(CNN) – For some, the universal human desire for wholeness may only be fulfilled by an act of excision.
New science is showing how specific parts of the brain may differ in those with the rare but fascinating disorder known as body integrity dysphoria, or BID.
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Those with BID appear by objective standards to have healthy bodies, but they cannot shake an insatiable feeling of incompleteness: In order to be their full selves, they must remove or amputate a limb or other body part.
The condition is believed to be driven by a mismatch between someone's mental picture of their body image and their actual physical self.
One way to explain it physiologically is that in BID, the part of the brain associated with body image has less functional connectivity with other parts of the brain, according to a study published Thursday in the journal Current Biology.
Researchers recruited 16 men who want to remove their healthy left legs from a support group website for people with BID and compared scans of their brains to 16 men without the condition.
Those with BID have less brain connectivity in specific areas
"We show clear associations between a mental state and changes in brain structure and functionality," said Peter Brugger, a professor of neurology and neuropsychiatry at the University of Zurich.
Those with BID had noticeably different neural architecture in the paracentral lobule, the part of the brain that controls how we feel and interpret feedback from the lower limbs. The area had reduced connectivity to other areas of the brain.
The right superior parietal lobule, an area used to construct one's total body image, also had reduced connectivity and a reduced density of gray matter, indicating less neuron activity.
"The feeling that a limb belongs to us relies on the extent to which the sensorimotor limb area is functionally connected to all the other brain regions," said the study's main author Gianluca Saetta, a doctoral student at the University of Zurich.
Having enough gray matter in the right parietal region of the brain is necessary to create a healthy perception of the body in space.
"Interestingly, we found that the less gray matter in the right parietal region of the brain, the stronger the desire for amputation, and the more BID individuals acted as if they were amputees," Saetta said.
The disorder is rare
There are only about 200 cases of BID recorded in the medical literature, but it's expected to be included in the 11th edition of the International Classification of Diseases (ICD-11), as a "disorder of bodily distress or bodily experience," the researchers said. The new revision is set to take effect in January 2022.
Brugger first became interested in the disorder after observing a similar phenomena in which people have sensations of limbs that never fully developed.
"In the case of a woman born without arms and legs, we could show that arms and legs are nevertheless represented in the brain," Brugger said. "If such a thing is possible -- your brain contains the signature of a limb that has never been there -- then we thought it might also be possible that a brain may lack this signature despite regular physical development."
At the moment, there isn't a known cure or treatment, but Saetta noted that for those who did achieve amputations, they "only regret to not have it done sooner."
Yet many don't have the option for amputation, living through the discomfort.
"Some seek transient relief in pretending to be an amputee, for instance by using crutches or wheechairs," he said.
Those with BID commonly stay silent, and outside of people who join online forums self-identifying as sufferers, it's been difficult for researchers to get a read on how many people may have the condition.
"Any estimation of how common BID is, first requires the definition of precise diagnostic criteria," Brugger said. "Our findings will be helpful for the definition of these criteria. This will, in turn, assist the development of potential rehabilitative treatments."
He and Brugger hope to find non-surgical ways to treat those with BID, such as transcranial magnetic stimulation or "deep brain stimulation with implanted electrodes."
Stimulating brain cells has shown success in treatment-resistant depression, by activating parts of the brain that have diminished activity, according to the Mayo Clinic.