Pain

From birth to losing a loved one, the experience of pain is central to human life. Indeed, with the exception of the very few individuals born incapable of experiencing pain, it may be argued that people expend most of their time and effort in trying to avoid the pain of hunger, physical injury or emotional hurt. From this perspective, the phenomenon of pain has been the engine for much of technological progress and the advancement of civilisation. This article is a general introduction to the cycle of Citizendium articles on our current understanding of the phenomenon of pain, what it means, how it works, and where humanity may be heading in trying to come to terms with this most basic of experiences.

The meaning of pain

The experience of pain is universally and intuitively recognised, but its definition remains controversial. Most people would consider that, by saying that they "have pain", and by giving some indication of where and how severe the hurt is, they are making their experience clear to the listener. The idea that such a bland subjective communication tells us what the person is experiencing presents difficulties for those who wish to study pain from either a physical scientific or a philosophical perspective. The scientist and philosopher would like to be able to define what the experience of pain is, so that it may be analysed, discussed, researched, understood, and hopefully relieved. Without such an agreed definition, a Tower of Babel like confusion could result.

The difficulty stems from the fact that pain is an observation about something that arises inside the sufferer's own body. This is very different from external sense experiences such as vision, hearing or taste, where the stimulus is a clearly definable physical or chemical entity, correlated in a very specific way with the word which we use for our perception of that external event (e.g. blue, G-flat, or sweet). For pain, the only observation that might correlate with the experience is injury to body tissue, yet although an injury may be obvious to an observer, the feeling of pain is not. Pain may be deduced from physical observations, but can be confirmed only by the sufferer.

The International Association for the Study of Pain (IASP) recognised these unique and enigmatic qualities of the pain experience when, in 1994, its committee on taxonomy formulated the IASP definition of pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. This definition clearly applies to pain in humans only, as science is not able to evaluate the emotions of animals. There are other definitions of pain, and recent advances in imaging of the brain in pain (e.g. fMRI, SPECT), as well as the identification of biochemical markers of nociceptor nerve activity (e.g. C-fos) may allow more specific descriptions of the phenomenon of pain. At present there is no consistent explanation for how and why persons perceive pain in different situations, and in different ways. The idea that some anaesthetists propound that the stress response, spinal metabolic changes and autonomic changes are sufficient to diagnose "pain", when the person is under general anaesthesia, indicates that even in scientific circles there is no consensus about the difference between nociception and pain.

Nociceptive pain

Pain often has a physical cause, an injury to the body outside of the nervous system. In these cases, pain is initiated by mechanical, thermal or chemical changes in non-nervous tissues; this causes activation of specific nerves which relay to spinal centres concerned with the detection of injury, and thence to the thalamus and cortex, as well as to the reticular system. This hard-wired injury detection mode for pain is called "nociception", meaning detection of harm, while the nerves which detect the damage are called nociceptor nerves ("nociceptors" for short). Common causes of nociceptive pain include traumatic injury (fractures, torn tissue and burns), degenerative conditions such as osteoarthritis, infections and inflammatory conditions such as abscesses or sunburn, and cancers causing tissue breakdown.

Neuropathic pain

A second physical origin for pain is damage to the nociceptor nerves themselves; this is important because it is very difficult to treat, tends to be long standing, and may not be diagnosed easily. In these cases there is no damaged tissue, and no heat, pressure, or release of pain nerve stimulating chemicals at the site where the brain perceives the pain to be coming from, i.e. there is no actual or potential tissue damage in "the area which hurts". It is the spontaneous activity of the damaged and dysfunctional nerves which convey impulses to the spinal cord nociceptor nerve structures, and thence to the higher centres. This mode of the experience of pain is called neuropathic, implying pathology or disease of the nerves themselves. Neuropathic pain may follow injury to a nerve, occurring at the same time as a more general tissue injury, so that nociceptive and neuropathic pain may initially co-exist, the combination often changing to more "pure" neuropathic pain as the tissue injury heals, but the nerves remain dysfunctional. A nerve may be traumatically injured in isolation, or a more generalised nerve injury may result from metabolic diseases such as diabetes, the effects of alcohol abuse, or neurotrophic (nerve) infections such as shingles (zoster). During its normal development, the brain " learns" to associate the activity of a specific set of pain nerves with injury to a specific body part, so neuropathic pain is felt in the area that would normally be innervated by the damaged nerve, i.e. the person does not perceive the nerve itself to be sore.

Central pain

The third type of pain which persons may experience is caused by damage to the central nervous system, including the spinal cord, structures at the base of the brain (notably the thalamus) and the brain itself. While this may correctly be called neuropathic (pathology of nerve tissue), the clinical and prognostic implications of these pain states has lead to the term "central pain" being applied to these very stubborn pain syndromes. Examples of such syndromes include spinal cord injury pain and post stroke pain. The central nervous system itself is insensitive to pain (does not contain nociceptor nerve fibres) and the pain is felt by the sufferer to be located somewhere else in the body, as is the case with peripheral nerve injury.

