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Analyses of data from baseline structural imaging studies performed in two large prospective, observational glaucoma cohort trials suggest inter-eye asymmetry in optic disc topography parameters and average retinal nerve fiber layer thickness may provide an additional tool to identify eyes with early signs of glaucoma.
Orlando, FL-Analyses of data from baseline structural imaging studies performed in two large prospective, observational glaucoma cohort trials suggest inter-eye asymmetry in optic disc topography parameters and average retinal nerve fiber layer (RNFL) thickness may provide an additional tool to identify eyes with early signs of glaucoma, reported researchers at the annual meeting of the American Academy of Ophthalmology.
The analyses were based on data from 1,043 participants in the African Descent and Glaucoma Evaluation Study (ADAGES) and the Diagnostic Innovations in Glaucoma Study (DIGS), of which 519 were healthy controls, 146 had suspected glaucoma with glaucomatous optic nerve head damage but normal visual fields, and 378 were patients with glaucoma and glaucomatous optic nerve head damage and repeatable visual field loss.
Compared with patients with suspected glaucoma, patients with glaucoma had significantly smaller mean disc area and worse disease severity measured by mean average (average of left and right eyes) visual field mean deviation.
In addition, more than twice as many patients with glaucoma as those with suspected glaucoma had average RNFL thickness asymmetry values outside the normal range (~25% versus ~10%, respectively). However, there were no significant differences between patients with suspected glaucoma and those with glaucoma in the proportions with abnormal inter-eye asymmetry values for any of the optic disc topography parameters.
Disc area, disc area asymmetry
Results of multivariable analyses performed to identify whether inter-eye asymmetry in the CSLO and SLP values varied by demographic and other ocular factors showed increased cup area, cup volume, rim area, and average RNFL thickness asymmetries were all significantly associated with worse glaucoma disease severity. In addition, disc area asymmetry and average disc area correlated with increases in most optic disc topography asymmetries but not average RNFL thickness asymmetry.
"These findings indicate that disc area and disc area asymmetry should be considered when evaluating asymmetry in the optic disc topography parameters. However, because average RNFL thickness asymmetry was only associated with worse disease severity but not larger disc area or greater disc area asymmetry, we believe the average RNFL thickness asymmetry may be a more clinically relevant measure of asymmetry in [patients with glaucoma and those with suspected glaucoma]," said Grant H. Moore, speaking on behalf of the study group.
Moore is a medical student, New York University School of Medicine and University of California, San Diego (UCSD), and received a Research to Prevent Blindness Medical Student Research Fellowship Grant for the project. Linda M. Zangwill, PhD, professor of ophthalmology, Hamilton Glaucoma Center, UCSD, is the corresponding author.
Moore noted results of previous research have shown that inter-eye asymmetry in cup:disc ratio is an early sign of glaucomatous damage and also predictive of conversion from ocular hypertension to glaucoma.
"SLP and CSLO allow objective measurement of RNFL thickness and optic disc topography, but there is limited information on possible associations between asymmetry of these features and severity of visual field damage and other ocular factors, such as disc area," he said.
"With that in mind we sought to determine if inter-eye asymmetry in optic disc topography and in average RNFL thickness differed between groups of patients with glaucoma with documented repeatable evidence of visual field damage and patients with suspected glaucoma with glaucomatous- appearing optic discs without visual field damage and to identify whether inter-eye asymmetry in the imaging measurements varied by other ocular factors," he added.
In addition to disc area asymmetry, mean disc area, and disease severity (visual field mean deviation), other factors considered for their potential to influence inter-eye asymmetry in the CSLO and SLP parameters were the HRT image standard deviation, mean K, K value asymmetry, race, age, and axial length. None of the latter variables was consistently associated with changes in inter-eye asymmetry Moore observed that the study has several potential limitations. Because the inclusion criteria of DIGS and ADAGES limited enrollment to individuals with visual acuity of 20/40 or better, the population analyzed included a limited number of patients with advanced glaucoma.
"The clinical importance of asymmetry is likely strongest in patients who are in the early stages of glaucomatous disease," he said.
As another limitation, there was the potential for biased selection of patients with suspected glaucoma who might have been referred to the UCSD tertiary ophthalmology department based on asymmetry of optic nerve head appearance. In addition, the analyses did not account for anisometropia, which may be important considering that myopic eyes have relatively thinner RNFL thickness and that both DIGS and ADAGES excluded patients with high myopia and hyperopia.
Linda M. Zangwill, PhD
Dr. Zangwill has received research equipment from Carl Zeiss Meditec, Heidelberg Engineering, Optovue, and Topcon Medical Systems.
Grant H. Moore
Moore has no financial interest in the subject matter.