In part II of this discussion, we will pick up where we left off by diving into how chronic pain can alter the way an individual sees themselves and how they interpret the world around them. If you haven’t already, please read part I of this discussion where we go into detail about how chronic pain can alter one’s proprioceptive feedback. Part II will be focused on “exteroreception” and “interoreception”.
Exteroreception: Tactile Stimuli
Exteroreception deals with the more familiar senses of the body: touch, smell, taste, sight, and hearing. Just like previously discussed, these senses are integrated and understood by the body through a complex neural loop involving both peripheral nerves and central processing of the stimuli. Individuals with chronic pain have been shown to have decreased two-point discrimination sense as well as location sense. Two-point discrimination is tested by using a caliper that is gradually spread apart until the individual can sense two distinct points of contact using only tactile sensation. For those with chronic pain, the calipers were spread farther than those without chronic pain before two distinct points of contact were felt. In addition, when asked to locate a tactile stimulus on the body, those with chronic pain commonly report the wrong location [1]. These changes in the processing of tactile information have been linked to the reorganization of central processing in the somatosensory cortex.
Exteroreception: Sight and Sound
Visual input and processing changes have also been observed in individuals with chronic pain. Individuals with chronic pain were asked to identify whether images of their hands or feet belonged to the left or right side of their body. When the image corresponded with the affected limb it took them longer and they were less accurate in determining the laterality of the image [2]. Those with chronic pain also may perceive the affected limb to be much larger than it is. Further evidence of a link between visual stimuli and chronic pain is the ability to reduce pain by manipulating the visual and motor inputs. The use of mirror box therapy and prism adaptation to re-establish sensory and motor feedback has been shown to increase the range of motion and function, reduce swelling, and restore subjective body mid-line for those with CRPS and phantom limb pain due to limb loss [3].
Those with chronic pain also may perceive the affected limb to be much larger than it is. Click To Tweet
In addition, lower tolerance to auditory inputs has been demonstrated in those with chronic pain, specifically those with fibromyalgia. There is a proximity of auditory and nociceptive processing regions of the brain. It is thought that an overall deficit in systemic inhibition of overstimulation can be playing a role as well. While most of the information available is specific to fibromyalgia, it is believed a similar mechanism may play a role in chronic pain [4].
Exteroreception: Taste and Smell
While studies of chronic pain and sensory processing of tastes and odors are decidedly more limited than the previously discussed senses, there is still evidence to suggest a link between them. Increased sensitivity to bitter tastes has been correlated to increased pain during the injection of the anesthetic drug propofol, while sweet tastes have been shown to have analgesic effects in neonates [5, 6]. In addition, individuals with chronic temporomandibular disorders have a higher rate of taste disturbances such as a reduced flavor sensitivity [7]. Lastly, individuals with fibromyalgia have been shown to have a greater sensitivity to unpleasant odors. While this may relate back to the idea that fibromyalgia may stem from decreased inhibitory potentials, it speaks to the importance of sensory processing and its relation to the perception of pain [8].
Interoception:
Interoception has to do with how the body feels to an individual. It is mediated largely by the autonomic nervous system rather than the sympathetic. It is responsible for our heart rate, thermoregulation (body temperature), and plays a role in swelling amongst other roles. A non-invasive and therefore frequently used method of measuring autonomic activity is heart rate variability (HRV). This measurement takes into account the time between each heartbeat and measures how much that interval changes from beat to beat. A lower variability indicates greater autonomic impairments because the body is not responding properly to stimuli. For example, individuals heartbeat should be lowest at rest and increase with activity or other excitable stimuli, therefore creating greater variability. It has been shown that individuals across various chronic pain populations (CRPS, low back pain, fibromyalgia, and phantom limb pain) have a lower HRV and increased resting heart rate on average indicating autonomic dysfunction [9].
As mentioned earlier interoreception deals with how a person feels. The way this has been quantified in the past is by asking the individual to draw human figures of themselves under the direction to characterize how their pain makes their body feel. Consistently these drawings show distorted images either leaving out the painful body part or portraying it as larger or misshapen. The clear limitation of this technique is the variability in each individual’s artistic ability. New technology has helped this by using 3D digital modeling tools to standardize the process of conveying body image. The results were the same that those with chronic pain portrayed themselves in a distorted manner [10].
Those with chronic pain commonly report the wrong location when asked to locate a tactile stimulus. Click To Tweet
Summary:
In conjunction with part I of this discussion, it is clear to see that there is a link between sensory processing and chronic pain. This helps to illustrate the idea that chronic pain is a very complex process that spans over various body systems. There is no one cure or one treatment that can alleviate chronic pain, especially if it involves changes to the central processing of stimuli. Efforts are continuously put forth to better understand chronic pain, and in doing so the ability to treat and diminish the pain will become more effective.