Purpose:
In primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) damage of retinal ganglion cells may continue to the linked optic radiations. We hypothesized that the axonal integrity (i.e. fractional anisotropy [FA]; mean diffusivity [MD]) and demyelination (i.e. radial diffusivity [RD]) of the optic radiation may be associated with impaired visual function in glaucoma and that there are differences between NTG and POAG.
Methods:
In this prospective observational study 24 patients with healthy optic nerve head (mean age 50.9±14.5 yrs), 16 NTG patients (mean age 60.8±12.1 yrs), and 35 POAG patients (mean age 62.2±12.1 yrs) underwent diffusion tensor imaging (DTI) inclusive semi-automatically optic radiation outlining. The spatial-temporal contrast sensitivity was determined by the frequency doubling test (FDT score, normal value ≤ 5). The mean values of the two eyes and two optic radiations were calculated. The patient groups were separated each by their median of the FDT score. The resulting 6 groups were compared using the unpaired t-test.
Results:
In the two control groups (FDT score 0±0 vs. 1.8±2.6, p=0.001) age and DTI parameters did not differ significantly. Same results were found for the two NTG groups (FDT score 1.6±2.3 vs. 22.1±16.9, p=0.001). In the two POAG groups (FDT score 4.6±4.6 vs. 32.2±11.1, p<0.001) FA in the optic radiation was significantly different (p=0.029), but not MD, RD, or age. The NTG and POAG groups with normal/ borderline FDT scores (1.6±2.3 vs. 4.6±4.6, p=0.077) did not differ in age or DTI parameters. In contrast, the NTG and POAG groups with pathological FDT scores (22.1±16.9 vs. 32.2±11.1, p=0.122) showed differences of FA (p=0.002), MD (p=0.036), and RD (p=0.007), but not of age. The POAG patients had the worse DTI values in the optic radiation.
Conclusions:
Despite comparably advanced impairment of the spatial-temporal contrast sensitivity the extent of damaged axonal integrity/ demyelination of the optic radiation is different between NTG and POAG patients. POAG patients show the stronger optic radiation damage. If impairment of the spatial-temporal contrast sensitivity is normal/ borderline this difference is not verifiable.
Keywords: imaging/image analysis: clinical • contrast sensitivity • visual impairment: neuro-ophthalmological disease