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HandWiki. Clouding of Consciousness. Encyclopedia. Available online: (accessed on 22 June 2024).
HandWiki. Clouding of Consciousness. Encyclopedia. Available at: Accessed June 22, 2024.
HandWiki. "Clouding of Consciousness" Encyclopedia, (accessed June 22, 2024).
HandWiki. (2022, November 21). Clouding of Consciousness. In Encyclopedia.
HandWiki. "Clouding of Consciousness." Encyclopedia. Web. 21 November, 2022.
Clouding of Consciousness

Clouding of consciousness (also known as brain fog or mental fog) is 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".

brain fog consciousness clouding

1. Background

The term clouding of consciousness has always denoted the main pathogenetic feature of delirium since physician Georg Greiner[1] pioneered the term (Verdunkelung des Bewusstseins) in 1817.[2] The Diagnostic and Statistical Manual of Mental Disorders (DSM) has historically used the term in its definition of delirium.[3] 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.[4] Clouding of consciousness may be less severe than delirium on a spectrum of abnormal consciousness.[5][6][7] Clouding of consciousness may be synonymous with subsyndromal delirium.[8]

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.[9] The significant clinical features of subsyndromal delirium are inattention, thought process abnormalities, comprehension abnormalities, and language abnormalities.[9] The full clinical manifestations of delirium may never be reached.[8] 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)[8] or have a decreased post-discharge level of functional independence vs. the general population but still more independence than full delirium.[9]

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".[10]

2. Psychopathology

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.[5][11] Various etiologies disturb this regulating part of the brain, which in turn disturbs the "overall level" of consciousness.[12] This system of a sort of general activation of consciousness is referred to as "arousal" or "wakefulness".[11]

It is not necessarily accompanied by drowsiness, however.[13] Patients may be awake (not sleepy) yet still have a clouded consciousness (disorder of wakefulness).[14] Paradoxically, sufferers declare that they are "awake but, in another way, not".[15] 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.[16][17]

The sufferer experiences a subjective sensation of mental clouding described in the patient's own words as feeling "foggy".[18] One sufferer described it as "I thought it became like misty, in some way... the outlines were sort of fuzzy".[15] Others may describe a "spaced out" feeling.[19] 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.[20] 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.[21] Clouding of consciousness is not the same thing as depersonalization even though sufferers of both compare their experience to that of a dream. Psychometric tests produce little evidence of a relationship between clouding of consciousness and depersonalization.[22]

This may affect performance on virtually any cognitive task.[21] As one author put it, "It should be apparent that cognition is not possible without a reasonable degree of arousal."[5] 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.[9] The extent of the impairment is variable because inattention may impair several cognitive functions. Sufferers may complain of forgetfulness, being "confused",[23] or being "unable to think straight".[23] 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).[24]

3. In Diseases

The emerging concept of sluggish cognitive tempo has also been implicated in the expression of 'brain fog' symptoms.[25]

Patients recovering from COVID-19 report experiencing 'brain fog', which can reflect a wide variety of neurological and psychological symptoms linked to COVID-19.[26]


