Classifying Pain

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Pain, a universal human experience, presents a significant challenge to both individuals and healthcare systems worldwide. Its complexity necessitates a structured approach to understanding and management, for which classification is paramount. This article will delineate the fundamental principles and systems of pain classification, primarily drawing upon the authoritative framework established by the International Association for the Study of Pain (IASP).

The IASP Definition of Pain and Its Significance

The IASP, a leading global organization dedicated to pain research and management, provides a cornerstone definition that guides clinical practice and scientific inquiry. In 2020, following extensive review and consideration of new knowledge, the IASP updated its widely accepted 1979 definition. Pain is now defined as “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage”. This revised wording, particularly the phrase "or resembling that associated with," replaced the former "or described in terms of such damage." This change was implemented to be more inclusive, acknowledging that individuals who cannot verbally articulate their pain, such as infants, the elderly, or those with cognitive impairments, as well as nonhuman animals, can still experience pain.

The IASP definition formally acknowledges pain as a multifaceted experience encompassing both sensory and emotional components, moving beyond a purely biomedical interpretation. This definition underscores that while pain is often associated with tissue damage, it is not exclusively dependent on it.

The initial definition in 1979 was pioneering in its time, establishing a common language. The subsequent revision, latest being 2020 at the time of writing, addressed its limitations, particularly concerning non-verbal populations, thereby reflecting a commitment to greater inclusivity and accuracy in defining a fundamental human experience.

Accompanying this definition, the IASP provides six key notes that further elaborate on the nature of pain:

Pain is always a personal experience that is influenced to varying degrees by biological, psychological, and social factors.
Pain and Nociception are different phenomena. Pain cannot be inferred solely from activity in sensory neurons. Nociception refers to the neural process of encoding noxious stimuli, an observable activity from a third-person perspective, whereas pain is the subjective, first-person experience that results from brain activity. Nociceptive signals may not always result in pain, and pain can occur without identifiable nociceptive input.
Through their life experiences, individuals learn the concept of pain. This note acknowledges the developmental and learned aspects of pain perception and expression. Early life experiences, for instance, can shape how individuals react to subsequent painful stimuli.
A person's report of an experience as pain should be respected. This emphasizes a patient-centered approach, mandating that a clinician accept a patient's self-report of pain as valid.
Although pain usually serves an adaptive role, it may have adverse effects on function and social and psychological well-being. Acutely, pain often acts as a protective warning system. However, when pain becomes chronic, it can lose this adaptive function and become a debilitating condition in itself.
Verbal description is only one of several behaviors to express pain; inability to communicate does not negate the possibility that a human or a nonhuman animal experiences pain.

The Importance of Pain Classification

A systematic approach to classifying pain is indispensable for several reasons. Pain classifications serve as vital tools in the assessment and diagnosis of patients, providing a common language for healthcare professionals and researchers. This common lexicon facilitates clearer communication, which is essential for collaborative care and for the consistent application of diagnostic criteria.

Furthermore, accurate classification guides clinical decision-making regarding the most appropriate treatment plans. By understanding the type and underlying mechanisms of a patient's pain, clinicians can select more targeted and effective therapies. For instance, pain predominantly driven by inflammatory mechanisms may respond well to anti-inflammatory medications, whereas neuropathic pain often requires different pharmacological approaches, such as anticonvulsants or antidepressants.

In the realm of research, robust classification systems are crucial for designing studies, identifying homogenous patient groups, and comparing outcomes across different investigations. This, in turn, strengthens the evidence base for various interventions and helps in understanding the specific mechanisms driving different pain states. The development of the International Classification of Diseases, 11th Revision (ICD-11) codes for chronic pain, for example, aims to improve diagnostic coding and thereby enhance epidemiological research and healthcare statistics.

Ultimately, effective pain classification is a step toward personalised pain medicine. As the understanding of pain mechanisms deepens and classification systems become more nuanced, treatments can be increasingly tailored to the individual patient. This targeted approach holds the promise of improving therapeutic efficacy, minimizing adverse effects, and reducing the reliance on less specific or potentially harmful interventions. For example, the recognition that opioids are often ineffective and potentially detrimental for certain types of pain, such as some nociplastic pain conditions, is a direct outcome of better mechanistic understanding and classification.

The Biopsychosocial Model of Pain

The understanding of pain has evolved from a purely biomedical model, which primarily focuses on physical pathology, to a more comprehensive biopsychosocial model. This model posits that pain is a dynamic and multifactorial experience arising from the interplay of biological, psychological, and social factors.

Biological factors include genetics, neurophysiological processes (like nociception and sensitization), inflammation, tissue damage, and structural changes.
Psychological factors encompass thoughts (e.g., catastrophizing, fear of movement), emotions (e.g., anxiety, depression, anger), coping strategies, beliefs about pain, and past experiences.
Social factors involve cultural influences, socioeconomic status, work environment, family and social support, and access to healthcare.

