Pain Overview

Physical Pain

What is Pain?

Pain is defined as an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage. It is the body’s warning system, signaling that something is wrong. There may not be evident or actual tissue damage for the pain to occur, which is especially true in chronic pain.

The American Pain Foundation describes pain as the fifth vital sign after blood pressure, pulse, respiration and temperature. Vital signs are clues that physicians use to diagnose and treat illnesses. Pain is a symptom of a wide variety of injuries, conditions and diseases.

Pain is a function of the nervous system, which is made up of nerve cells (neurons) and fibers that send and receive messages in the form of electrical currents and chemical interactions.

There are three major categories of nerves:
  • Autonomic nerves. Maintain unconscious body processes, such as breathing, heart rate, blood pressure and perspiration.
  • Motor nerves. Control muscle movement.
  • Sensory nerves. Govern sensation. For instance, these nerves allow a person to feel an object while touching it.

Pain results from an interaction between several components of the nervous system, including:

  • Peripheral nerves. Nerves that extend from the spinal cord to the skin, muscles and internal organs. Some peripheral nerves are designed to sense temperature, touch, pressure and vibration. Others are designed to detect potential or actual damage. These types of nerves are called (nociceptors). They are located throughout the skin, bones, joints and muscles but are concentrated in areas more prone to injury, such as fingers and toes.
  • Spinal cord. Nerve fibers that carry messages enter the spinal cord in an area called the dorsal horn (a part of the spinal cord that processes messages). The nerve fibers release neurotransmitters (chemicals) that activate nerve cells in the spinal cord. These nerve cells process the information and send it to the brain.
  • Brain. Nerve messages enter the area of the brain called the thalamus (located in the center of the brain, it is formed of gray matter and serves as a relay station for nerve impulses). The thalamus sends the messages simultaneously to three areas of the brain – the physical sensation region (somatosensory cortex), the emotional feeling region (limbic system) and the thinking region (frontal cortex).

The body may respond to pain messages in several ways. Chemicals released by spinal nerve cells may increase the strength of the pain signal that reaches the brain. This process is called wind-up or sensitization. The brain may block pain messages from traveling to higher parts of the brain, or it may send nerve signals that release natural painkillers (called endorphins or enkephalins) that diminish pain messages.

Other substances in the body cause the opposite reaction. A protein called substance P stimulates nerve endings at the site of the injury and spinal cord. This increases pain messages and worsens the pain sensation.

Sometimes people experience pain from a stimulus that normally causes no discomfort, such as light touch or a breeze. This condition, allodynia, results when neurons in the central nervous system become unusually excitable. Allodynia is a common feature of disorders including migraines and fibromyalgia.

Sleep disturbances and fatigue often accompany pain. These symptoms can in turn cause additional disorders such as headaches and muscle pain (myalgia). The many pain conditions that involve fatigue include arthritis, fibromyalgia, lupus, chronic fatigue syndrome, polymyalgia rheumatica, myositis, sarcoidosis, sickle cell anemia and Lyme disease.

In recent years, the medical community has recognized that pain is not simply a physical response to an unpleasant external stimulus (noxious stimulus). Rather, there are a number of biological, social and psychological factors that interact in complex ways to influence the experience of pain.

A theory of pain was developed in the 1960s by psychologist Ronald Melzack and anatomist Patrick Wall called the gate control theory of pain. Under the theory, pain is “gated” or framed by past experience.

Gender and age also appear to play a role in pain. Research indicates that men and women respond differently to pain. Women are believed to seek help more quickly, are more likely to use a variety of support resources to deal with pain and recover more quickly from pain. The sex hormones estrogen (in women) and testosterone (in men) help account for the difference.

Individuals’ experiences of pain vary greatly. The greatest level of pain a person can endure is called the pain tolerance level. The lowest intensity of stimulation at which a person experiences pain is known as the threshold. People often reveal their response to pain through pain behaviors, such as grimacing, guarding the affected area, limping or avoiding activity.

According to the American Pain Foundation, pain is the number one complaint of older Americans, and one in five older Americans regularly use painkillers. Elderly patients are often treated using a special set of guidelines developed by the American Geriatrics Society. Children are also treated using special guidelines, in part because they often lack the verbal skills necessary to describe the pain they are experiencing.

Types and differences of pain

There are two types of pain, nociceptive pain and neuropathic pain. Nociceptive pain occurs when the peripheral nerve receptors that sense actual or potential tissue damage (nociceptors) are stimulated by an unpleasant sensation (noxious stimulus). Trauma and injury are common examples of noxious stimuli.

Nociceptive pain can be somatic or visceral. Somatic pain is usually constant and concentrated in one location of the body. It is sometimes described as sharp, aching, throbbing or gnawing. Visceral pain is usually not concentrated in one area and is often described as deep, aching or squeezing pain.

Neuropathic pain is caused by damage to the peripheral or central nervous system. It is often described as burning, electric, tingling or shooting.

Pain may also be described as acute or chronic. Acute pain is triggered by tissue damage from trauma, illness, injury or surgery. Acute pain may be mild or severe. It may pass quickly or last for months. Acute pain is usually localized to one area of the body, and its cause is usually easy to identify. For more information, see Trauma Pain.

Chronic pain is long-term pain due to a chronic condition or pain that persists after the normal course of recovery of an illness or injury. It may remain constant or may come and go. The definition of some conditions includes a time frame, such as pain lasting at least three months for fibromyalgia or six months for chronic fatigue syndrome. Unlike acute pain, the cause of chronic pain is often difficult to diagnose. In some cases, chronic pain may occur without any indication of injury (e.g., fibromyalgia).

