Pain: An Overview 2

By Dr. Ranjith Pallegama

 

Variability in report of pain

     Different pain thresholds are reported among individuals. As such, there can be individual variations from nociceptive mechanisms to pain perception. If pain is induced using a stimulus of known strength in different individuals, they will perceive it differently and rate different pain intensities. In addition to the variation among individuals there are day to day or time to time variations within individuals. This variability in pain expression has often been attributed to variations in biological, psychological, sociocultural and personality factors. For example, a child brought up in one society may withstand more pain compared to another one brought up in different sociocultural values. Evidence of genetic variations in pain expression and existence of ‘pain genes’ is relatively new to pain medicine.

 

Pain genetics

     As it was mentioned in the previous section, a large component of the sex difference in pain experience can be attributed to their early socialization and upbringing. However, considerable amount of new evidence suggests that genetic factors contribute to this difference as well. Starting from the indication that received from a familial disorder that exhibited congenital insensitivity to pain, pain genetics has evolved and been a popular field of research making a large body of new findings available.

     A rapid expression of certain genes such as c-fos gene in the spinal cord in response to peripheral stimulations that give rise to pain has been observed as a "molecular memory for nociceptive pain stimulation". This shows the potentials to be used as an objective marker of nociceptive pain at least in pain research.

     An observation that has a clinical application is the genetic differences in responding to analgesic drugs (pharmacogenetics). This may partly explain the individual variations in response to different drugs especially those used as adjuvant drugs in pain medicine (please see the next chapter). Pharmacogenetics promises many future developments such as individualized pain control.

     Marshal Devor in writing a review in Pain 2010-An Updated Review: Refresher Course Syllabus for the 13^th International Congress on Pain has defined a pain gene as “a gene for which there are one or more polymorphisms (i.e., variations in the sequence of DNA base-pairs) that affect the expression or the functioning of its protein product in a way that affects pain response”. Alleles of these genes are thought at least partly to be responsible as to why two different individuals encounter noxious stimuli of identical strength report different levels of pain. In addition, there are genes that cause or predispose an individual to certain painful diseases to different degrees. Already available information in the field of pain genetics is enormous and more will be available in the future paving ways to better understand pain mechanisms and making many tools available for pain control in the future.

 

Management of pain

     Acute pain can be basically tackled with analgesics such as non-steroidal anti-inflammatory drugs, acetaminophen and drugs such as codeine combinations. An appropriate choice depend on multiple factors related to pain such as, the source, character, location, duration, patients general physical condition, patient’s associated medical conditions contraindications etc. Local anesthetics and topical applications (LA, capsaicin, topical cooling) may also be of use. Agents that excite larger afferent fibers (activating gate control mechanisms) may also be effectively used. General anesthesia and block anesthesia are commonly used in surgery and opioid derivatives (e.g., morphine) is effectively used in post-surgical pain management. A comprehensive chapter on pharmacological management of pain is available .

     Management of chronic pain condition requires meticulous assessment of the condition and proper diagnosis. This requires careful examination, assessment of pain and special investigations. Basically, medications, local anesthesia, physiotherapy, counseling and behavioral management, occupational therapy and managing related medical conditions are used in combination depending on the diagnosis and patients’ related factors. Techniques of alternative medicine such as acupuncture have effectively been used in many parts of the world. There are several hypothesized mechanisms to explain the effectiveness of such techniques that are discussed in next sections.

 

Placebo effect

     Placebo means ‘I shall please’ in Latin. The term placebo is used to indicate a treatment that does not affect the specific mechanisms of a disease but given to patients with specific objectives either in research or treatment. Placebos are extensively used in clinical trials. Placebo effect means a perceived favorable response that a treatment or an intervention elicits on patients.

     Placebo effects may influence patients’ outcome after any treatment and both the clinician and patient may misattribute these effects on treatment effects. But in certain specific situations placebo effect is objectively used in treatment, especially in conditions that has a natural history of regression. Models have been developed to explain placebo effect and those emphasize the role of expectations, and learning and hence termed ‘expectation induced placebo effect’. Yet, much remained to be understood in relation to the placebo effect and certain subsequent chapters contain the details in relevant sections.

 

References

Caastro-Lopes J. Current topics in pain. IASP press. Seattle; 2009.

Hicks CL, von Baeyer CL, Spafford P, van Korlaar I, Goodenough B. The Faces Pain Scale - Revised: Toward a common metric in pediatric pain measurement. Pain 2001;93:173-183.

Jensen MP, Karoly P, O'Riordan EF, Bland F, Burns RS. "The subjective experience of acute pain. An assessment of the utility of 10 indices". Clin J Pain.1989 June; 5 (2): 153–9.

Justin DM. Pain 2005- An updated Review: Refresher course syllabus. IASP press. Seattle: 2005.

Linton  SJ. Understanding Pain for Better Clinical Practice: A psychological perspective.London: Elsevier; 2005.

Melzack R and Wall PD. Pain Mechanisms: A New Theory. Science 1965 Nov; 971-979.

Merskey and Bogduk  1994. H Merskey and N Bogduk Editors. Classification of Chronic Pain. Descriptions of Chronic Pain Syndromes and Definitions of Pain Terms (2nd ed.) IASP Press; Seattle: WA 1994.

PD Wall, R Melzack (eds). Textbook of Pain (4th edition). R Melzack eds.Textbook of Pain, London: Churchill Livingstone, 1999.

Warfield  CA, Fausett HJ. eds., Manual of Pain Management, 2nd Edition. Lippincott Williams and Wilkins; Philadelphia:2002.