After an amputation, it is not rare for patients to still have sensations or to feel extreme pain in their missing limb. In this case, the situation can be crippling. This neuropathic pain has multiple origins, in both the peripheral and central nervous systems. This is not yet well understood. There has been some evidence for a reorganization of the cortex linking inputs from other body parts to the brain region associated with the missing one. But a recent study shows that phantom pain is correlated with a preservation of the cortical area corresponding to the missing hand. This finding could yield new therapeutic approaches.
Source : WikiCommons/Edward M. Hubbard.
After an amputation, some patients can still feel their missing limb as a phantom sensation (tingling, hot or cold, numbness…) or phantom pain (stabbing, shooting, burning, cramping…). The latter phenomenon occurs in 50 to 80% of amputees. It is often localized in the extremities of the missing limb. These feelings are mostly transient, but can be excruciating. Their frequency ranges from rare to constant… This pain is difficult to control but can decrease with time. It can compromise a person's life physically as well as psychosocially.
Ghost pain is neuropathic; meaning is a chronic pain resulting from injury to the nervous system. As such, it is very difficult to treat. It represents a real medical challenge, as each patient must be managed individually. Indeed, they will respond differently to the available treatments: pain medication, electrode or magnetism stimulation, surgery and mirror therapy among others. This pain has multiple origins, as it involves both branches of the nervous system: the peripheral (from the end of the finger to the spinal cord) and the central (including the spinal cord and the brain). For example, in the absence of peripheral nerves, some pain signals are not inhibited and thus, are continuously transmitted to the brain via the spinal cord neurons. Furthermore, at the brain level, various studies, including the first by V. S. Ramachandran in the nineties, showed that an amputation was followed by a reorganization of the primary sensorimotor cortex. This zone processes the touch and/or pain inputs to send instructions to the muscles.
This maladaptive plasticity model is based on the brain’s ability to adapt to new conditions. In this case, the loss of input in the deprived limb area of the primary sensorimotor cortex leads to its remapping and its responsiveness to inputs from cortical neighbors. For example, after the arm amputation, its phantom sensation was linked to lip movements. More remapping is linked to more pain, and a correlated reduction in representation of the missing limb in the sensorimotor cortex.
However, based on several lines of evidence suggesting that mapping of the cortex is preserved, T. R. Makin et al. from Oxford University and the Hospital for Sick Children in Toronto, looked directly into the cortical hand area. Their results, published in Nature communications on March 5, 2013, supported this hypothesis, and opened up a potential new therapeutic approach.
The researchers used functional MRI to examine brain activity in three groups: amputees, patients born with only one arm, and adults with two full limbs. The scans allowed them to locate the stimulated brain areas when subjects moved their hands in reality or by imagining it for the missing one.
They notably observed that ghost pain was associated with a preservation of the representation of the hand in the brain and a reduction of the connectivity between the phantom cortex and the sensorimotor system. For T. R. Makin, “the aim of this study was to be able to directly target the right area in the brain to re-couple the representation of the phantom hand with the external sensory environment. This smart rehabilitation is currently under clinical trial by using the non-aggressive transcranial direct current stimulation technique developed at Oxford University to act directly at the neuron level”.
For Dr Alain Serrie, chief of the service douleur at Lariboisière Hospital in Paris, these patients must be treated with various techniques in a global approach (physical and psychological for example). At present, 30 to 40% of patients can manage their pain and regain a certain quality of life, allowing them to work and maintain their social interactions. Concerning the transcranial stimulation tested according to the results of the T R. Makin study, he believes that even if it cannot treat amputees, it could be used to select them for a stimulation of the premotor cortex by a neurologic pacemaker requiring surgery.