Polarimetry of the retinal nerve fiber layer

Retinal nerve fiber layer (RNFL) examination is a useful tool in the diagnosis and follow-up of glaucoma. Because the clinical techniques to date have been subjective, there is a definitive need to develop instruments that are capable of making accurate and reproducible measurements of the RNFL. Introduction of new quantitative methods, such as confocal scanning laser ophthalmoscopy and optical coherence tomography raises high expectations in clinical practice. In the Nerve Fiber Analyzer (Laser Diagnostic Technologies, San Diego, CA) the measurement technique is based on the assumption that the RNFL has birefringent properties. The change in the polarization state, called retardation, can be quantified by determining the phase shift between polarization of light returning from the eye with the known state of polarization of the illuminating laser beam. The retardation shows a typical double-hump pattern of the RNFL thickness around the optic disk. After elimination of artifacts produced by the anterior segments of the eye, the retardation is related linearly to histopathologic thickness measurements of the RNFL in monkey and human eyes. Prior to large-scale implementation of this technique into clinical practice, however, several questions need to be answered.