The relative contribution of each of these domains to an individual's pain experience can vary significantly from one person to another and can also change over time for the same individual. For example, in acute pain following an injury, biological factors may be predominant. However, if the pain persists and becomes chronic, psychological factors like fear-avoidance beliefs and social factors like job loss or social isolation can become increasingly important in maintaining and exacerbating the pain experience.

The adoption of the biopsychosocial model necessitates a holistic approach that considers the whole person, not just the site of physical injury or pathology. This model of pain is often used as an argument for multidisciplinary treatment (MDT). MDT involves an integrated approach where multimodal treatment is delivered by a team of healthcare professionals from various disciplines (e.g., physicians, psychologists, physical therapists, occupational therapists). This team collaborates in assessment and treatment, utilizing a shared biopsychosocial framework and common goals to address the multifaceted nature of the patient's pain.

While this sounds nice in theory and an MDT approach may well be effective, the effect sizes are very small. Furthermore MDT is expensive. Using a biopsychosocial model is not restricted to MDT, individual clinicians can view and manage chronic pain through this lens.

Key Mechanistic Descriptors

A cornerstone of modern pain classification is the differentiation of pain based on its underlying neurophysiological mechanisms. The IASP recognizes three primary mechanistic descriptors:

Nociceptive Pain: Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors. This is the most common form of pain and occurs with a normally functioning somatosensory nervous system.
Neuropathic Pain: Pain caused by a lesion or disease of the somatosensory nervous system. This implies damage or pathology within the nerves, spinal cord, or brain regions involved in sensory processing.
Nociplastic Pain: Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain. This category was introduced more recently to account for pain states where central nervous system sensitization and altered pain processing are believed to be the primary drivers.

These three mechanistic descriptors are not always mutually exclusive; a patient can experience pain that involves a combination of these mechanisms. Understanding the predominant mechanism(s) at play is crucial for guiding treatment choices.

Table 1: IASP Core Pain Terminology and Mechanistic Descriptors

Term	IASP Definition
Pain	An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.
Allodynia	Pain due to a stimulus that does not normally provoke pain. Note: Involves a change in the quality of sensation; the original modality is nonpainful, but the response is painful.
Hyperalgesia	Increased pain from a stimulus that normally provokes pain. Note: Represents an augmented response in a specific mode (pain); the quality of sensation is not altered.
Analgesia	Absence of pain in response to stimulation which would normally be painful.
Nociceptive Pain	Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors. Note: Occurs with a normally functioning somatosensory nervous system.
Neuropathic Pain	Pain caused by a lesion or disease of the somatosensory nervous system. Note: Requires a demonstrable lesion or disease meeting established neurological diagnostic criteria.
Nociplastic Pain	Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.

Fundamental Pain Classifications

Understanding the multifaceted nature of pain requires a systematic approach to its classification. Beyond the primary mechanistic descriptors, pain can be categorized based on several fundamental principles, including its duration, etiology, and anatomical location. Furthermore, comprehending specific neuroanatomical concepts such as Dermatomes, Myotomes, Sclerotomes, and Referred Pain

Classifying Pain: Core Principles

Pain is a heterogeneous experience, and its classification can be approached from multiple angles to provide a comprehensive understanding for clinical and research purposes.

Classification by Duration

One of the most fundamental ways to classify pain is by its duration. The IASP distinguishes between acute and chronic pain:

Acute Pain: This type of pain is generally of recent onset and limited duration. The IASP defines acute pain as pain that lasts from a few seconds to three months and is usually associated with actual or threatened tissue injury. It serves a biological purpose, acting as a warning signal of injury or disease, and typically resolves as the underlying tissue damage heals or the threat is removed. A key feature of acute pain is that it is self-limiting.
Chronic Pain: In contrast, chronic pain persists beyond the expected period of healing or recurs for an extended duration. The IASP defines chronic pain as pain that lasts or recurs for more than three months. Unlike acute pain, chronic pain often serves no apparent biological purpose and can become a condition in itself, significantly impacting an individual's physical function, emotional well-being, and quality of life. It can persist for several years.

IASP/WHO ICD-11 Classification of Chronic Pain (Primary and Secondary)

Recognizing the significant burden of chronic pain, the IASP, in collaboration with the World Health Organization (WHO), developed a new classification system for chronic pain, which has been incorporated into the 11th Revision of the International Classification of Diseases (ICD-11). This classification distinguishes between Chronic Primary Pain and Chronic Secondary Pain.

Chronic Primary Pain: This is defined as chronic pain in one or more anatomical regions that is characterized by significant emotional distress (e.g., anxiety, anger, frustration, or depressed mood) or functional disability (interference in daily life activities and reduced participation in social roles). The diagnosis of chronic primary pain is appropriate independently of identified biological or psychological contributors, unless another diagnosis would better account for the presenting symptoms. In essence, for chronic primary pain, the pain itself can be conceived as a disease.

