In this study, we evaluated the presence of cognitive dysfunction in patients with COVID-19 reporting cognitive complaints that persisted after the acute phase. We found a lower than expected performance in several cognitive tests, which is consistent with the existence of cognitive dysfunction in this subgroup of COVID-19 patients. Two different procedures support these findings. On the one hand, a battery of standard neuropsychological tests administered by a trained neuropsychologist in person and using normative data from a large multicentre normative available in our country. On the other hand, a computerized battery by comparison with a matched healthy control group recruited in our centre. In this regard, several cognitive tasks were impaired two, three or four times more than expected in healthy controls. Similarly, statistically significant differences were observed in many tests in comparison with healthy controls. These findings confirm that patients reporting cognitive complaints after COVID-19 actually showed lower performance on cognitive testing.
Another remarkable finding was the analysis of the specific cognitive tests impaired. COVID-19 patients showed a diminished performance on several tests evaluating the following cognitive functions. First, attention and executive function, with alterations in processing speed (SDMT, TMT-A), divided attention (TMT-B), selective attention (WAF and Stroop), visual vigilance, intrinsic alertness, working memory (span, N-back), and inhibition (Inhibition test, Stroop). Second, episodic memory (FCSRT, FGT). And third, visuospatial processing (JLO, VOSP, visual tasks of WAF battery).
Importantly, effect sizes were generally small for most cognitive tests. This finding suggests that, on average, the magnitude of cognitive deficits was generally small. However, considering that this performance is detected in young patients, it could have a high socio-economic impact.
One of the most interesting findings of our study is the low correlation of cognitive tests with neuropsychiatric scales. Although according to previous studies the prevalence of anxiety and depression in our sample was also important [2], scores in depression and anxiety questionnaires did not significantly correlate with cognitive performance. This suggests that depression and anxiety do not explain cognitive findings in these patients and supports that the cognitive disorder is not secondary to psychological aspects. As expected, depression was weakly correlated with some attentional tasks, especially the N-back, which has been previously suggested as a cognitive signature in depressed patients [20]. Most of the patients included in our study did not require hospitalization, and ICU admission was performed in 10% of cases, suggesting that cognitive complaints also occurred in patients with mild forms of acute COVID-19.
Olfactory dysfunction was correlated with cognitive performance, as has been recently suggested using the MoCA test [21]. Specifically, the inhibition test was the most highly correlated test with olfactory dysfunction and was the only one showing a moderate effect size. This might suggest that both hyposmia and inhibition could share pathophysiological mechanisms. In this regard, both olfactory dysfunction and inhibition impairment have been associated with orbitofrontal damage in other disorders [22–23]. This finding is worthy of investigating in future studies with neuroimaging correlations.
Our study has some limitations. First, we only enrolled patients reporting cognitive complaints after COVID-19 without any previous potential cause of cognitive dysfunction. Hence, our findings are limited to these patients and not to all patients with COVID-19. Second, we did not have previous neuropsychological assessments of patients enrolled in this study. Consequently, it is not possible to draw definitive conclusions about a causal relationship between COVID-19 and cognitive dysfunction. However, we tried to reduce the impact of this limitation using strict inclusion and exclusion criteria. In addition, we used two methods of analysis from two independent control groups (comparison with a healthy control group and use of normative data at country level) with consistent findings. Third, we did not examine potential associations between cognitive deficits and clinical, demographic, or neuroimaging characteristics. Future studies with larger samples are necessary to evaluate these features, which are essential to understand the pathophysiology of cognitive dysfunction in COVID-19 patients [24]. Patients in our study were evaluated 9.42 ± 3.54 months after COVID-19 onset of symptoms, which excluded patients in an acute confusional state and it hints that cognitive dysfunction may be detected several months after the acute stage.
In conclusion, our study shows that patients with COVID-19 reporting cognitive symptoms showed a reduced cognitive performance, especially in the attention-concentration and executive functioning, episodic memory, and visuospatial processing domains. Cognitive performance was correlated with olfactory dysfunction and sleep quality to a lesser extent, but not anxiety or depression. Self-perceived cognitive functions were correlated with both cognitive performance and mood. Future studies combining cognitive assessment with a multimodal evaluation (such as neuroimaging, immunological measurements, serum or CSF biomarkers) and longitudinal follow-up are necessary to disentangle the specific mechanisms associated with COVID-19 cognitive dysfunction.