: Development of a visual field screening test using a visual field analyzer. In Proceedings of the Sixth International Visual Field Symposium, Doc Ophthalmol Proc Series, Vol 42, Heijl A and Greve EL, editors. Dordrecht, Dr W Junk Publishers, 98, pp Brenton RS and Phelps CD: The normal visual field on the Field Analyzer. Ophthalmologica 93:6, Interzeag AG: Visual Field Atlas. Schlieren, Switzerland. Druckerei Geber Publisher 978, pp Downloaded From: on 9//28