Phantom limbs are a well-known phenomenon where sensations and feelings are still experienced from a missing limb. In rare cases after brain injury, an additional phantom limb can appear – causing the sensation of a phantom third hand, arm or leg.
The drawings on the left are from two case studies of people with these ‘supernumerary phantom limbs’ recently published in the journal Neurology. They show an artist’s impression of the body sensations of two patients who suffered brain stem strokes.
Both patients had the experience of having a third arm and a third leg, although the male patients had the leg ‘appear’ along the midline of the body, while the female patient seemed to experience it ‘superimposed’ upon an existing leg.
One distressing element for the female patient was that although the patient could ‘move’ the phantom arm voluntarily, “she described occasional loss of control and feeling strangulated by the phantom arm around the neck”.
Two earlier case studies from neuropsychologist Peter Halligan and colleagues reported similarly disembodied extra limbs, but this time after damage to the right hemisphere of the brain.
As is more common after right hemisphere damage, these tended to have a delusional quality, so they weren’t just sensations – the patients genuinely believe their additional limbs existed.
In this last case, the patient reported actually ‘seeing’ the additional limb, similar to this case study of a gentleman who believed he had a third leg protruding from his left knee after suffering a stroke that affected the thalamus:
He consistently maintained that the phantom leg was attached to his knee with a “bone plate” that “had no flesh on it”. However, he reported that the phantom limb itself looked normal and had a shinbone and a foot. It usually “appeared” in the morning when he was helped to put on his trousers. The patient stated that the phantom limb prevented him from turning over in bed, but did not adversely affect him otherwise.
When asked about how he knew about this leg he said that he could see it (despite his severe visual impairment) and feel it with his hand. He believed that the phantom limb belonged to him, although he readily accepted that it was not “normal” to have three legs. Initially he reported that the “leg” was growing from his own knee, but then reasoned that (given its size) he would have noticed it before the stroke.
At other times he believed the leg was attached to him by the nursing staff, but could not explain why. The patient was aware of phantom limb phenomena as his wife was an amputee. He was also aware that a stroke may affect perception and cognition. He did not believe either issue applied in his case.
The experience of a ‘supernumerary phantom limb’ is usually the result of a brain injury and typically resolves over time.
Phantom limbs are thought to arise because the somatosensory cortex, the part of the brain that represents the body’s sensations and feelings, reorganises so that the area previously used to represent the limb is partially ‘re-used’ for other functions, meaning the sensations sometimes get activated when these other functions are active.
Nevertheless, supernumerary phantom limbs are still mysterious, largely due to the small number of cases and diverse brain areas involved.
There is some suspicion that they might be caused because of disrupted communication between parietal lobes, which are known to represent body image, and the sensory feedback from the nerves in the body.