Influence of disc size on optic nerve head versus retinal nerve fiber layer assessment for diagnosing glaucoma

Purpose: To explore and compare the influence of optic disc size on the diagnostic accuracy of retinal nerve fiber layer (RNFL) thickness and optic nerve head (ONH) quantitative assessment.

Design: Observational, cross-sectional evaluation of diagnostic tests.

Participants: We included 120 eyes from 50 normal subjects and 70 glaucomatous patients classified by the presence of a repeatable visual field defect for the analysis.

Testing: The RNFL thickness was measured by scanning laser polarimetry with variable corneal compensator (GDx-VCC, Carl-Zeiss Meditec, Dublin, CA) and spectral-domain optical coherence tomography (Cirrus HD-OCT, Carl Zeiss Meditec, Inc). We obtained ONH imaging by means of confocal scanning laser ophthalmoscopy (HRT3; Heidelberg Engineering, GmbH, Dossenheim, Germany).

Main outcome measures: Sensitivity and specificity for normative classifications, sensitivity at fixed specificity and area under the receiver operating characteristics curve (AUC) for continuous parameters. A logistic marginal regression model and coefficients of variation (CoV) have been used to test and quantify the influence of optic disc size on the diagnostic accuracy of the 3 technologies under investigation.

Results: Among continuous parameters average RNFL thickness for Cirrus HD-OCT, nerve fiber indicator for GDx-VCC and cup shape measure for the HRT3 showed the best diagnostic accuracy with an AUC of 0.97, 0.94, and 0.94, respectively. Among normative classifications, the highest sensitivity and specificity were found for OCT average RNFL thickness (75.8% and 94.7%), for GDx superior thickness (77.1% and 97.5%), for HRT3 Moorfields regression analysis result (89.4% and 73.7%) and for HRT3 GPS global (92.3% and 76.5%). The diagnostic performance of HRT3 parameters seemed to be significantly influenced by optic disc size, although the same was not true for Cirrus HD-OCT and GDx VCC. The most steady performers for each imaging device across disc size groups were Cirrus HD-OCT average thickness (CoV, 1.6%), GDx-VCC inferior thickness (CoV, 2.5%), and HRT3 GPS temporal and nasal (CoV, 21.4%).

Conclusions: The diagnostic accuracy of quantitative RNFL assessment as performed by Cirrus HD-OCT and GDx-VCC is high and virtually unaffected or only minimally affected by the size of the optic disc and may provide more consistent diagnostic outcomes across small and large discs than ONH assessment as performed by HRT3.