Factors Affecting Interocular Differences in Retinal Nerve Fiber Layer Thickness

I have read, with great interest, the recent article regarding glaucoma diagnostic ability of interocular differences in circumpapillary retinal nerve fiber layer (RNFL) thickness derived from optical coherence tomography (OCT) temporal-superior-nasal-inferior-temporal (TSNIT) maps.¹ This study reported that comparisons of TSNIT RNFL thickness curves between eyes have the best diagnostic capability to detect photographic RNFL defects. Therefore, the authors concluded that interocular comparisons of TSNIT RNFL thickness curves may be useful for detecting glaucoma. 

I would like to address two points regarding this conclusion. According to my clinical observations, there is a significant interocular asymmetry in RNFL thickness among healthy eyes. When interocular differences in RNFL thickness of 1234 healthy eyes of 617 subjects were analyzed, the right eye had a thinner RNFL than the left eye in the superior quadrant. Additionally, the right eye had a thicker RNFL than the left eye in all other quadrants examined.² Other studies also have reported similar findings.^3–5 Thus, when evaluating interocular differences, the laterality of the eye should be considered. 

Another important factor in assessing the TSNIT RNFL thickness curve is the location of the RNFL peak. In general, circumpapillary RNFL thickness has two peaks: one in the inferior-temporal area and one in the superior-temporal area. When the location of the RNFL peak is physiologically asymmetric between eyes, it may be misinterpreted as an abnormal finding. My unpublished data shows that healthy eyes have nine types of normal interocular differences in RNFL thickness peak location asymmetry, as investigated in 600 healthy eyes of 300 subjects (Fig.; Table). Although 51.8% of eyes had symmetric RNFL peak locations, the remaining eyes had RNFL peak location asymmetries. As shown in Figures D and F, RNFL peak location asymmetry in the inferior area may be misinterpreted as an abnormal finding. Park et al.¹ also commented that in patients with different RNFL thickness profiles between eyes, using interocular differences in the TSNIT curve may be limited. Therefore, when assessing the interocular RNFL thickness differences, physiologic asymmetry in RNFL peak location should be considered.1552 

Figure

 

The nine types of interocular differences in the RNFL thickness peak locations (right eye, solid vertical lines; left eye, dotted vertical lines). Subjects were classified into nine subgroups (A–I) based on RNFL peak location differences between eyes (right eye − left eye). If the interocular difference in RNFL peak location was within 1 SD of the mean, it was expressed as a single vertical line. If the RNFL peak location was lower or higher than 1 SD from the mean, it was expressed as separate vertical lines.

Table

 

Classification of the Interocular Difference in RNFL Peak Location (Right Eye − Left Eye) Based on the Mean ± SD Range