  1. Georg Friedrich Christoph Greiner (1817). Der Traum und das fieberhafte Irreseyn: ein physiologisch-psychologischer Versuch. F. A Brockhaus. OCLC 695736431. 
  2. Augusto Caraceni; Luigi Grassi (2011). Delirium: Acute Confusional States in Palliative Medicine. Oxford University Press. pp. 2. ISBN 9780199572052. 
  3. George Stein; Greg Wilkinson (April 2007). Seminars in General Adult Psychiatry. RCPsych Publications. pp. 490. ISBN 978-1904671442. 
  4. Dan G. Blazer; Adrienne O. van Nieuwenhuizen (2012). "Evidence for the Diagnostic Criteria of Delirium". Curr Opin Psychiatry 25 (3): 239–243. doi:10.1097/yco.0b013e3283523ce8. PMID 22449764. 
  5. Plum and Posner's diagnosis of stupor and coma. Oxford University Press. 2007. pp. 5–6. ISBN 9780199886531. 
  6. Anthony David; Simon Fleminger; Michael Kopelman; Simon Lovestone; John Mellers (April 2012). Lishman's Organic Psychiatry: A Textbook of Neuropsychiatry. John Wiley & Sons. pp. 5. ISBN 9780470675076. 
  7. Fang Gao Smith (April 2010). Core Topics in Critical Care Medicine. Cambridge University Press. pp. 312. ISBN 978-1139489683. 
  8. Sébastien Ouimet et al. (2007). "Subsyndromal delirium in the ICU: evidence for a disease spectrum". Intensive Care Med 33 (6): 1007–1013. doi:10.1007/s00134-007-0618-y. PMID 17404704.
  9. David Meagher et al. (2012). "Features of subsyndromal and persistent delirium". The British Journal of Psychiatry 200 (1): 37–44. doi:10.1192/bjp.bp.111.095273. PMID 22075650.
  10. Augusto Caraceni; Luigi Grassi (2011). Delirium: Acute Confusional States in Palliative Medicine. Oxford University Press. p. 11. ISBN 9780199572052. 
  11. Augusto Caraceni; Luigi Grassi (2011). Delirium: Acute Confusional States in Palliative Medicine. Oxford University Press. pp. 19–21. ISBN 9780199572052. 
  12. Yudofsky & Hales (2008). The American Psychiatric Publishing textbook of neuropsychiatry and behavioral neurosciences. American Psychiatric Pub. pp. 477. ISBN 978-1585622399. 
  13. Roger A. MacKinnon; Robert Michels; Peter J. Buckley (2006). The Psychiatric Interview in Clinical Practice 2nd edition. American Psychiatric Publishing, Inc.. pp. 462–464. 
  14. Plum and Posner's diagnosis of stupor and coma. Oxford University Press. 2007. pp. 8. ISBN 9780198043362. 
  15. G Sorensen Duppils; K Wikblad (May 2007). "Patients' experiences of being delirious". Journal of Clinical Nursing 16 (5): 810–8. doi:10.1111/j.1365-2702.2006.01806.x. PMID 17462032.
  16. Lipowski ZJ. (1967). "Delirium, clouding of consciousness and confusion". Journal of Nervous and Mental Disease 145 (3): 227–255. doi:10.1097/00005053-196709000-00006. PMID 4863989.
  17. William Alwyn Lishman (1998). Organic Psychiatry: The Psychological Consequences of Cerebral Disorder. John Wiley & Sons. pp. 4. 
  18. Augusto Caraceni; Luigi Grassi (2011). Delirium: Acute Confusional States in Palliative Medicine. Oxford University Press. pp. 82. ISBN 9780199572052. 
  19. Fred Ovsiew, M.D. (1999). Neuropsychiatry and Mental Health Services. American Psychiatric Press, Inc.. pp. 170. ISBN 0880487305. 
  20. Simon Fleminger (2002). "Remembering delirium". The British Journal of Psychiatry 180 (1): 4–5. doi:10.1192/bjp.180.1.4. PMID 11772842.
  21. Barbara Schildkrout (2011). Unmasking Psychological Symptoms. John Wiley & Sons. pp. 183–184. ISBN 9780470639078. 
  22. G. Sedman (1970). "Theories of Depersonalization: A Re-appraisal". The British Journal of Psychiatry 117 (536): 1–14. doi:10.1192/s0007125000192104. PMID 4920886. 
  23. John Noble; Harry L. Greene (1996). Textbook of Primary Care Medicine. Mosby. pp. 1325. 
  24. Plum and Posner's diagnosis of stupor and coma. Oxford University Press. 2007. pp. 7. ISBN 9780199886531. 
  25. Russel A. Barkley (2013): Two Types of Attention Disorders Now Recognized by Clinical Scientists. In: Taking Charge of ADHD: The Complete, Authoritative Guide for Parents. Guilford Press (3rd ed.), p.150. ISBN:978-1-46250-789-4.
  26. Koralnik, Igor J.; Tyler, Kenneth L. (July 2020). "COVID-19: A Global Threat to the Nervous System". Annals of Neurology 88 (1): 1–11. doi:10.1002/ana.25807. ISSN 1531-8249. PMID 32506549. PMC 7300753. 
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