The Definition of “Brain Fog” is Clouded by a Lack of Consensus.


By Ronessa Dass & Dr. Tara Packham

What is Brain Fog?

Brain fog is a phenomenon often reported by people with post-acute challenges resulting from SARS-CoV-2 infection (long COVID), with estimates ranging from 5 to 33% (1–4). Long COVID occurs when symptoms of COVID-19 persist past a typical period of time and are not explained by other previous conditions (1). Brain fog is the most common side effect of long COVID and more seriously affects women, older adults, and racialized minorities (3,5–7). Symptoms of brain fog in long COVID have been reported to last approximately four months; however, symptoms can last for years (8). Brain fog currently has no definition and is instead identified by a broad range of fluctuating symptoms including, mental fatigue, memory impairment, word finding issues, and slowed cognitive processing speed (1,7–10). Brain fog may also impact physical performance: albeit, this has not been extensively investigated (11,12). The exact mechanism of brain fog in long COVID remains unknown, partially due to difficulties in recruiting participants for studies and challenges in measuring its associated outcomes (1,2,5). Most theories suggest that inflammation in the brain and spinal cord may play a large role; however, some studies posit that this is not sufficient to produce symptoms of brain fog and another mechanism may be involved (13). For example, environmental stressors, sleep impairments, and mental health co-morbidities may aggravate symptoms of brain fog (6,14). The phenomena of brain fog remains poorly understood and, to date, no definition or key constructs of brain fog have been established.


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What challenges do individuals experiencing brain fog face?

Recent studies investigating brain fog in people with long COVID demonstrate brain fog’s impact on cognitive and emotional health. Persons experiencing brain fog reported feelings of depression and frustration as they have increased difficulty performing everyday tasks such as formulating sentences or remembering events (4,11,13). The inability to perform these tasks can have a cumulative effect on cognitive performance, and affect an individuals’ ability to work (4,11,15–17). Such challenges can lead to feelings of incompetence and may also decrease an individuals’ self-confidence and perception of self (4,11,15).

Brain fog has been linked to physical challenges. Improved physical health may aid in mitigating symptoms of brain fog (7,12), although, individuals experiencing brain fog find physical activities (e.g., exercise) challenging, partially due to fatigue and/or muscle weakness (11,18).

Brain fog is seen as an ‘invisible impairment’, and is poorly recognized or understood by the general public and researchers (11). This contributes to societal stigma, and may deter individuals from participating in social activities (17), leading to self-isolation and loss of social support (3,6,11,16,19). Persons experiencing brain fog may be reluctant to seek help from their healthcare professionals out of fear of being misunderstood or not taken seriously (3). Combined, the complex challenges associated with brain fog have been linked to higher rates of mental health comorbidities in persons with long COVID and brain fog (4).


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Brain fog in the field of rehabilitation

Brain fog produces profound challenges in multiple areas of one’s life. A clear definition and standardized assessment of brain fog would have three overall benefits to the field of rehabilitation: (1) helping healthcare professionals provide optimal care to persons experiencing brain fog, (2) facilitating research on brain fog, and (3) improving public awareness of brain fog.

Implications for healthcare professionals

Detailed and comprehensive assessments are useful in providing information regarding an individual’s unique experiences and to provide personalized treatments (16). Brain fog affects multiple aspects of one’s overall health; thus, long-term interdisciplinary rehabilitation may be necessary (20). Further, patient/client education is a key part of interventions used across rehabilitation disciplines (21). Brain fog can create issues with education-based interventions, as individuals may have increased challenges with attention and memory (1,7–10). Exercise is another important component of rehabilitation treatment, which may also be hindered by symptoms of brain fog (11,18). Therefore, rehabilitation healthcare professionals (e.g., occupational therapists, speech-language pathologist, and psychologists.) must carefully adapt their approaches to account for the obstacles in the client’s understanding and treatment participation (11,22). To summarize, both a comprehensive and standardized definition will support healthcare professionals in providing effective care by minimizing the cumulative cognitive burden of rehabilitation interventions (16). This need has been further supported by Bosch and Turkstra, a new study which investigated personalized strategies targeting cognitive symptoms experienced by persons with long COVID (23).

Further, due to the complex nature of brain fog and the overlap with other cognitive and fatigue disorders, its clinical importance may be undermined by healthcare professionals (3). For instance, some cognitive researchers and psychologists believe brain fog is a distinct experience (3,8,11), while others argue that it is simply a subset of cognitive dysfunction (3,7). Due to a lack of consensus, brain fog is not an official diagnosis, and many individuals may be incorrectly categorized as experiencing, or not experiencing, brain fog. This demonstrates the role that rehabilitation researchers must play in brain fog research.

Implications for researchers

Validating outcome measurements is a crucial step in improving the quality and overall confidence in research findings to help researchers understand the mechanisms of brain fog (14,24).

Currently, the effects of brain fog in long COVID are inconsistent across studies, partially due to the wide variety of assessments used (4,8,13). Many studies employ a binary measurement of brain fog, by asking whether or not a particular trait (e.g., cloudiness) is present or absent (1). This is limiting as brain fog may manifest differently in individuals and is a heterogenous experience that cannot be accurately captured by binary measurements (1,8–10,16). Other studies have used existing measures of cognition to assess brain fog; however, these may not be designed to measure its symptoms (25). For example, a study investigating the effectiveness of the Montreal Cognitive Assessment (MoCA) tool for screening cognitive impairments in COVID patients found the questions did not detect the cognitive changes reported by participants (25).

Research validating assessments is necessary to design and test the effectiveness of interventions that will help individuals overcome the challenges associated with brain fog (14,24). To illustrate, the true efficacy of a cognitive behavioral therapy intervention or an exercise intervention would be uncertain without the use of a comprehensive assessment (7–10,14,24). If an assessment is not sensitive enough, the changes produced by the treatment (e.g., improved memory) may not be accurately represented (7–10,14,24). Similarly, if using a binary assessment, the perceived changes may be too subjective (7–10,14,24).


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Public awareness

Establishing a better understanding of brain fog will increase awareness in the general public and decrease societal stigma. Increasing awareness is important to help individuals understand their symptoms and share them with their healthcare professionals. This is helpful for optimizing collaborative and informed care, increasing autonomy, and sense of control over their condition (26). Public discourses normalizing brain fog can help individuals feel more comfortable participating in their daily routines and meaningful activities, as they may feel less judged or misunderstood by others (17).

“Brain fog is the most common side effect of long COVID and more seriously affects women, older adults, and racialized minorities.”


Although the concept of ‘brain fog’ has garnered increasing interest since it began appearing in the public discourse through surges of COVID-19 cases, it is not a new phenomenon and has been described across numerous conditions (27). Consensus regarding this complex phenomenon is long overdue. A clear definition of brain fog and standard assessments will aid in improving the quality of research, and correspondingly the quality of life of persons experiencing brain fog.


Featured illustration by Virginia (Ginny) Chiu for rehabINK.

To refer to this article, it can be cited as:

Dass R & Packham T. The definition of ‘brain fog’ is clouded by a lack of consensus. rehabINK. 2023:Issue#14. Available from: https://rehabinkmag.c


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