It seems that almost daily, I get a question from some patient somewhere wondering why their (insert body part here) hurts when they’ve had a nerve injury despite the fact that the area feels numb to the touch. This phenomenon can be seen in patients suffering from diabetic neuropathy (most commonly noted in the lower extremities), amputees with phantom limb pain and anyone with a sensory nerve injury anywhere else (e.g. the head/neck region). I will qualify my remarks below by saying that this topic is a huge one and cannot be covered in its entirety in a brief post or even a book chapter. There are whole journals published monthly devoted to the study of such clinical dilemmas. The goal here however is to provide a general understanding of why one might have these types of sensations and as a launch point for discussion with your treating physician about what can be done. I will also use phantom limb pain as the template for understanding this problem as it is one of the most common manifestations of this problem and the one most conceptually accessible to a non-physician.
First of all, what is phantom limb pain? Simply put, it is the sensation of pain from a body part that no longer exists. For example, a right below-knee amputee feels as if the right foot is being squeezed and is painful, even though that very foot was removed a long time ago. But how is this possible? Phantom limb pain has traditionally been hypothesized to occur as a consequence of abnormal mutability of signals within the brain (specifically the cerebral cortex) as a result of lost input from a limb. Translating from medicalese, since the sensory input from a limb no longer exists, the neurons within the brain that used to map to that part of the body re-organize themselves in an abnormal way thus leading to the perception of pain. Another potential mechanism is that the nerve ends from those nerves that used to go to the foot and now reside in the amputation stump are irritated in some way, but still go to that part of the brain which mediated right foot sensation. Therefore, again, when those peripheral nerves fire, the patient perceives that they have right-sided foot pain even though there is no right foot because those signals ultimately still end up in the right-foot-part of the brain (which of course still exists). This situation might occur if you strike the nerves within the stump (e.g. while wearing an ill-fitting prosthesis) of if they are neuromatous. It might also occur if a nerve end that has been implanted into a muscle in the neck is “tweaked” by that muscle. There are other theories as well which state that nerves within the spinal cord that receive sensory input from an absent limb fire abnormally, thus ultimately sending messages to the brain that one is experiencing pain. So which theory is correct?
Well, as with many things in life this problem is not a zero-sum game. In other words it’s not that one theory is absolutely right and the others are all wrong. The overall pain sensations are likely due to a combination of factors. In fact, I was just reviewing an article in a prominent pain journal in which they demonstrate that blocking a peripheral nerve in an amputation stump leads to some persistence of phantom limb pain, whereas blocking nerves in the spinal cord leading to that limb resulted in temporary, but complete cessation of said pain. This result would suggest that it is these spinal nerves that mediate this pain. However, the authors then go on to admit that electric charges emanating from peripheral nerves within a stump are likely responsible for the sensation of phantom pain when a person bears weight, such as while wearing their prosthesis. My take home message from this paper is therefore that there are several components to this phantom pain. One component may occur at rest or at night when no pressure is placed on the stump. This component of the phantom pain is important and may be treated by addressing those spinal nerves. However, if you are an amputee, you’ll likely want to walk using a prosthesis at some point. If so, those peripheral nerves at the stump also need to be addressed so that this component of phantom pain gets better allowing the patient to ambulate. Indeed, this latter mechanism is the partial rationale behind targeted muscle re-innervation in the extremities. Therefore, in any individual patient, the optimal pain relief will probably only be achieved by several specialties working together to attack the problem from a number of angles.