Pain as a Neurodegenerative Disorder

Alzheimer, Parkinson, Lou Gehrig. All famous names, all poorly understood neurological diseases for which there is only palliative treatment. Although they represent very different pathologies and possibly very different etiologies, they share among themselves a progressive damage to the central nervous system. We don’t blame these poor souls for their predicament; we don’t chastise them for getting sick. As human beings, we feel sorry for their plight; as doctors, we feel impotent to cure their illnesses.

Why is it then that society treats people with mental illness and people with chronic pain differently? Why is it that our attitude changes, for instance, when facing a case of Parkinson’s where dopamine is missing from the substantia nigra, as compared with an intermittent decrease in serotonin, as found in depression? Even as medical students and doctors, we are many times more likely to make an off-color remark about a patient with a psychiatric illness than we would ever dare make about a patient with Lou Gehrig’s disease.

Our behavior regarding chronic pain has been molded during medical training by the many patients whom we believe have pain out of proportion to their complaints, by the close association of chronic pain and mental illness, and by a certain number of malingerers and drug seekers we all have met during our ER rotation.

The body of evidence is, however, growing for chronic pain as a form of neurological disorder, and both animal and human studies point to the spinal cord, specifically the dorsal horn, as the main culprit in the genesis of chronic pain. The process is complex, poorly understood, but we know it involves the activation of specific nerve cells, which are responsible for a type of pain super sensitivity. Moderate amounts of pain can be dealt by the nervous system in a straightforward fashion, but severe or constant pain can lead to a barrage of action potentials that arrive to the spinal cord, and overwhelm it. That barrage of arriving impulses releases toxic amounts of the neurotransmitter glutamate, starting a cascade that involves intracellular calcium, nitric oxide, substance P and eventually leads to altered gene expression of these cells.

That’s right. A process such as the neurotransmission of pain impulses, which lasts a few milliseconds, may change certain nerve cells permanently, in a process similar to what is thought to happen in post traumatic stress disorder when the experience of a severe trauma can lead to permanent neurological changes. When continuous, pain is thought to induce not only alteration of cell receptors, but also disappearance of certain synapses and spouting of others, in a vast reorganization of the apparatus involved in pain transmission.

The corollary to the above is that the responsible treatment of any pain which has the potential of becoming chronic, such as shingles or post thoracotomy pain, has to rely on the reduction of the peripheral stimulus reaching the spinal cord. A clear example can be seen in the different incidence of phantom pain that occurs when a lower limb amputation is done under general anesthesia alone, or when epidural anesthesia is utilized. The patient is unaware of the stimulus of the surgery dissection in either instance, but patients subjected to epidural anesthesia have roughly half the incidence of phantom pain twelve months post operatively.

The brain has an important role in modulating the arrival of pain impulses. This is due, at least in part, to the capacity of our supratentorial structures to block the pain arising in the spinal cord, so called descending inhibition. Some people are fortunate enough to have been born with an enhanced capacity to deal with pain. Others, and that probably include people with mental illnesses, are less able to do so, and therefore suffer more than their counterparts. This is compounded by the fact that they possibly receive a lesser quality of medical care because doctors can’t tell whether they are “really hurting”, or if it’s all a figment of their imagination.

Next time you feel you’re losing your cool with a chronic pain patient, take a step back and just consider that maybe, one day, we’ll be using a big important name like Alzheimer or Parkinson to describe a condition we really don’t know much about.

© Dr. Moacir Schnapp and Dr. Kit Mays