“The aim of the wise is not to secure pleasure, but to avoid pain.” — Aristotle
Pain Defined
Pain is defined as: “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [1].”
Pain can be classified according to how long it has been present:
Acute pain: 0-6 weeks,
Subacute pain: 6-12 weeks,
Chronic pain: 12 weeks and beyond.
Which stage of pain a person is in depends on the severity of the injury. It can also fluctuate back and forth depending on the circumstance.
A more useful way to classify pain is by its mechanism [2]:
Nociceptive pain: arises from actual or threatened damage to organs, skin, joints, and muscle tissues.
Nociceptive pain that originates from internal organs and blood vessels is called ‘visceral’ pain.
Nociceptive pain coming from the skin, joints and muscle tissues is called ‘somatic’ pain. Examples include most forms of neck and back pain, sprains/strains, osteoarthritis, and the like. Given that I’m a Chiropractor by trade, somatic pain will be the focus in these articles.
Neuropathic pain: originates in the nerves themselves, rather than in damaged organs or tissues.
Neuroplastic pain: pain that occurs WITHOUT damage to the nerves or other tissue.
We’ll dive deeper into these terms later. But to understand pain in its context, we first have to understand some basics about the system the body uses to perceive pain, the nervous system.
Nervous system – Anatomy
The neuron, or nerve cell, is the most basic functional unit of the nervous system. There are billions of neurons in the human body.
Neurons come in different shapes and sizes but carry out the same basic function, which is to transmit information to other cells throughout the body. Information can be transmitted to parts of the body, or it can be received from parts of the body and outside world. The information itself is simply electrochemical signals that we often term ‘impulses’. It’s up to our brain to interpret what these impulses actually mean.
The axons (the long skinny part of a neuron) form into bundles, which we call nerve fibers. The nerve fibers bundle to together to form what we simply call ‘nerves’.
Nerves are like strings of electrical wire. The brain is a big ball of wires. Attached to the brain is the spinal cord, a long cylindrical tube of wires bunched together. Off from the spinal cord the nerves branch, and branch, and branch again into smaller nerves until they’ve covered the entire human body.
Nervous system – Function
Functionally the nervous system is broken down into two parts:
Central Nervous System - Brain and spinal cord
Peripheral Nervous System - everything else
The Peripheral Nervous System can be broken down further:
Autonomic Nervous System - Involuntary control of the function of internal organs.
Somatic nervous system - Voluntary control of muscles, and sensory nerve fibers from the skin, muscles, and joints.
When sensory nerve fibers get stimulated, they send impulses to the brain where it interprets what is happening inside and around us.
Receptors
Different sensory nerves contain different receptors on their surface.
These receptors get excited by a specific stimulus (this is important).
Nerves are activated (or ‘fire’) when a specific stimulus reacts with the nerves receptor. There are many sensory receptors [3]. For now, these are the ones we'll be interested in:
Mechanoreceptors (respond to touch, pressure or distortion) like with exercise, massage, and stretching.
Thermoreceptors (respond to change in temperature) as in ice packs or hot packs.
And...
Nociceptors (respond to harmful things that could cause pain)
Nociceptive Pain (Acute)
When a nociceptor is activated it triggers an impulse along its nerve. The impulse is sent to the spinal cord, and then to the brain, where the impulse is interpreted as an actual or potential threat to the body. If the brain decides the threat is credible, the experience of pain is perceived so we can undergo evasive action and reduce potential injury. This is ultimately the purpose of pain, to prevent injury of the host organism.
Some nociceptors (Aδ fibers) are responsible for sending impulses to the brain during acute pain situations, like pricking your finger with a push pin. Some nociceptors (C fibers) are responsible for sending impulses in non-acute conditions, like after the push pin has been removed from your finger but it still hurts.
Neuropathic Pain (Acute)
Neuropathic pain is similar to nociceptive pain, but with a few distinct differences. Instead of pain originating in organs or tissues, neuropathic pain is derived from the actual nerves themselves. There can be damage to a single nerve (mononeuropathy) or multiple nerves (polyneuropathy).
The underlying lesion can be mechanical in nature, such as a ‘pinched’ nerve in the back causing Sciatica, or irritation to a nerve in the wrist that causes Carpal Tunnel Syndrome. Comparable to muscles, tendons, and ligaments, a nerve can undergo injury by means of laceration, compression, or stretch as well.
There are systemic diseases that also cause nerve pain: diabetes, cancer, stroke, autoimmune disease, multiple sclerosis and other neurodegenerative diseases. Just to name a few.
It is estimated that anywhere from 3-10% of the population is affected by neuropathic pain [4].
Neuropathic pain appears to be less prevalent that nociceptive pain, but more troublesome. It often becomes chronic, is more severe, and responds less favorably to medications [5].
