Photogrammetric measurement of nerve fiber layer thickness

To measure retinal nerve fiber layer (NFL) thickness, three sets of simultaneous stereophotographs of the optic disc, taken with the Donaldson fundus camera, were measured at 5 degrees intervals in a stereoplotter (Kern PG2) at the surface of the internal limiting membrane (ILM) and the retinal pigment epithelium (RPE) along the disc margin. The NFL thickness was expressed as a ratio of the vertical disc radius. The reproducibility of the method showed that the percent coefficients of variation (CV%) for 15 normal, 10 ocular hypertensive, and 10 glaucomatous subjects were 5.2%, 6.0%, and 3.9%, respectively. The presence of a halo or scleral crescent had no significant effect on reproducibility. The photogrammetric technique for measuring NFL thickness appears to be highly reproducible and sensitive and may be a valuable quantitative method for: (1) early detection of optic nerve damage from glaucoma and other optic neuropathies and (2) monitoring progression of retinal NFL defects.