Pain – The basic concepts

The International Association for the Study of Pain (IASP) defines pain as ‘an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage’.¹⁵⁸

Acute pain is experienced from actual or threatened damage to tissue and is due to activation of nociceptors. Chronic pain describes persistent or recurrent pain (typically three months or more) and includes multiple pain conditions including CNCP (eg fibromyalgia, non-specific back pain, osteoarthritis, headache), chronic cancer pain and CPSP.^123,159

The understanding of pain has broadened from a simple stimulus–response model (biomedical) to encompass the whole person. Each individual’s pain experience is affected by a complex combination of biomedical, psychological and social factors. The social and psychological factors are commonly the main determinants, especially of CNCP. To reflect this, Australian authorities have formally moved to a biopsychosocial framework. Note that the term ‘biopsychosocial’ is used in this guide without implying an order of importance.

The degree of pain and disability experienced in relation to similar physical injury varies. Similarly, there is individual variation in response to methods used to alleviate pain. Communication of the pain experience can be difficult. Patients are often frustrated and distressed by the limitations of language in expressing their pain experience.

When planning management and interventions for pain, these factors need to be considered. Additional insights and effectiveness of treatment can be gained by exploring and integrating patients’ wishes and their past experiences.

In clinical practice, we have traditionally classified pain based on time: acute or chronic. This has limitations, one of which is that it perpetuates the misconception that chronic pain is just ‘unhealed’ acute pain.^160–162 Instead of classifying pain in terms of time, it is far more important to understand the underlying pathophysiology of pain.

The classification of pain based on pathophysiology is dynamic.¹²³ A change to the definition of neuropathic pain occurred in 2011, and a third descriptor for pain is currently being debated.¹⁶³

Nociceptive pain

Nociceptive pain arises from actual or threatened damage to non-neural tissue and is predominantly due to activation of nociceptors.^158,164 Nociceptive pain guards against tissue injury (eg warns of potentially damaging stimuli such as heat) and supports healing and repair (eg increases pain to normally innocuous stimuli to aid protection/immobilisation of injured tissue). It requires a normally functioning somatosensory nervous system.^163,165

Nociceptive pain can be further classified depending on the location of nociceptors. ‘Visceral’ pain results from stimulation (due to stretch, inflammation or ischaemia) of nociceptors within the viscera. Pain experienced by stimulation of nociceptors in the musculoskeletal system is sometimes referred to as ‘somatic’ pain.

Typically, nociceptive pain only lasts in the presence of continual noxious stimuli and disappears after resolution of the tissue injury. However, certain diseases may generate recurrent or ongoing noxious stimuli to produce chronic nociceptive pain (eg rheumatoid arthritis).^163,165

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system. It is not a diagnosis but a clinical description of the pain resulting from an injury or damage to either the peripheral or central nervous system by mechanical trauma, metabolic diseases, neurotoxic chemicals, infection, or tumour invasion.^163,165 To satisfy the diagnostic criteria of neuropathic pain, the lesion or disease must be demonstrable through:

• an appropriate history (eg episode of shingles within the past six months, limb amputation)
• the presence of signs of neurological deficit
• diagnostic interventions (eg imaging, neurophysiology)
• confirmation of underlying cause (eg stroke, type 2 diabetes, multiple sclerosis).

Chronic neuropathic pain is not a single entity, but refers to a heterogeneous group of pain conditions. Nociceptive pain and neuropathic pain may occur together. For example, a study of patients with prolonged low back pain found that about 37% had a predominant neuropathic pain component.¹⁶⁶

Unfortunately, current treatment of neuropathic pain is less than ideal, with fewer than 50% of patients experiencing satisfactory pain relief and tolerable side effects.¹⁶⁷

Nociplastic pain

Patients initially experiencing nociceptive pain, such as osteoarthritis, may develop alterations in nociceptive processing manifested as altered descending pain inhibition^168,169 accompanied by spread of hypersensitivity.^170–172 Clinicians working in the field of pain have recognised the need for a word to describe pain that arises from altered nociception despite no clear evidence of:

• actual or threatened tissue damage causing the activation of peripheral nociceptors (nociceptive pain)
• disease or lesion of the somatosensory system causing the pain (neuropathic pain).

