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Corneal polarization affects some retinal imaging technologies, study suggests
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SAN DIEGO — Corneal polarization values vary widely, possibly affecting the results of retinal imaging with scanning laser polarimetry, according to a study here.
Findings of the study “must be addressed” for scanning laser polarimetry to best detect and monitor glaucoma, the study authors advise.
The GDx scanning laser polarimeter (Laser Diagnostic Technologies) uses a narrow-band corneal compensator that assumes a fixed magnitude and axis of corneal polarization in order to separate corneal birefringence from the birefringence of the retinal nerve fiber layer. This study, by Robert Weinreb, MD, and colleagues, found that there is a normal distribution of both the axis and magnitude of corneal polarization, and therefore many eyes have different values from those assumed by the GDx.
Dr. Weinreb and colleagues measured both the corneal polarization axis (CPA) and magnitude (CPM) in 55 eyes with glaucoma and 51 healthy eyes using a modified GDx with an experimental CPA and CPM compensator. Each eye had three sets of measurements taken. Nasally upward CPA values were recorded as negative; nasally downward CPA values were recorded as positive.
The CPM and CPA measurements were normally distributed. CMP measurements ranged from 7 nm to 91 nm. CPA measurements ranged from -13º to 73º. When all eyes were combined, there was a significant effect of age on CPA (P < .001). There were no differences in CPM or CPA between healthy and glaucomatous eyes after adjusting for age. No effects of corneal thickness on CPM or CPA or of corneal curvature were observed.
A downside to the technology, Dr. Weinreb noted, was that macular imaging was used to determine both sets of values. “Some macular abnormalities that disrupt the Henle layer and/or macular birefringence may affect the ability of this method to measure CPM and CPA,” Dr. Weinreb noted.
Laser Diagnostic Technologies has been investigating adding custom corneal compensation to its GDx to account for the variation found in this study.
The article is published in the July issue of Archives of Ophthalmology.