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Comparison of Colvard pupillometer and infrared digital photography for measurement of the dark-adapted pupil diameter
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
Comparison of Colvard pupillometer and infrared digital photography for measurement of the dark-adapted pupil diameter Jay C. Bradley, MD, Justin E. Anderson, MD, Ke Tom Xu, PhD, Sandra M. Brown, MD
PURPOSE: To investigate the accuracy of pupil diameter measurement using the Colvard pupillometer and to determine the learning curve for inexperienced examiners. SETTING: Texas Tech University Health Sciences Center, Lubbock, Texas, USA. METHODS: In this population study, subjects with normal pupillary behavior were tested by 1 of 2 investigators (examiner A, examiner B). After 5 minutes of dark adaptation at 1 lux, digital infrared pupil photography of the right eye was performed, followed by measurement of the horizontal pupil diameter and vertical pupil diameter with the Colvard pupillometer. The photographs were digitally analyzed to determine the horizontal and vertical pupil diameters. During phase I of the study, examiners were masked to the results of infrared pupil photography; during phase II, they reviewed the infrared pupil photography results after each testing session. Bland-Altman plots were created to detect measurement bias; results were graphed by subject test sequence to assess learning. A test difference of less than G0.5 mm was considered clinically acceptable. RESULTS: Fifty-nine subjects were tested in phase I, of whom 39 had adequate infrared pupil photography for analysis; 40 were tested in phase II, of whom 34 were included. The mean age of the analyzed subjects was 27 years (range 18 to 44 years). For all subjects, the infrared pupil photography median horizontal pupil diameter was 7.09 mm G 0.75 (SD) (range 5.44 to 8.79 mm); the median vertical pupil diameter was 7.22 G 0.79 mm (range 5.45 to 9.10 mm). Examiner A initially had a negative bias (Colvard pupillometer value less than infrared pupil photography value) for both horizontal and vertical pupil diameter measurements, which resolved during phase I after 23 subjects were tested; 18 of the final 19 subjects tested (11 phase I, 8 phase II) showed a test difference of less than 0.5 mm for all readings. The pupil diameter did not affect the bias. Examiner B had a strong positive bias that persisted throughout the study. Testing 26 subjects in 5 sessions during phase II did not improve the accuracy. During the final testing session, 3 of 8 subjects had a test difference of 0.5 mm or more in at least 1 dimension. The pupil diameter did not affect the bias. CONCLUSION: The Colvard pupillometer is susceptible to user errors causing unidirectional bias and seems to have a steep and variable learning curve. J Cataract Refract Surg 2005; 31:2129–2132 Q 2005 ASCRS and ESCRS
Because of its relatively low cost, portability, and apparently simple function, the Colvard pupillometer (Oasis Medical Inc.) is probably the most frequently used pupil-measurement device in the field of refractive surgery. The goal of preoperative pupil diameter evaluation is not simply to measure the pupil in the dark, but rather to determine the patient’s physiologic dark-adapted pupil diameter; that is, the largest diameter the pupil Q 2005 ASCRS and ESCRS Published by Elsevier Inc.
is likely to achieve in a very-low-light environment under binocular, ‘‘free viewing’’ conditions at distance fixation. Unless the examiner attends carefully to full dark adaptation, assures patient alertness, instructs the patient to maintain distance fixation to prevent accommodation, and minimizes parallax, no measurement taken with the Colvard pupillometer (or any other method) will be accurate. 0886-3350/05/$-see front matter doi:10.1016/j.jcrs.2005.04.041
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MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Table 1. Infrared pupil photograph measurements of all subjects.
Examiner A
Number of subjects Mean (mm) Median (mm) Range (mm) SD (mm)
Examiner B
All Subjects
HPD
VPD
HPD
VPD
HPD
VPD
29 6.88 6.93 5.44–8.47 0.69
27 7.02 7.14 5.45–8.60 0.71
44 7.16 7.21 5.70–9.10 0.78
42 7.32 7.27 5.58–9.53 0.82
73 7.05 7.09 5.44–9.10 0.75
69 7.20 7.22 5.45–9.53 0.79
HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter Values refer to pupil diameters measured after a minimum of 5 minutes of dark-adaptation at a constant environmental illumination of 1 lux.
