Definitions and clinical phenomena

Derealisation

Definition

A feeling that the external world is unreal, dreamlike, distant or visually altered, while reality testing is usually preserved.

  • Merck: patients keep the knowledge that the experience is a feeling rather than real (intact reality testing), which distinguishes derealisation from psychosis, where insight is lacking (1).
Assessment

Clarify onset, duration, triggers and insight; assess panic, trauma, depression, substances, sleep deprivation, migraine, seizures and neurological symptoms. Ask about depersonalisation and psychosis.

  • Merck: depersonalisation/derealisation can occur in many mental disorders and in physical disorders such as seizure disorders (ictal or postictal); diagnosis is by symptoms after other causes are excluded (1).
Red flags

New neurological deficit, seizure, intoxication, delirium, loss of reality testing, suicidality or inability to function requires urgent assessment.

Clinical context

Can occur transiently during severe anxiety, panic, trauma or fatigue, or as part of depersonalisation-derealisation disorder. It is not the same as a delusion.

  • Merck: 25–75% of the general population have at least one transient episode in their lifetime, but only about 1–2% meet criteria for depersonalisation/derealisation disorder, which is often triggered by severe stress (1). See Dissociative disorders and Depersonalisation pages.
Management

Explain the symptom, reduce triggers, use grounding and paced breathing, and treat underlying anxiety, trauma or mood disorder. Persistent or impairing symptoms warrant mental-health review.

  • Merck: treatment is psychotherapy plus medication for any comorbid depression or anxiety (1).
References
  1. Merck Manual Professional Version. Depersonalization/derealization disorder. https://www.merckmanuals.com/professional/psychiatric-disorders/dissociative-disorders/depersonalization-derealization-disorder

Last updated from Sen’s notes on 9 October 2026.