Dissociation is a phenomenon in which there is a lack of connection in a person’s thoughts, memories, feelings, actions, or sense of identity. During the period of dissociation, certain information is split off from other information with which it is normally connected. Dissociative experience is probably a continuum, from complete absorption in a task with total unawareness of surroundings, to fugue states, to total amnesia.

Dissociation can be interpreted as an “emergency defense,” or a “shut off mechanism.”

It may be an evolutionarily adaptive mechanism designed to prevent overwhelming flooding of consciousness at the time of trauma. Once the individual has learned to dissociate in the context of trauma, he or she may subsequently transfer this response to other situations and it may be repeated thereafter arbitrarily in a wide variety of circumstances. The dissociation therefore may destabilize adaptation and becomes pathological (Allen and Smith 1993).

Patients who receive treatment interventions that address their trauma-based dissociative symptoms show improved functioning and reduced symptoms (Gentile et al. 2013).

Dissociation is closely related to conversion syndrome (hysteria, hysterical dissociation), and some consider the latter to be a subset of dissociation syndrome. Hypnosis is a widely used technique to induce dissociation. There is evidence that identical functional brain changes occur in conversion paralysis and hypnotically induced paralysis of the lower limb (Halligan et al. 2000). Dissociation is an important symptom in posttraumatic stress disorder (PTSD), as well as in the borderline personality (see Chap. 25). Conversion, PTSD, and borderline personality disorder, however, are not classified under the rubric of dissociative disorders in DSM-5. Syndromes included in the DSM-5 as dissociative disorders are dissociative identity disorder (multiple personality), dissociative amnesia, dissociative amnesia with dissociative fugue, depersonalization/depersonalization disorder, and other specified or unspecified dissociative disorder.

Depersonalization refers to a psychological state in which the perception or experience of the self feels detached or unreal. One feels as if one is an outside observer of one’s mental processes or body, as if in a dream. Depersonalization is accompanied by feelings of disembodiment and subjective emotional numbing. It has been proposed that depersonalization is caused by a fronto-limbic (particularly anterior insula) suppressive mechanism—presumably mediated via attention—which manifests subjectively as emotional numbing, and disables the process by which perception and cognition normally become emotionally colored, giving rise to a subjective feeling of ‘unreality’. Depersonalization syndrome patients show increased prefrontal activation as well as reduced activation in insula/limbic-related areas to aversive, arousing emotional stimuli. Parietal mechanisms may underlie feelings of disembodiment (Reutens et al. 2010; Sierra and David 2011)

Derealization is an alteration in the perception or experience of the external world so that it seems strange or unreal. In depersonalization, there is increased alertness that may be associated with an activation of prefrontal attentional systems (right dorsolateral prefrontal cortex) and reciprocal inhibition of the anterior cingulate, leading to the experiences of “mind emptiness” and indifference to pain that are often seen in depersonalization. In derealization, there may be a left-sided prefrontal inhibition of the amygdala resulting in dampened autonomic output, hypoemotionality, and lack of emotional coloring, resulting in feelings of unreality or detachment. Derealization and depersonalization may be conceptualized as a syndrome of corticolimbic disconnection (Sierra and Berrios 1998). Depersonalization and derealization may serve an evolutionarily adaptive function of intensifying alertness and dampening potentially disorganizing emotion (Stein and Simeon 2009).

Depersonalization and derealization experiences often occur in normal people in situations of severe anxiety, as in medical settings where a serious diagnosis or medical procedures may be discussed. Furthermore, many drugs, particularly analgesics and sedatives, as well as mild delirium that may be associated with a medical condition, may predispose patients to depersonalization/derealization. Specific neurological conditions such as partial complex seizures as well as encephalopathies and strokes may be associated with these phenomena. In the CL setting, psychological support and reassurance may alleviate the frightening aspect of these experiences. Reduction or change in a medication that might be associated with the condition, as well as treatment of delirium and the underlying medical condition may be therapeutic.

When there is functional impairment due to depersonalization or derealization, depersonalization or derealization disorder may be diagnosed. According to DSM-5, the lifetime prevalence of depersonalization/derealization disorder is 3 %, equally in males and females.

Depersonalization and derealization are common features of other psychiatric conditions, particularly borderline personality and posttraumatic stress disorder (PTSD).