In this study, we sought to determine how often and by how much the results of Colvard pupillometer testing would differ from infrared pupil photography. We considered a deviation of more than 0.5 mm in either direction to be clinically unacceptable if the measurement was to be used for refractive surgery. SUBJECTS AND METHODS This study was approved by the institutional review board, and all subjects provided informed consent. Individuals with no history of eye disease or injury affecting pupil diameter were recruited from department staff, acquaintances of the investigators, and medical students. Refractive error was not determined. The subjects were nonrandomly recruited and tested by 1 of 2 investigators (examiner A [J.C.B.], a first-year ophthalmology resident; examiner B [J.E.A.], an intern who will be an ophthalmology resident), neither of whom had previous experience with the Colvard pupillometer; their only instruction was in the user’s guide provided by Oasis Medical. Participants donned wraparound ‘‘cataract’’ sunglasses for a minimum of 5 minutes; ambient illumination was not controlled during this period. The subject was then formally dark-adapted at 1 lux for 5 minutes. After adaptation, multiple still digital infrared photographs of the right pupil were taken using experimental techniques previously described.1,2 The subject was instructed to gaze at the far end of the room ‘‘as though looking into the distance.’’ After photography, the horizontal and vertical pupil diameters of the right eye were measured using the Colvard pupillometer; the subject was instructed to avoid looking at the examiner or the pupillometer and to continue to gaze at a far wall. The left eye was not occluded. The examiners were careful to minimize parallax. The clearest infrared photograph was selected for analysis. Using digital image software, the horizontal and vertical pupil Accepted for publication April 11, 2005. From the Cabarrus Eye Center (Brown), Concord, North Carolina, and the Departments of Ophthalmology and Visual Sciences (Bradley, Anderson) and Family and Community Medicine (Xu), Texas Tech University Health Sciences Center, Lubbock, Texas, USA. No author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Sandra M. Brown, MD, 201 LePhillip Court NE, Concord, North Carolina 28025, USA. E-mail: sbrownmd@carolina. rr.com.
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diameters were calculated in millimeters using a 5.0 mm segment of the photographed ruler as the length standard. Each photograph was independently measured by 2 investigators (S.M.B., J.C.B.) and the results averaged. During phase I (masked), the results of infrared pupil photography were not compared with those of the Colvard pupillometer until all testing was completed; this prevented biased modification of the examiners’ Colvard pupillometer technique. During phase II (unmasked), examiners compared infrared pupil photography with Colvard pupillometer values at the conclusion of each day of testing to encourage learning. RESULTS
Fifty-nine subjects were recruited for phase I. Examiner A tested a total of 33 subjects, of whom 21 were included in the final analysis; examiner B tested a total of 26 subjects, of whom 18 were included. Exclusions were due to inadequate infrared photographs caused by the shallow depth of focus of the camera or upper lid shadowing that prevented vertical pupil diameter measurement. Forty subjects were recruited for phase II. Examiner A tested a total of 11 subjects, of whom 8 were included; examiner B tested 29 subjects, of whom 26 were included. The mean age of all subjects was 27 years (range 18 to 44 years); 84% percent were in their third decade. The results of infrared pupil photography of all subjects are given in Table 1. The mean infrared pupil photography horizontalto-vertical pupil diameter ratio of all subjects was 0.98. Results are displayed graphically for examiner A (Figure 1) and examiner B (Figure 2). The Colvard pupillometer reticule extends to 13.0 mm horizontally and vertically (Figure 3). Both examiners noted that the reticule was suboptimal due to central crowding from the graduations on the horizontal axis that obscured the pupil center and lack of 0.5 mm graduations on the vertical axis. DISCUSSION
In our study, 2 inexperienced but educated examiners who understood accommodation and parallax tested a cohort of cooperative subjects with the Colvard pupillometer in an optimized experimental environment. When
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Bland Altman Graph CP minus IRPP (mm)
CP minus IRPP (mm)
Bland Altman Graph 1.00 0.50 0.00 -0.50 -1.00 -1.50 5.50
6.00
6.50
7.00
7.50
8.00
8.50
1.50 1.00 0.50 0.00 -0.50 -1.00 5.50
6.00
6.50
Learning Effect 0.50 0.00 -0.50 -1.00 -1.50 12
17
22
27
32
37
42
Subjects by Test Sequence HPD masked
HPD unmasked
VPD masked
7.00
7.50
8.00
8.50
9.00
9.50
Mean of CP and IRPP (mm)
VPD unmasked
CP minus IRPP (mm)
CP minus IRPP (mm)
Mean of CP and IRPP (mm)
Learning Effect 1.50 1.00 0.50 0.00 -0.50 -1.00 4
14
24
34
44
54
Subject Test Sequence HPD masked
HPD unmasked
VPD masked
VPD unmasked
Figure 1. Results for examiner A. After testing approximately 22 subjects while masked to the infrared photography results, this examiner achieved a consistent clinical test agreement of G0.5 mm. Additional practice with feedback on the infrared pupil photography results at the end of each testing session did not improve his performance. The Bland-Altman graph shows that bias was not a function of pupil diameter. (CP Z Colvard pupillometry; IRPP Z infrared pupil photography; HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter; masked Z no knowledge of the result of infrared pupil photography testing; unmasked Z knowledge of the result of infrared pupil photography testing).
