By Gail F. Schwartz, MD
This study published in the Archives of Ophthalmology in September describes the authors' use of modified software protocols for the measurement of retinal thickness with spectral-domain optical coherence tomography (SD-OCT; Heidelberg Engineering, Carlsbad, Calif.) to diagnosis various stages of glaucoma. They found the software offers excellent reproducibility, which has been a rate-limiting factor in previous imaging systems.
Specifically, they customized the retinal thickness protocol to acquire detailed retinal thickness measurements of the central 20 degrees of the posterior pole that allow the detection of small losses of retinal thickness and asymmetry between the eyes and the two hemispheres in each eye. They tested this strategy in four patients, comparing retinal thickness measurements between eyes and between the hemispheres of each eye. The opposite eye was used as a control because asymmetry is considered a strong clinical sign of glaucoma and a normative database of retinal thickness in the posterior pole was not available. The resulting custom maps are displayed in a compressed color scale that reveals small losses in retinal thickness.
SD-OCT has been used to measure macular thickness in retinal diseases, but aside from measuring the retinal nerve fiber layer, it is not believed to have been used for glaucoma. Imaging continues to evolve in its resolution and ability to detect glaucoma and progression, and retinal thickness analysis has been of interest for several years. The authors say that the SD-OCT used in the study offers more flexible software for measurement and display than alternative technologies, and that the benefits of high-speed, detailed retinal thickness measurement by SD-OCT in glaucoma diagnosis have not yet been fully realized.
They say that the detection of glaucoma may become easier by combining the diagnostic potential of RNFL thickness with the modified macular thickness map. However, in cases of symmetrical loss between the eyes, as well as when there is symmetrical loss in both hemispheres of the same eye, asymmetry plots will have minimal value. A normative database that incorporates age, sex, refractive status and ethnicity is being assembled and will be required to use this technology fully.
These measurements may also be useful in patients with end-stage glaucoma in whom visual fields are almost completely extinguished, making it difficult or impossible to use visual fields for monitoring. Despite extensive tissue loss, residual retinal thickness in the macula may permit monitoring of such patients for progression. Macular thickness measurements, however, may be limited in patients with glaucoma who have coexisting retinal conditions, such as epiretinal membranes, diabetic macular edema and macular degeneration.