Depersonalization Disorder (including Derealization)
Depersonalization Disorder is categorized in the DSM-5 as a Dissociative Disorder and has been combined with derealization. Depersonalization disorder involved an individual feeling detached from their environment, in particular emotionally detached.
Depersonalization is a symptom found in many psychiatric disorders, but in Depersonalization Disorder it is the primary disorder.  It's origin seems to be in that brain regions involved with these emotions are less activated. 
DSM-5 Diagnostic criteria
The DSM-5 includes the following criteria for the combined disorder Depersonalization/Derealization Disorder:
A.An individual consistently has a feeling of both or either depersonalization or derealization. :156
- Depersonalization: Experiences of unreality, detachment, or being an outside observer with respect to one's thoughts, feelings, sensations, body, or actions (e.g.,perceptual alterations, distorted sense of time, unreal or absent self, emotional and/or physical numbing.)" :157
- Derealization: "Experiences of unreality or detachment with respect to surroundings (e.g., individuals or objects are experienced as unreal, dreamlike, foggy, lifeless, or visually distorted." :157
B. "During the depersonalization or derealization experiences, reality testing remains intact." :156
C. "The disturbance is not attributable to the physiological effects of a substance (e.g., a drug of abuse, medication or other medical condition (e.g., seizures)." :157
E. "The disturbance is not better explained by another mental disorder." :157
As with any dissociative disorder, the symptoms must be severe enough to cause clinically significant distress or significantly impair the individual's functioning in at least one major area of life (eg, work or social life).
ICD-10 Diagnostic Criteria
F48.1 Depersonalization-derealization syndrome
"A rare disorder in which the patient complains spontaneously that his or her mental activity, body, and surroundings are changed in their quality, so as to be unreal, remote, or automatized. Among the varied phenomena of the syndrome, patients complain most frequently of loss of emotions and feelings of estrangement or detachment from their thinking, their body, or the real world. In spite of the dramatic nature of the experience, the patient is aware of the unreality of the change. The sensorium is normal and the capacity for emotional expression intact. Depersonalization-derealization symptoms may occur as part of a diagnosable schizophrenic, depressive, phobic, or obsessive-compulsive disorder. In such cases the diagnosis should be that of the main disorder."
Depersonalization sometimes referred to as DPD (depersonalization disorder) is frequently described as is feeling as if one is outside of their own body or that their own body is unreal, but is a complex disorder with a consistent set of symptoms being reported for over a hundred years.:69 Psychiatrists Sierra and Berrios analyzed historic reports of depersonalization and determined the following two clusters of symptoms, with the core symptoms are the most frequently reported and the most distressing.:70
Nonchanging core symptoms
- visual derealization
- altered body experience (for example vision, hearing, smell, taste, touch or pain):74
- emotional numbing
- loss of agency feelings
- changes in the subjective experiencing of memory:70
- unreality experiences not related to vision
- mind-emptiness (subjective inability to entertain thoughts or evoke images)
- heightened self-observation
- altered time experiences
This is frequently described as feeling detached from one's environment.
SCI-DER, Cambridge Depersonalization Scale and Steinberg Depersonalization Test
The SCI-DER, CDS and Steinberg Depersonalization Test are all questionnaires designed to assess depersonalization and derealization in order to assess symptoms, although these should not be used for a definitive diagnosis.
This is a structured clinical interview, which is the diagnostic tool for depersonalization/derealization disorder and all dissociative disorders.
Depersonalization is believed to be "a response that is intended to distance the self from overwhelmingly painful or conflictual impulses or feelings", and makes sense as a "defense mechanism against over-whelming traumatic stress".:74
Links to other disorders
Depersonalization and derealization disorder cannot be diagnosed if the symptoms are transient rather than persistent and reoccuring, or if they only occur within another mental health or physical health disorder. Experiences of depersonalisation and derealization may occur during acute stress disorder, posttraumatic stress disorder, other specified dissociative disorder, dissociative identity disorder and others.
- Phillips, Mary L; Medford, Nicholas; Senior, Carl; Bullmore, Edward T; Suckling, John; Brammer, Michael J; Andrew, Chris; Sierra, Mauricio; Williams, Stephen C.R; David, Anthony S. Depersonalization disorder: thinking without feeling. Psychiatry Research: Neuroimaging, volume 108, issue 3, 1 December 2001, page 145–160. (doi:10.1016/S0925-4927(01)00119-6)
- APA, (2013). Diagnostic and Statistical Manual of Mental Disorders-5.
- World Health Organisation, (2010). ICD-10 Classification of Mental and Behavioural Reaction to severe stress, and adjustment disorders.
- Simeon, Daphne (2006) (coauthors: Abugel, Jeffrey). Feeling Unreal: Depersonalization Disorder and the Loss of the Self. ISBN 0199766355.