We usually interpret pain as meaning that some part of us is injured or inflamed and needs rest or a particular treatment to promote healing, unfortunately this view of pain is misleading in cases of chronic pain. When pain has been present on a daily basis for 3 months or more, it is called chronic pain, (this expression does not in fact refer to the severity but simply means that the pain is persistent). Chronic pain is often difficult to control because ordinary pain killers can lose their effect over time, and as a separate problem surgical interventions may not completely abolish the pre-existing pain problem. The reasons as to why chronic pain occurs remain poorly understood, but clearly pain will persist if the inflammatory mechanisms that generate pain through chemical messages fail to subside in the apparently normal way.
We are familiar with the throbbing pain that accompanies injury to, or infection of, a finger or thumb. This sensation of throbbing arises because the nerves are made more sensitive as a direct result of the associated inflammation that follows disruption of tissues. Normally we would not even be aware of the pulsing of blood into our finger each time our heart pumps, but now we feel each throb and it is uncomfortable. In the absence of inflammation, blood still pulses into each of our fingers with every heartbeat yet we do not notice it. This phenomenon is easily demonstrated by a pulse oximeter. This device should be familiar to anyone who has been in hospital in recent years, recalling the probe that is conveniently placed on a finger to identify that a patient is carrying a normal amount of oxygen in their blood. There is usually a screen which displays a wave passing across the screen as peaks and troughs, each peak representing the increase in the amount of blood in the finger tip produced by each heartbeat. Normally there is enough room for this tiny extra volume of blood, but when the finger is swollen there is correspondingly less room for extra blood so the pressure rises with each pulsation or throb.
The nerves, having become extra-sensitive due to chemical messages released from inflamed tissues, respond to a small increase in pressure in a way that would not excite them under normal circumstances, and in addition, this message acquires an unpleasant or noxious character which we call pain. This uncomfortable situation can only improve when the swelling and sensitivity subside, which is of course what normally happens. In chronic pain, however, the increased sensitivity of the nerves seems to persist in the absence of inflammation, and some individuals may experience unprovoked swellings from time to time. Also, once the nerves have become more sensitive, then a light touch that under normal circumstances would not be painful is found to have acquired an unpleasant character. The nearby nerves also become altered as immediately adjacent areas, which although they are not actively inflamed, also become sensitive. It is not exactly known how inflammation subsides, but typically pain after surgery is troublesome for 2 to 3 days. Should the enhanced sensitivity persist, pain can even arise without provocation and adjacent nerves can become similarly affected. These are typical patterns of chronic pain. The pain may also acquire a burning, stabbing or shooting character which is the hallmark of a nerve pain ( neurigenic or neuropathic type). These nerve pains are best treated by specialists who use pain-killers that are specifically intended to damp down the responses of these abnormally sensitive nerves. Sometimes it is appropriate to carry out nerve blocks to identify the territory of the pain, or indeed because of the phenomenon of phantom pain, clarify whether it is possible to modify the pain in a limb at all.
The fact that a person can experience pain in a part of their of their body which has been surgically removed, is clearly very strange, and represents one of the best examples of how the nervous system can be misleading in its handling of pain messages. The most common phantoms result from amputation of a limb or a finger or toe. The phantom sensations are not always painful, but when pain already existed in that part, the chances of the pain continuing as a phantom must always be anticipated. This means that the symptom of chronic pain alone does not immediately indicate that surgical removal is guaranteed to cure the pain.