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Effect of corneal polarization axis on assessment of retinal nerve fiber layer thickness by scanning laser polarimetry
CORRESPONDENCE Effect of Corneal Polarization Axis on Assessment of Retinal Nerve Fiber Layer Thickness by Scanning Laser Polarimetry EDITOR:
head tilted 45 degrees on either side of the vertical—in both cases the thickness measurements would increase. Obviously, more research is needed into the issue of corneal birefringence and how to accurately compensate for it. Dr. Greenberg and co-workers are to be congratulated for their research, which is clearly a step in the right direction. NEIL T. CHOPIN, MD
I COMMEND THE AUTHORS OF THIS ARTICLE ON THEIR
excellent work in developing a method to determine corneal polarization axis. This technique helps our understanding of the corneal compensation mechanism built into the only commercially available scanning laser polarimeter, the GDx® Nerve Fiber Analyzer (Laser Diagnostic Technologies, Inc., San Diego, CA). However, I believe there is an error in their conclusion regarding Case 1, Figure 5. This example shows low retardation signals from the nerve fiber layer, and the authors conclude that this is from “low corneal polarization axis.” They state that “eyes with low corneal polarization axis are characterized by weak macular and retinal nerve fiber layer retardation,” and that “apparent nerve fiber layer thickness in such eyes is artifactually decreased.” In fact, Case 1, Figure 5 shows a completely normal corneal polarization axis (18 degrees OD and 14 degrees OS) and absent signal from the macula. These corneas are precisely aligned with the corneal compensator, and the reduced nerve fiber layer signals from these eyes must be from an attenuated nerve fiber layer. Furthermore, the corneal compensator will always add retardation (data on file, Laser Diagnostic Technologies, Inc). This is because the compensator is oriented in such a manner as to neutralize the birefringence of the anterior segment; there is no way for the compensator itself to reduce the signal. Any misalignment (beyond 10 –15 degrees) of the cornea and the compensator will result in higher signals, as shown in cases 2 and 3 of Figure 5. The suggestion that there are corneal polarization axes that could result in an apparent decrease in the signal from the nerve fiber layer comes, in part, from figures 6 and 7, implying a linear relationship between corneal polarization axis and thickness measurements. Although a line has been fit to the data using linear regression, in reality the relationship is bimodal, with the lines tailing up from the 15-degree nasally downward axis. This could easily be demonstrated if the patient were to be imaged with the 528
©
2001 BY
San Diego, California
AUTHOR REPLY EDITOR:
We appreciate Dr. Choplin’s interest in our article and are grateful for the opportunity to respond to his thoughtful comments. Dr. Choplin is troubled by our observation that eyes with corneal polarization axis (CPA) measurements closely aligned to the compensator’s axis (for example, Case 1, Figure 5) are characterized by weak macular and peripapillary retardation, resulting in lower measured retinal nerve fiber layer (RNFL) thickness.1 He states that the patient in this illustration “shows a completely normal CPA” and “the reduced nerve fiber layer signals from these eyes must be from an attenuated nerve fiber layer.” Because the anterior segment compensator device assumes all individuals to have a fixed slow axis of corneal birefringence of 15 degrees nasally downward,2,3 we first determined the distribution of normal CPA measurements. Although the mode of our distribution did in fact align with the axis assumed by the compensator, considerable inter and intra-individual variability was identified (range from 54 degrees nasally upward to 90 degrees downward). Thus, the concept of a “normal” CPA is of limited usefulness. Rather, we emphasized the effects of alignment between CPA and the compensator axis. There is no evidence to suggest that the retardation in eyes such as Case 1 “must be from an attenuated nerve fiber layer.” The patient illustrated had completely normal achromatic and short wavelength automated perimetry and had healthy optic nerves with a vertical and horizontal cup-disk ratio of 0.1. Scanning laser polarimetry, however, demonstrated four retardation parameters outside 90% confidence intervals (considered borderline based upon the instrument’s normative database) and another four parameters outside 95% confidence intervals (considered abnormal). Such a paradox is indeed surprising but, we have