Figure 2. Results for examiner B. This examiner maintained a strong positive bias through both phases of the experiment. Additional practice with feedback on the infrared pupil photography results at the end of each testing session did not improve his performance. The Bland-Altman graph shows that bias was not a function of pupil diameter. (CP Z Colvard pupillometry; IRPP Z infrared pupil photography; HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter; masked Z no knowledge of the result of infrared pupil photography testing; unmasked Z knowledge of the result of infrared pupil photography testing).
compared with infrared pupil photography, examiner A showed a significant learning effect (Figure 1) and consistently recorded Colvard pupillometer measurements within G0.5 mm of the infrared pupil photography values after testing approximately 20 subjects. Between subjects 22 and 32, there was 1 clinically significant outlier with Colvard pupillometer measurements almost 1.0 mm smaller than infrared pupil photography values; the final 10 subjects fell within the clinically acceptable range with no outliers. The Colvard pupillometer measurements of examiner B were erratic (Figure 2), with most being larger than the infrared pupil photography values; no improvement was seen during phase II. One previous study3 compared the Colvard pupillometer with infrared videography and flash digital photography. We have criticized the results4 because the reported mean dark-adapted pupil diameter values (video 6.28 mm, photograph 6.24 mm) seemed too small for a study cohort with a mean age of 36 years. This study also used 2 examiners who found mean horizontal pupil diameter Colvard pupillometer values of 6.21 mm and 5.84 mm, respectively, after dark adaptation at less than 0.63 lux for
an unspecified length of time. These mean values are too small5,6 for this population and could not represent measurement of the true dark-adapted pupil diameter. Based on the results of our experiment, several conclusions can be reached regarding the use of the Colvard pupillometer for measurement of the dark-adapted pupil diameter under typical clinical circumstances. Foremost, the Colvard pupillometer is only as good as the user, and the inherent design of the device does not act to prevent the introduction of strong unidirectional bias. Thus, when Colvard pupillometer measurement is delegated to support staff or junior residents, it is important that they be properly trained and their accuracy assessed. Second, it is possible to obtain accurate measurements with the Colvard pupillometer. However, measurement errors greater than 0.5 mm can occur due to user error resulting from (1) failure to properly dark-adapt the patient, (2) failure to recognize accommodative miosis or decreased alertness, (3) failure to avoid parallax, (4) failure to read the reticule precisely, or (5) inaccurate recollection of the 4 reticule readings and/or incorrect subtraction. Third, because even experienced and generally accurate
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
2131
MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Figure 3. Two alternate reticules for handheld pupillometers. Upper Left: Photograph taken through the Colvard pupillometer showing scale from zero to 13.0 mm with 0.5 mm graduations. Upper Right: Inner graduation mark is at 2.5 mm. Lower Left: Inner graduation mark is at 2.0 mm and there is a pupil centering target (open circle). The reticules match the scale of the external ruler. The reticules give readings of 7.0 mm horizontally and 7.5 mm vertically. (Reticule designs copyright 2004, Sandra M. Brown, MD)
examiners can be wrong (as evidenced by examiner A’s subject 32), it is essential to be aware of population mean dark-adapted pupil diameter values5,6 so an atypically small measurement (ie, 5.0 mm in a 36-year-old7 or 31-year-old8 patient) can be rechecked. Finally, because the vertical pupil diameter is larger than the horizontal pupil diameter in most patients,2 Oasis Medical should give consideration to modifying the Colvard pupillometer reticule (Figure 3) to allow easier performance of vertical measurements.
