A new population-based study establishes normative ranges for interocular differences in retinal nerve fiber layer (RNFL) thickness and cup-to-disc ratio (CDR) using optical coherence tomography (OCT). The findings, published recently in Translational Vision Science & Technology, are expected to support clinicians in identifying early signs of unilateral or asymmetric glaucoma.
The cross-sectional study analyzed data from 522 participants in the Framingham Heart Study cohort. The mean age was 74.5 years (SD, 6.9), with the majority being female (59.4%) and white (88.3%). RNFL thickness and CDR values were collected for both eyes, and differences were calculated by subtracting left eye measurements from right eye measurements.
The mean peripapillary RNFL thickness was 88.1 µm in the right eye and 87.7 µm in the left eye. The resulting mean interocular difference of 0.4 µm was not statistically significant (SD, 6.1; P=.18). However, the 2.5th and 97.5th percentile values for interocular RNFL difference spanned from −12.7 µm to 12.7 µm, suggesting that differences within this range may be considered physiologically normal.
Similar percentile ranges were calculated for CDR. The interocular difference in average CDR had a 95% central range from −0.19 to 0.21 units.
Multivariable linear regression analysis was used to explore factors associated with RNFL asymmetry. The most significant predictors were differences in rim area (β=8.06/mm²; P<.001) and differences in signal strength (β=1.37/unit; P<.001). These findings suggest that image quality and structural optic nerve head differences may influence perceived asymmetry in RNFL measurements.
The authors, led by Louay Almidani, MD, MSc, of the Wilmer Eye Institute at Johns Hopkins University School of Medicine in Baltimore, noted that previous normative datasets used to assess bilateral symmetry in OCT metrics were limited by small sample sizes or restricted to device manufacturer data. By leveraging the Framingham Heart Study—a long-standing epidemiologic cohort—the current study provides population-based benchmarks that may improve clinical interpretation of asymmetry in RNFL and CDR.
The study’s findings may be especially useful in cases where a clinician suspects early or asymmetric glaucomatous damage but lacks sufficient historical data to make a definitive diagnosis, the researchers noted. OCT-based asymmetry, when used in conjunction with other indicators such as rim area and image quality, may guide decisions regarding further diagnostic evaluation or closer follow-up. The study also highlights the importance of considering technical artifacts, such as differences in signal strength, when interpreting OCT data. The significant association between signal strength and interocular RNFL difference underscores the need to verify that both eyes are imaged under comparable conditions.
According to the authors, defining a 95% central range of interocular RNFL and CDR differences may help reduce overdiagnosis or misclassification, especially in borderline cases. The results should not be used in isolation but rather incorporated into a comprehensive assessment of optic nerve health. GP