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New Developments in Scanning Laser Polarimetry
1Kremmer S., 1Selbach J. M., 2Steuhl K. P.,
1Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde (Essen)
2Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde, Abt. für Erkrankungen des vorderen Augenabschnitts (Essen)
In the last years, scanning laser polarimetry is of increasing importance in the diagnosis of glaucomatous nerve fiber damage. During the development of five device generations (NFA I, NFA II, GDx, GDx Access und GDx NFA) several improvements of hard- and software took place. Whereas first generation polarimeters (NFA I) were markedly influenced by brightness and reflections this problem was solved by second generation instruments (NFA II), simultaneously recording polarization and reflection measurements for every pixel. The connected computer takes both data into consideration for the change of polarization and the calculation of retinal nerve fiber layer thickness. Reproducibility was markedly improved. Then hardware tolerances were narrowed, and a new software enabled the reduction of the influence of blood vessels and more pixels were used for routine evaluation (GDx). An age and ancestry specific normative database supported the diagnostic evaluation especially at first exams. But one problem was still existing: not only the nerve fiber layer but also the cornea has birefringent properties on polarized light. Therefore, the first four generations of polarimeters had a fixed compensator eliminating these influences if corneal polarization properties were normal (slow axes of polarization 15° nasally downwards, magnitude 59.8 µm). Differences caused errors. Because in adults the birefringent properties of the cornea are stabile over time follow-up measurements were not influenced. 5-7 year follow-up results are compared to perimetry and scanning laser tomography. Additionally, the low influence of cataract surgeries with implantation of different material IOLs is of clinical importance in the follow-up of glaucoma patients. Measuring a macular reference and using a variable, automatic compensation of anterior segment birefringent properties (especially the cornea) the new fifth generation polarimeters (GDx NFA) are almost uninfluenced by anatomical variations. Each measurement of the retinal nerve fiber layer is individually compensated for the measured birefringent properties of the anterior eye segment. Even the influence of refractive surgery and keratoplasties can be compensated by devices of the new generation. Enhanced resolution of less than 3 µm enables more precise measurements and the age depending normative database can be further improved. In clinical use, SLP has several advantages because pupil dilation is not required and good quality images can be obtained even after the use of miotics. Additionally, the scans are acquired rather rapidly (< 1 second per measurement) reducing the influence of concentration and cooperation of the patients.
Supported by Alfried Krupp von Bohlen und Halbach-Stiftung