Pain can occur throughout the body:

  • Headaches affect millions of people. The most common types of headaches include tension headaches (sometimes described as a tight band around the head) migraines (characterized by throbbing pain and other symptoms such as nausea) and cluster headaches (characterized by excruciating, piercing pain on one side of the head).
  • Orofacial pain, dental pain, TMJ disorder, eye pain and ear pain are other types of head pain.
  • Neck pain can result from trauma such as whiplash, improper posture and ergonomics or infectious diseases such as meningitis or encephalitis.
  • Back pain is among the most common medical conditions in the United States and other developed nations. Its many causes include strain or sprain, sciatica, osteoporosis, scoliosis, pinched nerves, herniated disc, degenerative disc disease, spinal stenosis and ankylosing spondylitis.
  • Shoulder pain often stems from osteoarthritis, tendinitis, bursitis, dislocation, separation, torn rotator cufftorn rotator cuff or frozen shoulder.
  • Elbow, wrist and arm pain has many possible causes, including fractures, rheumatoid arthritis, carpal tunnel syndromecarpal tunnel syndromecarpal tunnel syndrome, tennis elbow and golfer elbow.
  • Chest pain can be due to disorders ranging from heartburn to heart conditions to costochondritis to esophageal spasms.
  • Abdominal pain can result from indigestion, ulcers, appendicitis, kidney stones or other problems involving internal organs.
  • Pelvic pain includes sexual pain and testicular and scrotal pain.
  • Leg pain, knee pain and foot pain have many possible causes including overuse, traumatic injury, gout and other types of arthritis, gait disturbances, diabetic neuropathy, complex regional pain syndrome and heel spurs.
  • Joint pain is not synonymous with arthritis. It can also be due to conditions including fibromyalgia, lupus, chronic fatigue syndrome, polymyalgia rheumatica, sarcoidosis, myofascial pain syndrome, sickle cell anemia and Lyme disease.

Causes of pain

Pain can be caused by many injuries, conditions and diseases. Pain may also be caused by infection. For example, encephalitis (abnormal swelling or inflammation of the brain) is usually caused by viral infection.

Acute pain is most often caused by disease, infection, inflammation, trauma or injury to tissues and is usually easy to diagnose. Some causes of acute pain include:

  • Injuries. These include sprains, strains, bruises, tears, dislocations and fractures. These commonly occur while playing sports, at home, in the workplace or while driving. Some people who have had a spinal cord injury develop a condition called central pain syndrome. They can experience intense pain ranging from tingling to burning.
  • Burns. Depending on the injury, pain caused by burns can be excruciating. Patients can still experience pain at the location of the burn even after it has healed.
  • Infection. Invasion of the body by a pathogen such as a virus, bacterium or fungus. Infections are often the cause of acute inflammatory conditions such as appendicitis. Some infectious diseases, including Lyme disease and meningitis, can cause chronic problems.
  • Surgery. Control of pain associated with surgery includes presurgical preparation and monitoring of the patient before, during and after the procedure.

The cause of chronic pain is not always evident. It can result from an acute condition that has not resolved within the expected recovery time, such as a fracture that has not healed properly or a wound that has become infected. However, in many cases it is caused by chronic conditions, such as:

  • Arthritis (inflammation of the joints). Millions of Americans have arthritic conditions such as osteoarthritis, rheumatoid arthritis, ankylosing spondylitis and gout. These conditions are characterized by joint pain.
  • Repetitive stress injuries. Muscular conditions that result from repeated motions performed during work or other activities. Some examples are tendinitis, bursitis, carpal tunnel syndrome and tennis elbow.
  • Fibromyalgia. A chronic pain illness characterized by musculoskeletal aches, pain and stiffness, soft tissue tenderness, general fatigue and sleep disturbances.
  • Myofascial pain syndrome. A condition characterized by chronic pain in the muscle tissues.
  • Chronic fatigue syndrome. A disorder marked by exhaustion, pain and other symptoms.
  • Systemic lupus erythematosus. An autoimmune disease that can affect many parts of the body, including the skin, joints and kidneys.

Assessment of pain

When patients are experiencing pain, physicians may take a number of steps to assess the pain and attempt to determine its cause. Evaluation typically begins with a medical history and physical examination.

Physicians may ask the patient to describe the pain, including its location, severity and duration. Physicians may use a pain diagram (a picture of the human body) to evaluate pain. Patients mark the area(s) where pain is being experienced. Physicians may also use pain scales, which help quantify the pain, or other types of pain assessment.

To identify the cause of pain, physicians may conduct a number of tests. Some tests are definitive for a disease, but in many cases a test can only narrow the list of possible causes, rule out certain conditions, show a nonspecific problem such as inflammation or indicate the need for additional testing. Categories of tests include:

Lab tests

Examples of tests that depict internal structures include:
  • Blood tests. Samples of blood can reveal abnormal amounts of hormones, enzymes or other substances. Types of blood tests include complete blood count, C-reactive protein test, enzyme test, rheumatoid factor test, sedimentation rate and waste product test.
  • Urine tests. These reveal substances that can indicate diseases ranging from gout to prostatitis.
  • Spinal tap (lumbar puncture). Spinal fluid is extracted from the lower back and tested to determine pressure and contents. It is performed to diagnose conditions including meningitis and encephalitis.
  • Biopsy. A sample of tissue is taken and analyzed. Biopsies reveal cancer and can be used in revealing organ damage caused by disorders including lupus.

Imaging tests

Assessments that involve analysis in a laboratory include:
  • X-ray. Produces an image of a body part, organ or bodily system on film or fluorescent screens.
  • MRI (magnetic resonance imaging). Uses powerful magnets to produce images on a computer screen and film.
  • CAT scan (computed axial tomography). Allows for multiple x-rays to be taken from different angles around the patient.
  • Ultrasound. Uses high-frequency sound waves to create images on a computer monitor.
  • