Short stories on pain (PartA):
Intuitive idea behind the objective assessment of pain.
Written by Hui-Ting Hong
Aug. 28, 2019
During the past two years, I’ve worked on the research topic related to multimodal learning especially in clinical practice on pain-level assessment. Although since this May, we’ve turned the application scenario to the prevailing media cultivated in our society, I think it would be nice to share something I’ve learned in modeling the in-the-wild problem settings these days. I hope others find this article useful and welcome to drop the comment.
What is pain?
Pain is an emotional experience we suffered quite frequently. It may originate from various parts of our body and there are multiple causes which account for the different extent of the pain we feel. Sometimes, I imagine it as the flame right burning on the snow, where the flame is the pathogen and snow is our body. As the pain stay longer, the frailer we became. However, the mechanism lying behind the pain we feel is such a maze, where the flame comes from does not imply the exact location we feel the pain comes from (usually happen in visceral pain). Moreover, the pathway of pain can vary from one another.
One example of the mechanism lying behind the pain we feel is starting from the tissue damage, moving on to the activation of pain receptor, electrical signal delivering, and finally sent to the thalamus. The thalamus could be imagined as a buffer, where commands the signal to be later sent to different parts of the brain. The reason why we might feel depressed or suffered from pain is due to the operation of our brain. Usually, the signal sent to the thalamus would later be delivered to the somatosensory cortex, frontal cortex and limbic system, which is responsible for our physical feelings, how we think and the emotion sensation respectively. Regardless of how we feel the pain, the response of every person is actually quite different from one another. So, what are the differences could it be? And where are these different responses come from? Here I think it might be nice to divide into two aspects to make the idea more clear: 1) Different types of pain we suffered. 2) Different intrinsic characteristics of individuals.
Complicated influential factors of pain
1) Different types of pain we suffered
Different locations or different kinds of damages on our body could lead to distinct impact in our brain and also accounts for different pathways of the pain, which, as a result, produce the various sensation of pain. In general, the pain could be categorized into acute pain vs. chronic pain or nociceptive pain vs. neuropathic pain. While there is much more genre of pain, I think I would simply cover the two mentioned above, which is also more related to the research we’ve conducted.
The acute pain comes quite quickly and got a limited duration, and in most cases, the acute pain attributes to the injury with an obviously defined cause. Rather than the goes away of acute pain after the healing of illness, the chronic pain, usually defined as the pain last for over 6 months, could persist even after the injury has no longer existed. Especially when the chronic pain is caused by the nerve damage, since nerves may regrow (peripheral nervous system, PNS) but could function abnormally [1,2,3,6]. Aside from the duration of pain, the pain could also be categorized base on how they were detected: nociceptive pain vs. neuropathic pain. The nociceptive pain is similar to what we mentioned in the third paragraph. In most cases, somatic pain and visceral pain are included in the nociceptive pain and while the former one usually caused by the damage of muscles, bones, and tissues, the latter one attributes to the illness of organs or blood vessels. The general feelings of aching might result from nociceptive pain and on the other side, the feeling of neuropathic pain, caused by the damage of nerves, might be more shooting and tingling[1,4,5].
In the emergency department (ED), where is exactly the place we collected our data, patients usually suffered from the acute pain, and it could cover both nociceptive pain and neuropathic pain.
[Note]: Whether the patients are suffered from acute pain or chronic pain is not included in the record sheet. We retrieve the possible messages from the conversation record (audio-video recordings). Moreover, there is no possible for us to identify the nociceptive or neuropathic pain from the record sheet or video, which leaves us an open door to investigate the type of pain from the patient’s expressive behavior (We will cover in the following posts).
2) Different intrinsic characteristics of individuals
Aside from the actual pain each individual suffered, the sensation of pain actually builds a connection with our intrinsic characteristics, including age and gender. The mechanism behind the gender difference in pain responses is yet entirely revealed. While it might be the consequence of potential social norms or stereotypes, the sex hormones, intrinsically different from female and male, is considered to be one of the influential factors in the perception of pain[9,10]. In most cases, women are more sensitive to pain and report a higher intensity of pain, compare to men. The treatment responses, therefore, would also vary from female to male[11,12]. On the other hand, as the age grows, the tolerance of pain will decrease. The age difference, to some extent, even results in the modification of medical treatment due to the degeneration of sensory and cognition, especially in elderly.
