People who have xenomelia usually don’t disclose their symptoms—even to their doctors.

The condition, also known as body integrity identity disorder, causes an extreme desire to have one of your own, perfectly healthy limbs amputated. Aware of how unusual that request would seem to a doctor, people feeling it usually keep it to themselves.

As a result, the medical history of xenomelia is meager. In a review (paywall) paper published this month, researchers summarized all the scientific literature they could find on it—which turned out to be just 11 papers. A lot of these were single case studies; the largest had 15 subjects. In medical science research, a reliable sample size is usually hundreds of patients. But there just aren’t that sort of data available for xenomelia.

“The reason I’m doing this work is not because I think there’s gonna be an outbreak of people with xenomelia,” says Mihir Upadhyaya, a psychiatrist-in-residency at the non-profit research group the Everest Foundation in Los Angeles, California and lead author of the paper. He estimates the prevalence of the disorder is probably less than even one in 100,000 people around the globe. For comparison, the EU defines a disease as “rare” when it affects fewer than five in 10,000. Upadhyaya himself has encountered two patients with the condition—more, he says, than most healthcare professionals will see in a lifetime.

Despite the scant evidence and rareness of the disease, Upadhyaya and his co-author, Henry Nasrallah, a neuroscientist at the St. Louis School of Medicine in Missouri, argue that understanding xenomelia is important for health care professionals. It teaches physicians that patients who ask to have a healthy limb amputated may have a valid medical reason for the request. Essentially, it provides a lesson in compassion: sometimes sympathy requires actions that would otherwise be considered harmful. Treating patients with true xenomelia means safely amputating the offending limb, giving them a physical disability. As Upadhyaya and Nasrallah write, “the word [patients] most often use to describe how they feel after removal of an unwanted limb is ‘complete.’”

Hardware problem

“The brain is hardware, and the mind is software,” Nasrallah says. “The number one job of the brain is to create the mind.” Any changes to the physical structure of the brain may change the mind as a result.

Take the most famous case in neuroscience: Phineas Gage, a 19th century foreman on a railroad construction site (he shows up in most psych textbooks, according to Smithsonian). In 1848, an explosive accident shot an iron rod straight through his eye and skull. Even with a chunk of his brain missing, he survived, but his personality appeared to change. He became flippant and forgetful, and the railroad company fired him when it was clear he was no longer able to do his job.

Concussions are a much more common injury that can lead to physical changes in the brain: they can both compress and stretch brain tissue, which can lead to memory loss, dementia, and depression.

Then there are genetic mutations, which can cause the brain to form incorrectly in utero, like with microcephaly, a condition where the brain is abnormally small. But even if the brain appears to be physically typical, they may contain genetic mistakes not visible to our current imaging technologies. Sometimes, these mutations are benign, but other times they can cause conditions like anxiety, depression, bipolar disorder, or schizophrenia.

In the case of xenomelia, researchers aren’t totally sure what has physically gone wrong to cause patients to fail to recognize their own limbs. In some cases, it seems to be triggered by head injuries. In others, it’s a lifelong struggle Nasrallah thinks could be related to schizophrenia—and caused by physical changes in the parietal lobe.

The parietal lobe is a large part of the brain that recognizes the body and takes in sensory input. In patients with schizophrenia, parts of the parietal lobe are misshapen (paywall). They also sometimes have difficulty recognizing themselves or others, says Nasrallah—a symptom that can appear similar to the way a patient with xenomelia won’t recognize a limb as their own.

Further evidence: Nasrallah and Upadhyaya have found that in most cases of xenomelia, patients want to have their left leg removed. The brain’s ability to recognize the left leg is specifically located the right parietal lobe—which is why Nasrallah and Upadhyaya others (paywall) now believe a malformed right parietal lobe may be responsible for xenomelia.

Hidden history

It would be much easier to confirm these suspicions if there were more data to work with. But after one report of xenomelia from the 18th century, in which a patient forced a surgeon at gunpoint to remove his leg—he later sent the surgeon a thank-you note and payment—the condition largely disappeared from the literature. It did pop up in a 1972 Penthouse article, featuring a man who was said to have a fetish called apotemnophilia—the sexual desire to be an amputee. (Not to be confused with acrotomophilia, which is a fetish characterized attraction to amputees.)

For a while, it was misconstrued as a form of paraphilia—a condition causing extreme and abnormal sexual urges, usually involving dangerous activities. That created “a stigma, an embarrassment to having this condition and talking to doctors about it,” says Upadhyaya.

That’s one of the reasons Upadhyaya is so interested in studying xenomelia. Usually, when patients feel strongly they are experiencing symptoms that doctors can’t detect—like with Morgellons—they firmly believe they are rational, and continue to seek treatment. But patients with xenomelia typically understand the irrationality of the treatment they seek, and so they tend to keep it to themselves.

Healthcare professionals are not in the business of inducing physical disabilities. “We want people to have two arms and two legs,” says Upadhyaya.

His two xenomelia patients both went overseas for their operations out of desperation, because they couldn’t find a surgeon who would help them in the US. Even if they did, they almost certainly wouldn’t take insurance. Although xenomelia was recognized in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM) IV, it was not in the DSM V.

Patients with “extra” limbs can become so distraught that they resort to drastic removal measures. One man described in Upadhyaya’s review surrounded his leg in dry ice, hoping it would freeze off; it ended up having to be surgically amputated anyway. Another anonymous patient, now 32, told Vice in 2015 that he had dreamed about staging a “bike accident” near a railroad track so that a train would run over his leg. At the time of the interview, he had successfully removed his leg without surgery, although he wouldn’t tell the publication exactly how. Afterward, he said, “I was awash with relief. It was over and I was free.”

A new form of compassion

Patients with xenomelia need treatment. Upadhyaya and Nasrallah agree that the best approach would be to somehow alter the brain itself to recognize all four limbs. But until scientists have a better idea of what specifically causes xenomelia, the most compassionate course of action for patients is surgical amputation of the “extra” limb.

Upadhyaya once met a xenomelia patient post-amputation who said that before the surgery, he could draw a line just above his knee to show exactly where he didn’t recognize his limb. He said it wasn’t quite pins and needles, and it wasn’t quite numbness—it was like a sack of sugar that needed to go. “He felt it was an extra add-on that he was burdened to carry around, even though he couldn’t walk without it,” Upadhyaya says.

“It sounds like a terrible thing to do, but it actually is the only thing they can do to live a normal life,” Nasrallah says.

The whole purpose of medicine is to make people feel better so they can live their best lives. Sometimes, what is best for the patient hasn’t yet been published in medical textbooks. In these cases, doctors have an obligation to try to understand the patient’s concerns. Ignoring or dismissing those as crazy can actually do more harm than good.

Of course, candidates for limb amputation should go through an intense psychiatric evaluation to determine that their xenomelia isn’t a temporary symptom of a larger psychiatric illness, like schizophrenia, and to ensure that they really are mentally fit to be making this permanent decision. But if they are, perhaps doctors should be prepared to broaden the scope of their medical empathy to include even the sorts of requests—like that of a xenomelia patient—they would normally chalk up to simple craziness.

Meanwhile, xenomelia provides an opportunity for scientists to learn about how the brain interprets the body that carries it around—and as they uncover more, perhaps they’ll also develop new ways to help patients keep and accept all of their limbs.