Dissociative Identity Disorder
The most severe dissociative disorder in the DSM-5, and the ICD-10 psychiatric manuals is dissociative identity disorder (DID). The ICD-10 has not updated their manual in the last few decades, and today still uses the historic, and misleading name multiple personality disorder (MPD). Unlike this name suggests, multiple personality disorder is not a personality disorder, nor does it mean that an individual has more than one personality. Both other specified dissociative disorder and dissociative identity disorder are complex dissociative disorders, with similar causes and shared treatment guidelines.
Dissociative identity disorder is understood to be an outcome of overwhelming physical and psychological trauma, and disturbed caregiver interactions, in which a unified sense of self has yet to be obtained, due to integration failure of self-states during early childhood.(citation needed) If trauma is experienced often enough during the earliest years of life, then self-states can endure, until they eventually evolve into states so dissociated they become "altered states of consciousness" or alters for short. Alters are only found in individuals with dissociative identity disorder. These highly dissociated parts of the personality are unique in their way of being, and research shows both psychobiological and physiological differences exist between many alters.
The experts who are leading researchers into dissociative disorders agree that the cause of dissociative identity disorder is early and severe childhood abuse (Chu, 2011), usually from a trusted caregiver, leaving the child no one to turn to. Psychological trauma and dissociation are entwined closely. When early childhood abuse is chronic and severe, it can result in dissociation and profoundly affect personality development and change brain structure. (Clark, 2010) Dissociation can be defined as a failure in the integration of consciousness, memory, awareness of body and of the environment, and identity.
There are three etiological models accepted by the International Society for the Study of Trauma and Dissociation (ISSTD), and all three models report that dissociative identity disorder does not develop from a single "core personality", but instead is a failure to integrate self-states, and obtain a unified sense of the self, which normally occurs at a young age. No one has an "original personality," because the personality is something that must be constructed and blended over time.
A diagnosis of dissociative identity disorder is given when two or more distinct, highly dissociated parts present, alternating control, accompanied by amnesia in the usually presenting part, which is often called the host. More often than not, symptoms of dissociative identity disorder overlap, meaning they have (comorbidity), with other disorders. Dissociative disorders and posttraumatic stress disorder (PTSD) are typically comorbid with DID. These comorbid disorders are all are subsumed under the dissociative identity disorder diagnosis. The overt symptoms required by the DSM-5, exist in only 5-6% of individuals, who dissociative identity disorder expert Paul F. Dell, in his 2006 landmark study, reported having dissociative identity disorder. Until the recent changes in the DSM-5, individuals without overt symptoms at the time of assessment are likely to have been diagnosed with unspecified dissociative disorder, but according to Dell, may in fact have dissociative identity disorder.
Therapy is the primary treatment method for DID rather than medication; the best results are obtained when trauma memories are "processed", allowing "dissociative boundaries" to abate and a unified sense of self to thrive, thereby reducing the dissociative symptoms of trauma. Phase-oriented treatment guidelines for adults, and a separate set for children and adolescents are offered by the ISSTD; this phase-oriented approach follows a similar method to the treatment guidelines for complex posttraumatic stress disorder published by the International Society for Traumatic Stress Studies.
Research is limited due to the etiology of dissociative identity disorder, which would involve isolating a very young child for years in an experience of constant, severe psychological and physical trauma, therefore dissociative identity disorder research understandably focuses on a clinical and case study approach, and also due to a lack interest from pharmaceutical companies. Once traumatology was introduced to the field of mental health, dissociation was better understood and the diagnosis of dissociative identity disorder increased. Prior to this knowledge, people with dissociative identity disorder were often misdiagnosed. Epidemiology research today reports that between 1% and 3% of the general population has dissociative identity disorder. 
Dissociative Identity Disorder
- Overt: According to the DSM, dissociative identity disorder is a disorder of mental states, where an individual has amnesia due to switching. (Dell, 2009, p. 319-321) Individuals with overt dissociative identity disorder may experience all the (generalized, systematized, localized, continuous, selective) known five types of amnesia, but although not defined well in the DSM, what is looked for is recent and recurring amnesia, including amnesia for mundane events. (Dell, 2009, p. 776)
- Covert: DES scores are lower in those with covert dissociative identity disorder than with overt dissociative identity disorder. These individuals are skilled at hiding overt symptoms. The overwhelming majority of individuals with dissociative identity disorder have this version. (Dell, 2009, p. 424)
- Subtle: This group of individuals has less frequent and severe dissociation when compared to individuals with either overt or covert dissociative identity disorder. (Dell, 2009, p. 424)
- Polyfragmented: Most individuals with dissociative identity disorder have less than a dozen alters,(citation needed) however at the far end of the spectrum is those individuals with polyfragmented dissociative identity disorder, having many alters, even one-hundred or more distinct alters. (Howell, 2011, p. 57) "In general, the complexity of dissociative symptoms appears to be consistent with the severity of early tramatization." (Chu, 2011) Large systems of alters are often broken into subsystems within the main system as a method of self-preservation and organization of trauma memories. Polyfragmentation often develops in children who suffered very early and extreme abuse, either at home or from ritual abuse.
