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COVID-19 Brain Fog May Be Caused By Bone Marrow Cells Invading The Brain


Ben Taub

Freelance Writer

clockFeb 25 2021, 15:33 UTC
Brain Fog

We may finally know what causes brain fog in COVID patients. Image: fran_kie/

Brain fog is a form of delirium reported by large numbers of people infected with SARS-CoV-2, often lingering for a period of time after the viral infection has been overcome. Scientists are still unable to explain the cause of this worrying symptom, although new research suggests that it may arise as a result of bone marrow cells migrating to the brain, obstructing blood flow within the cerebral cortex.

Most commonly experienced by older COVID-19 patients, brain fog is characterized by a sense of disorientation, impaired awareness, and general sluggishness. Surprisingly, however, autopsies performed on those who died after experiencing such symptoms have revealed no chronic inflammation in the brain or neural changes typically associated with viral infection.


In an attempt to solve the brain fog riddle, researchers from Johns Hopkins University and Brigham and Women’s Hospital in Boston analyzed brain tissue from 15 deceased COVID-19 patients, most of whom had displayed some form of neurological symptomology. Using a microscope, the study authors observed large cell nuclei lurking within the cortical capillaries of five of these patients.

Revealing their findings in the journal JAMA Neurology, the researchers explain that these nuclei resembled the structure of megakaryocytes, which are large bone marrow cells that produce blood platelets. Immunohistology then confirmed the presence of certain markers associated with megakaryocytes, corroborating the presence of these misplaced cells within the brains of deceased COVID-19 patients.

Although these bone marrow cells were only found in five of the 15 patients, the researchers point out that autopsies were conducted on small portions of each individual’s brain, which means the true prevalence of megakaryocytes may well be much larger than was observed.

Autopsies were also conducted on the brains of two people who had not contracted COVID-19, neither of which were found to contain megakaryocytes.


According to the authors, megakaryocytes have never previously been found in blood vessels in the brain, and it is not known how they ended up there in this instance. Nonetheless, their appearance may well explain COVID-19-related brain fog, as these large cells are likely to have restricted the flow of blood through capillaries in the cerebral cortex, resulting in impaired cognition.

“We suspect that SARS-CoV-2 damages lung tissue, leading to the release of chemical signals that induce the megakaryocytes to travel there from the bone marrow,” explained study author David Nauen in a statement. “When that happens, these large cells somehow find a way to pass through the lung capillaries and get to the brain.”

“We don’t yet know if the megakaryocytes we found in the brain are just the result of blood flow carrying them there or if a specific change occurs in the brain vessels that trap them,” he added.

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