Eye Movements, , , and Neuro- Entrance Size Predicts Retinal Illumination in Darkly Pigmented Eyes, But Not Lightly Pigmented Eyes

Randy H. Kardon,1 Sungpyo Hong,2 and Aki Kawasaki3

1Department of Ophthalmology and Visual Science, University of Iowa Hospitals and Clinics and Iowa City VA Medical Center, Iowa City, Iowa 2Daegu City, South Korea 3Hopitalˆ Ophtalmique Jules Gonin, Lausanne, Switzerland

Correspondence: Randy H. Kardon, PURPOSE. We determined the effect of entrance pupil size on retinal illumination. The Department of Ophthalmology and influence of unilateral on the magnitude of the pupil light reflex was studied to Visual Science, University of Iowa ascertain how a clinically significant anisocoria influences the relative afferent pupil defect Hospitals and Clinics, Iowa City, IA (RAPD). 52242; [email protected]. METHODS. Miosis was induced by topical 1% in the right eye of 14 healthy subjects Submitted: April 29, 2013 with normal eyes. The interocular difference in retinal illumination was assessed by Accepted: July 15, 2013 computerized pupillometry from the stimulus response curve of the right and left eyes. The main outcome measure was the RAPD, determined by computerized pupillography, at Citation: Kardon RH, Hong S, Kawa- baseline and after pilocarpine-induced anisocoria. saki A. Entrance pupil size predicts retinal illumination in darkly pigment- RESULTS. Induced anisocoria produced a significant change in RAPD from baseline (mean ¼ ed eyes, but not lightly pigmented 1.60 dB in the miotic eye, P ¼ 0.007). However, anisocoria correlated with RAPD only in eyes. Invest Ophthalmol Vis Sci. subjects with darkly pigmented irides (Pearson correlation coefficient 0.793, P ¼ 0.05). 2013;54:5559–5567. DOI:10.1167/ iovs.13-12319 CONCLUSIONS. In darkly pigmented eyes, entrance pupil size significantly influenced the retinal illumination. However, retinal illumination of lightly pigmented eyes is relatively independent of entrance pupil size, presumably due to extrapupillary transmission of light through the and . This has important implications in understanding the potential influence of anisocoria on the RAPD and also greater susceptibility of lightly pigmented eyes to light toxicity. Keywords: , retinal illumination, light damage, pupil light reflex

he primary function of the pupil is to modulate retinal With our instrument, both pupils could be recorded simulta- Tillumination.1 Based on this concept and assuming equal neously while stimulating either the right or left eye to estimate retinal adaptation, unequal pupil size (anisocoria) would be the response function over a large range of stimulus light expected to create unequal retinal illumination between the intensities. The pupil light reflex was used in this study because two eyes. In addition to the effect of pupil size on the amount it is an objective, biological light meter whose magnitude of light entering the eye, pupil size also may influence the reflects effective retinal illumination.24–26 effectiveness of photoreceptor function based on the angle of light rays with respect to the photoreceptors, also known as the Stiles-Crawford effect.2 METHODS In the clinical setting, unequal retinal illumination between We tested by computerized pupillometry 14 healthy subjects the two eyes can be observed as a relative afferent pupillary with no history of ocular disease or trauma, after informed defect (RAPD) and typically the cause is a pathologic lesion of consent was obtained according to the tenets of the the or . Because asymmetric entrance pupil Declaration of Helsinki. Approval for this study was obtained size may create unequal retinal illumination as well, this may through the Institutional Review Board of the University of increase or decrease artificially the estimate on a pathology- Iowa. Each subject was tested at baseline and at 90 minutes 3 related RAPD. Pupil size also has been considered an important after the instillation of 1% pilocarpine into the right eye. factor in other clinical tests, such as the scotopic threshold test Between testings, the subjects were instructed to maintain the for glaucoma4,5 and the Ganzfeld ERG test.6,7 In addition, the same level of adaption to the indoor lighting level of the role of pupil size is also important in understanding the ophthalmology clinic. cumulative pathologic effects of light on the internal structures Pupil responses to light stimuli were recorded using a of the eye in terms of formation8,9 and retinal computerized infrared pupillometer (Visual Pathways, Inc., degeneration.10–20 Therefore, in any study of light on the eye, Prescott, AZ) which presented a 308 radius light stimulus to including its role in disease, understanding the role of pupil size each eye in non-Maxwellian view. Each subject was adapted to on effective retinal illumination has great importance.21–23 a 3.1 apostilb background light for 30 seconds, then a sequence The purpose of our study was to understand better the of light stimuli was presented alternately to each eye at varying effect of asymmetric entrance pupil size (anisocoria) on retinal intensities above the background level. During the test, the illumination and function using computerized pupillometry. stimulated eye was allowed to foveate on a small central cross