Pain syndromes

There are some miscellaneous pain conditions which may best be described as "pain syndromes". While people suffering from a specific pain syndrome share a defined set of symptoms and signs, the causes for these distinctly characteristic disease patterns are poorly understood. Typically, the pain syndromes are chronic painful diseases where there is much speculation about the mechanisms and interactions between mind, neuropathology and peripheral tissue abnormalities. They tend to have significant impact on the sufferer's quality of life, and to be poorly understood by medical professionals and lay persons alike. Diseases which are at present still known as syndromes include fibromyalgia syndrome, myofascial pain syndrome, complex regional pain syndrome, failed back syndrome, and post-whiplash injury syndrome.

Psychogenic pain

There is no consensus that psychogenic (in the sense of imaginary) pain exists. If a person were to experience pain as a result of a purely psychiatric disturbance, then presumably the central pain localising and "pain as suffering" paths of the brain would be activated in the same way as they would in cases of "real" nociceptive or neuropathic pain, making the condition subjectively indistinguishable from "real physical disease" pain. On the other hand, there are psychological disturbances where persons may complain of pain, act as if in pain, expect others to respond to them as if they are suffering pain, and are not experiencing pain as such, but another feeling such as stiffness or itch. Such illusions of pain are extremely rare. Because pain is apparently so alike in all humans, this semantic error seldom occurs, and the clinical picture tends to be sufficiently inconsistent that the diagnosis would not be difficult to make after a period of conversation about the experience. It should be noted that a person who suffers hallucinations of pain (as opposed to illusions), is really experiencing pain (as hallucinations of voices are subjectively real to the person), so that the complaint would then be consistent, and would demand appropriate treatment to reduce the pain. Finally, the issue of pain complaints as malingering remains a social, medical and legal problem. In the cases where the suffering of pain would lead to real benefits for the person, be it psycho-social or financial, involved persons tend to make the diagnosis without necessarily observing the course of the condition adequately. In clinical practice, malingering for financial or personal secondary gain reveals itself if the person who complains is followed up adequately, as a pattern of inconsistent, irreconcilable, and conflicting actions and complaints. The psychological diagnosis of a "pain disorder" usually presents as persistent and excessive complaints of pain, with no obvious benefits to the person, but frequently associated with gross, persistent and intractable complaints about painful conditions which most persons would consider minor.

Pain in animals

It is important to consider the problem of pain in animals for two quite different reasons. The first is the rather utilitarian consideration that the vast majority of research that is done in an effort to advance our understanding and treatment of pain in humans is done on experimental animals. If animals do not experience pain as humans do, then this work becomes rather inconsequential. The second is that a large proportion of humanity has come to accept that the relief of suffering is an obligation which we have not only to our own kind, but to a varying extent to the animals with which we share this world. In this regard, experimental work on pain, which uses animals as subjects, carries with it the obligation of high ethical standards. At the same time, however, that which is learned about the manifestations and treatment of pain in animals can be used to pursue the ideal of helping fellow living creatures who suffer pain - the very same species which are used for these experiments. It is presently accepted that all vertebrates do possess the mechanisms for nociception, analogous to those of humans, and that these animals can and do suffer pain. The latter is true also for the Cephalopoda, though the pain sensing and processing systems differ from those of vertebrates.

Articles on pain

The reader is referred to the articles listed below - and to their sub-articles - for information on the different aspects of pain touched upon in this essay.

• Acute pain
• Back pain
• Biological pain markers
• Cancer pain
• Central pain
• Chronic pain
• Congenital absence of pain
• Definition of pain
• Diseases characterised by pain
• Economics of pain
• Headache
• Invasive pain treatments
• Neuropathic pain
• Nociceptive pain
• Pain and complementary and alternative medicine
• Pain and emotional suffering
• Pain and gender
• Pain and religion
• Pain and substance abuse
• Pain and placebo
• Pain assessment
• Pain biochemistry
• Pain clinics
• Pain genetics
• Pain imaging
• Pain in animals
• Pain in art and literature
• Pain in metabolic disease
• Pain in the child and fetus
• Pain in the older population
• Pain in the viscera
• Pain measurement
• Pain medications
• Pain physical treatments
• Pain physiology
• Pain rehabilitation
• Pain syndromes
• Pain treatment
• Pain, the state and civilization
• Pain: culture and ethnic factors
• Pelvic pain
• Philosophy of pain
• Psychology of pain
• Referred pain

Please add as required. Some could be fully self-sufficient articles, others sub-articles, others sections in an article. That can be sorted out as we get there. At this stage one would like to know that there is not some important aspect left out.