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Scientists have identified another possible long-term effect of Covid-19 to add to the growing list: eyesight problems.
A new study, led by a team based out of Linköping University in Sweden, found that even a mild case of the respiratory virus can leave people struggling with their vision months or even years later.
Researchers examined 100 people with eye problems who had contracted Covid-19 but had not been hospitalised. Overall, the group had suffered from eye problems between three months and three years, and a third were on full or part-time sick leave.
Among those affected were people who were unable to continue their studies or go to work because they could no longer read text.
They also reported experiencing a lot of discomfort, sensitivity to light and in many cases severe eye pain and struggled to focus their gaze.
What made the cohort’s situations unusual was that standard tests did not reveal anything wrong with their eyes, meaning that healthcare professionals could not provide an explanation for their symptoms nor a diagnosis.
The researchers compared the affected group with a control group of 32 people, who had also had mild Covid-19 but had not been left with eye problems.
The team instead used two different diagnostic tests and discovered chronic inflammation and deterioration of several eye functions controlled by the nerves in the brain.
The study found changes to nerves in the eye that couldn’t be detected with standard tests
‘We found that the problems experienced by those affected were not detectable by standard tests. We had to perform specialised examinations to detect deviations,’ said lead author Neil Lagali, professor of experimental ophthalmology at Linköping University.
‘The puzzle pieces then fell into place, and we found explanations for the symptoms.’
One of the advanced tests involved proteomics, where a variety of proteins in the tear fluid were analysed.
Upon examination, researchers found an abnormal pattern of proteins that regulate nerves and immune cells which corresponded with those seen in severe and fatal Covid cases.
The other test used measurements from instruments usually found in eye clinics in larger academic hospitals, rather than opticians’ offices.
Many of the people in the study reported that they had become highly sensitive to light which can now be explained because nerve damage means that their pupils now let in too much light.
The findings, published in the journal Nature Communications, explained that this impaired pupil function then led to headaches, difficulty reading text and focusing.
The study concluded that some people have suffered a severe reaction to Covid-19 that has manifested in their eyes, leaving them with long-term inflammation and damage to the nerves that control multiple eye functions.
Nerve damage was also cited as the reason why people were finding that they had become ‘cross-eyed’ where the two eyes don’t work in unison, medically known as strabismus.
The researchers hope that their findings can be rolled out on a larger scale, noting that they only looked at a small sample of Swedish patients.
‘I think it’s very important that the problem is recognised and that we show that it can be measured by objective testing. Patients don’t have access to that today. We’ve studied people in Sweden, but believe that many people around the world are experiencing these problems.
‘These people are really struggling in their daily lives. Now we know what’s wrong with their eyes, and have several clues as to how Covid-19 may have caused these problems.
‘We hope that the findings can lead to effective treatment and that in the long run their problems will ease, but unfortunately we don’t know yet.’
Around 2 million people in the UK are currently estimated to be living with long Covid, according to official Office for National Statistics (ONS) figures released in 2024.
Studies have found it causes changes in the gut, heart and even the brain.
Research published in the journal Brain, Behavior and Immunity found that people with long Covid and ‘clinically meaningful cognitive impairment’ had changes in the thickness in the front part of the brain.
