The variety of terms used to describe the duration of pain can be imprecise and confusing, particularly because clinicians (e.g., veterinarians) and pain researchers differ in their vocabulary. We present the terms here and explain how they are used in this report.
Acute pain is used by pain researchers to refer to pain that is momentary, such as associated with a needle stick (e.g., drug injection, venipuncture) or an experimentally applied noxious stimulus that does not produce noticeable tissue damage (e.g., pinch, mild electric shock). These experimental manipulations may generate a withdrawal reflex or vocalization. However, this pain is of very short duration (seconds to tens of seconds, perhaps minutes when assessing pain tolerance; see further discussion in text) and consequences to the subject are minimal and brief. In this document, momentary pain refers to this kind of brief, transient pain.
Acute pain is also used in both human and animal clinical medicine to label the pain typically associated with procedures or surgery. Tissue injury is a usual consequence of such procedures and thus the pain induced is considerably longer lasting than momentary (e.g., lasting for days to more than a week). In this document, pain of this nature is referred to as postprocedural or postsurgical pain.a
Persistent pain refers in this report to pain states that can last for days or weeks but that are caused by different mechanisms than momentary or postprocedural pain.b To study these mechanisms numerous animal models have been developed that are commonly known as “persistent pain models.” These are described in Appendix A.
Chronic pain, of long duration (weeks, months, or years), can be difficult to manage in both human and animal clinical settings.c These pain states are distinct from postprocedural or persistent pain in that they are typically associated with tissue degenerative and destructive diseases (e.g., osteoarthritis, cancer) and do not improve or resolve over time. In the context of laboratory animal medicine, chronic pain is most commonly a byproduct of non-pain-related research (e.g., aging, disease research).
The committee recognizes that the term “acute pain” is commonly used by human and animal clinicians/veterinarians to refer to postprocedural pain or “sharp” pain. However, “sharp” pain can be of both short or long duration (usually undefined), and “acute” means different things to different people. The committee therefore abstains from using the terms “acute” or “sharp” in favor of the terms “momentary” and “postprocedural” or “postsurgical” as defined above.
A common synonym for “persistent” is “tonic,” a description commonly used in pain research, that characterizes pain evoked for as long as nociceptors are stimulated.
Chronic pain in humans is usually defined as pain lasting beyond the expected course of normal healing, often arbitrarily set at 6 months or beyond. Such duration is not appropriate to apply to laboratory animals with much shorter lifespans than humans or early developmental stages. Recurring or constant pain that lasts beyond the expected course of normal healing (which differs per species and per insult/injury) may merit consideration as “chronic pain.” The committee urges pain researchers, veterinarians, animal care staff, and IACUCs to recognize the influence of lifespan on the definition of chronic pain.
From: 1, Pain in Research Animals: General Principles and Considerations
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