Nociplastic pain is described as pain caused by altered function of nociceptive pathways in the peripheral or central nervous system. These terms reflect the underlying pathophysiological change (sensitisation) in the nervous system.¹⁶³

This classification describes the pain of conditions including fibromyalgia, CRPS, ‘non-specific’ chronic low back pain, and functional visceral pain disorders (eg irritable bowel syndrome, bladder pain syndrome, chronic abdominal pain, and chronic pelvic pain).

The assessment of pain using a biopsychosocial (or sociopsychobiomedical) framework takes into account the multiple factors (social, psychological, biological) that influence the pain experience. It completes a more complex understanding of the patient’s pain perspective as well as identifying other targets for management. Factors do not always neatly fit into one category; for example, depression may have physical and social aspects as well as being a psychological factor influencing pain. A comprehensive pain assessment includes a:

• general assessment and pain-specific history (which explores the pain type, severity, functional impact, context, and the patient’s meaning of pain, expectations and fears)
• physical examination (assessing for signs of tissue damage or disease that might indicate nociceptive and/or neuropathic and/or nociplastic mechanisms of pain)
• psychological assessment (the pain experience is affected by mood, stress, coping skills, fear avoidance, and catastrophising).

This will inform the selection of treatment options most likely to be effective. The assessment may need to be repeated reasonably frequently, particularly while establishing a diagnosis and appropriate pain management.

The following descriptions provide more detail on the pain assessment including links to useful assessment templates and questionnaires.

General assessment and pain history

The standard medical assessment is as important in pain as it is in any other medical or psychological condition. Particularly with pain, history taking will explore multiple aspects such as any injury, details about the pain experience and impact, and attempted treatments (Table 1). The purpose of the assessment is to establish a possible physical mechanism of pain (whether it is nociceptive, neuropathic, nociplastic or a combination of aetiologies) and if there are underlying conditions or psychosocial issues.

When appropriate, a pain screening questionnaire may assist in the understanding of the patient’s pain experience. These include the Brief pain inventory, McGill pain questionnaire, DN4 neuropathic pain questionnaire and the Örebro musculoskeletal pain screening questionnaire.

Social assessment

Social assessment identifies factors in the patient’s environment that influence the pain experience. These include family and other relationships, work, life events, housing, sleep, activity and nutrition. Pain may be influenced by culture (eg seen as a way to strengthen the body, purify the soul or deepen the spirit with the idea of ‘no pain no gain’).

Pain influences interactions with others, occupational performance and self-care. Pain can be socially isolating, which can contribute to the pain–depression cycle. The patient should be questioned about the impact of pain on function (social and physical) and overall quality of life (Table 2). Specific questions might include the following:¹⁷³ • Social and recreational functioning

• How often do you participate in pleasurable activities, such as hobbies, going out to movies or concerts, socialising with friends, and travel?
• Over the past week, how often has pain interfered with these activities?

• Mood, affect, and anxiety (for more information, refer to 2.3.3 Psychological assessment) – Has pain interfered with your energy, mood, or personality?
  • Are you readily tearful?
• Relationships
  • Has pain affected relationships with family members, significant others, friends or colleagues? • Occupation
  • Has the pain required that you modify your work responsibilities and/or hours?
  • When was the last time you worked, and (if applicable) why have you stopped working? • Sleep
  • Does pain interfere with your sleep? How often over the past week?
• Exercise
  • How often do you do some sort of exercise?
  • Over the past week, how often has pain interfered with your ability to exercise?

To assess the true impact of pain on the patient’s life, it helps to have familiarity with the patient and/or obtain input from their family, friends and support systems.

