Clouding of consciousness

"Brain fog" redirects here; not to be confused with Brain fag or Mild cognitive impairment.

Clouding of consciousness, also called brain fog or mental fog,^[1][2] occurs when a person is conscious but slightly less wakeful or aware than normal.^[3] The term "brain fog" is used to represent a subjective condition of perceived cognitive impairment. It is defined as "a phenomenon of fluctuating states of perceived cognitive dysfunction that could have implications in the functional application of cognitive skills in people's participation in daily activities".^[4] They are less aware of time and their surroundings, and find it difficult to pay attention.^[3] People describe this subjective sensation as their mind being "foggy".^[5]

Background

The term clouding of consciousness has always denoted the main pathogenetic feature of delirium since physician Georg Greiner^[6] pioneered the term (Verdunkelung des Bewusstseins) in 1817.^[7] The Diagnostic and Statistical Manual of Mental Disorders (DSM) has historically used the term in its definition of delirium.^[8] The DSM-III-R and the DSM-IV replaced "clouding of consciousness" with "disturbance of consciousness" to make it easier to operationalize, but it is still fundamentally the same thing.^[9] Clouding of consciousness may be less severe than delirium on a spectrum of abnormal consciousness.^[3][10][11] Clouding of consciousness may be synonymous with subsyndromal delirium.^[12]

Subsyndromal delirium differs from normal delirium by being overall less severe, lacking acuteness in onset and duration, having a relatively stable sleep-wake cycle, and having relatively stable motor alterations.^[13] Subsyndromal delirium's significant clinical features are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities.^[13] Delirium's full clinical manifestations may never be reached.^[12] Among intensive care unit patients, subsyndromal subjects were as likely to survive as patients with a Delirium Screening Checklist score of 0, but required extended care at rates greater than 0-scoring patients (although lower rates than those with full ICU delirium)^[12] and had a decreased post-discharge level of functional independence compared to the general population.^[13]

In clinical practice, no standard test is exclusive and specific; therefore, the diagnosis depends on the physician's subjective impression. The DSM-IV-TR instructs clinicians to code subsyndromal delirium presentations under the miscellaneous category "cognitive disorder not otherwise specified".^[14]

Psychopathology

The conceptual model of clouding of consciousness in psychopathology is that of a part of the brain regulating the "overall level" of consciousness, which is responsible for awareness of oneself and of the environment.^[3][15] Various etiologies disturb this regulating part of the brain, which in turn disturbs the "overall level" of consciousness.^[16] This system of a sort of general activation of consciousness is called "arousal" or "wakefulness".^[15]

It is not necessarily accompanied by drowsiness.^[17] Patients may be awake (not sleepy) yet still have a clouded consciousness (disorder of wakefulness).^[18] Paradoxically, affected individuals say that they are "awake but, in another way, not".^[19] Lipowski points out that decreased "wakefulness" as used here is not exactly synonymous with drowsiness. One is a stage on the way to coma, the other on the way to sleep, which is very different.^[20][21]

The affected person has a sensation of mental clouding described in the patient's own words as "foggy".^[5] One patient said, "I thought it became like misty, in some way... the outlines were sort of fuzzy".^[19] Others may describe a "spaced-out" feeling.^[22] Affected people compare their overall experience to that of a dream, because, as in a dream, consciousness, attention, orientation to time and place, perceptions, and awareness are disturbed.^[23] Barbara Schildkrout, a clinical instructor in psychiatry at the Harvard Medical School, described her subjective experience of clouding of consciousness, which she also called "mental fog", after taking a single dose of chlorpheniramine (an antihistamine for her allergy to cottonwood) on a cross-country road trip. She described feeling "out of it" and being in a "dreamy state". She described a sense of not trusting her own judgment and a dulled awareness, not knowing how much time had passed.^[1] Clouding of consciousness is not the same thing as depersonalization, though people affected by both compare their experience to that of a dream. Psychometric tests produce little evidence of a relationship between clouding of consciousness and depersonalization.^[24]