REFERENCES 1. Brown SM, Khanani AM, Xu KT. Day to day variability of the darkadapted pupil diameter. J Cataract Refract Surg 2004; 30:639–644
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2. Khanani AM, Brown SM, Archer SM. Horizontal versus vertical darkadapted pupil diameters in normal individuals. J Cataract Refract Surg 2004; 30:2557–2558 3. Twa MD, Bailey MD, Hayes J, Bullimore M. Estimation of pupil size by digital photography. J Cataract Refract Surg 2004; 30:381–389 4. Brown SM, Bradley JC, Khanani AM. Duration of pupillary constriction in response to a photographic flash [letter]. J Cataract Refract Surg 2005; 31:455–456 5. Borgmann H. Grundlagen fu¨r eine klinishe Pupillographie. II. Abha¨ngigkeit des Pupillendurchmessers in Dunkelheit vom Lebensalter. Albrecht von Graefes Arch Klin Exp Ophthalmol 1972; 184:300–308 6. Loewenfeld IE. Pupillary changes related to age. In: Thompson HS, Daroff R, Frise´n L, et al, eds, Topics In Neuro-Ophthalmology. Baltimore, Williams & Wilkins, 1979; 124–150 7. Brown SM, Campbell CE. Systematic underablation in laser in situ keratomileusis: ablation pattern identified by advanced topographical analysis. J Cataract Refract Surg 2003; 29:1621–1625 8. Kohnen T. Consultation section: refractive surgery problem. J Cataract Refract Surg 2004; 30:1392–1395
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
Comparison of Colvard pupillometer and infrared digital photography for measurement of the dark-adapted pupil diameter Jay C. Bradley, MD, Justin E. Anderson, MD, Ke Tom Xu, PhD, Sandra M. Brown, MD
PURPOSE: To investigate the accuracy of pupil diameter measurement using the Colvard pupillometer and to determine the learning curve for inexperienced examiners. SETTING: Texas Tech University Health Sciences Center, Lubbock, Texas, USA. METHODS: In this population study, subjects with normal pupillary behavior were tested by 1 of 2 investigators (examiner A, examiner B). After 5 minutes of dark adaptation at 1 lux, digital infrared pupil photography of the right eye was performed, followed by measurement of the horizontal pupil diameter and vertical pupil diameter with the Colvard pupillometer. The photographs were digitally analyzed to determine the horizontal and vertical pupil diameters. During phase I of the study, examiners were masked to the results of infrared pupil photography; during phase II, they reviewed the infrared pupil photography results after each testing session. Bland-Altman plots were created to detect measurement bias; results were graphed by subject test sequence to assess learning. A test difference of less than G0.5 mm was considered clinically acceptable. RESULTS: Fifty-nine subjects were tested in phase I, of whom 39 had adequate infrared pupil photography for analysis; 40 were tested in phase II, of whom 34 were included. The mean age of the analyzed subjects was 27 years (range 18 to 44 years). For all subjects, the infrared pupil photography median horizontal pupil diameter was 7.09 mm G 0.75 (SD) (range 5.44 to 8.79 mm); the median vertical pupil diameter was 7.22 G 0.79 mm (range 5.45 to 9.10 mm). Examiner A initially had a negative bias (Colvard pupillometer value less than infrared pupil photography value) for both horizontal and vertical pupil diameter measurements, which resolved during phase I after 23 subjects were tested; 18 of the final 19 subjects tested (11 phase I, 8 phase II) showed a test difference of less than 0.5 mm for all readings. The pupil diameter did not affect the bias. Examiner B had a strong positive bias that persisted throughout the study. Testing 26 subjects in 5 sessions during phase II did not improve the accuracy. During the final testing session, 3 of 8 subjects had a test difference of 0.5 mm or more in at least 1 dimension. The pupil diameter did not affect the bias. CONCLUSION: The Colvard pupillometer is susceptible to user errors causing unidirectional bias and seems to have a steep and variable learning curve. J Cataract Refract Surg 2005; 31:2129–2132 Q 2005 ASCRS and ESCRS
Because of its relatively low cost, portability, and apparently simple function, the Colvard pupillometer (Oasis Medical Inc.) is probably the most frequently used pupil-measurement device in the field of refractive surgery. The goal of preoperative pupil diameter evaluation is not simply to measure the pupil in the dark, but rather to determine the patient’s physiologic dark-adapted pupil diameter; that is, the largest diameter the pupil Q 2005 ASCRS and ESCRS Published by Elsevier Inc.