DSM-5 (Diagnostic and Statistical Manual of Mental Disorders)
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published by the American Psychiatric Association (APA), (released at the APA's 2013 Annual Meeting in San Francisco, CA), presented changes to the dissociative disorder category.  Historically, even if dissociative identity disorder is overtly evident, a diagnosis of dissociative identity disorder was not given until a switch between the host alter and another alter was observed by a psychiatrist or therapist, and the switch had to result in "amnesia" for the host alter. The DSM-5 no longer requires time-loss to be observed by the person responsible for diagnosis, so now a client, a person supporting him/her or another professional can report the signs and symptoms caused by the switch between identities.  Other mental health diagnoses can also be made by self reporting of symptoms rather than needing separate observation.
The DSM-5 has five criteria which must be met:
A The presentation of two or more distinct personality states/alters must present, and each must have their own way of being. In other words these two parts have not obtained a unified sense of the self. Prior to the DSM-5 this symptom must have been observed by the a professional qualified to make the diagnosis, and could not be diagnosed based on reports by mental health nurses on an in-patient ward (for example).
"Disruption of identity characterized by two or more distinct personality states, which may be described in some cultures as an experience of possession. The disruption of marked discontinuity in sense of self and sense of agency, accompanied by related alterations in affect, behavior, consciousness, memory, perception, cognition, and/or sensory-motor functioning. These signs and symptoms may be observed by others or reported by the individual. ":155
B Amnesia is still a requirement in the DSM-5, but the type of amnesia is more clearly defined. Amnesia between identities is generally understood to mean that the host alter, experiences time-loss in the present and has no memory of the periods of time during which another alter takes the place of the one the host alter. The amnesia requirement is described as (Note that even though the DSM does not make it clear that they mean current [amnesia], 20% of the population who appears to not be traumatized, does not remember their childhood.) 
"Recurrent gaps in the recall of everyday events, important personal information, and/or traumatic events that are inconsistent with ordinary forgetting."
C An individual must be distressed by the disorder or have an impaired ability to function in a major area of life as a result. This is described as follows:
"The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning." :155
D Normal cultural or religious practice is excluded, and fantasy play in children are excluded.
The disturbance is not a normal part of a broadly accepted cultural or religious practice. (Note: In children, the symptoms are not attributable to imaginary playmates or other fantasy play). :155
E DID cannot be diagnosed if symptoms are attributable to substance use or other medical conditions.
"The symptoms are not attributable to the physiological effects of a substance (e.g., blackouts or chaotic behavior during alcohol intoxication) or another medical condition (e.g., complex partial seizures).":155
"Dissociation allows memories, skills, affects and other knowledge to be sectioned off and stored in less easily accessible ways." :95 Amnesia is a requirement in the DSM-5 for a diagnosis of dissociative identity disorder, reporting that at least two distinct and relatively enduring self-states must present,  alternating control, accompanied by the host alter's inability to consciously recall autobiographical information that should have been stored in memory and easy to recall. :836 Those with dissociative identity disorder often refer to the phenomena as time-loss, or state-dependent memory, because even though the host alter cannot recall information, the alter that was in control at the time can. When forgotten information is traumatic, it is often reveled as nightmares, flashbacks, and conversion symptoms.  Recent research has shown significant differences between dissociative identity disorder, dissociative disorder that are not specified, dissociative amnesia, depersonalization disorder, and nonclinical participants.  The host alter usually does not have knowledge of their own pathology and also lacks sufficient integration of trauma memories. In contrast, alters holding childhood trauma memories have conscious access to those memories, and they can recall them as personal experiences. 
Symptoms of other dissociative disorders commonly overlap with dissociative identity disorder, which manifests not only as intrusive thoughts or emotions, but also from the affects of dissociative boundaries.  The degree of symptom severity differs in people with dissociative identity disorder, ranging from a normal presentation, or even highly effective to those that are severely impaired. The main symptom, dissociation, reduces distress and acts as a coping mechanism, but at the same time causes its own distress. Keep in mind that the dissociative identity disorder diagnosis subsumes all symptoms of other dissociative disorders, and posttraumatic stress disorder, so they are not diagnosed separately.
