Delusional misidentification syndromes are a group of rare disorders that result in bizarre delusions. For the first time, neuroscientists have uncovered the neuroanatomy that underlies these strange experiences.
Identified around a century ago, delusional misidentification syndrome (DMS) is an umbrella term for a group of delusional disorders.
All DMSs include the belief that something - an object, person, or place - has been changed in some way.
In other conditions that involve delusions, such as schizophrenia, a large chunk of the patient's perception of the external world has changed. With DMS, however, it is only one particular object that becomes the focus of the delusions. Hence, DMS is referred to as a monothematic delusion.
One of the first DMSs to be documented was Capgras syndrome. In this strange condition, a patient recognizes a family member, but, at the same time, believes that there is something distinctly unfamiliar about them. This can lead them to conclude that the family member is, in fact, an imposter.
Another DMS, the Fregoli delusion, is the belief that strangers are actually family members in disguise. Known as the delusion of doubles, this DMS can also involve pets or places.
Despite being well documented, the neural basis of these delusions has proven evasive. Neuroscientists at Beth Israel Deaconess Medical Center (BIDMC), in Boston, MA, recently embarked on an effort to dig a little deeper, and they set out to understand which regions of the brain are at fault.
Investigating the delusional brain
Led by Dr. Michael D. Fox - director of the Laboratory for Brain Network Imaging and Modulation, and the associate director of the Berenson-Allen Center for Noninvasive Brain Stimulation at BIDMC - the team's findings are published in the journal Brain.
The researchers, which included lead author Dr. R. Ryan Darby, identified 17 patients with DMS and mapped them onto a brain atlas. Next, they used a lesion network mapping technique, recently developed by Darby and his colleagues.
All 17 of the patients were found to have lesions in brain areas that connected to the retrosplenial cortex, an area thought to be important in the perception of familiarity, episodic memory, navigation, and planning,
Additionally, 16 of the 17 had lesions connected to the right ventral frontal cortex, an area associated with evaluating beliefs.
When compared with brain maps of patients with delusional conditions other than DMS, differences were noted.
"Lesions causing all types of delusions were connected to belief violation regions, suggesting that these regions are involved in monitoring for delusional beliefs in general. However, only lesions causing delusional misidentifications were connected to familiarity regions, explaining the specific bizarre content - abnormal feelings of familiarity - in these delusions. In other words, lesions had to be connected to both regions to develop delusions like Capgras."
Dr. R. Ryan Darby
The authors of the study are quick to note the research's shortfalls. For instance, this mapping methodology does not involve brain imaging, such as functional magnetic resonance imaging (fMRI) scans. It relies on taking data from normal patients and determining the regions of the brain that are normally connected to the known lesions in the patient's brains.
Dr. Darby notes that the study would need to be replicated with a much larger sample. This condition is rare, however, and recruitment would therefore be challenging.
Shortfalls aside, the findings will still be useful in helping families cope with this strange phenomenon. The delusions can sometimes disappear as swiftly as they arrived, making dealing with the condition very difficult for loved ones.
Dr. Darby says: "The impact on the patient's family can be heartbreaking. I've seen patients who, thinking their homes were replicas, would pack their bags every night, hoping to return to their 'real' home. Patients who believe a spouse is an imposter often lose intimacy. In these cases, even just knowing that the delusion has a name and is part of a neurological disorder can be helpful for family members."
The study marks a leap forward in understanding DMS. As Dr. Fox says: "Understanding where these symptoms come from is an important step toward treating them."