RNFL loss of 17 percent identified as “tipping point” for detecting functional deficits

The authors conducted this study to identify the “tipping point” in glaucoma where functional deficits on visual field testing correlate with retinal nerve fiber layer (RNFL) loss as detected on spectral domain optical coherence tomography (SD-OCT). They prospectively examined glaucoma patients and healthy controls, and report in the January issue of the British Journal of Ophthalmology that there was an average NFL loss of 17 percent significantly associated with visual field loss. This information is valuable for examining glaucoma and glaucoma-suspect patients, and may help identify those with pre-perimetric glaucoma and assist with patient management.

The study included 40 glaucoma patients and 72 control subjects (112 eyes). They underwent visual field examinations (Humphrey field analyzer) and SD-OCT optic disc cube scans (Cirrus HD-OCT).

Comparison of global mean and sectoral RNFL thicknesses with visual field threshold values showed a plateau of threshold values at high RNFL thicknesses and a sharp decrease at lower RNFL thicknesses. The authors fit global and sectoral data to a “broken stick” statistical model to estimate the RNFL thickness “tipping point” where visual field threshold values become associated with the structural measurements. They computed the slope for the association between structure and function for data above and below the tipping point.

They found that the mean RNFL thickness threshold for visual field loss was 75.3 µm, reflecting a 17.3 percent RNFL thickness loss from age-matched normative value. Above the tipping point, the slope for RNFL thickness and threshold value was 0.03 dB/µm. Below the tipping point, it was 0.28 dB/µm. The difference between the slopes was statistically significant (P < 0.001). A similar pattern was observed for quadrant and clock-hour analysis.

This amount of NFL loss is not conserved in all quadrants. No "tipping point" was found in the nasal quadrant. Nevertheless, this study identifies the actual tipping point as detected by current SD-OCT analysis, whereas other studies that have examined structure versus function have hypothesized about its presence. However, the value identified in the current study is only a general benchmark and there may be variations between individuals, especially between those in different racial groups or with differing NFL thickness at baseline.

Based upon the results of the current study, the authors conclude that substantial structural loss of approximately 17 percent appears to be necessary for functional loss to be detectable using current testing methods.