Influence of corneal refractive surgery on scanning laser polarimetry
In order to improve the accuracy of glaucoma diagnostics, the use of the modern imaging technologies in routine practice has become necessary. Scanning laser polarimetry (SLP) is one of the widely-used modern imaging technologies. It was developed to measure the thickness of the retinal nerve fibre layer (RNFLT) around the optic nerve head, and to automatically compare the results with the corresponding normative database reference values. The method is based on retardation (slowing down) of the polarized illuminating laser light of the instrument along one axis (“slow axis”) by the birefringent retinal ganglion cell axons. Certain surgical interventions which involve the corneal tissue (such as LASIK) may have clinically significant influence on the corneal retardation, which may potentially lead to misinterpretation of the results. Since corneal refractive surgery has become a widely-used method to correct for ametropia, and many young people, who in future may develop glaucoma, undergo different types of refractive surgery, this issue gained great clinical significance. Following the introduction of GDx-VCC, the next generation of the GDx devices, the influence of corneal retardation became easily manageable, which helped to confirm that the virtual decrease of the post-LASIK polarimetric RNFLT was an artifact, and was not the sign of true RNFLT damage. Recently a new polarimetric software version (enhanced corneal compensation, GDx-ECC) was developed and investigated by different research groups for its ability to remove the LASlK-induced corneal retardation artifacts, proving the new method to be more accurate than GDx-VCC. It all led to a conclusion that since GDx-ECC is able to neutralize changes of corneal retardation induced by LASIK, this software may be even more suitable to long-term follow-up of eyes which undergo corneal refractive surgery.