The reason why people with long Covid can suffer from “brain fog” has been discovered by Irish scientists.

The breakthrough has profound importance for the understanding of brain fog and cognitive decline seen in some patients with the condition, according researchers at Trinity College Dublin.

It brings the possibility of new treatments for the condition, but also for other neurodegenerative illnesses such as multiple sclerosis (MS), they said.

The research, published in Nature Neuroscience on Thursday, shows disruption to the integrity of blood vessels in the brains of patients suffering from long Covid and brain fog.

From forgetfulness to difficulties concentrating, many people who have long Covid experience “brain fog”. Now researchers say the symptom could be down to the blood-brain barrier becoming leaky.

The barrier controls which substances or materials enter and exit the brain. “It’s all about regulating a balance of material in blood compared to brain,” said Prof Matthew Campbell, co-author of the research at Trinity College Dublin.

“If that is off balance then it can drive changes in neural function and if this happens in brain regions that allow for memory consolidation/storage then it can wreak havoc.”

Writing in the journal Nature Neuroscience, Campbell and colleagues report how they analysed serum and plasma samples from 76 patients who were hospitalised with Covid in March or April 2020, as well 25 people before the pandemic.