Multidimensional Nature of Pain

Original Editor - Alberto Bertaggia.

Top Contributors - Alberto Bertaggia, Michelle Lee and Jo Etherton  

Introduction

A definition of pain is provided by the International association for the Study of Pain (IASP) as follows[1]:

"An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage"

Pain is always subjective and everyone learns the use of this word through experiences related to injury in early life.

It is a sensation in a part or parts of the body, but it is also always unpleasant and therefore also emotional

Even in the absence of tissue damage or any likely pathophysiological cause, people still report pain; usually this happens for psychological reasons. In these cases, it is challenging to distinguish whether their experience arise from a damaged tissue or not, based only upon the subjective report[2].

It is important to underline that activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain[2], which is always the output of a widely distributed neural network in the brain rather than one coming directly by sensory input evoked by injury, inflammation, or other pathology[3].

In the following video Karen D. Davis tries to explain why do some people react to the same painful stimulus in different ways.


Relevant anatomy and physiology

Nociceptors

Nociceptors (from the latin nocere = to hurt) are sensory receptors which detect signals from damaged tissue or the threat of damage and indirectly also respond to chemicals released from the damaged tissue. There are free nerve endings present in many types of tissues,  and cell bodies located in the dorsal root ganglions or in the cranial nerve ganglia.

(A) Somatosensory neurons are located in peripheral ganglia (trigeminal and dorsal root ganglia) located alongside the spinal column and medulla. Afferent neurons project centrally to the brainstem (Vc) and dorsal horn of the spinal cord and peripherally to the skin and other organs. Vc, trigeminal brainstem sensory subnucleus caudalis. (B) Most nociceptors are unmyelinated with small diameter axons (C-fibers, red). Their peripheral afferent innervates the skin (dermis and/or epidermis) and central process projects to superficial laminae I and II of the dorsal horn. (C) A-fiber nociceptors are myelinated and usually have conduction velocities in the Aδ range (red). A-fiber nociceptors project to superficial laminae I and V. from: Dubin AE, Patapoutian A. Nociceptors: the sensors of the pain pathway. J Clin Invest. 2010 Nov 1;120(11):3760–72. Copyright © 2010, American Society for Clinical Investigation.

Nociceptors have unmyelinated (C-fiber) or thinly myelinated (A-fiber) axons[4]. C-fibers support conduction velocities of 0.4–1.4 m/s, while A-fibers support conduction velocities of approximately 5–30 m/s[5].

Nociception

Nociception is a mechanism which comprises the processes of transduction, conduction, transmission and perception[6].

  1. Transduction is the conversion of a noxious thermal, mechanical, or chemical stimulus into electrical activity in the peripheral terminals of nociceptor sensory fibers. This process is mediated by specific receptor ion channels expressed only by nociceptors.
  2. Conduction is the passage of action potentials from the peripheral terminal along axons to the central terminal of nociceptors in the central nervous system.
  3. Transmission is the synaptic transfer and modulation of input from one neuron to another.
  4. Projection neurons in the dorsal horn transfer nociceptive input to the brainstem, hypothalamus, and thalamus and then, through relay neurons, to the cortex. Here is where perception occur as a subjective experience.

Types of pain classification

Based upon the works of Woolf[6][7], this is a useful types of pain classification:

  1. Nociceptive pain. This kind of pain is concerned with the sensing of noxious stimuli and is a high-threshold pain only activated in the presence of intense stimuli. It has a protective role requiring immediate attention and responses (i.e. withdrawal reflex).
  2. Inflammatory pain. This second kind of pain is important to promote protection and healing of the injured tissues by creating an enviroment which suggests avoidance of movement and stress of the body parts. This is made possible by activation of the immune system causing inflammation.
  3. Pathological pain. This type of pain is uncoupled from noxious stimulii and even from tissue damage, it is not protective, and results from abnormal functioning of the nervous system (peripheral or central). To note, this is a low-threshold pain. This time pain is not a symptom, but rather a disease itself.  It occurs with peripheral sensitization and central sensitization.
Pain can be broadly divided into three classes. (A) Nociceptive pain represents the sensation associated with the detection of potentially tissue-damaging noxious stimuli and is protective. (B) Inflammatory pain is associated with tissue damage and the infiltration of immune cells and can promote repair by causing pain hypersensitivity until healing occurs. (C) Pathological pain is a disease state caused by damage to the nervous system (neuropathic) or by its abnormal function (dysfunctional). From: Woolf CJ. What is this thing called pain? J Clin Invest. 2010 Nov 1;120(11):3742–4. Copyright © 2010, American Society for Clinical Investigation.

Acute and chronic pain

Acute pain is caused by a noxious stimuli ad is mediated by nociception. It has early onset and serve to prevent tissues damages. It is also useful to learn to avoid threat of damage, because certain categories of noxious stimulii become linked to the sensation of pain. This is why this type of pain is defined as adaptive, it helps to survive and to heal[6]

Chronic pain is pain continuing beyond 3 months or after healing is complete[2]. It may arise as a consequence of tissue damage or inflammation or have no identified cause[8]. It can affect a specific body part (i.e. Complex Regional Pain Syndrome (CRPS), low back pain (LBP), pelvic pain) or be widespread (i.e. fibromyalgia). Chronic pain is a complex condition embracing physical, social and psychological factors, consequently leading to disability, loss of independence and poor quality of life (QoL)[9].

Psychological factors in pain

Anxiety

Health anxious individuals form dysfunctional assumptions and beliefs about symptoms and disease based on past experiences and become health anxious when these dysfunctional scheme are triggered by critical incidents[10]. Moreover, they will have a tendency to misinterpret somatic information as catastrophic and personally threatening[10].

