Classification of Acute Pain and Chronic Pain Syndromes

Definition of Pain

The International Association for the Study of Pain (IASP) updated the definition of pain in 2020. The previous definition remained unchanged for more than 40 years (originally released in 1979), but it provided a necessary and widely accepted description in the medical field. The 1979 definition of pain was as follows: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” One year later, with a better understanding of pain and new research, the IASP understood the necessity of modifying the definition. They created an expert task force comprising 14 members. After two years of discussion, the 2020 definition was updated to read: “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.” This new definition includes those unable to articulate their pain verbally, such as infants and the elderly.

It is also important to note a few items to make this definition complete:

• 1

  Pain is a personal experience influenced by biologic, psychological, and social factors.

• 2

  Pain cannot be inferred solely from activity in sensory neurons or nociceptors.

• 3

  Each individual will learn the concept of pain through life experiences.

• 4

  The report of an experience of pain should be respected.

• 5

  Despite having an important adaptive role, pain may have adverse effects on function and social and psychological wellbeing.

• 6

  A verbal description is only one of several behaviors to express pain, and the inability to communicate does not negate the possibility of experiencing pain.

Taxonomy and Classification

Taxonomy derives from the Greek words tasso (arrangement) and nomia (rules or law), and it is defined as an organization of concepts arranged using hierarchical relationships. The purpose of creating a medical classification is pragmatic and includes exchanging standardized information, facilitating statistical comparisons, and improving research at national and international levels. An ideal classification should be comprehensive, biologically plausible, reliable, and exhaustive, and each item should be independent and mutually exclusive. However, achieving all of these can be challenging in medical taxonomy.

Currently, there are multiple classifications, groupings, and descriptions of different medical disciplines. Perhaps the most well known is the International Classification of Diseases (ICD), which includes multiple diagnoses and is used worldwide. Classifications related to pain have also been developed. Currently, there is a classification for psychiatric conditions that can be found in the Diagnostic and Statistical Manual of Mental Disorders (DSM) , fifth edition (V). The categorization of headaches can be found in the International Classification of Headache Disorders. The classification for acute pain can be found in the ACTTION-APS-AAPM Acute Pain Taxonomy (see Section E, Acute Pain, for more on this workgroup). The classification of chronic pain was provided by the IASP.

International Classification of Diseases (ICD)

In 1793, Dr. Francois Bossier de Sauvages de Lacroix, a French physician and botanist, developed a categorization of 10 main pathologic conditions, subdivided into 2,400 unique diseases. This is believed to be the origin of the ICD. In 1853, a plan was made to create a worldwide system for classifying causes of mortality during the International Statistical Congress. This work was completed in 1893 and named the “International List of Causes of Death.” This list was adopted by multiple countries, including the United States, and was updated every decade until 1938. In 1948, it was renamed the ICD when the World Health Organization took over this classification system, expanding the list to include not only mortality but also morbidity. The ICD-6 was the first version to include psychiatric disorders. Starting with the ICD-9 in 1977, the list included 4-digit-level categories and optional 5-digit subdivisions, resulting in a better level of detail. This was advanced in 1992 with the ICD-10 when an alphanumeric code was introduced, dramatically improving specificity and expanding it from 17,000 to approximately 155,000 codes. In this version, the conditions were classified by causal agents, systems of the body, symptom pattern, and type of psychiatric illness. The implementation of the ICD-10 in the United States was officially approved in 2015 with clinical modifications.

The 11th and newest version of the ICD was released in 2018 and approved by the World Health Assembly in 2019. With the contribution of an IASP taskforce, this version is considered more flexible than its predecessor. It has vast advantages in terms of chronic pain definitions, given that in the past, this type of pain was associated with only a pathophysiologic mechanism, neglecting the psychological and social contributors. In practical terms, chronic pain is still defined as pain that persists over three months. An extremely relevant change is the differentiation of primary versus secondary pain syndromes, with the inclusion of the term “chronic primary pain.” This term is intended to group several poorly understood conditions with the important component of emotional distress and/or significant functional disability not accounted for by other diagnoses. ^, Chronic pain would then be the parent code of seven children codes that comprise the most relevant groups of chronic pain conditions: chronic primary pain, chronic cancer-related pain, chronic postsurgical or post-traumatic pain, chronic neuropathic pain, chronic secondary headache or orofacial pain, chronic secondary visceral pain, and chronic secondary musculoskeletal pain ( Fig. 2.1 ). ^, These changes are promising to improve the acquisition of accurate epidemiologic data, provide adequate billing, and potentiate the development and implementation of new therapies for chronic pain conditions.