Synonyms

Pain of Recent Origin; Persisting Pain; Subacute Pain; chronic pain

Definition

Acute pain is pain that has been present for less than three months (Merskey 1979; Merskey and Bogduk 1994). Chronic pain is pain that has been present for more than three months (Merskey 1979; Merskey and Bogduk 1994). Subacute pain is a subset of acute pain: it is pain that has been present for at least six weeks but less than three months (van Tulder et al. 1997).

Characteristics

Acute pain, subacute pain, and chronic pain are defined by units of time, but the concepts on which they are based are more fundamentally aetiological and prognostic.

Acute pain was first defined by Bonica, as “a complex constellation of unpleasant sensory, perceptual and emotional experiences and certain associated autonomic, physiologic, emotional and behavioural responses” (Bonica 1953). Bonica went on to say “invariably, acute pain and these associated responses are provoked by ... injury and/or disease ... or abnormal function.” Thus acute pain was originally defined as a biological phenomenon resulting from physiological responses to bodily impairment. Pain was recognised as playing the important pathophysiological role of making an individual aware of impairment so they could respond appropriately. Responses include withdrawal from the stimulus causing the pain, to avoid further impairment, and behaviours that minimise the impact of the impairment and facilitate recovery. For example, if a person suffers a fracture the resultant pain warns them to limit activities that might further deform the injured part. In this way, acute pain is fundamentally associated with the early stage of a condition, and with the healing process. It can be expected to last for as long as the healing process takes to restore the impaired tissue.

Chronic pain was defined by Bonica as “pain that persists a month beyond the usual course of an acute disease or ... (beyond the) time for an injury to heal, or that is associated with a chronic pathologic process.” The implication is that if pain persists beyond the time in which an impaired tissue usually heals, the condition involves more than a simple insult to the tissue. One explanation for persistent pain would be that the original insult caused damage beyond the capacity of the natural healing process to repair. Another explanation would be that the insult was recurrent, with each recurrence renewing and prolonging the time required for healing. Yet another would be that the condition involved a chronic pathological process that continues to impair tissue over a long period. Other possible explanations invoke exogenous factors, such as inappropriate interventions applied for treatment, and/or endogenous factors such as cognitions and behaviours that inhibit recovery. Recognition of these endogenous factors lead Engel to develop the biopsychosocial model of chronic pain (Engel 1977), which although originally intended by its author to refer to only some types of chronic pain, is nowadays applied inappropriately by many to chronic pain in general.

The time factor ascribed by Bonica, i.e. one month longer than the usual time of recovery, would vary from condition to condition. In order to standardise the definitions of acute and chronic pain, attempts were made to ascribe finite durations to them. In 1974, Sternbach (Sternbach 1974) suggested six months as an arbitrary limit, such that pain present for up to six months would be classed as acute, whereas that present for more than six months would be deemed chronic. Others felt six months was too long, and discussion ensued. The International Association for the Study of Pain (IASP) formed a committee chaired by Harold Merskey to consider such issues and it determined, in 1979 in a publication defining pain terms, that “three months is the most convenient point of division...” (Merskey 1979).

Thus, we have the current definitions of acute and chronic pain as pain present for less than, and more than, three months. The three month period is arbitrary, but it operationalises the definitions so that pains can be classified readily and systematically as acute or chronic.

The definition of subacute pain has not been addressed so deliberately. The term ‘subacute’ evolved to describe longer-lasting acute pain, and has been applied in the literature (van Tulder et al. 1997) to pain present for between six weeks and three months. As such, it forms a subset of acute pain. The main division between acute and chronic pain remains at three months.

The pragmatism of the time-based definitions should not be allowed to obscure the concept from which they were derived: that different types of condition give rise to acute and chronic pain. Acute pain should be considered primarily as pain due to a condition that is likely to resolve spontaneously by natural healing. Chronic pain should be considered as signifying a condition unlikely to resolve spontaneously by natural healing. The clinical significance of the three categories of pain flows from the implicit likelihood of spontaneous recovery, which is crucial to management and prognosis.

The management of acute pain is clear when the condition is understood and known to be likely to resolve within a short time by natural healing. By definition, no therapeutic intervention is necessary for recovery; so, rational management involves helping the patient understand the situation, reassuring them and simply allowing natural healing to proceed. The only active intervention that might be needed is something to ease the pain while healing occurs; and the least invasive measure for that purpose is to be preferred. Such an approach carries the least risk of iatrogenic disturbance of the healing process. It fits nicely with Hippocrates’s aphorism of “first, do no harm” (Hippocrates. Of the Epidemics, I; II: VI), to which doctors have (supposedly) subscribed for centuries. Cochrane promoted this approach in his farsighted work that lead to the formal development of evidence-based medicine; he wrote of “the relative unimportance of therapy in comparison with the recuperative power of the human body” (Cochrane 1977), and wondered “how many things are done in modern medicine because they can be, rather than because they should be” (Cochrane 1977). The effectiveness of the approach has been shown by Indahl et al. (1995) in the management of subacute low back pain, and by McGuirk et al. (2001) in the management of acute low back pain.

Rational management of chronic pain is quite different. As the circumstances giving rise to chronic pain will not resolve spontaneously, intervention is indicated in virtually every case. The key to the problem is accurate diagnosis. Psychosocial factors are important in chronic pain, but their roles are usually secondary to what began and often persists as a biological impairment. If the treating clinician can identify an underlying biological mechanism, many chronic conditions have specific treatments that will control the pain effectively (Lord et al. 1996; Govind et al. 2003). Nevertheless, psychosocial factors must always be considered as well, and addressed if necessary in the management of the condition, but not to the exclusion of the fundamental (biological) cause.

Pursuing the diagnosis of a disorder so as to address its cause seems obvious and is standard practice in other fields of medicine, but for some reason it is controversial in pain medicine. Chronic low back and neck pain, in particular, are rarely managed as if precise diagnosis is possible, which these days it is in the majority of cases (Bogduk et al. 1996). If specific treatment is applied and the pain is controlled, associated psychosocial problems can also be expected to remit. There is sound evidence (Wallis et al. 1997) to show this happens, but no sound evidence to show that when pain is controlled effectively, related psychosocial problems persist.