Self-alteration, derealization, depersonalization, flashbacks, trances, identity confusion, awareness of alters, voices, thought withdrawal and insertion, made impulses, feelings and actions, non psychotic auditory and visual hallucinations and Paul Dell's phenomenological model of dissociative identity disorder goes beyond the narrow definition of symptoms suggested in the DSM-5, explaining specific dissociative symptoms. Dell has documented evidence that comprises a pathognomonic pattern of dissociative symptoms that is unique to dissociative identity disorder including: state dependent amnesia. 
An alter is a dissociated self-state with their own way of being - complete with idiomatic autobiographical memory, procedural and general knowledge, behavior, physical sensations, emotions, their own separate centers of subjectivity, identity, autonomy, sense of personal history and skills unique to them. Alters are contained within dissociative boundaries, with the degree of dissociation experienced between parts as defining a dissociative disorder (Howell, 2011, p. 8). Together the alters are highly interrelated parts of a system that comprise one person (Howell, 2011, p. 55). They are what sets dissociative identity disorder apart from all other mental disorders; no other mental disorder has alters. There are various names for different types of alters, but it's important to note that those without a mental disorder also have multiple parts to their personality.
"Dissociative disorders are almost always the result of childhood trauma." :99 Dissociation is a disruption in normal integration of consciousness memory.  Psychological trauma and dissociation are tightly linked. When a young child is abused early in life, and that abuse is chronic and severe, it changes personality development. :73 Also a combination of trauma and chronic emotional neglect in early childhood, leads to attachment disorder. :14 
An infant is not born with a unified sense of self, instead the process takes years of experience to obtain.  To survive the type of abuse that causes dissociative identity disorder, a child detaches from both emotional and physical pain which can change memory encoding and storage. A child's memory can then become compartmentalized into different parts of the personality and memory retrieval is impaired. If trauma is repeated enough to a very young child, the result can be an individual who is unable to integrate the different parts of their personality into one unified sense of self. The dissociated parts of the personality can endure, until they become what is known as an alter. :73
As already explained, in infancy behavioral states (eating, crying, hunger, etc...) link together to form states, so the thinking is that states are present before they differentiate and become alters, so for the creation of most alters, at least the initial ones, the parts were always there, and due to an inability to integrate they become dissociated states or alters. Since all humans can create fragments throughout life, it is possible in the individual with dissociative identity disorder for a fragment to eventually evolve enough to become an alter. Each traumatic experience may be the cause of a new fragment,  :14 but it's important to understand that the creation of an alter is not an instant "splitting" or "fracturing". The process takes a great deal of time.
"The age of the individual at the time of the abuse is a critical component due to the developmental processes that, under other circumstances, would normally occur at that time. In addition, the age at the beginning and the ending of the abuse is significant as it encompasses the sequence of developmental stages spanned by the maltreatment and should influence which developmental tasks are most disrupted. Although there is no conclusive data in this area, it appears as if vulnerability to dissociation increases if the abuse occurs at earlier developmental stages." 
It is common that survivors of early childhood abuse experience a poor psychological outcome, but not everyone that suffers trauma during early childhood will have symptoms of that abuse that linger into adulthood.  The relationship between a child and their abuser(s) is believed to be highly significant.  In rare cases, it is thought that dissociative identity disorder can be found in a child that was never abused, but did experience an important and early traumatic loss in their life. 
All three etiological models accepted by the ISSTD  report that dissociative identity disorder does not develop from a single 'core personality', but instead it is the failure of integrating different parts of the personality at young age. :122-124 Children are not born with a unified personality, nor is the creation of a |unified personality a quick process. What a child has is "multiple personality states" which in normal personality development, integrate to work as one unified personality.
"No one has an original personality. A person's sense of Self and identity is built up and synthesized over time. Furthermore, the usually presenting part is, by definition, a part in relation to and in relationship with other parts of the total organization of the personality. People do not start out in life unified, but developmentally accomplish the joining and harmonious functioning of different behavioral and mental states." :59
It's important to understand that this does not mean that the normal personality is one unified thing. Instead, it is made up of many parts of the personality, but works as one unified personality.  In dissociative identity disorder, the process of normal integration is interrupted, resulting in multiple self-states, each with their own view of self and the world. :122-124
The DSM-5 requires that an adult, for non-physiological reasons, be recurrently controlled by two or more discrete identity or personality states, accompanied by memory lapse (amnesia) for important information. While otherwise similar, the diagnostic criteria for children also rules out fantasy. The ICD-10 places the diagnosis in the category of "dissociative disorders", within the subcategory of "other dissociative disorders", and list the condition as multiple personality disorder.
