For years, "long COVID brain fog" has been a real, widely-reported experience without a clear biological explanation — which made it easy to dismiss as vague or psychological. A study published in October 2025 changed that, by directly imaging what's actually different in the brains of people experiencing it.
What the researchers actually measured
A team led by Fujimoto and colleagues used a specialized PET imaging technique ([11C]K-2 tracer) to directly visualize the density of AMPA receptors — a type of receptor central to how neurons communicate — in the living brains of 30 long COVID patients with cognitive impairment, compared against 80 healthy volunteers.1
What they found
Long COVID patients showed widespread increases in AMPA receptor density throughout the brain. Critically, the degree of receptor increase tracked closely with how severe a person's cognitive symptoms were — this wasn't a diffuse, unrelated finding, it lined up with the actual brain fog people were reporting. Higher inflammatory markers were also associated with higher receptor density, pointing toward neuroinflammation as a plausible driver.
The degree of AMPA receptor increase tracked with symptom severity — a measurable biological finding lining up with what patients were actually reporting, not a diffuse, unrelated result.
Accurate enough to be a real diagnostic candidate
The imaging approach distinguished long COVID patients from healthy controls with 100% sensitivity and 91.2% specificity in this study — numbers strong enough to suggest this could become an actual diagnostic tool, not just a research finding, pending replication in larger and more diverse samples.
Why this matters beyond the mechanism itself
For a long time, "brain fog" without an obvious lab abnormality has been hard for patients to get taken seriously about, precisely because there wasn't a visible biological marker. This study is a real step toward establishing long COVID brain fog as a measurable, physical condition — with a specific receptor system as a potential treatment target for future drug development, not just a symptom to describe and wait out.
What this doesn't mean, yet
This is one study, with a modest sample size, and it hasn't been replicated at scale. It doesn't yet translate into an available clinical test or an approved treatment — AMPA receptor-targeting is a research direction, not a current therapy. If you're experiencing persistent cognitive symptoms after a COVID infection, the honest next step is a conversation with your doctor about what's currently available for evaluation and management, not waiting for this specific research to become clinical practice.
References
- Fujimoto Y, et al. Systemic increase of AMPA receptors associated with cognitive impairment of long COVID. Brain Communications, Vol 7, Issue 5, October 2025. academic.oup.com
This article is for informational purposes only and has not been evaluated by the FDA. It is not intended to diagnose, treat, cure, or prevent any disease, including long COVID. If you're experiencing persistent cognitive symptoms after a COVID infection, speak with a healthcare provider for evaluation.
Related reading: What Actually Causes Brain Fog? and Grief and Your Brain: What the Research on "Grief Brain" Shows
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