This article is more than 5 years old. Information may no longer be current.
Visual crowding affects glaucoma patients’ daily tasks
Visual crowding significantly affects patients with glaucoma and may have a high impact on daily tasks such as driving, reading and identifying objects, according to a study.
Visual crowding is a natural visual phenomenon that occurs when objects are too close together to be individually identified and are perceived as jumbled, the authors explained. Critical spacing (s-critical) is the minimum distance beyond which objects can no longer be individually perceived.
The study investigated the effects of glaucomatous damage on visual crowding and included 13 subjects with glaucomatous visual field loss and 13 healthy controls. Assessment of visual crowding was based on the quantification of the s-critical at the periphery of each quadrant of the field of view. The target was a letter T, oriented up or down and surrounded by distractors (the letter H) at varying distances from the target, on an LCD screen in a darkened room. Subjects were asked to fixate a central point and press a key on a computer keyboard when they saw the target letter. Each subject underwent a total of 600 trials.
Glaucoma patients had a significantly greater s-critical as compared with healthy subjects, indicating a significantly worse crowding effect. A significant association was found between s-critical and retinal nerve fiber layer thickness on OCT in each quadrant, suggesting that the magnitude of crowding effect is strongly associated with the amount of nerve tissue loss. No significant correlation was found between s-critical and visual acuity: When presented in isolation, the target letters were correctly identified, indicating that crowding has a more limiting impact on vision than visual acuity.
“As we live in a cluttered world, crowding limits the ability to perform many daily tasks, such as finding car keys on a cluttered desk,” the authors wrote.
It might also impact safety when driving, “since hazards that could be clearly detected in isolation may not be recognized and acted upon if presented in a cluttered situation,” they pointed out.
Tests on visual crowding may be critical for the understanding of how glaucoma patients are affected in their daily life and may provide a clinical means for the diagnosis and monitoring of neural loss in glaucoma, they concluded. – by Michela Cimberle
Disclosure: The authors reported no relevant financial disclosures.
Perspective
Back to Top
It is frequently said that in glaucoma, structural change (retinal ganglion cell death) precedes functional loss (visual field deterioration), making the structure-function correlation in early disease relatively (and frustratingly) weak. However, it is more correct to say that the detection of structural change precedes the detection of functional loss, a discrepancy that arises from our means of assessing the disease, not the disease itself. The logarithmic (decibel) scale of visual field analysis masks functional loss in early disease and amplifies it in advanced disease. Conversely, the linear measurement of retinal nerve fiber layer thickness is very sensitive to initial change but reaches a floor at approximately 50 microns.
Ogata and colleagues have demonstrated a strong structure-function correlation through the use of an alternate measure of function, visual crowding. Interestingly, while their glaucomatous cohort was characterized as having mild or undetectable visual field loss, their average mean deviation of -3 dB equates to a 50% loss of retinal ganglion cells.
In showing that visual crowding effects, which may impact critical activities of daily living including driving, are significantly worse in patients with early glaucoma, this investigation emphasizes the importance of timely detection and reminds us that early glaucoma really isn’t early at all.