The loss of nerve fibers and a rise in immune cells on the cornea, or surface of the eye, may be an identifying signal for long COVID-19. A new study suggests that scanning the cornea could help identify COVID-19 long haulers ― those who suffer symptoms for months after infection.

Researchers found that changes to the cornea, the transparent part of the eye that covers the pupil, iris and fluid-filled interior, were particularly evident in patients who suffered neurological symptoms, such as loss of taste and smell, headache, dizziness, numbness, and neuropathic pain, after contracting COVID-19.

As many as one-third of COVID-19 victims develop lingering symptoms, according to The New York Times. These can range from chest pain and fatigue to heart irregularities. Some victims are unable to return to work and may require long-term medical care.

According to Medical Xpress, scientists in the eye study used a non-invasive laser imaging technique called corneal confocal microscopy, or CCM, to evaluate the corneas of 40 people who had recovered from acute COVID-19. They found that patients who reported lingering neurological symptoms had greater corneal nerve damage and loss and higher dendritic (immune) cells than individuals who didn’t have COVID-19.

“To the best of our knowledge, this is the first study of its kind reporting corneal nerve loss and an increase in dendritic cell density in patients who have recovered from COVID-19, especially in subjects with persisting symptoms consistent with long COVID-19,” said the researchers, led by first author Dr. Gulfidan Bitirgen, of the department of ophthalmology at the Necmettin Erbakan University Meram Medical Faculty Hospital in Konya, Turkey. Their study was published in the British Journal of Ophthalmology.

According to Science Alert, while this was a small study, and an observational one at that, the results still add to the growing body of evidence on how COVID-19 may contribute to long-term neurological and neuropathic problems.

The study also revealed that patients with more severe cases of COVID-19 had greater corneal nerve damage, so the abnormalities noted by the researchers may correlate to how the disease presents in different patients.

While the researchers acknowledge that larger studies need to be conducted to investigate the actual cause and effect of COVID-19 on the cornea, they point out that their study is another indication of how our eye health is inexorably linked to our overall health, which is why techniques like CCM shows promise in future diagnostic uses.

“Corneal confocal microscopy may have clinical utility as a rapid objective ophthalmic test to evaluate patients with long COVID—19,” the researchers said, according to Science Alert.