What is the pathophysiology of pain in the elderly?
Although the pathophysiology of pain in the older patient continues to be studied, there are some known physiologic changes in both pain signaling and perception that influence pain in the elder population.
First, there are age-related functional and structural changes to the peripheral nerves. Many studies in the elder population have shown a decrease in the number of myelinated and unmyelinated fibers, as well as an increase in the number of damaged fibers, both of which can alter pain conduction. In addition, there is a decrease in substance P and calcitonin gene-related peptide (CGRP), and a decrease in the rate of CGRP transport through neuronal axons. Substance P and CGRP are neurotransmitters for primary afferent nociceptive fibers, and these findings suggest age-related deterioration of afferent sensory neurons.
In the central nervous system, there are degenerative changes in dorsal horn sensory neurons in the spinal cord. These changes include axonal involution, demyelination, decreased CGRP, substance P, and somatostatin, and increased loss of noradrenergic and serotonergic neurons, all of which are found to cause altered pain processing and function of descending modulatory pathways. These age-related changes also impair the pain inhibitory systems of the nervous system. There are two endogenous inhibitory systems, opioid-dependent, and non-opioid dependent, both of which show age associated deterioration in many studies. It is suggested that the decreased effectiveness of endogenous pain systems may cause an increase in severity of pain due to noxious stimulation.