Clouded consciousness is a term used in medicine to denote a milder and less severe impairment of the organization of the general level of consciousness than delirium. A patient with clouded consciousness experiences a subjective sense of mental clouding, described as "foggy." The term clouded consciousness has referred to the same basic pathophysiological features of delirium since it was proposed by the physician George Greiner in 1817. Historically, the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) has used the term in its definition of delirium. However, both the DSM-III and DSM-IV replaced the term "clouded consciousness" with "disorder of consciousness" to make it easier to understand, but it is essentially the same thing. Clouded consciousness on the spectrum of disorder of consciousness may be less severe than delirium, and clouded consciousness may be synonymous with borderline delirium. Borderline delirium differs from ordinary delirium in that it is generally less severe; it lacks the severity of onset and duration, and the sleep-wake cycle and motor changes are relatively stable. The clinical features of borderline delirium include inattention, thought process distortions, comprehension distortions, and language distortions, but the clinical features do not reach full-blown delirium. Patients with borderline delirium have similar survival rates in intensive care units to those with a score of 0 on the delirium scale, but they require more care than patients with a score of 0. Patients with borderline delirium have lower rates of functional independence after discharge from the hospital than the general population, but they are still more independent than patients with full-blown delirium. There is no specific, definitive standardized test in clinical practice, so diagnosis is based on the clinician's subjective impression. The DSM-IV-TR directs clinicians to record manifestations of borderline delirium under the diverse category of "cognitive disorders not elsewhere classified." The abstract, pictorial model of clouding of consciousness centers on the presence of a part of the brain that regulates the "general level" of consciousness, which is responsible for awareness of self and the environment. Various causes can disrupt this organizing part of the brain, which in turn can disrupt the "general level" of consciousness. The general activation of consciousness is referred to as “alertness” or “wakefulness.” The clouding of consciousness is not necessarily accompanied by drowsiness; the patient may be awake but his awareness and perception of reality are clouded. Ironically, sufferers respond that they are “awake, but otherwise, not.” Lipofsky points out that the decline in “alertness” 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, two very different things. The sufferer experiences a subjective sense of mental clouding described in the patient’s own words as a “foggy sensation.” One sufferer describes it in the following words: “I thought everything was blurry, somehow… the outlines were blurry.” Others may describe a feeling of detachment from everything. Sufferers compare their overall experience to that of a dream; as in dream consciousness, attention, orientation to time and space, perception, and awareness are disturbed. Barbara Schildkrout, a board-certified psychologist and clinical instructor in psychiatry at Harvard Medical School, described her own experience of clouding consciousness, or what she also called “mental fog,” after taking a single dose of the antihistamine chlorpheniramine for a cottonwood allergy during a cross-country road trip. Schildkrout described feeling detached from everything, as if in a “dream state,” and feeling uncertain about her judgment, that her consciousness was frozen, and that she had no sense of time. Clouding consciousness is different from depersonalization, although people with both conditions describe their experiences as similar to dreaming. Standard psychological tests provide little evidence of a relationship between clouding consciousness and depersonalization. Clouding consciousness affects performance on almost any cognitive task. “It should be obvious that cognition is not possible without a reasonable degree of alertness,” one of the authors says. Cognitive functions include perception, memory, learning, executive functions, language, reasoning abilities, voluntary motor control, attention, and mental speed. The extent of impairment and impairment is variable because inattention can impair many cognitive functions. Sufferers may complain of forgetfulness, "confusion," or "inability to think straight." Borderline delirium is distinct from mild cognitive impairment, despite similarities, the main difference being that mild cognitive impairment is a dementia-like disorder; it does not involve disturbances in alertness. The concept of "slowed cognitive activity" implies the same terms and symptoms as "mental clouding."