Brain fog may affect performance on virtually any cognitive task.^[1] As one author put it, "It should be apparent that cognition is not possible without a reasonable degree of arousal."^[3] Cognition includes perception, memory, learning, executive functions, language, constructive abilities, voluntary motor control, attention, and mental speed. Brain fog's most significant clinical features are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities.^[13] The extent of the impairment is variable because inattention may impair several cognitive functions. Affected people may complain of forgetfulness, being "confused",^[25] or being "unable to think straight".^[25] Despite the similarities, subsyndromal delirium is not the same thing as mild cognitive impairment; the fundamental difference is that mild cognitive impairment is a dementia-like impairment, which does not involve a disturbance in arousal (wakefulness).^[26]

Drug use and withdrawal

Use of and withdrawal from a number of recreational and prescription drugs has been shown to alter brain cognition and contribute to issues such as memory loss, impaired recall, emotional volatility, mood instability, and altered behavior.^[27] People may misuse substances as a means of self-medication for psychological/neuro-cognitive difficulties, masking symptoms during use periods but leading to a rebound or exacerbation of symptoms in the immediate post-abstinence period.^[28] This may result from the users' dependency on and subsequent restriction from the substance in question, or from neurocognitive impairment / neurodegeneration resulting from use.

Cannabis

The effects of cannabis on neurological function have become better known in recent years, with the overwhelming majority of research finding that cannabis impairs cognitive function, with the duration and frequency of use and quantity and concentration of product consumed correlating with the degree and duration of impairment.^[29][30][31] The immediate short term effects of cannabis use can include "impaired short-term memory and motor coordination, altered judgment, paranoia, and psychosis."^[32] Long term use of the substance can lead to "altered brain development, poor educational outcomes, cognitive impairment, [and] diminished quality of life," as well as an increased risk of suicide.^[32]

The rapid increase in the THC concentration of cannabis products in recent years has led to concern among physicians that it may contribute to adverse health effects among the general population, particularly adolescents and young adults.^{[30][31][33][34][35][36]}

Cannabis use disorder

Main article: Cannabis use disorder

A 40-year-long study in New Zealand followed roughly 1,000 people from age 3 to 45 to measure the impacts of cannabis usage on brain function and functional IQ. It found that long-term cannabis users (those who reported consistent or dependent usage of the drug at age 45 and at least one previous period of sustained heavy usage)^[a] experienced a mean 5.5-point drop in IQ from childhood to adulthood.^[29][37] These cognitive deficits "could not be explained by persistent tobacco, alcohol, or other illicit drug use, childhood socioeconomic status, low childhood self-control, or family history of substance dependence." Comparative groups fared significantly better; non-users of all substances were the only group to experience a positive change in IQ over the length of the study. Cannabis quitters (those with at least one previous instance of diagnosed cannabis dependency who reported no usage at the conclusion of the study) and recreational midlife users (those reporting use between 6 and 51 days per year in their 30s and 40s, with no history of weekly or dependent usage) experienced a smaller but nevertheless comparatively significant drop in IQ. Long-term users were the only group whose performance worsened on every cognitive benchmark assessment administered at the end of the study period.^[37]

Presentation in psychotic disorders

Numerous studies have shown that heavy cannabis users face a markedly higher risk of schizophrenia, bipolar disorder, and transient psychotic episodes than the general population, with up to 50% of those who experienced cannabis-induced psychosis going on to develop schizophrenia.^[28][38][39] But a 2010 meta-analysis of previous research in the Schizophrenia Bulletin showed evidence that those same people have a paradoxically lower risk of neurocognitive difficulties; use of the substance substantially increased the prevalence of positive symptoms, but was simultaneously associated with reduced neurocognitive deficits or, in some cases, boosted cognitive performance (e.g., visual memory, working memory, and executive function).^[38][40]

