Visual Crowding in Glaucoma: Structural and Functional Relationships

Abstract

Purpose : In the peripheral visual field, objects that are identifiable in isolation become impossible to recognize when surrounded in close proximity by other objects. This phenomenon is known as visual crowding, and the threshold spacing of objects required for correct identification of a target is referred to as critical spacing (S_critical). Visual crowding influences the performance of a wide array of daily visual tasks, notably visual search. Due to the effects of glaucoma on peripheral vision, we hypothesized that neural loss in the disease would lead to stronger effects of visual crowding.

Methods : The study included 44 eyes of 28 patients with glaucoma and 24 eyes of 12 healthy subjects. S_critical thresholds were determined for all four quadrants of the visual field for each eye, at 10 degrees eccentricity; 218 total quadrants were tested. A quadrant was excluded from analysis if the standard automated perimetry (SAP) mean sensitivity (MS) value for that quadrant fell below the 5^thpercentile of normal, as determined from an independent population of healthy eyes. Participants were presented with a target letter, flanked radially by two other letters. The QUEST threshold-seeking algorithm was used to determine S_critical. Each quadrant threshold test required 30 stimulus presentations and took roughly two minutes to complete. We investigated the relationship between S_criticaland measures of structural damage on the macula and retinal nerve fiber layer (RNFL), obtained with spectral-domain optical coherence tomography (SDOCT), as well as SAP MS.

Results : In glaucoma patients, the mean S_criticalwas found to be substantially greater than that of visually normal individuals (4.97±2.41 deg vs. 4.05±1.91 deg; p=0.034). Multivariable regression analysis revealed that S_critical thresholds were a significant predictor of RNFL thickness (p=0.009), ganglion cell-inner plexiform layer thickness (GC+IPL; p=0.045), and SAP 24-2 and 10-2 MS (p=0.016; p=0.043 respectively).

Conclusions : These results suggest that a test of visual crowding may differentiate visually normal individuals from those with glaucoma. Given the significant relationships found between S_critical and GC+IPL / RNFL thickness, the larger S_critical thresholds required for glaucoma patients may be explained by anatomical reorganization of remaining ganglion cells (i.e. larger receptive fields in response to neural damage caused by the disease).

This is a 2020 ARVO Annual Meeting abstract.