What is pain?

Causes and Symptoms

Not all causes of pain are known or understood. Some basic causes of the most commonly reported pain include inflammation, as in arthritis, rheumatism, and infection; work-related and sports-related injuries; stress and tension; nerve pain, as from shingles, diabetic neuropathy, and sciatica; and pain related to such diseases as osteoporosis and cancer.

People have similar pain thresholds but different levels of pain tolerance, or how much pain they can bear. One congenital anomaly actually inhibits or eliminates the perception of pain. Pain tolerance is therefore subjective and can be influenced by socioeconomic status, cultural background, and socialization, with disparities noted in who suffers pain, what type of pain a person suffers, and how pain is perceived by the individual.

The most commonly reported types of pain are associated with the lower back, with severe or migraine headaches, and with joint pain, particularly in the knees. Physiological pain is a response of the body that is associated with tissue damage or inflammation, typically serving as a warning system to alert the body to potential physical harm. Although pain may be produced without a defined stimulus, as in the case of emotional or psychological pain, physiological pain is transmitted through stimulation of nerve pathways, a process called nociception. Nociceptors are free, sensitive nerve endings located outside the spinal column; they are found in skin and on internal surfaces, such as on the joints. When stimulated, nociceptors send signals through sensory neurons to the posterior horn of the spinal cord that are then transmitted to other nerve fibers, which travel upward through the brain stem to the thalamus, the gateway to conscious action in the brain. There, information is coordinated and localized and then sent to the cerebral cortex, where a conscious reaction to the stimulus is produced.

Pain is said to be "referred" when it is experienced at a location other than its site of origin. This occurs when nerve fibers carrying pain messages enter the spinal cord at the same place as other nerve fibers from other parts of the body using the same pathways. The other nerve fibers may become stimulated and result in painful perceptions in healthy areas of the body, such as referred pain from the heart to the neck, arm, and stomach.

Among theories of pain transmission, the gate-control theory of Ronald Melzack and Patrick Wall helps explain the differing degrees of pain that people may suffer. It is related to the amount of substance P (SP), a peptide found in nerve cells throughout the body, that actually reaches the brain. The transmission of neurons is generally very rapid, as when touching a hot stove produces immediate action to protect the body from damage. Messages carried by substance P, however, travel more slowly, since they must pass through a special gateway in the spinal cord. At the same time, pain signals are also prompting the brain to release chemical endorphins, the body’s natural painkillers, which must also pass downward through the same gate. Thus, there is some competition for passage, and the fewer receptors for substance P that actually arrive in the brain and attach to nerve cells there, the lower the pain perception. With healing, the gate closes, but when chronic pain occurs, it remains open even after healing or without an identified underlying cause.

The two basic types of pain are chronic and acute. Acute pain comes on suddenly and, although extreme, is generally brief in duration. It is a warning to the body about damage or disease, is localized, and is more easily treated. Chronic pain occurs daily and lasts longer than would be common for a specific injury. It no longer serves to warn and is much more difficult to treat, although most sufferers can be helped. Chronic pain may last beyond resolution of an underlying cause, or it may grow out of an acute condition. In this case, it may become a learned response that no longer has a purpose but continues to hurt. Chronic pain may also occur without any apparent cause, creating disability, depression, and suffering.

Pain may be medically classified as either superficial or deep. Superficial pain, also called fast or cutaneous pain, is carried by nerve fibers on the skin and outer linings of the organs. These nerve fibers are plentiful in the intestines, cornea, and nose, for example, and pain messages are quickly delivered to the brain, such as when one is cut or burned. Also termed somatic pain, it is experienced as intense or burning. Kidney stones or acid reflux from the stomach may create waves of this burning pain. Deep pain, on the other hand, also referred to as slow or visceral pain, comes from nerve fibers located in muscles, bones, and tissues of the internal organs, and it travels more slowly, taking longer to reach the brain. It may be experienced as dull aching or throbbing pain. The two types of pain may occur at the same time.

Treatment and Therapy

The major treatment for pain in the United States has been analgesic medications, or drug therapy, with sufferers spending over an estimated $18 billion a year for relief in the form of both prescription and over-the-counter medications. There are no standard guidelines for the use of analgesics, since the degree of relief varies from one patient to another. These medications are classified as either narcotic, such as morphine and other opioid addictive drugs, or nonnarcotic, such as aspirin, ibuprofen, and acetaminophen. Because patients respond differently and many analgesics can carry significant side effects with cardiovascular, renal, and gastrointestinal toxicity, the lowest dose of the preferred medication is usually recommended to start. Painkillers must often be administered with other medications directed to the underlying cause of the pain, so they must be compatible.

One subcategory of nonnarcotic analgesics is nonsteroidal anti-inflammatory drugs (NSAIDs). Another alternative, acetaminophen, addresses pain but has no effect on inflammation. Cyclooxygenase-2 (COX-2) inhibitors are nonnarcotic analgesics that suppress the COX-2 enzyme, which triggers inflammation. Although these drugs are seemingly well tolerated and effective, many of them were found to endanger the heart, and several were withdrawn from the market.

