Imagine that one of your limbs did not feel as if it belonged to you or was a natural part of your body. In a weird way the “extra” limb makes you feel incomplete, less than your whole self. The limb functions, you can feel it, and its completely healthy – it’s just not yours. It is a constant irritant. Eventually you become obsessed with the idea of amputating the limb. You fantasize about amputation and imagine yourself without the offensive body part.

This desire to amputate a healthy limb was described and named apotemnophilia  by Money in 1977. It has recently been renamed Body Integrity Identity Disorder (BIID). This is a rare and interesting disorder – at first it was thought to be psychological, but it is more likely to be a neurological disorder.

Recent fMRI evidence suggests that people with BIID have decreased activation of their right superior parietal lobe when the “extra” limb is touched. This likely relates to the brain’s internal map or schema of the body.

We take for granted that we feel as if we occupy our bodies, that we own the various parts of our bodies, and that we control them, but each of these sensations is actively constructed by circuits in the brain. These circuits can be disrupted.

When those parts of the brain involved in the network that makes us feel as if we occupy our body are disrupted the result is an out-of-body experience. This active brain construction relies upon sensory information, which for sighted people is mostly visual, but also sensory. This fact can be exploited using virtual reality technology. Essentially a subject can view a virtual image of themselves being touched, while the corresponding body part is actually touched. This can trigger the brain to construct their physical location in the virtual image – they will feel as if they physically occupy their virtual self.

Control is a similar phenomenon. Circuits in the brain compare the intention to move a limb with the actual movement, monitored by sight and sensation (tactile and proprioception, which is the sense of where a body part is in three-dimensional space). When the intention to move and the actual movement match, the brain constructs the sensation that you control the relevant body part. If this circuit is disrupted the result is called alien-hand syndrome – sufferers feel as if they do not control a body part, which acts on its own as if it is under alien control.

Finally there can be a mismatch between the brain’s internal map of the body (through a network called the ownership module) and the body itself. This is most often seen with phantom limb syndrome – after losing a limb some people still feel its presence, because their brain is still mapping to the missing limb. There are also cases of supernumerary phantom limb, when stroke or other brain damage disconnects the ownership module from the paralyzed limb, and it temporarily creates an extra phantom limb to own.

Interestingly, although rare, congenital phantom limb syndrome has been described. This is very interesting for it raises the question (very relevant to BIID) of why someone’s brain would map to a limb that never existed. This is a bit of a controversy that has not yet been resolved.

The simplest explanation is that the brain’s body map is genetically determined and hard-wired. In other words – it is independent of the development of the body itself. This contradicts to some extent the notion that brain areas actively map to body parts during development, so-called somatotopic mapping, which requires sensory feedback from the body part. Without the sensory feedback, why would the brain allocate resources to a non-existent limb.

An alternate theory is that the brain does interactively map to the body, but it is not entirely dependent on internal sensation. The process may also involve seeing other people and mirror neurons mapping to the image of what people look like. Body image is therefore partly learned from experience.

Given this as background, BIID can be seen as fitting nicely into this set of disorders that represent a mismatch between the brain’s actively constructed model of self and the physical reality of self. In the case of BIID the body is whole but the brain’s map of it is missing a piece, so that body part does not feel like it belongs.

BIID is rare, and so we mostly have case reports to go on in understanding this disorder, with only preliminary fMRI studies. A review of published case reports does provide some useful information. They found among cases: low age of onset, male predominance, no preferred sexual orientation, an association with gender identity disorder, no pertinent family psychiatric history, no preferred side of the unwanted limb, association with exposure to an amputee at a young age, no history of trauma to the unwanted limb, frequent attempts at self-amputation, and association with certain personality disorders, such as borderline personality disorder.

In terms of treatment the reviewers found that selective serotonin reuptake inhibitors (SSRIs) and cognitive behavioral therapy (CBT) were sometimes effective in relieving the stress and depression associated with BIID, but they did not diminish BIID itself. The desire for amputation was unchanged. Further, in patients who did have the limb amputated, 70% had resolution of the desire for amputation, while 30% had a recurrence of the desire.

It is not yet clear what all this means, but there are some suggestions. Probably BIID is not one discrete disorder, but is the final result of several possible underlying causes that result in the brain-body mismatch.

The onset at young age and exposure to amputees is interesting, as these facts are compatible with the theory that body mapping occurs during young development and is partly due to experience. Perhaps, therefore, some people with BIID developed their internal body image based upon exposure to an amputee, and the missing limb therefore became the model of a whole body.  It’s also possible that exposure to the amputee was simply a trigger but the predisposition has to be present already.

Regardless of how it develops, it will likely prove difficult to “cure” BIID. We do not yet have the technology to alter the brain’s wiring in a way that would be necessary to change the internal body image. The brain does have plasticity (the ability to change its wiring), but this plasticity is limited. Some basic functions can only develop at a young age, and once the window of development closes the wiring cannot be changed.

Perhaps brain plasticity may be able to reduce or correct BIID, but I could not find any research even attempting such an intervention.

Current treatments focus on reducing the negative consequences of BIID rather than BIID itself, namely the distress and depression. This seems like a reasonable approach, and should at least be tried prior to amputating a healthy limb, an intervention with only a 70% success rate.

The ethics of amputating a healthy limb to treat BIID are unclear. It is not unreasonable, however, based upon the principle of autonomy, and it does appear to improve the quality of life in most BIID patients who have an amputation. But this should be considered a last resort, only after serious attempts at non-invasive treatment have been made.

It is time, however, for BIID to come out of the shadows. It should not be stigmatized. It is simply a neurological disorder. Even though we do not yet have anything close to a cure, it can be addressed medically.