Diagnosis is normally performed by a clinically trained mental health professional through clinical evaluation, interviews with family and friends, and consideration of other ancillary material. Specially designed interviews and personality assessment tools may be used for evaluation.  A diagnosis of dissociative identity disorder takes precedence over any other dissociative disorders. 
Patients with dissociative identity disorder are arguably the most severe of the dissociative disorders and may be among the most difficult of all psychiatric patients to treat. :14 Even highly experienced therapists have a substantial number of patients that do not achieve a unified sense of Self due to the many factors which affect prognosis. Despite this, a review of the literature shows that treatment including a focus on dissociative symptoms is effective in reducing a range of symptoms associated with dissociative disorders, including depression,and anxiety.  Historically is has been suggested that the overall prognosis for dissociative identity disorder is variable, but poor. 
A realistic and appropriate goal of treatment is to integrate adaptive responses to abuse, injury or other threats into the overall personality structure.  Common treatment methods include an eclectic mix of psychotherapy, insight-oriented therapies,  and EMDR. Therapy for dissociative identity disorder is generally phase oriented. The ISSTD  has published guidelines for adults,  as well as a separate guide for children and adolescents. Movement through the phases is often non-linear; patients in the second or third phase of treatment may need to go back to a previous phase to maintain safety or to process previously unprocessed material.
The first phase of therapy focuses on symptoms and relieving the distressing aspects of the condition, ensuring the safety of the individual, improving the patient's capacity to form and maintain healthy relationships, and improving general daily life functioning. Comorbid disorders such as substance abuse and eating disorders are addressed in this phase of treatment. The second phase focuses on stepwise exposure to traumatic memories and prevention of redissociation. The final phase considers the reduction of dissociative boundaries, allowing the person to finally have full use of their memories and experiences that were held by individual alters.
With trauma memory processing, those with dissociative identity disorder will feel less need for separate alters. The memories and feelings held by the various alters will begin to integrate - resulting in a reduction in the dissociative boundaries that divide them, leaving the parts less separate and distinct. 
Some with Dissociative Identity Disorder function with mild disturbance and impact on quality of life, while others might be so impaired they cannot retain one personality state in order to reach goals and function in life. :14 When untreated, Dissociative Identity Disorder is chronic and recurrent with symptoms varying over time. Changes in identity and loss of state dependent memory (dissociative amnesia) often lead to a chaotic life.  It is common for alters to be unaware of each other, stemming from the dissociative barrier between them.  The best prognosis is when trauma memories are processed, dissociative boundaries abate, resulting in a "unified sense of Self", thereby reducing the symptomology of trauma. "Across the case series and treatment studies, estimates of full integration range from 16.7% to 33% of Dissociative Identity Disorder patients with shorter follow-ups associated with lower rates of integration. Case series that have not used standardized measures have reported higher rates of integration." 
At least two-years of psychotherapy are needed (for adults) to allow time for integration.   Approximately two-thirds of patients improved with therapy focused on dissociative symptoms, where those without this focus, individuals face persistent and long-lasting effects. Suicidal tendencies, anxiety, and depression may remit prior to the reduction of dissociative symptoms.  Unifying the various alters is considered best for the patient, allowing them to finally operate as a "unified sense Self," and to be free of the crippling symptoms of Dissociative Identity Disorder. Some patients however, choose not to integrate and retain individual alters, but with reduced dissociative barriers, and increased communication between alters.
"Screening studies using diagnostic tools designed to assess dissociative disorders, yield lifetime prevalence rates around 10% in clinical population."  In children, rates among females and males are the same. In adults more women are known to have dissociative identity disorder than men. Dissociative identity disorder is not rare; it is thought to occur in 1% to 3% of the worlds general population. 
Dissociative identity disorder was once referred to in older versions of the DSM as multiple personality disorder and is still being referred to as such in the ICD-10. Furthermore, comparisons between schizophrenia and dissociative identity disorder have added to general public confusion as to what dissociative identity disorder is. Media sensationalism around popular movies such as Sybil or the Three Faces of Eve and the television show United States of Tara have perpetuated myths further. Rates of diagnosed dissociative identity disorder have been increasing, as knowledge of trauma related disorders spread, and training of both medical doctors and mental health professionals improve. Developments in the field of traumatology were made in the 1970's and 1980's, leading to an interest in the area of dissociation. Prior to this time, those with dissociative identity disorder were often misunderstood and misdiagnosed (Krakauer, 2001).
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