Examples include: Chronic Widespread Pain (e.g., Fibromyalgia), Complex Regional Pain Syndrome (CRPS, particularly Type I), Chronic Primary Headache and Orofacial Pain, Chronic Primary Visceral Pain, and Chronic Primary Musculoskeletal Pain (e.g., some forms of chronic low back pain).

Chronic Secondary Pain: This category includes chronic pain that is initially conceived as a symptom of an underlying disease or condition. The ICD-11 outlines six major categories of chronic secondary pain:

Chronic cancer-related pain: Pain due to cancer or its treatment (e.g., chemotherapy, radiotherapy, surgery).
Chronic post-surgical or post-traumatic pain: Pain that develops or increases in intensity after a tissue trauma (surgical or accidental) and persists beyond three months.
Chronic neuropathic pain: Pain caused by a lesion or disease of the somatosensory nervous system.
Chronic secondary headache or orofacial pain: Chronic forms of symptomatic headaches (primary headaches like migraine are classified under chronic primary pain if they meet criteria) and chronic secondary orofacial pain (e.g., chronic dental pain).
Chronic secondary visceral pain: Pain secondary to an underlying condition originating from internal organs, caused by persistent inflammation, vascular mechanisms, or mechanical factors.
Chronic secondary musculoskeletal pain: Pain in bones, joints, and tendons arising from an underlying disease classified elsewhere (e.g., inflammatory arthritis, osteoarthritis with clear structural damage).

The ICD-11 classification, particularly the introduction of "Chronic Primary Pain," marks a significant advancement. It formally recognizes that chronic pain can be a disease entity in its own right, not merely a symptom of another condition.

Classification by Aetiology

Aetiological classification aims to categorize pain based on its underlying cause. This approach is fundamental to directing treatment towards the root of the problem. Broadly, pain can be divided into cancer pain and non-cancer pain. The ICD-11 chronic secondary pain categories are also inherently aetiological, linking the pain to specific causes such as trauma, surgery, cancer, or lesions of the nervous system. Understanding the aetiology is often intertwined with other classification axes, such as duration and mechanism. For example, pain from diabetic neuropathy is chronic, non-cancer, and neuropathic in mechanism.

Classification by Anatomical Location

Classifying pain by its anatomical location is often the initial step in clinical assessment. This involves identifying the specific body region(s) where the patient experiences pain. The IASP's "Classification of Chronic Pain, Second Edition (Revised)" provides a detailed framework organized by anatomical sites, such as relatively generalized syndromes, localized syndromes of the head and neck, spinal pain (further divided by region), local syndromes of the upper and lower limbs, and visceral syndromes. This system helps in localizing the source of pain and communicating findings among healthcare professionals. For instance, specifying pain as "lumbar spinal pain" or "pain in the right shoulder" provides crucial diagnostic information.

Classification by Underlying Mechanism

As introduced earlier, classifying pain by its predominant underlying neurophysiological mechanism—nociceptive, neuropathic, or nociplastic—is a critical dimension. This approach seeks to understand how the pain is generated and maintained, which is paramount for selecting mechanism-based treatments. It is important to recognize that these classifications are not always mutually exclusive, and a patient may present with pain that involves a combination of mechanisms.

The various axes of pain classification provide a multidimensional framework for understanding and assessing pain. A single pain experience can often be described using multiple classifiers simultaneously. For example, a patient might present with chronic (duration), neuropathic (mechanism) pain in the lower limb (anatomical location) due to diabetic neuropathy (aetiology).

Table 2: Overview of Major Pain Classifications

Classification Axis	Sub-types / Examples	Key IASP Definitions/Notes
Duration	Acute Pain: < 3 months, related to tissue injury, self-limiting. Chronic Pain: > 3 months, persists beyond healing, may be a disease itself.	Acute:. Chronic:.
	Chronic Primary Pain: Fibromyalgia, CRPS Type I, chronic primary LBP.	Pain as a disease, significant emotional distress/functional disability.
	Chronic Secondary Pain: Cancer pain, post-traumatic pain, neuropathic pain, secondary visceral pain.	Pain as a symptom of an underlying disease.
Aetiology	Cancer Pain, Non-Cancer Pain (e.g., post-traumatic, degenerative, inflammatory).	Delineates the cause of pain.
Anatomical Location	Headache, Low Back Pain, Pelvic Pain, Right Arm Pain.	Specifies the body region affected.
Underlying Mechanism	Nociceptive Pain: (e.g., sprain, burn, appendicitis).	Arises from activation of nociceptors in non-neural tissue.
	Neuropathic Pain: (e.g., diabetic neuropathy, radicular pain, post-stroke pain).	Caused by a lesion or disease of the somatosensory nervous system.
	Nociplastic Pain: (e.g., fibromyalgia, some chronic LBP, IBS).	Arises from altered nociception without clear tissue/nerve damage.