Narcotic analgesics are the most effective, but long-term use can create dependency, and these drugs are stringently restricted in the United States by state and federal laws. Doctors have therefore been hesitant to use them for severe chronic pain, even in patients dying from cancer or other painful diseases, when other medications are not working. This situation appears to be changing.

Nondrug therapies include such techniques as transcutaneous electrical nerve stimulation (TENS), massage therapy, neurosurgery, physical therapy and exercise, and mind-body therapies such as guided imagery, meditation, relaxation, and hypnosis. These therapies attempt to alleviate chronic pain in various ways by stimulating blood circulation, blocking nerve-pain messengers, and enlisting the help of the brain, where pain messages are processed.

A combination of biomedical and nonbiomedical therapies also uses a number of alternative therapies for pain. Acupuncture and acupressure, the foundation of Chinese medicine, are thought to stimulate blood circulation and possibly the autonomic nervous system through insertion of very fine needles at crucial points in the body. Herbal medicine uses substances that are derived from plants with therapeutic or pharmacologic properties and benefits. Many modern medicines have ingredients that originated in plants and can be synthesized in the laboratory. Guided imagery, aromatherapy, creative arts therapy, magnet therapy, and therapeutic touch are often used as adjuncts to dealing with pain, but most have not been proved to be effective. Like analgesics, these therapies address the control and management of pain rather than offering a cure.

Although many of these complementary therapies are not biomedically sanctioned or recognized, many sufferers of chronic pain try some form of complementary medicine. Little or no research has been done on many of these therapies, but their popularity relates to the fact that chronic pain is closely connected with the brain, affecting emotions, attitudes, and psychological stability, which are not addressed by conventional medicine and treatment. Some of these therapies may work through the placebo effect, the phenomenon in which a patient's expectation that a treatment or therapy will be effective produces a successful result. Some approaches are backed by positive evidence, while others have been shown to have no effect. Very little evidence exists about how or why many of these therapies are successful, but combination therapies are vital in alleviating pain, however they may work.

Perspective and Prospects

The development of pain medicine and pain clinics devoted solely to the study and alleviation of pain is a fairly recent occurrence. Since pain was traditionally seen as a symptom rather than as a disease or condition in itself, the medical profession has historically focused on treating the cause, considering pain to be purely a diagnostic tool. The discovery and development of anesthetics for surgical procedures in the latter nineteenth century was a huge advance in medical care and treatment and was a precondition for the later development of pain medicine. In addition to traumatic and postoperative pain, anesthesiologists worked to refine techniques and develop expertise in management relating to other types of pain as well.

Anesthesiology progressed rapidly during World War II, with improved use of nerve blocking and analgesics. Anesthesiologist John Bonica contributed significantly to the development of pain medicine. He was faced with extreme, intractable, complex, and phantom-limb pain (the sensation of pain felt in a limb no longer there) in the injured during wartime and lacked the knowledge or methods to treat them. When pain persisted and physiological causes could not be identified, it became necessary to look elsewhere for the source of the pain. It became obvious that numerous specialists, including psychologists and psychiatrists, needed to consult and discuss their varied findings and opinions.

Practitioners of pain medicine mostly come from other medical fields most closely related to pain, such as neurology, anesthesiology, and rehabilitation. As defined by the American Academy of Pain Medicine, the specialty is concerned with the study, prevention, evaluation, treatment, and rehabilitation of people in pain. Many are certified as pain specialists through the American Board of Anesthesiology. While some pain clinics focus on specific types of pain, such as bone and joint, others address a broader spectrum of suffering and tend to use a variety of methods and treatments, including alternative therapies, to find something that works. Some pain cannot be eliminated but can be minimized or controlled enough to allow the patient to function.

The need to study and understand the causes and alleviation of pain has become more urgent. According to the National Center for Health Statistics, in 2012, 13.9 percent of adults reported that in the past three months, they had experienced neck pain lasting a full day or more; 14.2 percent, a severe headache or migraine lasting a full day or more; and 27.5 percent, lower-back pain lasting a full day or more. The same report found that between 1999 and 2010, consumption of opioid analgesics had increased approximately 300 percent. Pain is usually seen as a result of another physical condition, but considering the costs that accompany pain and resulting disability in terms of dollars and loss of individual function reflected in absenteeism in the workplace, pain places an increasing burden on the American health-care system. The general cost of pain and pain-related items is estimated to top $100 billion each year.

Research is being conducted into the origins and mechanics of pain in an attempt to identify new and more effective therapies. A study funded by the National Institutes of Health found that the perception of pain (the extent to which one feels pain) is inherited through a gene with a specific variant. This gene variant affects sensitivity to acute pain as well as the risk of developing chronic pain. Other genes may also play a role. This study opens up pathways for developing new treatments and approaches to pain.

Professional organizations such as the American Academy of Pain Medicine, the American Pain Foundation, the American Pain Society, and the International Association for the Study of Pain represent only a few of the growing number of resources available for the study of pain and pain management. Alternative approaches are represented by organizations for specific therapies and the National Center for Complementary and Alternative Medicine.

Bibliography

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