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".
Background 
The term clouding of consciousness has
always denoted the main pathogenetic
feature of delirium since Greiner first
pioneered the term in 1817. The
Diagnostic and Statistical Manual of
Mental Disorders 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,
subclinical Wernicke's encephalopathy,
candidiasis, Lyme disease, anaphylaxis,
and intestinal tapeworms. The condition
whereby intestinal faecal toxins bypass
the liver poisoning the brain causing
clouding of consciousness used to be
referred to as "autointoxication" but is
now referred to as "hepatic
encephalopathy". Minimal hepatic
encephalopathy reduces quality of life
by impairing work activities, social
interactions, and driving, but it does
not affect 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
neglect subsyndromal delirium because
they "expect" delirium to be severe and
they prefer to dedicate 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".
Psychopathology 
Neurologists conceptualize clouding of
consciousness in terms 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 yet still have a clouded
consciousness. Paradoxically, sufferers
declare 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 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 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 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.
Unscientific methods 
Practitioners of non-evidence-based
methods 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
or mild cognitive impairment. Not to be
confused, to expession Brain fag is also
a term popularly used these circles, 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.
Chemotherapy 
Chemotherapy can also cause "chemo
brain" or "brain fog". See: PCCI -
Post-chemotherapy cognitive impairment
See also 
Altered level of consciousness
Cognitive orthotics
Coma
Delirium
Depersonalization disorder
Excessive daytime sleepiness
Four boxes test
Idiopathic hypersomnia
Insomnia
Mental confusion
Mild cognitive impairment
Obtundation
Pumphead syndrome
Sleep inertia
Slow-wave sleep
Sluggish cognitive tempo
Somnolence
Stupor
References 
