Delusional Misidentification Syndrome

Delusional misidentification syndrome is an umbrella term, introduced by Christodoulou (book "The Delusional Misidentification Syndromes", Karger, Basel,1986) for a group of delusional disorders that occur in the context of mental or neurological illness. They all involve a belief that the identity of a person, object or place has somehow changed or has been altered. As these delusions typically only concern one particular topic they also fall under the category called monothematic delusions.

This syndrome is usually considered to include four main variants:

• The Capgras delusion is the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.
• The Fregoli delusion is the belief that various people the believer meets are actually the same person in disguise.
• Intermetamorphosis is the belief that people in the environment swap identities with each other whilst maintaining the same appearance.
• Subjective doubles, described by Christodoulou in 1978 (American Journal of Psychiatry 135, 249, 1978), is the belief that there is a doppelgänger or double of him or herself carrying out independent actions.

However, similar delusional beliefs, often singularly or more rarely reported, are sometimes also considered to be part of the delusional misidentification syndrome. For example:

• Mirrored-self misidentification is the belief that one's reflection in a mirror is some other person.
• Reduplicative paramnesia is the 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, despite this being obviously false.
• The Cotard delusion is a rare disorder in which people hold a delusional belief that they are dead (either figuratively or literally), do not exist, are putrefying, or have lost their blood or internal organs. In rare instances, it can include delusions of immortality.
• Syndrome of delusional companions is the belief that objects (such as soft toys) are sentient beings.
• Clonal pluralization of the self, where a person believes there are multiple copies of him or herself, identical both physically and psychologically.

There is considerable evidence that disorders such as the Capgras or Fregoli syndromes are associated with disorders of face perception and recognition. However, it has been suggested that all misidentification problems may exist on a continuum of anomalies of familiarity, from déjà vu at one end to the formation of delusional beliefs at the other.