Individuals who are on their smartphones for extended periods of time may have corneal astigmatism changes and vergence adaptation issues, a new study reports.

The investigation enrolled 29 healthy adults between the ages of 19 and 24 who had normal binocular and accommodative functions. Participants watched a movie for 30 minutes using a smartphone while either walking on a treadmill or sitting in a chair. The researchers measured corneal aberrations and near heterophoria before and after the task.

The results showed that smartphone use for 30 minutes while walking, but not sitting, significantly increased corneal horizontal/vertical (H/V) astigmatism. Compared with baseline, corneal H/V astigmatism after walking and using a smartphone increased by 0.11μm and was 0.10μm higher compared with the sitting group. The change in corneal H/V astigmatism in individuals who sat while watching the movie wasn’t significant, the investigators found.

However, near heterophoria showed a different pattern of vergence adaptation between the two groups. Although smartphone use didn’t cause a notable change in near heterophoria, the vergence adaptation showed an eso change of 0.79 in the walking group. Smartphone use while walking induced against-the-rule corneal astigmatism and shifted the near heterophoria to less exophoric when compared with the sitting group. Of note, these significant optical and binocular vision changes—although small in magnitude—were observed in young participants with normal vision.

“It would be important to investigate whether patients with binocular vision and accommodative anomalies would suffer from similar or even more effects,” the researchers wrote in their paper.