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FIGURE 1. Example of how a miotic pupil may decrease the amplitude of the pupil light reflex by reducing entrance of light through the smaller pupil opening. The stimulus light is alternated between the right and left eyes, and the movement of the left pupil is recorded as shown by the tracings. The beginning and peak of the pupil tracing are marked on the tracings with a thin vertical line by the software analysis program. Time (in seconds) is shown along with the square wave stimulus. In the baseline state before pilocarpine (top), the pupils are equal and the pupil light reflexes are of the same amplitude in response to right or left eye stimulation. At 90 minutes after 1% pilocarpine to the right eye (bottom), the pupil is miotic and immobile, and the left, untreated pupil responses are smaller with right eye stimulation compared to left eye stimulation, causing an RAPD.

before the ensuing light stimulus to control fixation. The after pilocarpine using a magnified infrared video camera with refractive power of the instrument was adjusted by entering millimeter rule in the picture so that the entrance pupil size the refraction of the subject into the instrument’s software so could be measured in all cases, even after profound miosis. that the focal point was set at infinity to control accommoda- This was necessary because at very small pupil sizes (<2.0 mm tion. A flat black metal septum separated the right and left eye diameter), the computerized infrared pupillometer was not optical pathways to minimize stray light scatter. The stimulus always able to track and measure the bright (infrared retro- duration was 0.2 seconds and the time between light stimuli illuminated) image of the pupil. was 3.3 seconds. The stimulus duration was well within the Pupillographic tracings were stored and analyzed (Fig. 1) latency time of the pupil light reflex so that the entrance pupil using a software program described in a previous report.27 The size was not affected during the time that the stimulus was on. effective retinal illumination was estimated from the stimulus Eight stimulus intensities were presented over a 3.5 log unit response curve (fit by the Naka Rushton equation)28–30 based range (35 dB range; 44 to 9 dB of attenuation above a on the pupil light reflexes recorded at different intensities, at background of 3.1 apostilbs; 44 dB attenuation ¼ 0.13 cd/m2, baseline, and after induction of unilateral miosis. The RAPD 0.9 dB ¼ 400.7 cd/m2). Each stimulus was repeated six times was determined at an intensity of 20 dB of attenuation (100 during the test in staggered fashion. The order of the stimuli apostilbs or 31.8 cd/m2 above a background level of 3.1 was from dim to brightest stimuli for the stimulus protocol, the apostilbs). The RAPD was calculated as the interocular right eye received the same intensity as the left eye for each difference of the stimulus response curves at this intensity intensity step. Each subject was tested with the same protocol. (Fig. 2). The effective retinal illumination over the entire range The order of the stimuli was the same for each subject. of stimulus intensities was assessed as the area under the Previous unpublished studies demonstrated that reversal of the stimulus response curve and the afferent asymmetry between stimulus order has no significant effect on the fitted response the two eyes was estimated from the area difference between function due to retinal adaptation. Because of the induced the two eyes (Fig. 2). miosis after instillation of pilocarpine, only the movement of Because the effect of anisocoria on the RAPD seemed to the untreated left pupil was recorded and analyzed (sampling vary widely among the subjects during the initial analysis, one rate ¼ 60 Hz) at the 90-minute posttreatment test time. The of the investigators (AK) was asked to rank each subject by the pupil diameter of the treated right eye was recorded before and degree of ocular pigmentation based on clinical observation of

