E3 PreliminaryPreliminaryPEM not requiredCross-SectionalPeer-reviewedReviewed
What is brain fog?
McWhirter, Laura, Smyth, Heather, Hoeritzauer, Ingrid et al. · Journal of neurology, neurosurgery, and psychiatry · 2023 · DOI
Quick Summary
This study looked at how people describe 'brain fog' by analyzing online discussions on Reddit. Researchers found that brain fog involves many different experiences—trouble remembering things, difficulty concentrating, feeling disconnected from reality, thinking slowly, trouble communicating, and fatigue. Brain fog appeared in discussions about many conditions, with long COVID being the most commonly mentioned cause.
Why It Matters
For ME/CFS patients and researchers, this study validates that cognitive symptoms are complex, multifaceted experiences beyond simple 'fogginess'—including dissociation, effort dysregulation, and concentration problems. Understanding the specific nature of these cognitive complaints helps clinicians better assess and communicate about these debilitating symptoms and may guide investigation of underlying pathophysiological mechanisms relevant to post-viral conditions like ME/CFS.
Observed Findings
- Brain fog descriptions included seven main categories: forgetfulness (51 posts), concentration difficulty (43), dissociative phenomena (34), cognitive slowness/excessive effort (26), communication difficulties (22), fuzziness/pressure (10), and fatigue (9).
- 50% of posts came from illness-related subreddits, with long COVID cited in 10% of posts with causal attributions (60/570).
- Brain fog appeared across diverse contexts including COVID-19, psychiatric disorders, neurodevelopmental conditions, autoimmune diseases, functional disorders, drug use/discontinuation, and other behavioral contexts.
- The term 'brain fog' is used heterogeneously to describe disparate cognitive and dissociative experiences rather than a single unified symptom.
Inferred Conclusions
- Brain fog represents a heterogeneous set of experiences including dissociation, fatigue, forgetfulness, and excessive cognitive effort rather than a single symptom.
- Cognitive complaints occur across multiple disease categories and health contexts, suggesting shared pathophysiological mechanisms may underlie brain fog across different conditions.
- Encouraging detailed first-person descriptions of specific cognitive experiences (rather than using the umbrella term 'brain fog') will facilitate better understanding of underlying pathophysiological mechanisms.
Remaining Questions
- What are the specific pathophysiological mechanisms underlying each category of cognitive symptom (dissociation vs. forgetfulness vs. slowness)?
What This Study Does Not Prove
This study does not establish causation or compare the prevalence of brain fog across different conditions. Social media data represents self-selected populations and may not reflect the full spectrum of experiences; the causal attributions reported by Reddit users (e.g., to long COVID) are user-generated claims, not verified clinical diagnoses. Cross-sectional design cannot determine temporal relationships between conditions and cognitive symptoms.
Tags
Symptom:Cognitive DysfunctionFatigue
Phenotype:Long COVID Overlap
Method Flag:No ControlsExploratory OnlyMixed Cohort
Metadata
- DOI
- 10.1136/jnnp-2022-329683
- PMID
- 36600580
- Review status
- Editor reviewed
- Evidence level
- Early hypothesis, preprint, editorial, or weak support
- Last updated
- 12 April 2026
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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