COVID-19 patients commonly report having headaches, confusion and other neurological symptoms but doctors do not fully understand how the disease targets the brain during infection.

And now, researchers have found severe brain inflammation and injury consistent with reduced blood flow or oxygen to the brain, including neuron damage and death. The findings, published in the journal Nature Communications, are said to be the first comprehensive assessment of neuropathology associated with SARS-CoV-2 infection in an experimental model.

The study also found small bleeds in the brain. Surprisingly, these findings were present in subjects that did not experience severe respiratory disease from the virus. "Because the subjects didn't experience significant respiratory symptoms, no one expected them to have the severity of disease that we found in the brain," said lead investigator Tracy Fischer from Tulane University in the US.

"But the findings were distinct and profound, and undeniably a result of the infection," Fischer added. The researchers said that the findings are also consistent with autopsy studies of people who have died of COVID-19, suggesting that nonhuman primates may serve as an appropriate model, or proxy, for how humans experience the disease.

Neurological complications are often among the first symptoms of SARS-CoV-2 infection and can be the most severe and persistent. They also affect people indiscriminately -- all ages, with and without comorbidities, and with varying degrees of disease severity.