Capgras delusion

# Overview
The Capgras delusion (or Capgras's syndrome) is a rare disorder in which a person holds a delusional belief that an acquaintance, usually a spouse or other close family member, has been replaced by an identical looking impostor. The Capgras delusion is classed as a delusional misidentification syndrome, a class of delusional beliefs that involves the misidentification of people, places or objects. It can occur in acute, transient, or chronic forms.
The delusion is most common in patients diagnosed with schizophrenia, although it can occur in a number of conditions including after brain injury and dementia. Although the Capgras delusion is commonly called a syndrome, because it can occur as part of, or alongside, various other disorders and conditions, some researchers have argued that it should be considered as a symptom, rather than a syndrome or classification in its own right.

# History
It is named after Joseph Capgras (1873-1950), a France|French psychiatrist who first described the disorder in a 1923 paper by Capgras and Reboul-Lachaux. They used the term l'illusion des sosies (the illusion of doubles) to describe the case of a French woman who complained that various "doubles" had taken the place of people she knew. However, the term illusion has a subtly different meaning from delusion in psychiatry so "the Capgras delusion" is used as a more suitable name.

# Presentation
This case is taken from a 1991 report by Passer and Warnock:
Mrs. D, a 74-year old married housewife, recently discharged from a local hospital after her first psychiatric admission, presented to our facility for a second opinion. At the time of her admission earlier in the year, she had received the diagnosis of atypical psychosis because of her belief that her husband had been replaced by another unrelated man. She refused to sleep with the impostor, locked her bedroom and door at night, asked her son for a gun, and finally fought with the police when attempts were made to hospitalize her. At times she believed her husband was her long deceased father. She easily recognized other family members and would misidentify her husband only.

# Etiology
Some of the first clues to the possible causes of the Capgras delusion were suggested by the study of brain-injured patients who had developed prosopagnosia. In this condition, patients are unable to consciously recognise faces despite being able to recognise other types of visual objects. However, a 1984 study by Bauer showed that even though conscious face recognition was impaired, patients with the condition showed autonomic arousal (measured by a galvanic skin response measure) to familiar faces, suggesting that there are two pathways to face recognition - one conscious and one unconscious.
In a 1990 paper published in the British Journal of Psychiatry, psychologists Hadyn Ellis and Andy Young hypothesised that patients with Capgras delusion may have a 'mirror image' of prosopagnosia, in that their conscious ability to recognise faces was intact, but they might have damage to the system which produces the automatic emotional arousal to familiar faces. This might lead to the experience of recognising someone, while feeling something wasn't 'quite right' about them.
In 1997 Hadyn Ellis and colleagues published a study of five patients with Capgras delusion (all diagnosed with schizophrenia) and confirmed that although they could consciously recognise the faces, they did not show the normal automatic emotional arousal response.
The same year, William Hirstein|Hirstein and Vilayanur S. Ramachandran|Ramachandran reported similar findings in a paper published on a single case of a patient with Capgras delusion after brain injury. Ramachandran also portrays this case in his book Phantoms in the brain. Since the patient was capable of feeling emotions and recognizing faces but couldn't feel emotions when recognizing familiar faces, Ramachandran hypothesizes that the origin of Capgras syndrome is a disconnection between the temporal cortex, where faces are usually recognized (see temporal lobe), and the limbic system, involved in emotions. Because the patient couldn't put together memories and feelings, he believed objects in a photograph were new on every viewing, even though they normally should have evoked feelings (e.g. a person close to him, a familiar object, or even himself). Ramachandran therefore believed there was a relationship between Capgras syndrome and a more general difficulty in linking successive episodic memories, since it is believed that emotion is critical for creating memories.  Also in Phantoms in the brain Ramachandran describes that this patient was also miserable at identifying direction of eye gaze, and during testing occasionally interjected that the subject in the photographs had changed (when in reality all that had changed was the direction of the subjects eye gaze).  Because of the important role the pathways between the temporal lobes and amygdala play in detecting eye gaze, this data further supports Ramachandrans theory.
It is likely that more than an impairment of the automatic emotional arousal response is necessary to form Capgras delusion, as the same pattern has been reported in patients showing no signs of delusions. Ellis and colleagues suggested that a second factor explains why this unusual experience is transformed into a delusional belief; this second factor is thought to be an impairment in reasoning, although no definitive impairment has been found to explain all cases.