is likely to achieve in a very-low-light environment under binocular, ‘‘free viewing’’ conditions at distance fixation. Unless the examiner attends carefully to full dark adaptation, assures patient alertness, instructs the patient to maintain distance fixation to prevent accommodation, and minimizes parallax, no measurement taken with the Colvard pupillometer (or any other method) will be accurate. 0886-3350/05/$-see front matter doi:10.1016/j.jcrs.2005.04.041
2129
MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Table 1. Infrared pupil photograph measurements of all subjects.
Examiner A
Number of subjects Mean (mm) Median (mm) Range (mm) SD (mm)
Examiner B
All Subjects
HPD
VPD
HPD
VPD
HPD
VPD
29 6.88 6.93 5.44–8.47 0.69
27 7.02 7.14 5.45–8.60 0.71
44 7.16 7.21 5.70–9.10 0.78
42 7.32 7.27 5.58–9.53 0.82
73 7.05 7.09 5.44–9.10 0.75
69 7.20 7.22 5.45–9.53 0.79
HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter Values refer to pupil diameters measured after a minimum of 5 minutes of dark-adaptation at a constant environmental illumination of 1 lux.
In this study, we sought to determine how often and by how much the results of Colvard pupillometer testing would differ from infrared pupil photography. We considered a deviation of more than 0.5 mm in either direction to be clinically unacceptable if the measurement was to be used for refractive surgery. SUBJECTS AND METHODS This study was approved by the institutional review board, and all subjects provided informed consent. Individuals with no history of eye disease or injury affecting pupil diameter were recruited from department staff, acquaintances of the investigators, and medical students. Refractive error was not determined. The subjects were nonrandomly recruited and tested by 1 of 2 investigators (examiner A [J.C.B.], a first-year ophthalmology resident; examiner B [J.E.A.], an intern who will be an ophthalmology resident), neither of whom had previous experience with the Colvard pupillometer; their only instruction was in the user’s guide provided by Oasis Medical. Participants donned wraparound ‘‘cataract’’ sunglasses for a minimum of 5 minutes; ambient illumination was not controlled during this period. The subject was then formally dark-adapted at 1 lux for 5 minutes. After adaptation, multiple still digital infrared photographs of the right pupil were taken using experimental techniques previously described.1,2 The subject was instructed to gaze at the far end of the room ‘‘as though looking into the distance.’’ After photography, the horizontal and vertical pupil diameters of the right eye were measured using the Colvard pupillometer; the subject was instructed to avoid looking at the examiner or the pupillometer and to continue to gaze at a far wall. The left eye was not occluded. The examiners were careful to minimize parallax. The clearest infrared photograph was selected for analysis. Using digital image software, the horizontal and vertical pupil Accepted for publication April 11, 2005. From the Cabarrus Eye Center (Brown), Concord, North Carolina, and the Departments of Ophthalmology and Visual Sciences (Bradley, Anderson) and Family and Community Medicine (Xu), Texas Tech University Health Sciences Center, Lubbock, Texas, USA. No author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Sandra M. Brown, MD, 201 LePhillip Court NE, Concord, North Carolina 28025, USA. E-mail: sbrownmd@carolina. rr.com.
