Clouding of consciousness

Clouding of consciousness, also called brain fog or mental fog, occurs when a person is slightly less wakeful or aware than normal. They are less aware of time and their surroundings, and find it difficult to pay attention. People describe this subjective sensation as their mind being "foggy".

Background
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. 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. Subsyndromal delirium's significant clinical features are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities. Delirium's full clinical manifestations 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, 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".

Psychopathology
The conceptual model of clouding of consciousness 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. 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 called "arousal" or "wakefulness".

It is not necessarily accompanied by drowsiness. 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 has a sensation of mental clouding described in the patient's own words as "foggy". One patient said, "I thought it became like misty, in some way... the outlines were sort of fuzzy". Others may describe a "spaced-out" feeling. 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. 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. 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.

Brain fog 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. Brain fog's most significant clinical features 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 people 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).

In diseases
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, decreasing the brain's ability to think effectively.

Many people with fibromyalgia experience cognitive problems (known as "fibrofog" or "brainfog"), which may involve impaired concentration,  problems with short - and long-term memory, short-term memory consolidation, working memory, impaired speed of performance, inability to multitask, 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 studies have linked the two conditions, to the point that a fibromyalgia diagnosis has been proposed as an indication to 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 problems, and other symptoms.

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". While it is unclear how the disease leads to brain fog, proper treatment with levothyroxine has been shown to reduce cognitive impairment.

The emerging concept of sluggish cognitive tempo has also been implicated in the expression of brain fog symptoms.

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. But adverse neurological health effects of mold exposure are controversial due to inadequate research and data, and more research is needed in this area.