Subacute Pain
There is no standard definition in the literature on what constitutes subacute pain. Chronologically it is defined anywhere between seven days up to six months. Subacute pain “is quite similar to acute pain in its etiologic and nociceptive mechanisms [6].”
Neurologically and biochemically, subacute pain is a blended middle ground between acute and chronic pain. Inflammation, which is a necessary function of healing, likely plays a role in the production of subacute pain.
Importance of Treating Pain
Pain-related problems account for up to 80% of visits to physicians [7].
The treatment of acute and subacute pain in a medical setting hasn’t advanced much in decades. Largely a pharmacological approach has been taken and the same drugs used to control pain back then are still used now. “Thus, techniques or drugs that were introduced and achieved widespread use for acute pain management within the past 20 years have produced no changes in scientometric indices that would indicate real progress and have failed to improve national outcomes for relief of acute pain [8].”
This is unfortunate. The management of acute/subacute pain has been widely considered inadequate [9]. It has become obvious that a multidisciplinary (both pharmacological and non-pharmacological) approach to treating acute and subacute pain is necessary.
One of the most important jobs for a clinician who treats pain is to make sure acute and subacute pain, both nociceptive and neuropathic, is managed in such a way that it doesn’t become chronic. Chronic pain is a much more difficult and expensive problem to manage.
For more on chronic pain, see our article Pain 102: Chronic Pain, The Software Problem.
Summary, Key Points:
Pain is a sensory experience, related to actual or perceived tissue damage.
Pain can be acute, subacute or chronic.
Pain can be nociceptive, neuropathic, or neuroplastic.
The treatment of acute pain hasn’t improved to a large extent.
It is important to not let pain become chronic.
Glossary
Sensory: of or relating to your physical sense. A physical feeling: something that your body experiences.
Impulse: a small amount of energy that moves from one area to another.
Receptor: a nerve ending that senses changes in light, temperature, pressure, etc., and causes the body to react in a particular way.
Stimulus: something that causes something else to happen, develop, or become more active.
Distortion: to change the natural, normal, or original shape, appearance, or sound of (something) in a way that is usually not attractive or pleasing.
Lesion:an injured or diseased spot or area on or in the body.
Systemic: of, relating to, or affecting the entire body.
Works Cited
1. “IASP Announces Revised Definition of Pain.” International Association for the Study of Pain (IASP), 16 July 2020, www.iasp-pain.org/publications/iasp-news/iasp-announces-revised-definition-of-pain/.
2. Hush, Julia M, et al. “Untangling Nociceptive, Neuropathic and Neuroplastic Mechanisms Underlying the Biological Domain of Back Pain.” Pain Management, vol. 3, no. 3, May 2013, pp. 223–236, 10.2217/pmt.13.11. Accessed 8 Nov. 2021.
3. “12.2A: Classification of Receptors by Stimulus.” Medicine LibreTexts, 20 July 2018, med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/12%3A_Peripheral_Nervous_System/12.2%3A_Sensory_Receptors/12.2A%3A__Classification_of_Receptors_by_Stimulus.
4. Valentin, Gitte H, et al. “Prognostic Factors for Disability and Sick Leave in Patients with Subacute Non-Malignant Pain: A Systematic Review of Cohort Studies.” BMJ Open, vol. 6, no. 1, Jan. 2016, p. e007616, 10.1136/bmjopen-2015-007616. Accessed 11 Nov. 2021.
5. Yawn, Barbara P., et al. “The Prevalence of Neuropathic Pain: Clinical Evaluation Compared with Screening Tools in a Community Population.” Pain Medicine, vol. 10, no. 3, Apr. 2009, pp. 586–593, 10.1111/j.1526-4637.2009.00588.x. Accessed 8 Nov. 2021.
6. Cole, B. Eliot. Pain Management: Classifying, Understanding, and Treating Pain. June 2002, www.turnerwhite.com. Accessed 11 Nov. 2021.
7. Voscopoulos, C, and M Lema. “When Does Acute Pain Become Chronic?” British Journal of Anaesthesia, vol. 105, Dec. 2010, pp. i69–i85, 10.1093/bja/aeq323. Accessed 10 Nov. 2021.
8. Kissin, Igor, et al. “No Evidence of Real Progress in Treatment of Acute Pain, 1993–2012: Scientometric Analysis.” Journal of Pain Research, Apr. 2014, p. 199, 10.2147/jpr.s60842. Accessed 10 Mar. 2021.
9. Sinatra R. Causes and consequences of inadequate management of acute pain. Pain Med. 2010 Dec;11(12):1859-71. doi: 10.1111/j.1526-4637.2010.00983.x. Epub 2010 Oct 28. PMID: 21040438.
Image: “Components of neuron” by Jennifer Walinga is licensed under CC-BY-SA-4.0
Image: “Overview of Nervous System” by Openstax is licensed under CC-BY-4.0
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