The glaucoma patients who passed the driving test showed increased visual exploration - for example, they made more head and eye movements.
Reporting in the journal Optometry and Vision Science, the team, including Dr. Enkelejda Kasneci of the University of Tübingen in Germany, explains how the drivers they tested increased visual scanning - mostly by moving the head and eyes more frequently - to compensate for loss of visual field due to glaucoma.
Dr. Anthony Adams, editor-in-chief of the journal, notes:
"Like most people, many glaucoma patients with binocular peripheral visual field loss can make useful adaptations for their condition."
Glaucoma is a group of conditions that damage the optic nerve, usually due to a build-up of pressure in the eye. It commonly develops in both eyes, but it can progress more quickly in one than the other.
There are different types of glaucoma, the most common being chronic open-angle glaucoma, which has few symptoms. People who develop it often do not realize it at first because it starts with loss of peripheral vision and slowly works toward the center.
Driving performance compared with head and eye movement
For the new study, the researchers invited participants to undertake a driving test in one of the world's most advanced driving simulators, situated at the Mercedes-Benz Technology Center in Sindelfingen, Germany. The moving-base simulator comprises a real car body, 360-degree visual projection and real acceleration.
- Glaucoma is the second leading cause of blindness in the world
- Estimates suggest over 3 million Americans have glaucoma, only half of whom know they have it
- Blindness from glaucoma is six to eight times more common in black people than white people.
The participants were six people with glaucoma that was causing vision loss in both eyes (binocular) and eight people with normal vision.
The researchers assessed fitness to drive according to the requirements of the official German driving test. They also analyzed other aspects of driving performance, such as lane keeping, steering stability and speed.
During the test, which lasts 40 minutes, the participant meets a variety of traffic conditions and hazards. These include pedestrians suddenly stepping out from behind parked cars or oncoming cars taking risky maneuvers. Failure to deal appropriately with any one of nine hazardous situations results in failing the test.
Assessment of the hazard situations and driving performance according to the German driving test were made by a certified driving instructor, who did not know which drivers had normal vision and which had glaucoma.
The researchers also took measures of the drivers' eye and head movements. They compared these with their performance on the test.
Three of the six participants with glaucoma passed the driving test, and their performance was no different from that of the participants with normal vision.
However, the glaucoma drivers who passed the test showed increased visual exploration - they made more head and eye movements - than the glaucoma drivers who did not pass. The drivers who passed the test also had a tendency for shorter fixation times and more fixations per minute. Dr. Kasneci notes:
"Such behavior indicates an increased scanning activity in glaucoma patients who passed."
Call for 'more individualized assessments of fitness to drive'
In contrast, the glaucoma patients who failed the test showed a tendency to fewer head and eye movements and more straight-ahead eye position, sometimes referred to as "tunnel vision."
The researchers note that the glaucoma patients who passed the test also tended to drive more slowly, giving them more opportunity to scan their surroundings.
They acknowledge that the study has limitations - the small number of participants being the main one. The reason for such a small number is mainly because of the high cost of using the simulator.
Nevertheless, the authors argue that their findings offer new evidence that some glaucoma patients adopt compensating strategies - like increasing visual scanning with more frequent head and eye movements - that help them drive safely despite their binocular vision loss. They conclude:
"This type of compensation improves traffic safety and may have practical implications in planning individualized driving fitness tests and driver rehabilitation programs."
In many European countries, glaucoma patients with binocular visual field defects would be ineligible for a driver's license. In other countries, including the US, they would be granted a license if they passed an on-road test.
Dr. Kasneci and colleagues suggest there should be more individualized assessments that take into account a glaucoma patient's ability to compensate for binocular visual field loss.
While as yet there is no cure for glaucoma, scientists appear to be making progress toward one. For example, in September 2014, Medical News Today reported on an international study that suggests targeting stiff cells that impede fluid drainage in the eye could lead to a cure. That finding is unique because it is the first time researchers have identified a mechanical feature of cells as a cause of the eye disease.