Brain Fog & Further Accompanying Symptoms / Subtypes

7. Brain fog & further accompanying symptoms / subtypes

Many people affected by Long COVID, ME/CFS and POTS suffer not only from fatigue and post-exertional malaise (PEM) but from a whole cluster of accompanying symptoms. These often occur together and can be grouped into overlapping subtypes (neuro-cognitive, autonomic/cardiovascular, immune-inflammatory, pain-dominant, gastrointestinal). The subtype grouping is a pragmatic ordering scheme, not an established, validated classification. The connecting hallmark symptom remains PEM; several of the phenomena described here are at the same time considered possible modifiable PEM triggers and are thus relevant for pacing.

Important note: this section serves as information and does not replace medical advice, diagnosis or treatment. All medications and therapies named are to be used only with medical guidance. No dosages are deliberately given and no promises of a cure are made.

Brain fog (cognitive dysfunction)

"Brain fog" refers to problems with memory, attention, word-finding and executive function. The largest study so far (Hampshire et al., NEJM 2024, around 113,000 participants with online cognition tests) made measurable differences visible: those who recovered with early-resolving symptoms showed an equivalent of about −3 IQ points, people with persisting symptoms (Long COVID, over 12 weeks) about −6, and intensive-care courses about −9 points [1][2]. Assessment: these are small group-average effects, not an individual diagnosis or a measure of personal intelligence. The study was cross-sectional; people whose symptoms had already resolved had smaller losses. A true time course ("normalisation") cannot be directly proven from this – but the data suggest that brain fog in many affected people recedes as symptoms improve and is not necessarily permanent. As possible mechanisms, neuroinflammation with activation of microglia/glial cells, viral persistence, disturbances of the endothelium and blood-brain barrier, and – much more disputed – fibrin microclots ("microclots") are discussed; the microclot hypothesis has so far not been independently confirmed and is judged controversially. Imaging and spectroscopic methods (among them PET-MRI, 7-tesla MR spectroscopy) investigate these processes; peripheral blood inflammatory markers do not reliably reflect what happens in the brain [18][19]. These findings are promising, but are not to be regarded as causally established.

Mast cell activation (MCAS)

In a proportion of those affected, symptoms compatible with mast cell activation are found (flushing, itching, hives, gastrointestinal and circulatory complaints, food/stimulus intolerances). In a case-control study (Weinstock et al. 2021, n=136 Long COVID, mostly women) such symptoms were significantly increased after SARS-CoV-2 infection and resembled those of untreated MCAS patients; before infection, those affected and controls barely differed [3]. Important: the study recorded symptom questionnaires, not a laboratory-confirmed MCAS diagnosis – whether this is a true MCAS in the sense of the diagnostic criteria is open. Reviews discuss activated mast cells with increased mediator/cytokine release [4]. Therapy (doctor only): as symptomatic options, H1/H2 antihistamines and mast-cell stabilisers are discussed; the principle is blocking or stabilising mast-cell-mediated reactions. H1/H2 blockers are in part available over the counter (OTC); their use against Long COVID-associated mast-cell symptoms, however, is off-label and rests on small, uncontrolled studies (weak evidence). Medical assessment is required before any use (also to distinguish other causes and because of possible side effects/interactions). This is expressly not a dosing, self-treatment or healing instruction.

Small-fibre neuropathy (SFN)

Damage to thin nerve fibres can cause burning pain, abnormal sensations and autonomic disturbances. A skin-biopsy-based case-control study (published 2024) describes newly occurring SFN after COVID; a large part of those affected reported PEM. Patients treated with intravenous immunoglobulins (IVIG) responded more often in this small series (reported as 9/9 vs. 3/7, p=0.02) [5][6][7]. IVIG is an immunomodulating therapy; its use in post-COVID SFN is off-label, cost-intensive, associated with relevant side-effect potential and, given the very small case numbers (low evidence level, ~Class III), only preliminarily supported. It belongs exclusively in specialised medical hands. The widespread claim that up to ~50 % of chronic pain/fibromyalgia cases are based on an SFN comes from the fibromyalgia field; the transfer to Long COVID is an extrapolation and not reliably quantified.

Autonomic dysfunction, POTS and HRV

Orthostatic intolerance and POTS occur frequently together with mast-cell symptoms and SFN (described in the literature as the "POTS-MCAS-SFN triad"), with autonomic small-fibre damage as a hypothetical common denominator – a plausible but unproven construct. One possible measurement parameter is heart rate variability (HRV): studies show reduced nocturnal parasympathetic (vagal) activity and relative sympathetic dominance (e.g. Sci Rep 2023, n=103) [13][14]. Wearable-based HRV monitoring is being trialled to estimate individual exertion/PEM thresholds in pacing; this is so far experimental and not clinically validated [15].

Unrefreshing sleep

Despite sufficient sleep duration, those affected wake up unrefreshed. This "unrefreshing sleep" is a core criterion of ME/CFS diagnostics (IOM 2015 / SEID) [9] and very common in Long COVID; EEG studies point to an altered sleep microstructure and links to a disturbed circadian rhythm [10].

Hormonal / HPA-axis changes

In Long COVID, some studies show a flattened salivary/serum cortisol daily rhythm or a lowered morning cortisol value (in one cohort n≈96 the morning median was markedly lower than in controls) – a pattern that has also long been described in ME/CFS [16]. The figures named are study-specific and are not to be understood as a generally valid reference. Important: a cortisol substitution cannot be derived from this; the findings are at most markers, not a treatment indication. Taking cortisone/cortisol on one's own can be harmful.

Sensory overload

Hypersensitivity to light, noise and smells forms, in a factor analysis (Front Neurol 2025, n≈2,313 ME/CFS + ~299 PASC), its own sensory-perceptual factor (high factor loadings for light, noise and smell) [17]. Sensory overload can favour PEM and should be taken into account in pacing.

Gastrointestinal complaints

Nausea, diarrhoea, abdominal pain and irritable-bowel-like complaints persist in a substantial proportion. As possible mechanisms, viral persistence in the gut, dysbiosis/microbiome changes and chronic mucosal inflammation are discussed [11][12]; causal evidence is still lacking.

Overlap and assessment

Long COVID, ME/CFS, fibromyalgia and irritable bowel syndrome overlap strongly (pain, fatigue, unrefreshing sleep, cognitive disturbance, stimulus hypersensitivity) and presumably share mechanisms such as central sensitisation and autonomic/small-fibre dysfunction; the diagnostic distinction remains difficult [8]. Evidence note: many of the subtype findings named here come from small to medium-sized, in part uncontrolled studies (mostly low evidence level, ~Class III). Large controlled cohorts and uniform case definitions are still missing – the results are promising but preliminary and must be assessed by a doctor in the individual case.