Study reveals the role of eye movements in Parkinson's disease

 Rapid side-to-side eye movements can help stabilize posture, avoid falls and maintain balance for people with Parkinson's disease, just as they can for healthy people. This seemingly counterintuitive conclusion was reached by researchers at São Paulo State University (UNESP) in Brazil and the University of Lille in France in a study supported by FAPESP. An article on the study is published in the journal Biomechanics.

Ten individuals diagnosed with Parkinson's and 11 neurologically healthy individuals participated in the study. All participants were over 60 and were submitted to tests that involved standing still and trying to maintain balance with their feet side by side and parallel, or one foot in front of the other (tandem stance), while making horizontal or vertical saccadic eye movements, or gazing steadily at a fixed target.

Horizontal saccadic eye movements rather than gaze fixation correlated with significantly reduced body sway for both the healthy participants and those with Parkinson's, regardless of the position of the feet (side-by-side or tandem), while vertical saccadic eye movements correlated with increased body sway for the latter group.

"It's not an automatic strategy to avoid losing balance and falling, since routinely moving the eyes rapidly from side to side is difficult. You can train horizontal saccadic eye movement, but this study was aimed at basic research rather than practical recommendations. We didn't expect people with Parkinson's to be capable of combining these two movements. Our findings furnish new knowledge of the disease, and of its motor and cognitive consequences," said Fabio Barbieri, first author of the article. He heads UNESP's Human Movement Research Laboratory (MOVI-LAB) and runs an outreach project called "Active Parkinson" offering patients physical activities twice a week free of charge on its Bauru campus. 

Unexpected benefit

When the researchers began the study, they thought eye movements could not help Parkinson's patients maintain stability and avoid falls.

Individuals with this disease have a postural deficit that impairs stability and sway control. They also have difficulty controlling their eye movements. Their blinking tends to be slow, and it's hard for them to pick information from the environment."

Fabio Barbieri, São Paulo Research Foundation

Postural improvement correlated with horizontal saccadic eye movements, while body stability worsened in conjunction with vertical saccadic eye movements. "For young adults, vertical saccadic eye movements are beneficial, but for older adults they don't reduce body sway. Although Parkinson's patients can make vertical saccadic eye movements, they can't adapt [by reducing sway] as well as they can to horizontal saccadic eye movements because up-and-down movements are more difficult in general and involve a lower degree of eye rotation, hindering integration between the sensory system [i.e. the eyes] and the postural system [i.e. the body]," said Sérgio Tosi Rodrigues, last author of the article and head of the Laboratory of Information, Vision and Action (LIVIA) at UNESP's Department of Physical Education in Bauru, where he is a full professor.

Tosi Rodrigues has conducted studies on postural stability and saccadic eye movements in different populations, such as healthy young and older adults, diabetics, and multiple sclerosis as well as Parkinson's patients.

The results of previous studies suggest the postural control system receives both visual information from images projected on the retina and inputs from the muscles that move the eyes, which help reduce body sway.

"Besides limitations due to a disease, such as Parkinson's, control of gaze and posture seems to vary with age. Generally speaking, the natural aging process leads to a deterioration of motor control and visual perception, among other alterations. Visual functions perform less well in older than younger people, for example, potentially making older people more susceptible to falls," Tosi Rodrigues said.

According to Barbieri, "combining static balance with vertical saccadic eye movements may be difficult for Parkinson's patients, increasing body sway as a result".

In healthy individuals, vertical and horizontal saccadic eye movements benefited postural stabilization more than fixed gaze. "This happens because postural control is backgrounded. To try to simplify what's complex, there's a change in attention. The subject shifts attention from posture to focus on eye movement, requiring more caution from the brain in order to control posture and stability so as to avoid a fall," Barbieri said.