# Violence Secondary to Capgras Syndrome
Epidemiologic studies have indicated a modest relation between interpersonal violence and the presence of an active psychotic disorder, with a higher incidence of
severe acts of violence in delusional disorders, compared with other psychiatric disorders . In a study of incident cases of schizophrenia in a London borough over a 20-year period, Wessely and others found that men with schizophrenia were nearly 4 times more likely to commit violent offences Shah and others found that, of 718 homicide convictions, 38% involved psychosis, with 6% having a lifetime history of schizophrenia . In a Finnish study that examined data on 1423 homicide offenders over a 12-year period, Eronen and others found that 93 (6.5%) suffered from schizophrenia. Calculations of the odds ratios (ORs) revealed that the risk of committing a homicide was about 10 times greater among men and women with schizophrenia, compared with the general population. Eronen and others also examined 693 homicide offenders with mental disorders over an 8-year period and found that schizophrenia increased the OR of homicidal violence by approximately 8 times in men and 6 times in women. Nearly one-half of the 63 offenders with schizophrenia suffered from the paranoid subtype. In contrast, the presence of affective disorder, anxiety disorder,dysthymia, or mental retardation did not significantly increase the OR. Numerous reports exist of cases of violent behaviour in CS sufferers. In summary, in the assessment of individuals with Capgras syndrome or its
misidentification variants, consideration needs to be given to its psychodynamics and delusional content, as well as whether the misidentified persons live with or are easily accessible to the delusional individual. Given its possible frequent association with physical harm, there should be aggressive clinical and clinicolegal interventions to reduce the danger of the Capgras individual's physically harming others. Even though studies have displayed equivocal results at best in the prognostication of future harmful misidentification syndromes present with a specific combination of psychodynamics and environmental variables that heighten the danger presented by these delusional individuals. Clinicians need to be alert to these danger signals of potential serious physical violence against the persons believed to be impostors.

# In fiction
Capgras Syndrome plays an important part in Richard Powers' 2006 novel The Echo Maker-, winner of the National Book Award.
In Shirley Jackson's short story "The Beautiful Stranger," the protagonist begins with the delusion that her husband has been replaced and is ultimately unable to recognize her own house.
Plot devices playing on this fear (typically in which protagonists know that their acquaintances have been replaced by Espionage|spies, Extraterrestrial life|aliens, or monsters, while others think the protagonist insane), appear in several works of horror fiction|horror and science fiction. Well-known examples include Invasion of the Body Snatchers, The Stepford Wives, Total Recall (film)|Total Recall, The Astronaut's Wife, Impostor (film)|Impostor and The Faculty.  The film adaptation of The Addams Family (film)|The Addams Family offers a unique perspective on this plot device when Uncle Fester is thought to be an impostor by all involved parties, but is actually the true individual who assumed he was playing a role as a result of amnesia and psychological manipulation.
The character of Debbie in Kate Atkinson's Human Croquet may suffer from this, and the syndrome is mentioned about halfway through the book.
An example of Capgras delusion (and possibly a parody of the latter genre) occurs in The Outward Urge, where an astronaut develops the delusion that his colleague is a Martian.
In an episode of CSI:NY, a man was found dead in his apartment with his sister, who claims she was attacked by a woman who had somehow entered their apartment twice before. It was eventually revealed that the woman had Capgras syndrome and that the "intruder" was actually her own reflected image in various surfaces. It was during the last "struggle" with the "intruder" that she accidentally killed her brother. This was later confirmed during interrogation when she tried attacking her own reflection in the interrogation room's two-way mirror.