Clouding of consciousness, also called brain fog or mental fog, occurs when a person is slightly less wakeful or aware than normal. They are not as aware of time or their surroundings and find it difficult to pay attention. People describe this subjective sensation as their mind being "foggy".
The term clouding of consciousness has always denoted the main pathogenetic feature of delirium since physician Georg Greiner pioneered the term (Verdunkelung des Bewusstseins) in 1817. The Diagnostic and Statistical Manual of Mental Disorders (DSM) has historically used the term in its definition of delirium. However, 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. Clouding of consciousness may be less severe than delirium on a spectrum of abnormal consciousness. Clouding of consciousness may be synonymous with subsyndromal delirium.
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. The significant clinical features of subsyndromal delirium are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities. The full clinical manifestations of delirium may never be reached. 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 delirium) or have a decreased post-discharge level of functional independence vs. the general population but still more independence than full delirium.
In clinical practice, there is no standard test that is exclusive and specific; therefore, the diagnosis depends on the subjective impression of the physician. The DSM-IV-TR instructs clinicians to code subsyndromal delirium presentations under the miscellaneous category of "cognitive disorder not otherwise specified".
The conceptual model of clouding of consciousness is that of a part of the brain regulating the "overall level" of the consciousness part of the brain, which is responsible for awareness of oneself and of the environment. Various etiologies disturb this regulating part of the brain, which in turn disturbs the "overall level" of consciousness. This system of a sort of general activation of consciousness is referred to as "arousal" or "wakefulness".
It is not necessarily accompanied by drowsiness, however. Patients may be awake (not sleepy) yet still have a clouded consciousness (disorder of wakefulness). Paradoxically, affected individuals say that they are "awake but, in another way, not". 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.
The affected person experiences a subjective sensation of mental clouding described in the patient's own words as feeling "foggy". One patient described it as "I thought it became like misty, in some way... the outlines were sort of fuzzy". Others may describe a "spaced out" feeling. Affected individuals 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. Barbara Schildkrout, MD, a board-certified psychiatrist and clinical instructor in psychiatry at the Harvard Medical School, described her subjective experience of clouding of consciousness, or what she also called "mental fog", after taking a single dose of chlorpheniramine (an antihistamine for her allergy to cottonwood) while 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 long time went by. Clouding of consciousness is not the same thing as depersonalization even 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.
This may affect performance on virtually any cognitive task. As one author put it, "It should be apparent that cognition is not possible without a reasonable degree of arousal." Cognition includes perception, memory, learning, executive functions, language, constructive abilities, voluntary motor control, attention, and mental speed. The most significant clinical features of brain fog, however, are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities. The extent of the impairment is variable because inattention may impair several cognitive functions. Affected individuals may complain of forgetfulness, being "confused", or being "unable to think straight". 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).
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”. Brain fog is a common symptom in many illnesses where chronic pain is a major component. Brain fog affects 15% to 40% of those with chronic pain as their major illness. In such illnesses, pain processing may use up resources and therefore decrease the brain's ability to think effectively.
Many people with fibromyalgia experience cognitive problems (known as "fibrofog" or "brainfog"), which may involve impaired concentration,[unreliable medical source?] problems with short- and long-term memory, short-term memory consolidation, working memory, impaired speed of performance, inability to multi-task, cognitive overload, and diminished attention span. About 75% of fibromyalgia patients report significant problems with concentration, memory, and multitasking. A 2018 meta-analysis found that the largest differences between fibromyalgia patients and healthy subjects were for inhibitory control, memory, and processing speed. Many of these are also common symptoms of ADHD (attention deficit hyperactivity disorder), and the two conditions have been linked via studies, to the point that a diagnosis of fibromyalgia has been proposed as an indication to also screen for ADHD. It is alternatively hypothesized that the increased pain compromises attention systems, resulting in cognitive problems.
In chronic fatigue syndrome, also known as myalgic encephalomyelitis, the CDC's recommended criteria for diagnosis include that one of the following symptoms must be present:
- 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)
Lyme disease's neurologic syndrome, called Lyme encephalopathy, is associated with subtle memory and cognitive difficulties, among other issues. Lyme can cause a chronic encephalomyelitis that resembles multiple sclerosis. It may be progressive and can involve cognitive impairment, migraines, balance issues, and extensive other issues.
Patients recovering from COVID-19 report experiencing 'brain fog', which can reflect a wide variety of neurological and psychological symptoms linked to COVID-19.
Brain fog and other neurological symptoms may also result from mold exposure. This may be due to mycotoxin exposure and consequent innate immune system activation and inflammation, including in the central nervous system. However, adverse neurological health effects of mold exposure are controversial due to inadequate research and data, and more research is needed in this area.
- Cannabis use disorder
- Cognitive orthotics
- Depersonalization disorder
- Excessive daytime sleepiness
- Four boxes test
- Idiopathic hypersomnia
- Mental confusion
- Mild cognitive impairment
- Postural orthostatic tachycardia syndrome (POTS)
- Post-chemotherapy cognitive impairment
- Pumphead syndrome
- Reactive hypoglycemia
- Sleep inertia
- Slow-wave sleep
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