Heart health and cerebrovascular deficits

Studies published over the past decade have shown that cannabis use (particularly in cases of heavy use or dosage) is associated with significantly increased risk of cardiovascular deficits and/or coronary events in people under age 50, particularly ischemic strokes and myocardial infarctions (heart attacks).^[41][42][43] This effect is especially pronounced in young children.^[44][45] Despite having an immediate short-term impact of lowering blood pressure, cannabis has been associated with hypertension in regular and heavy users.^[36] This, in turn, can restrict the flow of oxygen to the brain, leading to "forgetfulness, trouble with learning, memory and comprehension".^[46] Continued restriction can lead to cerebral small and large vessel disease later in life, which significantly increases the likelihood of dementia onset.^[47]

In diseases

Brain fog is a common symptom in many illnesses where chronic pain is a major component.^[4] Brain fog affects 15% to 40% of those with chronic pain as their major illness.^[48] In such illnesses, pain processing may use up resources, decreasing the brain's ability to think effectively.^[4]

Fibromyalgia

Many people with fibromyalgia experience cognitive problems^[49] (known as "fibrofog" or "brainfog"), which may involve impaired concentration,^{[50][51][unreliable medical source?][52]} problems with short-^[53] and long-term memory, short-term memory consolidation,^[53] working memory,^[54] impaired speed of performance,^[53] inability to multitask, cognitive overload,^[53] and diminished attention span. About 75% of fibromyalgia patients report significant problems with concentration, memory, and multitasking.^[55] A 2018 meta-analysis found that the largest differences between fibromyalgia patients and healthy subjects were for inhibitory control, memory, and processing speed.^[55] Many of these are also common symptoms of ADHD (attention deficit hyperactivity disorder), and studies have linked the two conditions, to the point that a fibromyalgia diagnosis has been proposed as an indication to screen for ADHD.^[56][57][58] It is alternatively hypothesized that the increased pain compromises attention systems, resulting in cognitive problems.^[55]

Chronic fatigue

In chronic fatigue syndrome, also known as myalgic encephalomyelitis, the CDC's recommended criteria for diagnosis^[59] include that one of the following symptoms must be present:^[59]

• Problems with thinking and memory (cognitive dysfunction, sometimes described as "brain fog")
• While standing or sitting upright, lightheadedness, dizziness, weakness, fainting, or vision changes may occur (orthostatic intolerance)

Chronic traumatic encephalopothy

Main article: Chronic traumatic encephalopathy

Lyme disease

Lyme disease's neurologic syndrome, called Lyme encephalopathy, is associated with subtle memory and cognitive difficulties, among other issues.^[60] Lyme can cause a chronic encephalomyelitis that resembles multiple sclerosis. It may be progressive and can involve cognitive impairment, migraines, balance problems, and other symptoms.^{[citation needed]}

Hypothyroidism

Brain fog is a common symptom of hypothyroidism, with a survey showing that 79.2% of those with the condition reported experiencing brain fog to some extent, either selecting the option for "frequently" or "all the time".^[61] While it is unclear how the disease leads to brain fog, proper treatment with levothyroxine has been shown to reduce cognitive impairment.^[62]

Cognitive disengagement syndrome

Cognitive disengagement syndrome was implicated in the expression of brain fog symptoms.^[63]

COVID-19

Patients recovering from COVID-19 have reported experiencing brain fog, which can reflect a wide variety of neurological and psychological symptoms linked to COVID-19.^[64][65] Patient symptoms can persist for months, even years, and are often characterized by fatigue, difficulties with "attention...memory recall, language and executive functioning," as well as depression and anxiety.^[66][67][68] The virus has also been shown to cause encephalopathy, though this is more common in elderly patients and those with pre-existing chronic conditions.^[69] The Alpha and Delta variants of the virus were characterized by severe cognitive impairment at the time of infection, followed by a slow recovery thereafter.^[66]

