Visual crowding occurs when objects that can be recognized in isolation are unrecognizable in clutter. The phenomenon can limit peripheral vision and can explain performance in a broad array of daily tasks. A recent study observed pronounced visual crowding in glaucoma patients, even in the presence of only mild visual field loss on standard perimetry. Also, the magnitude of the crowding effect was significantly correlated to the amount of nerve tissue loss as measured by optical coherence tomography (OCT).

In 13 glaucoma patients and 13 controls, researchers measured mean sensitivity with standard automated perimetry (SAP) and retinal nerve fiber layer (RNFL) thickness with OCT. To collect visual crowding measurements, participants had to discriminate the orientation of a target letter (T) when presented with surrounding flankers (the letter H) at varying distances from the target during the experiment. The distance at which flankers degraded the performance in recognizing the target was considered the critical spacing. Glaucomatous eyes had significantly greater (i.e., worse) critical spacing values than controls.

Researchers found that the critical spacing values were significantly associated with RNFL thickness measurements but not with mean sensitivity. They believe this finding suggests that assessing visual crowding may provide a sensitive measure of visual function that corresponds well to the degree of neural tissue loss in glaucoma, even before substantial visual field loss is apparent on SAP.

The researchers also suggest doctors should quantify crowding magnitude in glaucoma patients for tasks that impact safety (e.g., driving) and quality of life (e.g., reading speed). They conclude that the development and validation of a psychophysical test that could quickly assess visual crowding in glaucomatous patients could be a useful tool to assess functional performance in this population.