Pain is one of the most common symptoms for which patients seek treatment, and management of pain and relief of suffering should be at the core of the health care provider’s commitment to patients. However, most curricula devote little time to pain biology, and pain management is often neglected. We know that proper management of pain is essential, not only to bring relief of the primary symptom, but also to prevent the consequences of unrelieved pain. It is now recognized that unrelieved pain can delay healing and depress the immune system. Unrelieved pain can also cause stress, autonomic symptoms, and alterations in the peripheral nervous system (PNS) and central nervous system (CNS) that may result in persistent pain or chronic pain syndromes.
There is every medical and ethical reason to treat pain aggressively using all the evidence-based resources that are likely to benefit patients. This chapter defines pain and different terms used to discuss its features, and outlines how pain, including orofacial pain, can be classified. The first goal of definition and classification is to minimize, as much as possible, the confusion caused by using either different terms to name the same symptoms and conditions or, even worse, the same word to name different pain symptoms or conditions. By using the same terminology, clinicians and researchers can understand each other better, and consequently, the exchange of information is much improved. Standardization also helps to address a second goal: to constitute groups of subjects for clinical research studies whose outcomes can be compared worldwide.
What Is Pain?
The definition of pain
Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.1 Although we often refer to pain as a sensation, it is probably better described as a multidimensional or multifactorial experience encompassing sensory, affective (emotional), motivational, and cognitive dimensions. While there are certain sensory qualities of somatic sensations that are almost exclusively associated with pain, such as stinging, pricking, burning, and aching, pain is also an unpleasant emotional experience. It is because of this emotional dimension that the adjective painful is sometimes applied to other emotional experiences in the absence of sensory stimulation. Pain also has a strong motivational component, evoking both withdrawal reflexes and highly organized avoidance and escape behavior. The motivational aspect of pain is a primary function, and without it the organism will not survive. The cognitive component of pain refers to its meaning to the individual. For example, if the pain is believed to be due to a malignant tumor, its effect on the individual will be much greater than if the pain is believed to result from minor trauma due to a fall.
Theories of pain
Various theories of pain have been proposed. One of the oldest that still has some salience is that a noxious stimulus evokes a specific sensation (pain), which is basically similar to vision and touch, with hardwired lines from specific “pain receptors” to regions in the CNS that process only pain-related signals (specificity theory). Another group of theories proposes instead that noxious stimulation activates several different types of receptors, including tactile receptors, and that summation of the signals in the CNS leads to pain (intensive or summation theory). A third theory proposes that the pattern of signals produced by the noxious stimulus would be important for the recognition of pain and its distinction from other sensations (pattern theory).
More recently, evidence was produced that a large amount of interactions exist between nociceptive and non-nociceptive inputs to the CNS. A theory was formulated based on the potential for inhibition of nociceptive transmission in the CNS by low-threshold mechanosensitive afferent inputs to the CNS. This theory explains, for example, why rubbing an acutely injured body part can often, at least temporarily, produce some pain relief. A few years later, research demonstrated that the “gating” of nociceptive transmission in the spinal cord and brainstem could also be provoked by controls descending from brain centers located higher in the CNS and involved in stress, emotion, cognition, distraction, etc. While not all elements of this so-called gate control theory as originally proposed have held up to detailed scientific scrutiny, this theory has had a huge impact on the understanding of pain by provoking an intense research interest over the past 40 years. The resulting advances in understanding pain from anatomic, physiologic, pharmacologic, neurochemical, molecular, and behavioral research have pointed to the high level of neural integration and the multiple factors involved in pain.
Acute, persistent, and chronic pain
We have all experienced the pain of touching a hot kettle. The pain is sharp but soon subsides. We call this acute (or transient) pain, and it is protective; it warns us of impending tissue damage. A stimulus that is damaging or potentially damaging to tissues is considered noxious. Pain that lasts for a few days or a few weeks can follow athletic injuries of the elbow, knee, or elsewhere. We call this persistent pain; it can also be protective since it forces us to rest the injured part and avoid further damage. In some clinical conditions, however, pain persists long after an injury has apparently healed, possibly for months or years, resulting in chronic pain. This type of pain can be nonprotective. In this book, the terms persistent pain and chronic pain will be used interchangeably. In clinical terms, pain that lasts for at least 3 to 6 months is considered chronic. In contrast, persistent pain can refer to pain that lasts for just hours or days.
A number of terms are used to describe various features of pain. Box 1-1 is a glossary of terms customarily used to describe common aspects of acute, chronic, or persistent pain.
|Box 1-1 Pain glossary2|
|Algesia||Any pain experience following a stimulus|
|Allodynia||Pain or a painful sensation due to a stimulus that does not normally provoke pain or which is innocuous (eg, skin touch after a sunburn)|
|Causalgia||Pain after a trauma to a nerve that may be associated with vasomotor dysfunction|
|Habituation||A decrease or loss of response in nerve terminals or neurons following repetitive stimulations|
|Hyperalgesia||An increased pain response to a noxious stimulus in an affected area versus a control area|
|Hypoalgesia||A diminished pain response to a noxious stimulus in an affected area versus a control area|
|Hypoesthesia||A decreased sensitivity to stimulation that feels similar to the effect of local anesthesia|
|Neuroma||A mass of peripheral neurons formed by a healing scar at a damaged nerve that can cause hyperexcitability of neurons or spontaneous discharge (also termed ectopic discharge)|
|Neuropathic pain||Aberrant pain induced by an injury to a sensory nerve or neuron; may be evoked by thermal, mechanical, or chemical stimuli or may be secondary to a disease (eg, diabetes mellitus, postherpetic neuralgia); may also be central in origin; may occur spontaneously|
|Nociception||The reception and transmission of nociceptive messages|
|Pain||An unpleasant sensory, emotional, and motivational experience, associated with actual or potential tissue damage, that requires consciousness|
|Pain threshold||The lowest level of stimulation perceived as painful by a subject (> 50% of the time)|
|Pain tolerance||The highest level of pain a subject is prepared (able) to tolerate|
|Paresthesia/dysesthesia||An abnormal sensation that is termed dysesthesia when it becomes unpleasant|
|Sensitization||The increased excitability of nerve terminals or neurons produced by trauma or inflammation of peripheral tissues; can be peripheral or central or both|
|Sprouting||The extensive spread of regenerated nerve endings into surrounding tissue following nerve damage|
Methods Used in Classification
To properly manage orofacial pain, the clinician must be able to appreciate the underlying pain mechanisms. This includes a working knowledge of/>