A recent review of 49 brain imaging studies reveals that COVID-19 is associated with widespread structural and functional changes in the human brain. The findings indicate that the virus affects regions responsible for memory, emotion, and executive function, which may help explain the neurological symptoms many patients experience. The research was published in the journal Cerebral Cortex.
Following the initial outbreak of the coronavirus, many patients began reporting enduring neurological issues, such as brain fog, fatigue, and memory loss. A team of scientists led by researchers Li Chen, Huan Lan, and Wenxiong Liu synthesized data from existing medical studies to build a comprehensive picture of how the virus impacts the central nervous system. They focused on research utilizing magnetic resonance imaging, a technology that uses strong magnetic fields to generate detailed maps of the brain’s internal anatomy and activity.
To evaluate the extent of the damage, the researchers looked at different types of brain tissue. Gray matter consists of the brain’s neuron cell bodies, which process information, while white matter contains the nerve fibers that connect these processing centers. Functional imaging techniques measure blood flow or oxygen levels to track how different brain areas communicate in real time.
The research team conducted a systematic review, gathering 49 previously published studies that compared the brain scans of people who had contracted COVID-19 with those of healthy individuals. These studies included patients in various stages of the disease, ranging from acute infection to long-term recovery. Some of the individual studies were small, involving as few as 10 participants, while others evaluated more than 200 patients.
By pooling the results, the researchers identified consistent patterns of abnormalities across several key brain regions. The frontal lobe, which handles decision-making and cognitive control, frequently showed structural changes. The temporal and parietal lobes, areas involved in sensory processing and attention, also exhibited noticeable differences in patients who had recovered from the virus.
Structural changes in the frontal lobe might explain the loss of top-down cognitive control observed in some patients. The researchers note that the prefrontal cortex is highly sensitive to psychosocial stress. This means the stress of the pandemic itself, alongside the biological infection, might contribute to these structural shifts.
The temporal lobe includes areas like Heschl’s gyrus, a region that processes auditory information. Patients recovering from COVID-19 often experience difficulties processing sound, which can contribute to chronic fatigue as the brain works harder during daily listening activities. The parietal lobe, a region involved in visual and spatial attention, also showed structural changes linked to impaired attention allocation.
In many of these regions, COVID-19 patients displayed a reduction in overall gray matter volume and a thinning of the outer cortical layer. This tissue loss could result from reduced oxygen supply or severe immune system inflammation during the infection. Conversely, a few studies reported localized increases in gray matter volume. The researchers suggest this could represent temporary swelling caused by vascular injury, or it could be the brain’s attempt to compensate for damaged tissue by growing new neural connections.
The review highlighted extensive structural issues in the brain’s white matter. To measure this, scientists track the microscopic diffusion of water molecules along nerve fibers. In COVID-19 patients, this water movement was often abnormal, indicating that the protective coating around the nerve fibers had degraded. These microscopic changes were present even in patients who experienced only mild to moderate respiratory symptoms and had otherwise normal-looking gray matter on standard medical scans.
Functional imaging scans revealed altered patterns of spontaneous brain activity. When patients were simply resting in the scanner, their brains showed abnormal synchronization between different regions. The limbic system, a network of deep brain structures including the insula, hippocampus, and amygdala, frequently exhibited connectivity issues. Because these areas regulate emotion and memory, functional disruptions here often correlated with clinical symptoms like anxiety, depression, and post-traumatic stress.
When patients were asked to perform working memory tasks during their scans, their brains displayed altered activation patterns. This suggests that the nervous system had to reorganize its resources to maintain normal cognitive performance. Other functional scans focused on the olfactory network, the brain regions responsible for processing smell. Patients suffering from a persistent loss of smell showed disrupted connectivity in this specific network, which also correlated with lower scores on short-term verbal memory tests.
Tests measuring cerebral blood flow found lower than normal circulation in several brain areas, including the frontal and temporal lobes. This restricted blood supply limits the delivery of oxygen and nutrients, which might contribute to difficulties with attention, language processing, and executive function. The review found that deep subcortical nuclei, such as the thalamus, were particularly vulnerable to this reduced blood flow. These deep relay centers have high metabolic demands and rely on adjacent blood vessels that lack secondary backup circulation.
The reviewed studies rely heavily on cross-sectional data, meaning they capture a single snapshot of the brain after infection rather than tracking changes over an extended period. Because the vast majority of these studies lack pre-infection baseline brain scans, it is difficult to prove definitively that COVID-19 directly caused all the observed changes. Individual biological differences present before the pandemic might account for some of the variations in brain structure and function.
The researchers point out that the clinical status of the patients varied widely across the studies. Grouping together individuals with acute infections and those experiencing long-term recovery makes it challenging to isolate how the brain heals over time. Some studies also focused only on specific, predefined brain regions rather than scanning the entire brain. This targeted approach can artificially inflate the statistical differences between patients and healthy controls.
Future research utilizing unbiased, whole-brain analyses and long-term tracking could map the specific trajectory of these neurological changes. Additional follow-up investigations are required to clarify whether COVID-19-related brain alterations are permanent, or if they are reversible with therapeutic interventions and time.
The study, “Widespread structural and functional brain alterations in COVID-19: a systematic review of MRI studies,” was authored by Li Chen, Huan Lan, Wenxiong Liu, Chao Zuo, Graham J. Kemp, Song Wang, Qiyong Gong, and Xueling Suo.
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