Abstract
Purpose: :
To determine whether subjects with smaller optic disc area have a different structure-function (SF) relationship than those with larger optic disc area.
Methods: :
Glaucoma patients and suspects presenting to the clinic with a reliable visual field within one year, visual acuity of 20/60 or better and no retinal or neuro-ophthalmological disease that can potentially affect the visual field were invited to participate in the study. Three optic disc cubes (Cirrus HD-OCT, Carl Zeiss Meditec Dublin) were attempted in all eligible eyes from study participants. Average peripapillary retinal nerve fiber layer thickness (RNFL) from the scan with the highest quality was selected to represent structure for each eye. Mean deviation from 24-2 Sita Standard visual fields (Humphrey, Carl Zeiss Meditec Dublin) was used to represent function. Optic disc size was categorized into small and large by the sample median. Mixed models with linear splines were then fit to determine whether smaller optic discs modify the relationship between SF.
Results: :
Eighty-two eyes from 49 patients met eligibility criteria. Subjects were on average 69 years-old (SD=14.4), 65% being female. Small optic discs were on average 1.55mm2 (Range=1.23-1.81) and larger optic discs were on average 2.19mm2 (Range= 1.83-3.60).The larger and smaller optic disc patients were comparable with respect to proportion of females (60% vs 73%, chi2 p=0.18), mean age (68.0 vs 67.5 years, p=0.86), visual field mean deviation (-7.9 vs -8.2 dB, p=0.88), average peripapillary rnfl thickness (67.8 vs 72.1 um,p=0.19) and axial length (22.8 vs 23.1 mm, p=0.2).A model fitted with linear splines for RNFL thickness allowed for different slopes above and below 75 um. For RNFL thicknesses below 75 um, a 10um lower RNFL thickness was associated with a 7.44 (SE=1.19) and 6.00 (SE=0.98) dB lower MD for larger and smaller discs respectively (p value for difference in slopes = 0.33)For RNFL thicknesses above 75um, a 10 um lower RNFL was associated to a 1.57 (SE=2.4) dB higher MD in larger discs, while in smaller discs a 10 um lower RNFL was associated to a 2.56 (SE=1.30) dB lower MD. (p value for difference in slopes = 0.14)
Conclusions: :
The SF relationship in this group of glaucoma patients appeared steeper and less variable below than above 75 um of RNFL irrespective of disc size. Smaller disc size did not reach statistical significance as an effect modifier in the structure-function relationship tested.
Keywords: imaging/image analysis: clinical • visual fields • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)