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diameters were calculated in millimeters using a 5.0 mm segment of the photographed ruler as the length standard. Each photograph was independently measured by 2 investigators (S.M.B., J.C.B.) and the results averaged. During phase I (masked), the results of infrared pupil photography were not compared with those of the Colvard pupillometer until all testing was completed; this prevented biased modification of the examiners’ Colvard pupillometer technique. During phase II (unmasked), examiners compared infrared pupil photography with Colvard pupillometer values at the conclusion of each day of testing to encourage learning. RESULTS
Fifty-nine subjects were recruited for phase I. Examiner A tested a total of 33 subjects, of whom 21 were included in the final analysis; examiner B tested a total of 26 subjects, of whom 18 were included. Exclusions were due to inadequate infrared photographs caused by the shallow depth of focus of the camera or upper lid shadowing that prevented vertical pupil diameter measurement. Forty subjects were recruited for phase II. Examiner A tested a total of 11 subjects, of whom 8 were included; examiner B tested 29 subjects, of whom 26 were included. The mean age of all subjects was 27 years (range 18 to 44 years); 84% percent were in their third decade. The results of infrared pupil photography of all subjects are given in Table 1. The mean infrared pupil photography horizontalto-vertical pupil diameter ratio of all subjects was 0.98. Results are displayed graphically for examiner A (Figure 1) and examiner B (Figure 2). The Colvard pupillometer reticule extends to 13.0 mm horizontally and vertically (Figure 3). Both examiners noted that the reticule was suboptimal due to central crowding from the graduations on the horizontal axis that obscured the pupil center and lack of 0.5 mm graduations on the vertical axis. DISCUSSION
In our study, 2 inexperienced but educated examiners who understood accommodation and parallax tested a cohort of cooperative subjects with the Colvard pupillometer in an optimized experimental environment. When
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Bland Altman Graph CP minus IRPP (mm)
CP minus IRPP (mm)
Bland Altman Graph 1.00 0.50 0.00 -0.50 -1.00 -1.50 5.50
6.00
6.50
7.00
7.50
8.00
8.50
1.50 1.00 0.50 0.00 -0.50 -1.00 5.50
6.00
6.50
Learning Effect 0.50 0.00 -0.50 -1.00 -1.50 12
17
22
27
32
37
42
Subjects by Test Sequence HPD masked
HPD unmasked
VPD masked
7.00
7.50
8.00
8.50
9.00
9.50
Mean of CP and IRPP (mm)
VPD unmasked
CP minus IRPP (mm)
CP minus IRPP (mm)
Mean of CP and IRPP (mm)
Learning Effect 1.50 1.00 0.50 0.00 -0.50 -1.00 4
14
24
34
44
54
Subject Test Sequence HPD masked
HPD unmasked
VPD masked
VPD unmasked
Figure 1. Results for examiner A. After testing approximately 22 subjects while masked to the infrared photography results, this examiner achieved a consistent clinical test agreement of G0.5 mm. Additional practice with feedback on the infrared pupil photography results at the end of each testing session did not improve his performance. The Bland-Altman graph shows that bias was not a function of pupil diameter. (CP Z Colvard pupillometry; IRPP Z infrared pupil photography; HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter; masked Z no knowledge of the result of infrared pupil photography testing; unmasked Z knowledge of the result of infrared pupil photography testing).
Figure 2. Results for examiner B. This examiner maintained a strong positive bias through both phases of the experiment. Additional practice with feedback on the infrared pupil photography results at the end of each testing session did not improve his performance. The Bland-Altman graph shows that bias was not a function of pupil diameter. (CP Z Colvard pupillometry; IRPP Z infrared pupil photography; HPD Z horizontal pupil diameter; VPD Z vertical pupil diameter; masked Z no knowledge of the result of infrared pupil photography testing; unmasked Z knowledge of the result of infrared pupil photography testing).
