Prosthetic Categories

How Limb Loss Affects the Brain

    Unlike other animals, humans are constantly developing. This is why we need our parents’ protection in the first phase of life. This is also why our experiences have the power to make our brains adapt, change, and modify in function and structure throughout life. This phenomenon is called neuroplasticity, and it plays a part in how people suffering from phantom limb pain can overcome it.

     Mirror box therapy can retrain the brain to stop phantom limb pain.

    Phantom limb pain in history  

    Notable people throughout history have been critical to how scientists understand phantom pain today. The earliest notes discovered that describe phantom limb pain as “sensory ghosts” was by Ambroise Paré, a French doctor in the 16th century.

    But the first thorough description is by the father of American neurology Silas Weir Mitchell. Hospitals were filled with patients after the Battle of Gettysburg during the Civil War. The majority of these patients had their limbs amputated. During this time, Mitchell noted that of 90 amputees, 86 reported sensations—ranging from pain and itchiness to shrinking and heat—in their missing limb.  

    Literature is rife with references to these sensory ghosts. For example, the poet Walt Whitman’s stories. Whitman went to Fredericksburg, Virginia, in 1863 to look for his wounded brother. He then spent the next three years volunteering at a field hospital, accompanying and assisting wounded men. During this time, Whitman gathered many stories of amputee soldiers who told of the ghosts of their flesh.

    Furthermore, in the 108th chapter of the classic novel Moby Dick, readers can find Captain Ahab’s descriptions of the sensations in his amputated limb.

    Phantom limb pain in science   

    But how does science explain these sensory ghosts? It’s all in the brain.

    Inside our brains are areas that regulate distinct functions, which is why specific brain injuries can cut off our sense of taste and make us lose the ability to move a particular body part or speak. In the same way, there are areas in the brain that correspond to sensory information from a specific arm or leg.

    When that arm or leg is amputated, the areas in the brain that receive the signals from the amputated limb remain active due to the time it has been used. 

    This is because when the brain has learned to direct and locate a body part, the brain continues to think that it’s still there, even if that body part ceases its function.

    And when the brain stops receiving signals from the limb, the part of the brain that loses its function generates spontaneous discharges that people interpret as discomfort, itchiness, or pain. Then the neural circuits responsible for processing input and producing movement in the missing limb lose their role, making it prone to entangling with other neural networks, like the one responsible for perceiving pain. Scientists call this theory the stochastic entanglement theory.

    Treating phantom limb pain through the brain  

    Traditionally, physicians tried to treat phantom limb pain by treating the residual limb. However, this is not where the pain resides. Certain pain medicines may work, but chronic pain requires a different approach.

    Vilayanur S. Ramachandran, an Indian doctor, found a way to treat phantom limb pain by tricking the brain. He started with the idea that patients whose limbs were paralyzed before an amputation were more likely to experience phantom pain. This implied that the patients tried to move the limb for a time without success, which created a sensory feedback loop that the brain remembered.

    The phantom pain was due to how the brain interpreted that it should be trying to move the paralyzed limb. 

    Following this idea, Ramachandran thought that therapy would require brain retraining to eliminate this assumed paralysis. So, he devised a system using a box with a mirror in which the sound side limb, through its reflection, mimicked the missing limb. This allowed the brain to interpret that the missing limb was no longer paralyzed.

    When done consistently for a time, the mirror box therapy has been successful, but many studies that describe its use don’t have a sufficient methodology. This is why the theory remains controversial. However, this theory does shed some light on a centuries-long mystery and the crucial part our brains play in treating phantom limb pain.