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FIGURE 2. Example of stimulus response curves plotted for baseline state before pilocaripin (top) and after miosis (bottom) was induced in the right eye by pilocarpine (same patient as example in Fig. 1). The stimulus response curve is shown for the left, untreated eye’s pupil contraction amplitude when the right eye was stimulated (as a function of light intensity) and also for the left eye’s pupil contraction for left eye stimulation. As the stimulus light was made brighter, the pupil contracted more, producing a sigmoid-shaped response curve (see Methods). The area under this curve (shaded) essentially was the same no matter which eye was stimulated in the baseline state, as shown in the top graph. When the right eye was made miotic, the area under the stimulus-response curve decreased compared to pretreatment and compared to the fellow eye stimulation. The lightly shaded area in the bottom graph reflects the decreased response of the stimulated right eye due to the reduced entrance size of the pupil. The RAPD was estimated by quantifying the intensity difference between the two curves depicted by the distance between the vertical dashed lines (in this case at the stimulus intensity level of 20 dB on the stimulus-response curve of the untreated eye). For example, in the bottom graph,a decrease of intensity of 2 dB (0.2 log units) was needed to diminish the pupil reaction from the normal eye to produce a contraction equal to that produced by stimulation of the treated eye with the miotic pupil.

the iris color of each subject. This investigator was not told the testing of all subjects was completed by visual inspection of reason for this at the time, so as to maintain an unbiased their iris color during the same day by memory. Iris estimate of iris color. The subjects were ranked in order of pigmentation was correlated with the change in RAPD, change increasing iris pigmentation from 1 to 14. Since all of the in the area difference under the stimulus-response curves of subjects were available in the immediate vicinity of the study the two eyes, and the degree of induced anisocoria using a location, their ranking by iris pigmentation was done after the Spearman rank correlation test.

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FIGURE 3. The relationship between the anisocoria produced by pilocarpine in the 14 subjects and the expected RAPD, predicted by the difference in retinal luminance calculated from the difference in pupil entrance size. The high linear correlation is anticipated, because the expected RAPD values are based on mathematical calculations. The correlation was not a perfect line for the expected RAPD because the effect of anisocoria on retinal illumination also is dependent upon the pupil size (area) at a given level of anisocoria (see Results).

For each patient, the entrance pupil size and degree of reflexes to an alternating light stimulus are shown before (top) anisocoria were tabulated at the light intensity of 20 dB and after (bottom) the production of miosis. The pharmaco- attenuation, the intensity at which the RAPD was measured logically constricted pupil caused a smaller amplitude pupil (see above). The expected retinal illumination (in dB) of each light reflex when the miotic right eye was stimulated eye was derived mathematically based on the entrance pupil compared to the untreated left eye. The results from the same size measured at each intensity of stimulus from the following subject are shown in Figure 2, in which the amplitude of the equation: Retinal illumination (in log units) ¼ log (area of left (untreated) pupil light reflex was plotted as a function of pupil) þ log (stimulus intensity) under conditions of clear the stimulus light intensity for the right and left eye ocular media and a standardized axial eye length.31–33 Since stimulation. Before the right eye was treated with pilocarpine, retinal illumination is being calculated in log units, log retinal there was no difference in the area under the stimulus illumination ¼ log (pupil area times stimulus intensity). The response curves for right and left eye stimulation. After miosis expected retinal illumination was calculated with and was induced in the right eye, the stimulus response curve from without correction for the Stiles-Crawford effect,2,34 which right eye stimulation fell short of the left eye’s stimulus takes into account the effect of pupil size on the response of response curve at all intensities and, hence, the area under the the photoreceptors, as influenced by the angle of light rays curve was less. This result was anticipated because of the entering the pupil with respect to the orientation of the difference in entrance pupil size that was induced between the photoreceptors. This effect is not apparent when pupil two eyes; however, as will be reported, this was not found in diameters are 6 mm or less and did not change the all of the subjects. calculations significantly for the ‘‘expected RAPD’’ over the The predicted retinal luminance for each subject was range of pupil sizes studied in our experiments. The expected calculated based on the stimulus light intensity and pupil area RAPD (in dB units) was the difference of the calculated retinal at the time of the stimulus onset (see Methods). The calculated illumination between the two eyes for a measured amount of difference in retinal luminance (expected RAPD) was plotted anisocoria. as a function of the degree of anisocoria produced by Two of the subjects (one darkly pigmented, subject 14, pilocarpine in each of the 14 subjects (Fig. 3). There was a and one lightly pigmented, subject 1) also were tested at significant linear correlation between the anisocoria produced various degrees of miosis by using increasing concentrations and the expected RAPD (correlation coefficient r2 ¼ 0.92). The of pilocarpine in the right eye on the same day (1/8%, 1/4%, correlation was not a perfect line for the expected RAPD. This 1/2%, 1%). This was done to establish the relationship was because the effect of anisocoria on retinal illumination also between pupil size and effective retinal illumination within is dependent upon the actual pupil size at a given level of a given subject (as measured by the RAPD) to compare two anisocoria. That is, 1 mm anisocoria for a 5 and 6 mm pupil of a subjects with widely different degrees of iris pigmentation. subject would not have the same effect as a 3 and 4 mm pupil in the same subject, because a difference in pupil size based on diameter measurements does not reflect the difference in area, RESULTS which increases as pupillary diameter increases for any given An example of pupil light reflexes recorded from the mobile, level of anisocoria. In this case, the area difference was greater untreated pupil of one subject with darkly pigmented irides is for a subject with the 5 and 6 mm pupils compared to a subject shown in Figure 1. In the figure, the subject’s pupil light with the 3 and 4 mm pupils.

