COVID-19 is associated with persistent cognitive deficits, including the acceleration of Alzheimer's disease symptoms, researchers have found.

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In addition to the respiratory and gastrointestinal symptoms that accompany COVID-19, many people with the virus experience short- and long-term neuropsychiatric symptoms, including loss of smell and taste, and cognitive and attention deficits, known as "brain fog." Initial findings from Greece and Argentina by an international, multidisciplinary consortium suggest that older adults frequently suffer cognitive impairment, including persistent lack of smell, after recovery from SARS-CoV-2 infection. The consortium includes scientific leaders, including the Alzheimer's Association and representatives from nearly 40 countries -- with technical guidance from WHO -- to evaluate the long-term consequences of COVID-19 on the central nervous system.

The findings were presented at The Alzheimer's Association International Conference 2021, held between July 26-30 in Denver, US. Other key findings by the consortium suggest that biological markers of brain injury, neuroinflammation and Alzheimer's correlate strongly with the presence of neurological symptoms in COVID-19 patients. Individuals experiencing cognitive decline post-COVID-19 infection were more likely to have low blood oxygen following brief physical exertion as well as poor overall physical condition, the researchers said. "These new data point to disturbing trends showing COVID-19 infections leading to lasting cognitive impairment and even Alzheimer's symptoms," said Heather M Snyder, Alzheimer's Association vice president of medical and scientific relations.

Gabriel de Erausquin of the University of Texas Health Science Center along with colleagues from the Alzheimer's Association-led global SARS-CoV-2 consortium studied cognition and olfactory senses in a cohort of nearly 300 older adults from Argentina who had COVID-19. Participants were studied between three and six months after COVID-19 infection.

More than half showed persistent problems with forgetfulness, and roughly one in four had additional problems with cognition including language and executive dysfunction, the researchers said. These difficulties were associated with persistent problems in smell function, but not with the severity of the original COVID-19 disease, they said. "We're starting to see clear connections between COVID-19 and problems with cognition months after infection," Erausquin said. Certain biological markers in blood are indicators of injury in the brain, neuroinflammation and Alzheimer's disease. To study the presence of these blood biomarkers, neurodegeneration and neuroinflammation in older patients who were hospitalised with COVID-19, Thomas Wisniewski, a professor at New York University, and colleagues took plasma samples from 310 patients with COVID-19. 

Of the patients, 158 were positive for SARS-CoV-2 with neurological symptoms and 152 were positive for SARS-CoV-2 without neurologic symptoms. The most common neurological symptom was confusion due to toxic-metabolic encephalopathy (TME). In patients who were initially cognitively normal with and without TME related to COVID-19 infection, the researchers found higher levels of blood biomarkers in COVID-19 patients with TME compared to COVID-19 patients without TME. "These findings suggest that patients who had COVID-19 may have an acceleration of Alzheimer's-related symptoms and pathology," Wisniewski said.

"However, more longitudinal research is needed to study how these biomarkers impact cognition in individuals who had COVID-19 in the long term," he said. George Vavougios, a postdoctoral researcher for the University of Thessaly (UTH), and colleagues studied cognitive impairment and related health measures in 32 previously hospitalised mild to moderate COVID-19 patients two months after discharge from the hospital. Among them, 56.2 per cent presented with cognitive decline.

Short-term memory impairments and multi-domain impairment without short-term memory deficits were the predominant patterns of cognitive impairment, they noted. Worse cognitive test scores correlated with higher age, waist circumference and waist-to-hip ratio. After adjusting for age and sex, worse memory and thinking scores were independently associated with lower levels of oxygen saturation during the 6-minute walk test, which is commonly used to assess the functional capacity of people with cardiopulmonary disease. "A brain deprived of oxygen is not healthy, and persistent deprivation may very well contribute to cognitive difficulties," Vavougios added.