dear editor, A high rate of ‘brain fog’or cognitive impairment in post-coronavirus disease 2019 (post-CoVid-19) patients, greater than 50% in all investigations, has been reported by numerous studies so that cognitive impairment was more frequent in critically ill patients with CoVid-19 [1, 2]. the etiology of ‘brain fog’or cognitive impairment is not yet clear. however, the proposed etiologies are discussed. Since the hippocampal distribution of microglial activation has been interrelated to virus-induced cognitive impairment [3], this finding may explain why some CoVid-19 survivors show neuropsychiatric symptoms, such as memory disturbances, somnolence, fatigue and insomnia [1]. in this regard, long-standing cognitive impairment and neurodegeneration, with associated hippocampal atrophy [4], have been previously shown to cause difficulties in systemic inflammation associated with severe sepsis [5]. this may also be the case in acute respiratory distress syndrome (ArdS), a common clinical observation in CoVid-19 patients, as it is also associated with cognitive impairment and neurodegeneration [6]. hypoxia or vascular injury can clarify cognitive dysfunction, particularly in those CoVid-19 cases with severe acute illness demanding intensive care and/or respiratory help [1]. endothelial dysfunction, which causes microvascular damage, has also been proposed [7]. Nevertheless, patients with mild CoVid-19 also experience cognitive complaints [1]. in this respect, other aetiologies, including immunological dysregulation, chronic inflammation and dysfunction of peripheral organs, have been proposed [1]. other investigations have coupled cognitive complaints with anxiety and depression [8].

Additionally, the cerebral white matter, which is vital for cognitive function, is principally vulnerable to ischaemic injury caused by CoVid-19 [9]. there is growing evidence that cerebral hypoperfusion increases the speed of amyloid-beta (Aβ) accumulation and is coupled with tau and tdP-43 pathology [9].