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FIGURE 4. This scattergram shows the lack of a relationship between the anisocoria produced by pilocarpine and the change of measured RAPD derived from computerized pupillography in the same 14 subjects shown in Figure 3. The method of RAPD determination for the values used in this graph consisted of determining the dB attenuation needed in the more responsive eye to reduce its pupil response to that of the opposite eye (see Methods and Fig. 2).

The Table summarizes the effects of unilateral miosis on the or no RAPD was produced, even though a significant miosis difference in RAPD between the baseline condition and after (anisocoria) was present. pharmacologic miosis. As seen from the mean values at the To try to understand this discrepancy, we plotted a bottom of the Table, there was an overall increase in the RAPD scattergram of the relationship between the pharmacological- ly-induced anisocoria and the change of measured RAPD from of 1.60 dB (0.160 log units) following induced miosis (P ¼ baseline, as shown in Figure 4. Unexpectedly, there was no 0.007, mean anisocoria was 2.2 mm). However, there did not significant linear correlation (P ¼ 0.84). The lack of correlation seem to be a consistent effect of anisocoria on the RAPD between anisocoria and change of RAPD was not anticipated among subjects. In some subjects, there was a definite and, accordingly, a reason was sought for this result. decrease in the pupil light reflex in the eye with miotic pupil, One possibility that was considered was that retinal thus producing an RAPD. However, in other subjects, very little illumination during the light stimulation might not have been

TABLE. Summary of Characteristics and Pupil Data From Normal Subjects Before and After Receiving 1% Pilocarpine in the Right Eye

Subject No. Ranked Baseline Anisocoria, Post-Pilocarpine Anisocoria, Change in by Iris Pigment Pupil Size, mm mm Pupil Size, mm mm, Post- Post- RAPD Post From Least Pigmented Baseline Baseline Pilocarpine Pilocarpine Pilocarpine- to Most Pigmented OD OSOS–OD RAPD,* dB OD OS OS–OD RAPD,* dB Baseline, dB

1 4.76 4.62 0.14 1.44 2.21 5.26 3.05 3.3 1.86 2 5.33 4.68 0.65 3.13 4.00 4.96 0.96 3.03 0.1 3 5.45 5.36 0.09 4.76 4.00 5.37 1.37 4.9 0.14 4 4.47 4.44 0.03 0.95 2.00 4.71 2.71 0.71 0.24 5 3.68 3.60 0.08 3.31 3.00 4.47 1.47 5.1 1.79 6 5.55 4.56 0.99 1.68 5.00 5.32 0.32 3.28 1.6 7 5.42 5.54 0.12 0.42 2.00 5.55 3.55 2.28 1.86 8 4.85 4.51 0.34 3.22 2.50 4.51 2.01 4.15 0.93 9 6.46 6.12 0.34 4.32 4.50 6.15 1.65 0.38 3.94 1 4.91 4.69 0.22 1.06 3.00 4.85 1.85 3.74 4.8 11 5.31 5.73 0.42 0.43 2.50 5.73 3.23 1.08 0.65 12 6.06 5.71 0.35 0.40 2.00 6.06 4.06 2.17 2.57 13 4.95 4.65 0.30 4.10 2.50 4.78 2.28 2.66 1.44 14 5.99 6.00 0.01 0.12 3.74 5.99 2.25 4.22 4.34 Mean 5.23 5.01 0.21 0.47 3.07 5.27 2.20 2.07 1.60 SD 0.71 0.73 0.34 2.68 1.00 0.57 1.04 2.61 1.85 * RAPD in dB (positive value is a right RAPD, negative value is a left RAPD); log units ¼ dB/10. RAPD was determined by two different methods using computerized pupillography. The RAPD values in this Table were determined by determining the dB attenuation needed in the more responsive eye to reduce its pupil response to that of the opposite eye (see Methods and Fig. 2).

