Long COVID patients who endure dizziness, headaches and lost senses may have immune cells on the surface of their eyes, research suggests.
Not everyone who overcomes the coronavirus returns to a clean bill of health, with at least one in 10 people enduring lingering complications after supposedly fighting off the infection itself.
Long COVID can be wide ranging, with patients experiencing everything from fatigue and brain fog to palpitations and even organ damage.
With symptoms often vague, and the coronavirus only identified at the end of 2019, scientists are somewhat in the dark on long COVID.
To learn more, medics from Weill Cornell Medicine in Qatar analysed 40 people between one and six months after they overcame a confirmed coronavirus infection, using a questionnaire to reveal any long COVID symptoms.
Results reveal those who endured neurological symptoms four weeks after fighting off the coronavirus itself had a higher number of dendritic cells – which initiate part of the immune response – on the surface of their eyes, compared with people who were never infected.
They also had greater nerve damage to their cornea, the clear outer layer of the eye, the study found, as published in the British Journal of Ophthalmology.
It comes after the US's Biden administration said long COVID could qualify as a disability.
Long COVID has been defined as "signs and symptoms that develop during or following an infection consistent with COVID-19 and which continue for more than four weeks, and are not explained by an alternative diagnosis".
The UK's National Institute for Health and Care Excellence (Nice) recommends using the term "post-COVID syndrome" be used when symptoms persist beyond 12 weeks.
Although it is unclear, the coronavirus may linger in some of the body's tissues, despite being cleared from the airways. The immune system may also over-react to the coronavirus, triggering lasting inflammation that damages otherwise healthy tissues and organs.
Alternatively, long COVID may be caused by damage to small nerve fibres.
To learn more, the medics used a high-resolution imaging laser technique called corneal confocal microscopy. This has previously identified nerve damage and inflammatory changes linked to multiple sclerosis and chronic pain.
The 40 participants completed a Nice questionnaire on long COVID, where they rated the severity of a range of symptoms from zero to 28.
More than half (55%) of the participants endured neurological discomfort for four weeks post-infection.
Corneal scans revealed they had greater nerve fibre damage and higher dendritic cell numbers, compared with the scans of 30 healthy people who had never had the coronavirus.
The long COVID patients who did not endure neurological symptoms had similar numbers of nerve fibres in their cornea as the healthy controls, but more dendritic cells.
Watch: What is long COVID?
The scientists stressed it was an observational study, and therefore does not prove cause and effect. They also pointed out the study's relatively small number of participants and short duration.
Nevertheless, the scientists added: "To the best of our knowledge, this is the first study 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.
"We show patients with long COVID have evidence of small nerve fibre damage, which relates to the severity of long COVID and neuropathic, as well as musculoskeletal, symptoms.
"Corneal confocal microscopy may have clinical utility as a rapid objective ophthalmic test to evaluate patients with long COVID."