Several techniques for clinical NFL evaluation have been described. Ophthalmoscopic examination using red-free light and NFL photography have been used for many years, but these techniques are limited to eyes with clear media and to good cooperation of the patient and are difficult to perform and interpret. The slit-lamp biomicroscopic NFL examination and NFL photography evaluation are subjective and depend on the experience of the examiner (and also the photographer, as in the case of NFL photography).
Other technologies, such as scanning laser polarimetry, have been developed to measure the thickness of the NFL. This device is based on changes in polarization of light and retardation, which is related to the NFL thickness. Unfortunately, the NFL is not the only birefringent structure in the eye, and it remains unclear what the effects of corneal and lenticular birefringence and aging are on polarimetric measurements. Polarimetric measurements can be influenced by peripapillary atrophy or chorioretinal scars, which may occur in healthy eyes or eyes with disease other than glaucoma.34
Optical coherence tomography enables cross-sectional visualization and direct measurement of NFL thickness. Low coherence light is applied through a noncontact, non-invasive instrument and depicts the retinal microstructure with a resolution of 10 μ
m. We found good correlation of NFL measurements by OCT to histopathologic analysis (Huang LN, Schuman JS, Kloizman TP, et al. Poster presentation at the Association for Research in Vision and Ophthalmology Annual Meeting, 1997; and Schuman JS, Pedut–Kloizman T, Pieroth L, et al. Oral presentation at the Association for Research in Vision and Ophthalmology Annual Meeting, 1996). The reproducibility of OCT measurements compares well with other currently available ophthalmic instrumentation designed for optic nerve and NFL analysis.23,35–38
In cross-sectional studies, focal defects are found in approximately 20% of glaucomatous eyes examined.15
Quigley et al2
divided the types of NFL defects of patients who developed a visual field defect within a period of 5 years into upper and lower NFL zones. He found bundle defects in 5% superiorly and 3% inferiorly.
The gender distribution in our study did not correlate with that of previously described reports of bundle defects of the NFL, as the majority of our study patients were female. This distribution would not be expected to influence NFL measurements, as reported previously.22
It may indeed be that focal defects actually occur more frequently in women than in men; however, the number of study subjects in our study was not high enough to address this question. It is likely that this gender imbalance merely reflected the nature of our patient pool.
There are discrepancies in the literature about the pathogenesis of focal and diffuse defects of the NFL. Samuelson and Spaeth39
found a significantly lower maximal pretreatment IOP among patients with focal visual field defects than among those with diffuse visual field loss. Most of the patients analyzed also had focal NFL defects. Vascular, structural, and mechanical factors may increase laminar susceptibility, independent of IOP.
We chose to differentiate focal NFL defects from focal visual field defects because visual field changes in the earlier stages of glaucoma are difficult to track and can show a heterogenous pattern.1–4,13,40
Based on findings in the studies cited above, we conclude that evaluation of the NFL may be superior to visual field testing in the earliest stages of glaucoma, because of its high sensitivity.
Future studies will investigate in more detail which longitudinal changes in NFL thickness occur in patients with glaucoma. Furthermore, we expect additional refinements in our diagnostic criteria as more normative OCT data become available.
Localized NFL defects seem to occur in the early stages of glaucoma and are sometimes associated with optic disc hemorrhages. The stages of glaucoma in our study correlated with previously described focal defects of the NFL.14,16,17,21,41
Most case subjects in our study had early glaucoma. In this study, it is not possible to determine whether the focal defects preceded the development of diffuse defects.
The location of the focal NFL defects in this study also correlated with previously described focal defects. The focal NFL defects were located in the inferotemporal and superotemporal regions of the NFL. Jonas et al42,43
similarly found that focal NFL defects occur significantly more often in the inferior fundus region, followed by the superior areas and the temporal region.
We found a specificity of 81% and a sensitivity of 65% of detecting focal defects solely through statistical analysis of OCT measurements (i.e., without clinical interpretation of the tomographic OCT scan). Sensitivity may improve when more normative data are available.
Optical coherence tomography permits us to detect focal defects independent of the visibility of the NFL. Thus, the chances of detecting these defects in areas of physiologic decreased visibility are higher using OCT than compared to conventional clinical methods.44
We found that focal defects are often demarcated by blood vessels (, , and ); it is possible that blood vessels provide structural support for the NFL from the periphery toward the ONH.
Optical coherence tomography is an objective, reproducible, and sensitive tool to track localized defects of the NFL and may provide early detection of glaucomatous damage.18,45
Although clinical NFL examination has been recommended as a most sensitive indicator of glaucoma,3,4
clinical examination is difficult and subjective, and NFL photography requires high technical skill. It is critical that accurate NFL assessment be performed by all clinicians; OCT not only facilitates NFL evaluation, but it permits quantitative, objective, and reproducible tissue measurements.