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FIGURE 5. Anisocoria versus RAPD is replotted using change from baseline of the interocular area under the stimulus–response curve after pilocarpine as a measure of RAPD. In addition, the subjects were divided equally into two groups based on their rank of pigmentation; those with lightly pigmented eyes (rank 1–6) and those with darkly pigmented eyes (rank 9–14). With this subdivision, there was a significant linear correlation for the subjects with darkly pigmented eyes (closed circles), but no correlation for the subjects with lightly pigmented eyes (open circles).

solely determined by pupil size, but also by light penetration through the wall of the eye. It was hypothesized that the extrapupillary pathway for light entry into the eye would be more significant in subjects with less ocular pigmentation, regardless of pupil entry size. This hypothesis was explored by

first ranking the subjects according to iris pigmentation, from FIGURE 6. (A) Linear correlation between anisocoria and RAPD change least to greatest by one of the investigators (AK), who was in the most darkly pigmented subject (subject 14) who had different masked to the hypothesis when asked to rank the subjects. degrees of anisocoria induced by increasing concentrations of Rank iris pigmentation then was correlated with the degree of pilocarpine on the same day. The RAPD that actually was measured effective retinal illumination as determined by pupil responses showed a very high correlation with the amount of anisocoria and this between the treated and untreated eye that was induced by line (broken line, R2 ¼ 0.7206) was almost exactly the same as the miosis. RAPD that would have been expected based on mathematical calculations of pupil entrance area (solid line, R2 ¼ 0.9828). (B) Iris pigmentation rank did not correlate significantly with Similar graph as (A), but now for the most lightly pigmented subject the change in RAPD after induced miosis in the 14 subjects (subject 1). In contrast to (A), this subject’s change in RAPD over (correlation coefficient r ¼ 0.169, P ¼ 0.552), nor was there a different degrees of anisocoria induced by increasing concentrations of significant correlation between pigment rank and degree of pilocarpine showed no relationship to the degree of anisocoria (broken miosis produced by pilocarpine (r ¼ 0.359, P ¼ 0.201). line, R2 ¼ 0.0959). The line was almost flat compared to the line of A correlation between anisocoria and the change of the expected RAPD change based on calculations (solid line, R2 ¼ 0.9906). interocular difference of the area under the curve from baseline was replotted; the most darkly pigmented eyes (rank on the same day, starting with the most dilute solution. This 9–14) were plotted separately from the lightly pigmented eyes enabled us to ascertain the effect of varying degrees of miosis (rank 1–6), as shown in Figure 5. A significant correlation was (and, hence, anisocoria) on retinal illumination. The results of found for the more darkly pigmented eyes (Pearson correlation this experiment are shown in Figure 6. Two linear correlations coefficient 0.793, P ¼ 0.05), but not for the lightly pigmented are shown for each subject. In one correlation, the measured eyes (Pearson correlation coefficient 0.112, P ¼ 0.8). Because of the apparent correlation between ocular change in RAPD is correlated with anisocoria. In the second pigmentation and effective retinal illumination, we explored correlation, the expected change in RAPD that would be this concept even further in two subjects, one of whom was predicted by the entrance pupil size (see Methods) was darkly pigmented (subject 14) and the other who was lightly calculated and graphed. The measured change in RAPD in the pigmented (subject 1). We hypothesized that in the darkly darkly pigmented subject was almost the same as that pigmented subject, pupil entrance size would have a greater predicted from the calculations according to entrance pupil effect on retinal illumination and in the lightly pigmented size that was recorded, and the linear correlations were nearly subject it would have a much lesser effect. To confirm this, we identical. In contrast, for the lightly pigmented subject there repeated the initial experiment using increasing concentra- was almost no effect of anisocoria on the measured change in tions of pilocarpine (1/8%, 1/4%, 1/2%, and 1%) to the right eye RAPD.

