Patients with the most severe post-Covid smell and taste disorders are likely to perform worst in cognitive tests, especially when these involve memory, finds a study strengthening the hypothesis that loss of smell could be an early sign of the impending onset of Alzheimer's disease.
In the study, a group of Brazilian researchers analysed clinical data from 701 patients hospitalised for moderate or severe Covid-19.The results, reported in European Archives of Psychiatry and Clinical Neuroscience, showed a moderate or severe reduced sense of taste was the most frequently reported sensory complication (20 per cent), followed by a moderate or severe olfactory deficit (18 per cent), a moderate or severe deficit of both smell and taste (11 per cent), and parosmia (9 per cent) or distortion of olfactory perception, so that a previously enjoyed smell becomes unpleasant, for example.
Olfactory hallucinations (perceiving smells of which others were unaware) were reported by 12 of the participants, and gustatory hallucinations (perceiving tastes without eating anything) by nine. In both cases, most said the hallucinations occurred only post-Covid.
People reporting parosmia (smell loss associated with Covid) were found to have more memory problems than the rest, while those with a moderate or severe loss of taste performed significantly worse. Those who reported moderate or severe loss of both smell and taste were also found to have significantly impaired episodic memory.
"We didn't find any psychiatric symptoms (such as anxiety or depression) to be associated with loss of smell and taste, but as expected we observed that attention and episodic memory were more impaired in patients with more chemosensory alterations," said Rodolfo Damiano, doctoral candidate at from the University of Sao Paulo's Medical School (FM-USP) in Brazil.
"This finding corroborates the hypothesis that Covid affects cognition, and that damage in this area doesn't only have psychosocial or environmental causes."Covid-related loss of smell derives from the inflammation triggered by SARS-CoV-2 in the olfactory mucosa, the team said.
The cognitive losses observed in Alzheimer's and post-Covid syndromes derive from different pathogenic processes, but these may overlap, explained psychogeriatrician Orestes Forlenza, Professor in FM-USP's Psychiatry Department.
"This is particularly the case in older people who already have primary cognitive symptoms and contract Covid. There is preliminary evidence that this overlapping of pathogenic factors may accelerate or intensify the progression of cognitive losses," he said.
However, the exact mechanism leading to cognitive damage in Covid patients is unknown."Our hypothesis is that the virus causes neuroinflammation, which leads to cognitive impairment. We don't yet know if the damage is permanent. We'll continue to follow the patients to see if these complications clear up," Damiano said.