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0002-9394/01/$20.00 PII S0002-9394(00)00939-9
head tilted 45 degrees on either side of the vertical—in both cases the thickness measurements would increase. Obviously, more research is needed into the issue of corneal birefringence and how to accurately compensate for it. Dr. Greenberg and co-workers are to be congratulated for their research, which is clearly a step in the right direction. NEIL T. CHOPIN, MD
I COMMEND THE AUTHORS OF THIS ARTICLE ON THEIR
excellent work in developing a method to determine corneal polarization axis. This technique helps our understanding of the corneal compensation mechanism built into the only commercially available scanning laser polarimeter, the GDx® Nerve Fiber Analyzer (Laser Diagnostic Technologies, Inc., San Diego, CA). However, I believe there is an error in their conclusion regarding Case 1, Figure 5. This example shows low retardation signals from the nerve fiber layer, and the authors conclude that this is from “low corneal polarization axis.” They state that “eyes with low corneal polarization axis are characterized by weak macular and retinal nerve fiber layer retardation,” and that “apparent nerve fiber layer thickness in such eyes is artifactually decreased.” In fact, Case 1, Figure 5 shows a completely normal corneal polarization axis (18 degrees OD and 14 degrees OS) and absent signal from the macula. These corneas are precisely aligned with the corneal compensator, and the reduced nerve fiber layer signals from these eyes must be from an attenuated nerve fiber layer. Furthermore, the corneal compensator will always add retardation (data on file, Laser Diagnostic Technologies, Inc). This is because the compensator is oriented in such a manner as to neutralize the birefringence of the anterior segment; there is no way for the compensator itself to reduce the signal. Any misalignment (beyond 10 –15 degrees) of the cornea and the compensator will result in higher signals, as shown in cases 2 and 3 of Figure 5. The suggestion that there are corneal polarization axes that could result in an apparent decrease in the signal from the nerve fiber layer comes, in part, from figures 6 and 7, implying a linear relationship between corneal polarization axis and thickness measurements. Although a line has been fit to the data using linear regression, in reality the relationship is bimodal, with the lines tailing up from the 15-degree nasally downward axis. This could easily be demonstrated if the patient were to be imaged with the 528
©
2001 BY
San Diego, California
AUTHOR REPLY EDITOR:
We appreciate Dr. Choplin’s interest in our article and are grateful for the opportunity to respond to his thoughtful comments. Dr. Choplin is troubled by our observation that eyes with corneal polarization axis (CPA) measurements closely aligned to the compensator’s axis (for example, Case 1, Figure 5) are characterized by weak macular and peripapillary retardation, resulting in lower measured retinal nerve fiber layer (RNFL) thickness.1 He states that the patient in this illustration “shows a completely normal CPA” and “the reduced nerve fiber layer signals from these eyes must be from an attenuated nerve fiber layer.” Because the anterior segment compensator device assumes all individuals to have a fixed slow axis of corneal birefringence of 15 degrees nasally downward,2,3 we first determined the distribution of normal CPA measurements. Although the mode of our distribution did in fact align with the axis assumed by the compensator, considerable inter and intra-individual variability was identified (range from 54 degrees nasally upward to 90 degrees downward). Thus, the concept of a “normal” CPA is of limited usefulness. Rather, we emphasized the effects of alignment between CPA and the compensator axis. There is no evidence to suggest that the retardation in eyes such as Case 1 “must be from an attenuated nerve fiber layer.” The patient illustrated had completely normal achromatic and short wavelength automated perimetry and had healthy optic nerves with a vertical and horizontal cup-disk ratio of 0.1. Scanning laser polarimetry, however, demonstrated four retardation parameters outside 90% confidence intervals (considered borderline based upon the instrument’s normative database) and another four parameters outside 95% confidence intervals (considered abnormal). Such a paradox is indeed surprising but, we have
ELSEVIER SCIENCE INC. ALL
RIGHTS RESERVED.
0002-9394/01/$20.00 PII S0002-9394(00)00939-9