Research from the UK Biobank in 2022 showed comparative signs of neurodegeneration and cognitive decline amongst a group of 401 middle-aged and elderly COVID-19 patients (importantly, excluding 15 patients who had been hospitalized with the illness over that period) over roughly five months. The study focused mainly on imaging of the limbic system, with evidence that the disruption of the olfactory system played a critical role in the disease's neurodegenerative effects. The researchers found that, in comparison with a control, there were decreases in total brain size, evidence of cognitive decline, "reduction[s] in grey matter thickness and tissue contrast in the orbitofrontal cortex and parahippocampal gyrus", and markers of tissue damage in regions of the brain connected to the primary olfactory cortex.^[70] Over the observation period, those who had tested positive for COVID-19 lost 0.2% to 2% more brain matter than those who had not contracted the illness, the latter equivalent to 10 years worth of normal age-based degeneration.^[69][71][72] COVID-19 patients also showed disrupted contact between different regions of the brain.^[69]

Initially, it was thought that the presentation of brain fog in long COVID shared molecular features with Alzheimer's disease.^[73] This was disproved in 2024 by researchers from Rutgers University. 68% of COVID-19 patients who participated in their study had reported "a decline in thinking or memory", but comparison of cognitive testing scores between patients found similarities between the reported experiences of those with proven cognitive impairment and those whose experience was deemed subjective (i.e greater "fatigue, sleepiness, depression, and anxiety" than long COVID patients who reported no persistent neuro-intellectual symptoms), leading to uncertainty over the effects of long COVID on cognitive performance.^[74] Over two years, only half of patients involved in the study reported improvements in persistent cognitive difficulties.^[75][76] It was found that the persistence of symptoms was linked to auto-immune markers of viral infection, with the neurological changes associated with Alzheimer's not found in long COVID patients.^[75][74] Researchers believe that interferons, markers of a strong autoimmune response to infection, were a key aspect of symptom relief, with those whose autoimmune response was lacking showing few signs of improvement.^[76] Interferon-based therapies and antiviral drugs shown to be successful against the virus were found to be the best way to alleviate symptoms.^[75]

A 2025 study published in Nature found evidence that the SARS-CoV-2 virus was neuroinvasive, and could linger in the brain stem through slow replication regardless of the prevalence of infection in the body. Viral RNA and replicative virus were discovered in the brainstems of golden hamsters 80 days after initial infection. Subjects displayed signs of neurodegeneration, and "altered expression of genes involved in the dopaminergic and glutamatergic synapses, in energy metabolism, and in proteostasis."^[68]

A 2025 Rutgers Health study highlighted the need for greater attention to long COVID symptoms in young children, given their necessarily lesser ability to accurately report and explain their symptoms. Infants and toddlers diagnosed with long COVID were found to experience greater "difficulty sleeping, fussiness, and poor appetite", alongside other physical symptoms, while preschoolers were more likely to experience "daytime tiredness and low energy".^[77]

Mold exposure

Brain fog and other neurological symptoms may also result from mold exposure.^{[78][79][80][81][82]} This may be due to mycotoxin exposure and consequent innate immune system activation and inflammation, including in the central nervous system.^{[83][78][79][80][81][82]} But adverse neurological health effects of mold exposure are controversial due to inadequate research and data, and more research is needed in this area.^{[83][84][85][78][80][82]}

See also

• Psychiatry portal
• Psychology portal
• Medicine portal

• Altered level of consciousness
• Brain fag syndrome
• Cannabis use disorder
• Cognitive orthotics
• Depersonalization-derealization disorder
• Excessive daytime sleepiness
• Four boxes test
• Idiopathic hypersomnia
• Insomnia
• Mental confusion
• Mild cognitive impairment
• Obtundation
• Postural orthostatic tachycardia syndrome (POTS)
• Post-chemotherapy cognitive impairment
• Pumphead syndrome
• Reactive hypoglycemia
• Sleep inertia
• Slow-wave sleep
• Somnolence
• Stupor

Notes

1. Weekly or more (as reported at ages 18, 21, 26, 32, 38, and 45)^[29][37]