Pain is an unpleasant sensation signaling actual or possible injury.

Pain is the most common reason people visit their doctor. Pain may be sharp or dull, intermittent or constant, or throbbing or steady. Sometimes pain is very difficult to describe. Pain may be felt at a single site or over a large area. The intensity of pain can vary from mild to intolerable.

People differ remarkably in their ability to tolerate pain. One person cannot tolerate the pain of a small cut or bruise, but another person can tolerate pain caused by a major accident or knife wound with little complaint. The ability to withstand pain varies according to mood, personality, and circumstance. In a moment of excitement during an athletic match, an athlete may not notice a severe bruise but is likely to be very aware of the pain after the match, particularly if the team lost.

Spotlight on Aging
Pain is common among older people. However, as people age, they complain less of pain. The reason may be a decrease in the body's sensitivity to pain or a more stoical attitude toward pain. Some older people mistakenly think that pain is an unavoidable part of aging and thus minimize it or do not report it. The most common cause is a musculoskeletal disorder. However, many older people have chronic pain, which may have many causes. Effects of pain may be more serious for older people: Chronic pain can make them less able to function and more dependent on other people. They may lose sleep and become exhausted. They may lose their appetite, resulting in undernutrition. Pain may prevent people from interacting with others and from going out. As a result, they can become isolated and depressed. Pain can make people less active. Lack of activity can lead to loss of muscle strength and flexibility, making activity even more difficult and increasing the risk of falls. Older people are more likely than younger people to have side effects from pain relievers (analgesics), and some side effects are more likely to be severe. These drugs may stay in the body longer, and older people may be more sensitive to them. Many older people take several drugs, increasing the chances that a drug will interact with the analgesic, reducing the effectiveness of one of the drugs or increasing the risk of side effects. Older people are more likely to have disorders that increase the risk of side effects from analgesics. Having a heart or blood vessel (cardiovascular) disorder or risk factors for these disorders increases the risk of heart attack, stroke, blood clots in the legs, and heart failure when NSAIDs are taken. Having a kidney disorder, heart failure, or a liver disorder makes people more vulnerable to kidney damage from NSAIDs and less able to handle the fluid retention caused by the drugs. To reduce the risk of side effects, particularly when prescribing opioids, doctors give older people a low dose at first. The dose is increased slowly as needed, and its effects are monitored. Doctors also choose analgesics less likely to have side effects in older people. For example, acetaminophenSome Trade Names TYLENOL is usually preferred to NSAIDs for treating chronic mild to moderate pain without inflammation. Certain NSAIDs (indomethacinSome Trade Names INDOCIN and ketorolacSome Trade Names ACULAR ) and certain opioids (such as pentazocineSome Trade Names TALWIN ) are usually not given to older people because of the risk of side effects. Nondrug treatments and support from caregivers and family members can sometimes help older people manage pain and reduce the need for analgesics.

Acute Versus Chronic Pain: Pain may be acute or chronic. Acute pain begins suddenly and usually does not last long. Chronic pain lasts for weeks or months. Usually, pain is considered chronic if it does one of the following:

• Lasts for more than 1 month longer than expected based on the illness or injury
• Recurs off and on for months or years
• Is associated with a chronic disorder (such as cancer, arthritis, diabetes, or fibromyalgia) or an injury that does not heal

When severe, acute pain may cause anxiety, a rapid heart rate, an increased breathing rate, elevated blood pressure, sweating, and dilated pupils. Usually, chronic pain does not have these effects, but it may result in other problems, such as depression, disturbed sleep, decreased energy, a poor appetite, weight loss, decreased sex drive, and loss of interest in activities.

During treatment for chronic pain, many people experience a brief, often severe flare-up of pain. It is called breakthrough pain because it breaks through in spite of regularly scheduled pain treatment. Typically, breakthrough pain begins suddenly, lasts up to 1 hour, and feels much like the original chronic pain except it is more severe. Breakthrough pain may differ from person to person and is often unpredictable.

Chronic pain can make the nervous system more sensitive to pain. For example, chronic pain repeatedly stimulates the nerve fibers and cells that detect, send, and receive pain signals. Repeated stimulation can change the structure of nerve fibers and cells or make them more active and can thus increase pain transmission to the spinal cord and brain. As a result, pain may result from stimulation that might not ordinarily be painful, or painful stimuli may be felt as more severe.

When pain occurs repeatedly, people may anticipate it by becoming fearful and anxious. These emotions can stimulate the body to produce substances that make pain feel more intense. An example is prostaglandins, which make nerve cells more likely to respond to pain signals. Fear and anxiety can also reduce the production of substances that reduce the sensitivity of nerve cells to pain. An example is endorphins, the body's natural pain relievers. Fatigue can have the same effects on pain as fear and anxiety.

These changes in pain sensitivity partly account for pain that persists after its cause resolves and for pain that feels more severe than expected.

Did You Know... Chronic pain can physically change the nervous system in ways that make the pain worse and last longer.

Pain Pathways: Pain due to injury begins at special pain receptors scattered throughout the body. These pain receptors transmit signals as electrical impulses along nerves to the spinal cord and then upward to the brain. Sometimes the signal evokes a reflex response (see Symptoms and Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Reflex Arc: A No-Brainer). When the signal reaches the spinal cord, a signal is immediately sent back along motor nerves to the original site of the pain, triggering the muscles to contract without involving the brain. For example, when people inadvertently touch something very hot, they immediately pull away. This reflex reaction helps prevent permanent damage. The pain signal is also sent to the brain. Only when the brain processes the signal and interprets it as pain do people become conscious of the pain.

Pain receptors and their nerve pathways differ in different parts of the body. For this reason, pain sensation varies with the type and location of injury. For example, pain receptors in the skin are plentiful and capable of transmitting precise information, including where an injury is located and whether the source was sharp, such as a knife wound, or dull, such as pressure, heat, or cold. In contrast, pain receptors in internal organs, such as the intestine are limited and imprecise. The intestine can be pinched, cut, or burned without generating a pain signal. However, stretching and pressure can cause severe intestinal pain, even from something as relatively harmless as a trapped gas bubble. The brain cannot identify the precise source of intestinal pain, which is difficult to locate and is likely to be felt over a large area.

What Is Referred Pain? Pain felt in one area of the body does not always represent where the problem is because the pain may be referred there from another area. For example, pain produced by a heart attack may feel as if it is coming from the arm because sensory information from the heart and the arm converge on the same nerve pathways in the spinal cord.

Sometimes pain felt in one area of the body does not accurately represent where the problem is, because the pain is referred there from another area. Pain can be referred because signals from several areas of the body often travel through the same nerve pathways in the spinal cord and brain. For example, pain from a heart attack may be felt in the neck, jaws, arms, or abdomen. Pain from a gallbladder attack may be felt in the back of the shoulder.

Last full review/revision August 2007 by Russell K. Portenoy, MD