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 a common thread: a progressive damage to the central nervous system. We can't blame the people who suffer from these illnesses 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 should we regard a person with Parkinson's disease, which is caused by a reduced amount of dopamine in the substantia nigra of the brain, differently from someone who has a serotonin imbalance, as seen in depressed folks?

Even medical students and doctors are more likely to make an off-color remark about a patient with a psychiatric illness than they would ever dare make about a patient with Lou Gehrig's disease. This behavior regarding chronic pain can develop during medical training from encounters with patients deemed to 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 met in the emergency room.

The body of evidence is, however, growing regarding chronic pain as a form of neurological disorder. 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 and poorly understood, but we know it involves the activation of specific nerve cells responsible for pain super sensitivity. Moderate amounts of pain can be dealt by the nervous system in a straightforward fashion, with rapid recovery of the neurons involved in pain transmission; severe or constant pain can lead to a barrage of electrical impulses that reach the spinal cord and overwhelms it with toxic amounts of the neurotransmitter glutamate, and may lead to permanent changes in 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 happens in post-traumatic stress disorder when a severe emotional trauma is thought to 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 sprouting of others, in a vast reorganization of the apparatus involved in pain transmission.

The corollary to the above is that the treatment of any pain that has the potential of becoming chronic, such as shingles, has to rely on the reduction of the stimulus reaching the spinal cord. An example is seen in the reduction of phantom pain that occurs when a lower limb amputation is done under general anesthesia alone, or when epidural or local anesthesia is utilized, which prevents the nerve impulses from reaching the spinal cord. Several studies have shown that even though patients don't feel the pain of surgery in either case, patients subjected to epidural anesthesia have roughly half the incidence of phantom pain twelve months post operatively. The brain has another important role in modulating the arrival of pain impulses, by blocking the pain arising in the spinal cord, before it even has a chance to reach the brain.

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 person suffering from chronic pain, take a step back and consider that maybe, one day, we'll be using a big sounding name like Alzheimer or Parkinson to describe a condition that today we really don't know much about.


© Dr. Moacir Schnapp and Dr. Kit Mays


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