Some studies report an increase in pain correlated with increased levels of anxiety, but other suggests it has no effects, thus the effect of anxiety on pain may be dependent on attention[11].

Clinically, anxiety can compromise treatments as the practiotioners must expect patients to report catastrophization and greater pain during activities, thus there is need to target attentional focus and interpretation of sensations among health anxious clients[10].

Depression

There are strong evidencies of an established comorbidity of pain and depression[12][13]. Furthermore, when patients with pain have comorbid depression, they have greater pain, a worse prognosis, and more functional disability[14]. Additionally, chronic pain patients with co-morbid depression have higher health care costs compared to pain patients who do not have depression[15].

Pain and depression are associated by neurobiological, cognitive, affective and behavioral factors, thus the optimal treatment approach for comorbid pain and depression should simultaneously address both physical and psychological symptoms[16].

Expectation

Pain perceived when expected may vary based upon the types of cues received (i.e. cues indicates a more intense or damaging stimulus, then more intense pain is perceived, and viceversa) and even cues of an impending decrease in pain, for example the process of taking an analgesic, usually decrease pain[11]. Thus, expectation is thought to play a major role in placebo analgesia[17][18].

Attention and distraction

There is strong evidence that attention (and distraction) is highly effective in modulating the pain experience and demonstrate how cognitive processes can interfere with pain perception[19][20][21][22][23][24][25]. When a person is distracted with a cognitive task pain is perceived as less intense[19][22][26][27][28], even in chronic pain patients[29]. Conversely, pain increases when the pain is in the focus of attention[30]. Functional brain imaging and neurophysiological studies have shown that attention- and cognitive distraction-related modulations of nociceptive-driven activations take place in various pain-sensitive cortical und subcortical brain regions, accompanied by concordant changes in pain perception[19][22][23][24][26][28][31]. At present time, there are various hypothesis on the physiological bases of these phenomenons, although it is likely that a top-down modulation occur[32][33]. Previous studies on pain processing have demonstrated that key regions of the descending pain control system show enhanced responses during attentional distraction[19][22][26][25].

Fear

Pain-related fear is a general term to describe several forms of fear with respect to pain[34]. Fear of pain can be directed toward the occurrence or continuation of pain, toward physical activity, or toward the induction of (re)injury or physical harm[35].

Fear toward physical activity is also know as kinesiophobia, and is defined as “an excessive, irrational, and debilitating fear of physical movement and activity resulting from a feeling of vulnerability to painful injury or re-injury”[36][37].

Final Fear avoidance model.png
If pain, possibly caused by an injury, is interpreted as threatening (pain catastrophizing), pain-related fear evolves leading to avoidance behaviors, and hypervigilance to bodily sensations followed by disability, disuse and depression. This will maintain the pain experiences thereby fueling the vicious circle of increasing fear and avoidance[38].

In non-catastrophizing patients, no pain-related fear and rapid confrontation with daily activities is likely to occur, leading to fast recovery[38].

These concepts are explained by the Fear-Avoidance (FA) model, which was largely hypothetical in the beginning, but currently there is ample evidence to support the validity of the original FA model[39].

As of today, the FA model is considered to be a component in the development of disability in a variety of conditions, such as low back pain, chronic headache, whiplash disorder, osteoarthritis, knee injury pain, chronic-fatigue syndrome, fibromyalgia and neuropathic pain [40][41][42]

Social and cultural factors in pain

Culture is defined as "the beliefs, customs, arts, etc., of a particular society, group, place, or time" or "a particular society that has its own beliefs, ways of life, art, etc."[43].

Culturally-specific attitudes and beliefs about pain can influence the manner in which individuals view and respond both to their own pain and to the pain of others[44]. Cultural factors related to the pain experience include pain expression, pain language, lay remedies for pain, social roles, and expectations and perceptions of the medical care system[44].
Race/ethnicity, by virtue of their culturally-specific attitudes and beliefs, seems to have an impact on pain processing, including emotional and behavioural responses associated with chronic pain, larger in later stages[45][46].

Another psychosocial factor that may influence differences in pain sensitivity response is the gender role: individuals who considered themselves more masculine and less sensitive to pain have been shown to have higher pain thresholds and tolerances[47].

Socioeconomic factors (e.g. lower levels of education and income) seems to be correlated with a higher incidence of chronic pain diagnosis[48] and pain perception level[49].

The biopsychosocial model of pain

In the past, psychological and physiological (or patophysiological) factors were considered as separated components in a dualistic point of view. Later, the recognition that psychosocial factors, such as emotional stress, could impact the reporting of symptoms, medical disorders, and response to treatment lead to the development of the biopsychosocial model of pain[50].

The bio part is rapresented by the pathophysiology of the disease, or the mechanism of injury, and the relative nociception processes, considering the physiological aspects.

The psychosocial factors (as explaned above) involve both emotion (the more immediate reaction to nociception and is more midbrain based) and cognition (which attach meaning to the emotional experience)[50]. These could trigger additional emotional reactions and thereby amplify the experience of pain, thus perpetuating a vicious circle of nociception, pain, distress, and disability[50].

As of today there are evidencies of psychological factors, such as fear and anxiety, play an important role in the development of chronic pain[51].


Clinical implications

It has to be understood that there is an interaction among physiologic, psychological, and social factors[11], which perpetuates and may even worsen the clinical presentations[50].

  • There is the need to have sound knowledge of these interaction mechanism[11].
  • Targeting psychosocial factors should be a key component of physiotherapist-led intervention[52].
  • Treatment programs must be individually-tailored in order to specifically address the patients' attitudes and beliefs to improve treatment adherence and outcome[41].

Resources

Other Physiopedia Pages

Pain Course All Physiopedia pages with PAIN as their category. Psychological approaches to pain management

External links

Recent Related Research (from Pubmed)

References

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