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DISCUSSION be affected by the Stiles-Crawford effect, since these neurons are not known to have directional sensitivity. Our study originally was conceived to understand better the Among the 14 normal subjects tested, it was easy to relationship between asymmetric pupil size and the relative demonstrate that on the average, an induced pharmacologic afferent pupillary defect. The majority of previous studies have miosis in one eye produced a relative decrease in retinal used psychophysical means of assessing retinal illumination illumination in the treated eye compared to the fellow, untreated and light scatter based on how entrance pupil size and ocular eye. However, we were surprised to find that, overall, there was pigmentation affect the perception of light in terms of no significant relationship between anisocoria and retinal brightness or light flicker.35,36 The effect of entrance pupil illumination. A recent study has shown that with bright blue size on the pupillary light reflex provides an alternative means light (but not with red light), the postillumination sustained of measuring effective retinal illumination in an objective pupil contraction was reduced by a smaller pupil size, but iris manner. In some clinical situations, a unilateral or color was not specified.37 The initial contraction amplitude was miosis of the pupil may influence the clinical estimate of the unaffected by pupil entrance size, which may have resulted RAPD. Depending on which pupil is larger or smaller, the from the nonlinearity in pupil response at high intensity. Why RAPD can be underestimated or overestimated. One report were the RAPD results in this study so variable among the studied the effect of unilateral pharmacologic mydriasis on the normal subjects for the same degree of anisocoria under the log unit RAPD determined clinically using neutral density same carefully controlled conditions? filters.3 The results showed that unilateral dilation of the pupil We postulated that in lightly pigmented subjects an in normal subjects may induce an RAPD in the opposite eye, additional pathway exists for retinal illumination besides light but the relationship between amount of induced anisocoria entry through the pupil. Such an ‘‘extrapupillary’’ light and the magnitude of the RAPD was not as highly correlated as pathway would allow light to penetrate the sclera, , and/ was to be expected. In a subset of our study, patients with an or iris. We surmised that penetration through the eye to the existing RAPD had either the affected or unaffected eye dilated retina would be limited partly by the degree of melanin pharmacologically. The induced anisocoria did not produce a pigment in the eye. It would follow that large amounts of predictable effect on the RAPD in these patients, which was melanin pigmentation would constrain most of the light to difficult to explain. enter through the pupil and, therefore, darkly pigmented Because one pupil was fixed by a miotic agent, the subjects would show the most effect of pupil size on retinal untreated pupil was recorded during the alternating light test illumination. Conversely, lightly pigmented subjects would to determine the RAPD. It is known that some normal subjects allow light to pass through the sclera and uvea, and hence, have a greater direct than consensual pupil contraction, would be expected to show much less dependence of retinal depending on which eye is stimulated, termed contraction illumination on entrance pupil size. This was confirmed by anisocoria. Although contraction anisocoria theoretically may pharmacologically varying the entrance pupil size over a wide add or subtract from the calculated RAPD if only one pupil is range in two of the subjects, one darkly pigmented and the recorded, the effect does not change in a given person over other lightly pigmented. We found that the measured change in time. In the context of our study, a contraction anisocoria, if RAPD due to anisocoria was nearly exactly that which was present, would not affect the net change in RAPD induced by predicted in the darkly pigmented subject, indicating that entrance pupil size was the main, if not sole, determinant of changing the entry pupil size in one eye. During baseline the amount of effective retinal illumination. In contrast, the recording of both pupils before administration of the topical lightly pigmented subject showed no effect of anisocoria on miotic agent, we did not find, in fact, any significant the RAPD, indicating that light can penetrate the eye through contraction anisocoria in the normal subjects tested in this extrapupillary pathways when there is not sufficient ocular study. Since the pupil light reflex is a built-in objective light pigmentation to absorb it. meter of the eye, we chose to reexamine the relationship in a Although this concept seems obvious, we believe that it very controlled way by recording entrance pupil size carefully may be more difficult to demonstrate using standard psycho- and measuring the resulting effective retinal illumination using physical testing, since subjective light perception may not be the amplitude of the pupil light reflex. By using a computer- as accurate as using the pupil light reflex. In one study of ized pupillometer to provide a controlled stimulus and a ocular scatter of light, it also was found that a significant precise recording of pupil dynamic behavior, we felt that it degree of light presented as an annulus outside the confines of would be possible to confirm a relationship between the pupil could reach the retina and produce scatter in lightly anisocoria and effective retinal illumination, if one existed. pigmented subjects.36 The pupil light reflex is capable of Care was taken in the experimental design to control for the summating the area of retina stimulated much more effectively state of adaptation, and delivery of a repeatable and accurate than psychophysical tests of light perception, and also light stimulus over a range of intensities that could be given in responds effectively to the sum total of diffuse and scattered an alternating fashion to the right and left eye. A short duration light falling upon the entire retina. Therefore, the pupil light light stimulus was given (within the latency time of the pupil reflex may be an effective means of quantifying the transmis- light reflex) so that entrance pupil area would not be affected sion of light through the ocular wall by assessing the effect of by the light stimulus. pupil size (or lack thereof) on the amplitude of the pupil light The Stiles-Crawford effect was found to be minor in the reflex, as was demonstrated in this study. This may provide a context of our study. This is due to a number of reasons. First, means of assessing a patient’s risk of light-associated damage to the Stiles-Crawford effect is greatest with large sized pupils and the eye. reducing pupil size by a miotic tends to minimize this effect. There are a number of potentially important implications Second, the stimulus area of the retina is quite large and is even resulting from this study. First, it predicts that in less greater with brighter light stimuli due to light scatter, and not pigmented patients, the presence of anisocoria would have only would recruit rods (although rods do not contribute much less effect on the estimation of the RAPD. This could greatly to the Stiles-Crawford effect, since it occurs mainly become important in situations, such as acute trauma, where under photopic conditions), but also would recruit contribu- the presence and log unit amount of an RAPD may influence tions to the pupil light reflex from intrinsic activation of clinical decisions about the need for further evaluation and melanopsin containing retinal ganglion cells, which would not treatment. Ocular pigmentation, and hence penetration of

