"Ideally", says Dr. Quigley, "we would like to have a routine, quantitative method for measuring the extent of loss of nerve fibers." For eight years the Wilmer team has been working on such measures. Already in use in the Glaucoma Service are two instruments for estimating the number of nerve fibers: the Glaucoma-Scope and the Nerve Fiber Analyzer. Dr. Quigley and his colleagues are currently collecting patient outcome data to assess the validity and usefulness of these new diagnostic tools. Early glaucoma could be identified either by methods that generate images of structural change in the optic nerve head and nerve fiber layer (NFL), or by methods that determine that functional visual field loss had occurred. The Wilmer Glaucoma Service is pioneering methods that use each of the two approaches.
The structure of the optic disc and NFL is being evaluated clinically with both the Glaucoma-Scope and with the Nerve Fiber Analyzer. Both machines are able to image the retinal structures without dilation of the pupil. The Glaucoma-Scope has a simple design and may soon be produced at very low cost as a version that can be used with any standard slit-lamp. Its imaging system generates standard disc parameters such as cup/disc ratio and neural rim area, as well as more recently developed algorithms for monitoring change in disc structure. The latter include the mean position of the disc and the nerve fiber layer area, whose value has been demonstrated by laboratory studies performed by the Glaucoma Service team.
The Nerve Fiber Analyzer is the only instrument presently capable of directly measuring the actual thickness of the NFL, by using the principle of polarization shifting of laser light. The Glaucoma Service has the largest series of eyes that have been imaged by this method and is participating in producing its normal data base. The original research that showed that this machine actually measures the NFL thickness was carried out in our laboratory. These two methods promise to improve on the present standard NFL examination technique.
A number of new approaches are being developed at the Glaucoma service to test for functional glaucoma damage. A whole-field test of scotopic function has been proven to detect injury earlier than standard field tests, and takes only 2 minutes. Recent basic research has shown that motion detection is abnormal in glaucoma patients. These two methods came directly from research on the histology of human glaucoma. We are working directly with the Humphrey company to develop the next generation of Field Analyzers, which are already in use at our facility. The HFA II promises to carry out automated threshold testing at greater speed and more comfortably than the old Humphrey perimeter. Finally, the Dicon perimeter is being tested as an alternative in glaucoma screening and even as a replacement for present threshold test methods. |
