By Chin H. Huang, M.D.

(presented at the CAMS 1998 Annual Scientific Meeting)

since the beginning of the present century, theories of pain mechanism have evolved from specificity and summation models to the popular gate control theory. This latter pain theory, proposed by Melzack/Wall/Casey, has become the most important development in the field of pain management. More and more discoveries in recent years show that pain perception is no longer a straight forward afferent transmission of pain signal. It is a complex mechanism involving modulation coming from both peripheral and central nervous system. In the chronic pain state, pain signal generation can actually in the central nervous system without peripheral noxious stimulation.

Anatomically, there are numerous ascending excitatory and descending inhibitory pathways in pain signal transmission. Centralization (cephalad relocation in the central nervous system) of the pain signal generators occur spontaneously or after these neural pathways are interrupted, leading to totally unexpected pain syndromes. Advanced reflex sympathetic dystrophy, deafferentation pain and phantom pain phenomenon are just a few examples.

Traditionally, we believe that pain is an important biological reaction of defense and a fortunate warning to put us on our guard against diseases. Although this may be true in disease states such as appendicitis, fracture and angina, it does not explain the unnecessary pain in conditions such as migraine, post-therapeutic neuralgia and pain in labor and delivery. Scientific evidence shows that acute persistent pain eventually sensitizes wide dynamic neurons in the dorsal horn of the spinal cord ("wind-up phenomenon"), constituting the basis of developing chronic pain syndromes. Persistent and excessive pain has no biological function. It is actually harmful to our well being. Therefore, pain needs to be treated as early and as completely as possible, not to be left alone.

Practically, pain can be classified into five different types, i.e., visceral, somatic, referred, neuropathic and psychogenic, according to their origins of pain signal generation. Commonly, we see pain syndromes with different mixtures of these five types. In acute pain (predominantly nociceptive), visceral, somatic and referred mechanisms play important roles in the pain perception. In chronic pain (frequently non-nociceptive), neuropathic and psychogenic mechanisms prevail, resulting in protracted suffering and disability both physically and mentally.

We used to believe that destruction of the pathways of pain transmission could alliviate the pain. This has proven to be wrong. Due to the plasticity of our nervous system, the pain relief achieved by neuroablation is always short lived. In pain management, modulation of pain signal transmission is a far better choice than destruction.

Dr. Huang is Clinical Assistant Professor at UMDNJ-New Jersey Medical School.