Overview of Pain

By James C. Watson, MD, Mayo Clinic

Last full review/revision October 2018 

GET THE QUICK FACTS

 

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version

Topic Resources

• 3D Models (0)
• Audios (0)
• Calculators (0)
• Images (2)
• Lab Test (0)
• Sidebars (1)
• Tables (0)
• Videos (0)

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

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 and Pain Relievers 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. Such interactions may reduce the effectiveness of one of the drugs or increase the risk of side effects. Older people are more likely to have disorders that increase the risk of side effects from analgesics. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen, can have side effects. Risk of several of side effects is higher in older people, particularly if they have several other disorders or are taking NSAIDs in high doses. For example, older people are more likely to have a heart or blood vessel (cardiovascular) disorder or risk factors for cardiovascular disorders. For people with these disorders or risk factors for them, taking NSAIDs increases their risk of having a heart attack or stroke and of developing blood clots in the legs or heart failure. NSAIDs can damage the kidneys. This risk is higher for older people because the kidneys tend to function less well as people age. This risk is also higher in people with a kidney disorder, heart failure, or a liver disorder, which are more common among older people. Older people are more likely to develop ulcers or bleeding in the digestive tract when they take NSAIDs. Doctors may prescribe a drug that helps protect the digestive tract from such damage. These drugs include proton pump inhibitors (such as omeprazole) and misoprostol. When older people take NSAIDs, they should tell their doctor, who then evaluates them periodically for side effects. Opioids are more likely to cause problems in older people, who appear to be more sensitive to these drugs than younger people. When some older people take an opioid for a short time, it reduces pain and enables them to function better physically, but it may impair mental functioning, sometimes causing confusion. Opioids also increase the risk of falls and cause constipation and urinary retention, which tend to cause more problems in older people. Many older people are understandably concerned about the risks of opioid addiction. However, the risk is low when a person takes opioids as prescribed, low doses are prescribed, and opioid use is monitored by a single health care practitioner or team of practitioners who are providing coordinated care to the person. The risk is higher if people have been addicted or have a close family member who has been addicted to alcohol, opioids, or other 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, acetaminophen is usually preferred to NSAIDs for treating chronic mild to moderate pain without inflammation. Certain NSAIDs (indomethacin and ketorolac) and certain opioids (such as pentazocine) 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.

 

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 figure 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, cold, or itching. 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.

 

Reflex Arc: A No-Brainer

A reflex arc is the pathway that a nerve reflex, such as the knee jerk reflex, follows. 1. A tap on the knee stimulates sensory receptors, generating a nerve signal. The signal travels along a nerve to the spinal cord. 2. In the spinal cord, the signal is transmitted from the sensory nerve to a motor nerve. 3. The motor nerve sends the signal back to a muscle in the thigh. 4. The muscle contracts, causing the lower leg to jerk upward. 5. The entire reflex occurs without involving the brain.

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.

 

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 travel on the same nerve pathways in the spinal cord.

 

Acute versus chronic pain

Pain may be acute or chronic. Acute pain begins suddenly and usually does not last long (days, weeks, or sometimes a few months). Chronic pain lasts for many months or years.

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.

 

Causes

Different types of pain have different causes.

Nociceptive pain results from stimulation of pain receptors. It is caused by an injury to body tissues. Most pain, particularly acute pain, is nociceptive pain.

Neuropathic pain results from damage to or dysfunction of the brain or spinal cord (central nervous system) or the nerves outside the brain and spinal cord (peripheral nervous system). It may occur when

• Pressure is put on a specific nerve—for example, by a tumor or a ruptured disk in the spine, causing low back pain and/or pain radiating down the leg. Pressure on a nerve in the wrist can cause carpal tunnel syndrome.
  
  
• Nerves are damaged, as occurs in diabetes mellitus or shingles.
  
  
• The brain and spinal cord do not process pain signals normally or something disrupts this processing, as occurs in phantom limb pain, postherpetic neuralgia (pain after shingles), and complex regional pain syndrome.

Nociceptive or neuropathic pain or both may be involved in acute or chronic pain. For example, chronic low back pain and most cancer pain are caused mainly by ongoing stimulation of pain receptors (nociceptive pain). But in these disorders, pain can also result from nerve damage (neuropathic pain).

Psychologic factors can also contribute to pain. Psychologic factors often affect how people feel pain and how intense it seems, but these factors are rarely the only cause of pain.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version