To further reveal the underlying relation between age, gender and the intensity of pain, we’ve also conducted a series of statistical testing on verifying the variation of the acoustic signal across different groups of gender (male vs. female) and age (young adult, adult, elderly). We will cover this in the following posts.
In short, due to the individual variation, the treatment should be adopted for the needs of patients. And considering either kind of pain, it is essential to recognize the neurophysiology of pain, the amount of damage of body, and apply appropriate management to the key element in each situation. However, there are several problems remain concerned and this one might be listed at the top: The “self-reported pain level” is not perfectly reliable since it does not always build a connection with the actual pain patients suffered.
Where the notion of objective assessment of pain emerges today.
The attempt to objectively quantify the pain
Less connection built between what we feel and the actual amount of damage of body is a problem with no firm solution. We try to achieve consistency in measuring the level of pain by cooperating with doctors these days. The key concept in the whole process is, instead of replacing the evaluation done by nurses and doctors, playing a complementary role in improving the follow-up and treatment of pain. Hopefully, the objective assessment of pain could optimize the pain management and in the meanwhile increase the efficiency in medical resource delivering.
As we mentioned above, both the type of pain and the intrinsic characteristics variations affect the intensity of the pain we suffered. In the emergency department to be more specific, these concrete clinical situations do reveal the essentiality on the objective assessment of pain:
Inarticulate sounds from severely-injured patients
Gender disparity in the overestimation of pain
Barely accessed self-report pain scale in elderly with cognitive impairment
Apparently, there are much more cases than the three above, and studies need to be gathered to obtain further insight. With the fact that the self-report pain scale is currently the industrial-standard tools for doctors to have a clear idea on how uncomfortable a patient is, it is actually hard to reveal the underlying cause behind the pain we suffered. The following conversation would be familiar if we were sent to the ED: “How would you describe your pain on a scale of zero-to-ten, where zero is no pain and ten is a lot of pain?”, and the answer would be recorded by the nurse or doctor, which is exactly the Numeric Rating Scale (NRS) used nowadays for accessing the pain level for patients with capability on self-report. In most cases, multiple treatments have to be conducted, even we’ve got the information from self-report pain scale, until the discomfort is released. There is still room for improvement in the current emergency triage system, and research scholars are exploring different ways to improve the understanding of pain.
In the next post, I will cover some previous studies on pain measurement by mainly modeling the associated expressive behaviors of patients (facial/acoustic expressions). Also, several cutting-edge studies on either deconstructing acoustic information or embedding additional personal attributes in the automatic pain assessment will also be mentioned.
I hope this article would be helpful to those who are also interested in the research of pain-level assessment. Cheers!
Thanks to Matt for reading and giving me advice on the early version of this article.
 Johnson, Quinn et al. “Pain management mini-series. Part I. A review of management of acute pain.” Missouri medicine vol. 110,1 (2013): 74-9.
 Russo, Cathy M., and William G. Brose. "Chronic pain." Annual review of medicine 49.1 (1998): 123-133.
 Nicholson, Bruce. "Differential diagnosis: nociceptive and neuropathic pain." The American journal of managed care 12.9 Suppl (2006): S256-62.
 Packiasabapathy, Senthil, and Senthilkumar Sadhasivam. "Gender, genetics, and analgesia: understanding the differences in response to pain relief." Journal of pain research 11 (2018): 2729.
 Cairns, Brian E., and Parisa Gazerani. "Sex-related differences in pain." Maturitas 63.4 (2009): 292-296.
 Fillingim, Roger B., et al. "Sex, gender, and pain: a review of recent clinical and experimental findings." The journal of pain10.5 (2009): 447-485.
 Unruh, Anita M. "Gender variations in clinical pain experience." Pain 65.2-3 (1996): 123-167.
 Cole, Leonie J., et al. "Age-related differences in pain sensitivity and regional brain activity evoked by noxious pressure." Neurobiology of aging 31.3 (2010): 494-503.
 Petrini, Laura, Susan Tomczak Matthiesen, and Lars Arendt-Nielsen. "The effect of age and gender on pressure pain thresholds and suprathreshold stimuli." Perception 44.5 (2015): 587-596.
 Jones, Joshua, Tin Sim, and Jeff Hughes. "Pain Assessment of elderly patients with cognitive impairment in the emergency department: Implications for pain management—A narrative review of current practices." Pharmacy 5.2 (2017): 30.