compared with infrared pupil photography, examiner A showed a significant learning effect (Figure 1) and consistently recorded Colvard pupillometer measurements within G0.5 mm of the infrared pupil photography values after testing approximately 20 subjects. Between subjects 22 and 32, there was 1 clinically significant outlier with Colvard pupillometer measurements almost 1.0 mm smaller than infrared pupil photography values; the final 10 subjects fell within the clinically acceptable range with no outliers. The Colvard pupillometer measurements of examiner B were erratic (Figure 2), with most being larger than the infrared pupil photography values; no improvement was seen during phase II. One previous study3 compared the Colvard pupillometer with infrared videography and flash digital photography. We have criticized the results4 because the reported mean dark-adapted pupil diameter values (video 6.28 mm, photograph 6.24 mm) seemed too small for a study cohort with a mean age of 36 years. This study also used 2 examiners who found mean horizontal pupil diameter Colvard pupillometer values of 6.21 mm and 5.84 mm, respectively, after dark adaptation at less than 0.63 lux for
an unspecified length of time. These mean values are too small5,6 for this population and could not represent measurement of the true dark-adapted pupil diameter. Based on the results of our experiment, several conclusions can be reached regarding the use of the Colvard pupillometer for measurement of the dark-adapted pupil diameter under typical clinical circumstances. Foremost, the Colvard pupillometer is only as good as the user, and the inherent design of the device does not act to prevent the introduction of strong unidirectional bias. Thus, when Colvard pupillometer measurement is delegated to support staff or junior residents, it is important that they be properly trained and their accuracy assessed. Second, it is possible to obtain accurate measurements with the Colvard pupillometer. However, measurement errors greater than 0.5 mm can occur due to user error resulting from (1) failure to properly dark-adapt the patient, (2) failure to recognize accommodative miosis or decreased alertness, (3) failure to avoid parallax, (4) failure to read the reticule precisely, or (5) inaccurate recollection of the 4 reticule readings and/or incorrect subtraction. Third, because even experienced and generally accurate
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005
2131
MEASUREMENT OF THE DARK-ADAPTED PUPIL DIAMETER
Figure 3. Two alternate reticules for handheld pupillometers. Upper Left: Photograph taken through the Colvard pupillometer showing scale from zero to 13.0 mm with 0.5 mm graduations. Upper Right: Inner graduation mark is at 2.5 mm. Lower Left: Inner graduation mark is at 2.0 mm and there is a pupil centering target (open circle). The reticules match the scale of the external ruler. The reticules give readings of 7.0 mm horizontally and 7.5 mm vertically. (Reticule designs copyright 2004, Sandra M. Brown, MD)
examiners can be wrong (as evidenced by examiner A’s subject 32), it is essential to be aware of population mean dark-adapted pupil diameter values5,6 so an atypically small measurement (ie, 5.0 mm in a 36-year-old7 or 31-year-old8 patient) can be rechecked. Finally, because the vertical pupil diameter is larger than the horizontal pupil diameter in most patients,2 Oasis Medical should give consideration to modifying the Colvard pupillometer reticule (Figure 3) to allow easier performance of vertical measurements.
REFERENCES 1. Brown SM, Khanani AM, Xu KT. Day to day variability of the darkadapted pupil diameter. J Cataract Refract Surg 2004; 30:639–644
2132
2. Khanani AM, Brown SM, Archer SM. Horizontal versus vertical darkadapted pupil diameters in normal individuals. J Cataract Refract Surg 2004; 30:2557–2558 3. Twa MD, Bailey MD, Hayes J, Bullimore M. Estimation of pupil size by digital photography. J Cataract Refract Surg 2004; 30:381–389 4. Brown SM, Bradley JC, Khanani AM. Duration of pupillary constriction in response to a photographic flash [letter]. J Cataract Refract Surg 2005; 31:455–456 5. Borgmann H. Grundlagen fu¨r eine klinishe Pupillographie. II. Abha¨ngigkeit des Pupillendurchmessers in Dunkelheit vom Lebensalter. Albrecht von Graefes Arch Klin Exp Ophthalmol 1972; 184:300–308 6. Loewenfeld IE. Pupillary changes related to age. In: Thompson HS, Daroff R, Frise´n L, et al, eds, Topics In Neuro-Ophthalmology. Baltimore, Williams & Wilkins, 1979; 124–150 7. Brown SM, Campbell CE. Systematic underablation in laser in situ keratomileusis: ablation pattern identified by advanced topographical analysis. J Cataract Refract Surg 2003; 29:1621–1625 8. Kohnen T. Consultation section: refractive surgery problem. J Cataract Refract Surg 2004; 30:1392–1395
J CATARACT REFRACT SURG - VOL 31, NOVEMBER 2005