Abstract
Purpose: :
Even though a variety of functional tests are used to assess and follow visual loss in glaucoma, standard automated perimetry (SAP) remains the most widely used method. Fixation stability is important for the accuracy of SAP techniques which rely on the assumption that foveal fixation is maintained. We attempt to determine the fixation stability during microperimetry in eyes with glaucomatous scotomas splitting fixation but with residual foveal central vision.
Methods: :
Fifteen eyes of fifteen adult subjects with glaucoma were evaluated with confocal scanning laser opthalmoscopy-microperimeter (SLO-MP; Opko/OTI, Toronto, Canada). All eyes had split fixation and visual acuity (VA) of 20/30 or better. Eyes with retinal diseases were excluded. Patient fixation was evaluated during microperimetry with real-time tracking of the eye through registration of the blood vessels and automatic compensation for movement. Scatter plots of fixation positions were generated and bivariate contour ellipse area (BCEA values were calculated from raw fixation data. A smaller BCEA represents more precise fixation whereas a larger BCEA represents less stable fixation.
Results: :
In our patients (n=15 eyes), the BCEA, the elliptical area containing the eye’s fixation position 68% of the time, ranged from 391.4 minarc2 to 11368 minarc2, compared to a range of 100 min to 650 minarc2 in normal populations described elsewhere. Five patients had normal BCEA (mean 402±84.4 minarc2), 4 had intermediate (mean 1565±490 minarc2), and 6 had markedly abnormal (mean 9,132±2890 minarc2).Of our 15 patients, 11 had eccentric BCEA and 4 had central BCEA. Interestingly, we found no correlation between centricity of the BCEA and fixation stability.
Conclusions: :
The computerized fixation control when performing microperimetry with the SLO-MP allows precise determination of fixation stability in glaucoma patients. Our results suggest that fixation stability is compromised in some patients with advanced glaucoma even when the VA is foveal (20/30 or better). In these patients, fixation is not always foveal, nevertheless the SAP will still be mapped as though fixation were in the center of the field, and tested points will be shifted relative to their true retinal locations.