Phantom limbs are usually sensations that appear after an arm or leg has been amputated, but one case reports a phantom limb that appeared as an additional arm extending from the middle of the chest – despite all of the limbs being completely healthy.
The patient, reported in The Journal of Neuropsychiatry and Clinical Neurosciences, was a 31-year-old woman who didn’t have any limbs amputated but did have damage to her brain that altered how information was passed back and forth between her body and her ‘somatosensory maps’ – the areas of the brain that directly map on to and represent body parts.
During our observation period, she claimed to have the sensation of an extra arm arising from the middle of her upper chest. She explained that it was painless and of the same length as her real arms. She claimed that she was able to move the extra arm. She and her husband reported that the phenomenon occurred after the episode of tetraplegia [limb and torso paralysis]. This phenomenon continued for 14 months after her admission.
We have discussed a case of a ‘phantom third arm’ before, although this is somewhat different, as in the previous report the third arm seemed to ‘emerge’ from the patient’s shoulder, just as one of the existing arms did. In this case, though, the phantom arm seems to have been relocated to the chest.
The researchers suggest that the brain damage caused the re-organisation of the somatosensory maps for body parts and a disruption to the flow of normal sensations into these areas. Notably, on the brain’s body map (which you can see drawn out as a cortical homunculus) the arm and chest lie next to each other.
The areas that previously represented the shoulder/arm but had become disconnected began to receive the information from the neurons that represented the upper chest, blending the sensations and leading to the strange sensation of feeling a chest and what seemed like an additional arm at the same time.
It must be said, that while this is a likely explanation from what we already know about the brain and phantom limbs, it is just the researchers’ best guess as they didn’t test out the ideas any further.
Link to full text of case report.