Worldwide, about 450 million individuals have some sort of mental illness. Whereas such illnesses as anxiety disorder, depression, and eating disorders are widely known and seen more commonly in the population, there are a plethora of rare psychiatric illnesses that physicians may encounter. Although these disorders are seldom seen, it is important that physicians remain aware of them to ensure proper diagnosis and treatment for patients with these illnesses. These rare psychiatric syndromes range from reactions to overwhelming situations, delusions as a result of traumatic brain injury, and specific manifestations due to the presence of other psychiatric syndromes.

Capgras Syndrome

Capgras syndrome, named after the French psychiatrist who described "the illusion of doubles," is a delusion of misidentification. It is characterized by a person's delusional belief that someone whom they know, usually a spouse or other close family member, has been replaced by an identical-looking impostor or several doubles.

Capgras syndrome occurs most often in patients with schizophrenia, although it has also been reported in patients with dementia or epilepsy, as well as in patients who have experienced traumatic brain injury. Capgras syndrome is usually managed through treatment of the underlying disorders and typically involves antipsychotic medications. However, cognitive-enhancing treatments and nonpharmacologic strategies that diminish disorganization should be used in patients with dementia.

Although there are very few documented cases of patients with Capgras syndrome, two new cases emerged last year in patients with Parkinson disease. This has led researchers to believe that there may be a correlation between the development of Capgras syndrome and Parkinson disease with dementia.[1] Although the underlying neurologic cause of Capgras syndrome is unknown, some researchers theorize that it is caused by increased dopamine levels due to life stressors or medications, whereas others believe that brain lesions are a possible cause.[2,3]

Fregoli Syndrome

Fregoli syndrome is named after the Italian actor Leopoldo Fregoli, who was renowned for his ability to make quick changes of appearance during his stage act. Fregoli syndrome is essentially the inverse of Capgras syndrome. It is also a delusion of misidentification and is characterized by a person's belief that persecutors or familiar people can assume the guise of strangers, in that different people are in fact a single person who changes his or her appearance or who appears in disguise.

As in Capgras syndrome, Fregoli syndrome occurs most often in patients with schizophrenia, although it has also been reported in patients with dementia or epilepsy and those who have experienced traumatic brain injury. Therapy for Fregoli syndrome also involves treating the underlying disorders and usually involves antipsychotic medications. Cognitive-enhancing treatments and nonpharmacologic approaches that diminish disorganization should be used in cases of dementia.

A 2017 study used hypnosis in patients with Fregoli syndrome to test how strong their convictions were. Study participants were exposed to a stranger whom they had never met, but would believe that they knew this individual and they were in disguise. Researchers challenged this by telling the participant to focus on the voice of the stranger, and also tested whether the participants could identify the stranger in a photo. After hypnosis, it was found that over one half of the participants still claimed they knew the stranger and held true to their convictions.[4]