Occasionally, pain is used to gain sympathy, protection, benefits or medico-legal compensation.

Physical examination

A physical examination, including a thorough musculoskeletal examination, is particularly important in most of the chronic pain conditions and a relevant neurological examination is particularly important for neuropathic pain. A physical examination might include:

• assessing for signs of tissue damage/injury or disease that might indicate nociceptive and/or neuropathic mechanisms of pain (eg tissue deformity and cardinal signs of inflammation, or signs of neural disease or damage)
• careful evaluation for sources of referred pain (including viscera)
• looking for evidence of hypervigilance or guarding with particular movements. Compensatory postures and movements are important to analyse, as they might be placing extra load on sensitive tissues and addressing these early can avoid development of secondary issues
• looking for evidence of allodynia, hypoalgesia and hyperalgesia.

At the end of the examination, the GP should aim to establish a provisional diagnosis for the pain and the biomedical mechanism involved, as well as analysis regarding the disability level of the patient.

Psychological assessment

Psychological assessment explores the patient’s mood state, beliefs, thinking styles, coping skills, behaviours and responses that may contribute to the experience of pain, obstacles to recovery and treatment outcome.¹⁷⁴ Psychological factors that contribute to the experience and impact of pain can be amenable to change¹⁷⁵ and thus influence outcomes for the individual.¹⁷⁶ There are screening tools for many of these factors.

Experience of both acute and chronic pain is commonly accompanied and influenced by mood and anxiety disorders.¹⁷⁴ For example, anxiety is one of the most significant predictive factors for the severity of postoperative pain¹⁷⁷ and there is a consistent association between CPSP and depression.¹²⁵

Relevant beliefs include understanding of diagnosis and prognosis, and expectations about treatment, including willingness to be an active participant. Many people have a fear of pain, which contributes to the development of avoidance responses and can ultimately lead to disability. Thinking styles that are overly negative, ruminative and helpless (eg catastrophic thinking) are associated with more severe acute pain as well as persistent pain.¹⁷⁸

Measurement of pain and functional impact

There are several pain scoring systems. Verbal numerical rating scales are often preferred because they are simpler to administer and give consistent results.¹⁷⁹

The PEG scale may be useful in the general practice setting, particularly when assessing chronic musculoskeletal pain.¹⁰⁹ Scores (out of 30) give a reference point for the patient’s overall wellbeing and can be used to compare the same patient seen at different times or by different practitioners.¹⁰⁹

As so many factors influence the experience (and communication) of pain, it is not surprising that pain scores do not provide information about which patients are likely to respond to opioids.¹⁸⁰

Risk assessment

Opioids are often useful analgesics, but care needs to be taken when prescribing these drugs to limit the risks, including inappropriate use and diversion. Clinically, problematic opioid usage is more likely when used in:

• younger patients – substance use issues generally commence before 35 years of age
• patients without a definite patho-anatomic diagnosis
• patients with active substance use problems or who are in contact with patients with such problems
• patients with active psychiatric problems
• patients who use benzodiazepines – concomitant use of opioids substantially increases the risks of side effects, particularly cognitive impairment, sedation and respiratory depression.¹⁸¹

Comprehensive assessment addresses the risk of opioid misuse.^182,183 While screening for opioid risk has been recommended, at this point evidence of effectiveness is lacking. Additionally, treatment agreements and urine testing are also recommended but have not been shown to reduce overall rates of opioid prescribing, misuse, or overdose.^5,103

Patients with a history of SUD are at higher risk of harms. A check of state-based prescription monitoring systems is advocated. Those patients with a history of SUD should probably not be offered opioids in a general practice setting, but, rather, if pain control cannot be gained by other means, should be offered referral to specialist services.

A UDS may reveal evidence of substances of which the practitioner is not aware. Not all substances are routinely tested for (eg oxycodone testing needs to be specifically requested). If such drugs are found, whether illicit or legal, the patient should be referred for specialist assessment and management. Contacting your local pathology provider may be necessary.