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Clouding of consciousness, also known as brain fog or mental fog, is a term used in conventional medicine denoting an abnormality in the "regulation" of the "overall level" of consciousness that is mild and less severe than a delirium. The sufferer experiences a subjective sensation of mental clouding described as feeling "foggy".
The term clouding of consciousness has always denoted the main pathogenetic feature of delirium since Greiner first pioneered the term (Verdunkelung des Bewusstseins) in 1817. The DSM has historically used the term in its definition of delirium. Recently the DSM has replaced “clouding” with “disturbance” but it is still the same thing. Confusingly, however, there appears to be a trend among many doctors to now redefine clouding of consciousness to be less severe than delirium on a spectrum of abnormal consciousness. In this case, it can be said that clouding of consciousness is 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. Subsyndromal patients are likely to survive but require institutionalization or have a decreased post-discharge level of functional independence.
It is featured in such conditions as minimal hepatic encephalopathy (also known as subclinical hepatic encephalopathy or latent hepatic encephalopathy), subclinical Wernicke's encephalopathy, candidiasis, Lyme disease, anaphylaxis, and intestinal tapeworms. Minimal hepatic encephalopathy reduces quality of life by impairing work activities, social interactions, and driving, but it does not effect basic daily life activities such as dressing, personal hygiene, eating, shopping, answering the phone, or taking public transportation. Patients with MHE may even exhibit normal cognitive performances, but overall productivity may suffer from inattentiveness and fatigue secondary to attention abnormalities.
Conventional doctors understudy and neglected subsyndromal delirium because they "expect" delirium to be severe and they prefer to dedicated their medical resources to managing the more immediate "life-threatening" problems. They have a tendency to "psychologize" it and misdiagnose it as depression or apathy. In fact, most clinicians believe that minimal hepatic encephalopathy is "irrelevant" in spite of some evidence indicating that the diagnosis may be important. 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".
Neurologists conceptualize clouding of consciousness in terms of a part of the brain regulating the "overall level" of the consciousness part of the brain 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, sufferers declare that they are "awake but, in another way, not". Lipowski points out that "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 sufferer experiences a subjective sensation of mental clouding described in the patient's own words as feeling "foggy". One sufferer 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. Sufferers compare their overall experience to that of a dream because like 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 the antihistamine chlorpheniramine for her cottonwood allergy 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 both sufferers 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, 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. Sufferers 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).
Alternative medicine practitioners popularly use the term brain fog. It is not always clear whether they mean mental fog also known as clouding of consciousness or perhaps something else like feelings of depersonalization, mild cognitive impairment, or Brain fag. The difference between the word "brain" and the word "mental" is perhaps trivial just like the difference between the word "clouding" and the word "fog" is perhaps trivial. However, the difference between the word "fog" and the word "fag" is definitely not trivial. Brain fag is also a term popularly used in alternative medicine, but it is a culture-bound syndrome characterized by complaints of the brain being "fatigued" and is not the same thing as brain fog or mental fog.
Doctors have long said that autointoxication causes clouding of consciousness and that this autointoxication is what is responsible for hepatic encephalopathy. Ironically, modern conventional doctors label autointoxication itself as quackery while still maintaining that intestinal faecal toxins can poison the brain and cause hepatic encephalopathy (clouding of consciousness) if the liver fails to detoxify it. Intestinal candidiasis (also known as candida-related complex or the yeast syndrome) causing "brain fog" in alternative medicine parallels candidiasis causing clouding of consciousness in conventional medicine. Some alternative medicine practitioners say that mucoid plaque can indirectly cause "brain fog" by providing a haven for autointoxication causing bacteria and candida yeast as well as by acting as a reservoir for the toxins they produce. Yet other alternative medicine practitioners say that ongoing caffeine anaphylaxis can cause clouding of consciousness.