Misidentification syndromes are some of the most fascinating psychiatric disorders around.  I would say ‘cool’ but I’m sure that suffering from one can be very dispiriting, and will continue to use this adjective for trainers and indie bands only.  They involve a disturbance in the judgement of uniqueness of certain events.

First up, there’s Capgras Syndrome; also known as I’illusion de sosies (Illusion of doubles).  Here, a person is under the impression that someone close to them has been removed and replaced by an identical looking impostor.  It is named after Joseph Capgras, a French psychiatrist, who described this in a paper published in 1923.

Capgras Syndrome is associated with schizophrenia in more than half the cases. The theory behind it runs like this: when the eye sees a face, the brain processes the information in two parallel streams, which can be damaged independently.  Faces are at once explicitly identified via the temporal cortex and also more rapidly though the amygdala, which is involved with the limbic - emotional processing - system.  We can see an example of this rapid processing if we were to find ourselves running away from something without fully understanding what it is. 

If the temporal cortex path is damaged, the brain will have difficulty in recognising a face.  However, via extra-visual clues, emotional responses to familiar faces are preserved.  Therefore someone with prosopagnosia (inability to recognise faces) will still have an emotional response to the faces of people they know.  If the amygdala path is compromised then the person will still recognise the face, but the expected emotional response will be absent.  This could lead the feeling that a familiar face is ‘not quite right’ and to the erroneous conclusion that his is because of an impostor.

Subtly different is Fregoli Syndrome.  Rather than named after a pioneering psychiatrist, this disorder was named after Leopold Fregoli (1867-1936) who was an Italian actor and the greatest quick change artist of this day.  He was famous for his ability in impersonations and his quickness in exchanging roles, so much so that at times rumours spread that his act was in fact performed by more than one person.  This is also known as the illusion of a negative double, and the suffer believes that various people he or she meets is actually the same person in disguise.  This often has a paranoid flavour, with the sufferer believing that that are pursued by a someone that assumes different identities. 

A sort of combination of Capgras Syndrome and Fregoli syndrome is intermetamorphosis syndrome, first described by P Courbon and J. Tusques (1932); in this, the subject develops the delusional conviction that various people have been transformed physically and psychologically into other people.  This disorder involves false physical resemblance and false recognition. 

Almost there.  Reduplicative paramnesia was described by Pick in 1903 and is often seen in post traumatic brain injuries. With this, there is a belief that a familiar person, place, object or body part has been duplicated. For example, a person may believe that they are in fact not in the hospital to which they were admitted, but an identical-looking hospital in a different part of the country. 

Finally, there’s delusion of subjective doubles, in which a person believes there is a doppleganger or double accompanying the self.  Apparently meetings with doubles were a popular theme of 19th century romantic literature (see Dostoyevsky’s The Double).  It was believed that we each have a doppleganger who normally remains unseen; if we see our doppelganger then death is imminent…

Here’s a famous definition of delusion:

‘A belief held with unusual conviction that is unamenable to logic whose erroneousness is manifestly obvious to others’ - Jaspers (1959)

This came to mind the other day when I was reading about Mohamed Al Fayed’s peformance in court at the inquest into the death of Princess Diana.  Al Fayed spent time outlining the extent of the international conspiracy which had been involved her death and that of his son, stating that conspirators included Tony Blair, Robin Cook, MI5, MI6, the CIA, the French intelligence service and the French ambulance service, who drove to the hospital deliberately slowly so that she might die. 

This doesn’t strike me as very likely and this view appears to be shared by the inquiry’s coroner, who asked Mr Al Fayed: ‘Do you think that there is any possibility, however remote that your beliefs about conspiracies may be wrong and that the deaths of Dodi and Diana were in truth no more than a tragic accident?’

Fayed replied: ‘No way.  I am 100% certain’. 

I think this would count as ‘unusual conviction’.  In ICD-10, there is a diagnosis of ‘delusional disorder‘, which is defined by the presence of persistent, non bizarre, delusions.  A non-bizarre delusion is plausible; this is in contrast to a bizarre delusion which is not.  For instance a person who thinks that they are under survelliance by the security force may be delusional, but this does happen to a small number of people. This is non-bizarre; a person would hold a bizarre delusion if this had no chance of being true, for instance if they felt that there was a goat living on their head.  Delusions also need to be outside what is considered to be culturally accepted for instance, in isolation, some religious practices might be considered odd, but they are widely accepted and so not delusional. 

If you were to meet a person with a delusional disorder you might not notice anything obviously odd about them.  This is in contrast to someone who is suffering from a psychotic delusional disorder, when their behaviour may appear manifestly odd.  They are able to continue functioning normally, although may make some strange decisions based on their world view. 

Finally, sometimes people are labelled as being delusional, when in fact they are not.  This is called the Martha Mitchell Effect this is when a psychiatrist mistakes a patients perception of real events as delusional and misdiagnoses accordingly.  It is named after the wife of the attorney general in the Nixon administration who alleged that White House staff were engaged in illegal activities.  Her claims were attributed to mental illness, but the outcome of the Watergate scandal vindicated her.