Measurement of the retinal nerve fiber layer thickness in clinical routine

The possibilities and limitations of methods for indirect and direct nerve fiber layer thickness (NFLT) measurements, and a summary of our own clinical studies using Optic Nerve Head Analyzer (Rodenstock, Munich, Germany) (ONHA), Laser Tomographic Scanner (Heidelberg Engineering, Heidelberg, Germany) (LTS), Heidelberg Retinal Tomograph (Heidelberg Engineering, Heidelberg, Germany) (HRT), Nerve Fiber Analyzer (Laser Diagnostic Technologies, San Diego, CA) (NFA) and Optical Coherence Tomography (OCT), together with the literature of the review period, are highlighted. A retinal hemifield test with the new parameters, retinal asymmetry difference and retinal asymmetry index, is introduced. These relative nerve fiber layer thickness (NFLT) measurement values are found to be independent of age, reference plane, and disc size. This reduces the interindividual variation of the measurements and may improve screening possibilities for glaucoma. The detection probability for nerve fiber bundle defects was found to be stage-dependent using HRT and NFA. For laser polarimetry, a decreasing correlation between NFLT and visual field loss with advanced glaucomatous disease was found. In the lower retina, a 6.9% higher NFLT was evident compared with the upper peripapillary NFLT in healthy eyes using NFA and OCT. This is not in correlation with perimetric retinal sensitivity, which was found to be 3.9% higher in the lower hemifield compared with the upper hemifield. In general, quantitative NFLT measures (HRT, NFA, OCT) were correlated with visual field loss in glaucoma. The interindividual variability of NFLT measurements, among other things, induces limitations on distinguishing beginning glaucoma from normals. Clinical and scientific significance and future directions of NFLT measurements are discussed.