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light through the eye wall to the retina, would determine the 5. Congdon NG, Quigley HA, Hung PT, Wang TH, Ho TC, effect of either miosis or mydriasis on the RAPD. In either Glovinsky Y. Impact of age, various forms of cataract, and situation, a lightly pigmented eye would minimize the effect of visual acuity on whole-field scotopic sensitivity screening for anisocoria and a darkly pigmented eye would maximize the in rural Taiwan. Arch Ophthalmol. 1995;113:1138– effect. Second, in conditions, such as retinal degeneration or 1143. cataract development, which may be affected adversely by the 6. Armington JC. Pupil entry and the human electroretinogram. J total cumulative effect of a long-term exposure to ultraviolet Opt Soc Am. 1967;57:838–839. and visible light, persons with less ocular pigmentation may be 7. Hoffmann ML, Zrenner E, Langhof HJ. The effect of the pupil at greater risk for light toxicity,8–23 regardless of their pupil size as aperture and field stop on the various components of the in daylight. Finally, the results of our study suggested a human electroretinogram [in German]. Albrecht Von Graefes physiologic explanation for differences in subjective light Arch Klin Exp Ophthalmol. 1978;206:237–245. sensitivity and light scatter among dark-eyed and light-eyed 8. Bochow TW, West SK, Azar A, Munoz B, Sommer A, Taylor HR. persons. The effect of stray light entering the eye through an Ultraviolet light exposure and risk of posterior subcapsular extrapupillary pathway (transmission through iris/uvea) was . Arch Ophthalmol. 1989;107:369–372. found to be significant in lightly pigmented subjects, in terms 9. O’Keefe TL, Hess HH, Zigler JS Jr, Kuwabara T, Knapka JJ. of its effect on the pupillary light reflex regardless of pupil size. Prevention of cataracts in pink-eyed RCS rats by dark rearing. 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Decreasing stromal iris pigmentation as a risk factor for age- Acknowledgments related macular degeneration. Am J Ophthalmol. 1994;117: Supported by an unrestricted grant from Research to Prevent 19–23. Blindness (New York, New York), and the Division of Rehabilita- 19. Sandberg MA, Gaudio AR, Miller S, Weiner A. Iris pigmentation tion, Research and Development (Center of Excellence Grant) and extent of disease in patients with neovascular age-related from the Veterans Administration, Washington, DC (RHK), and a macular degeneration. Invest Ophthalmol Vis Sci. 1994;35: visiting scholarship from Kyungpook National University Hospital, 2734–2740. Taegu, Korea (SH). 20. Mitchell P, Smith W, Wang JJ. Iris color, skin sun sensitivity, and Disclosure: R.H. Kardon, None; S. Hong, None; A. Kawasaki, age-related . The Blue Mountains Eye Study. None Ophthalmology. 1998;105:1359–1363. 21. Herndon JH Jr, Freeman RG. Human disease associated with exposure to light. Annu Rev Med. 1976;27:77–87. 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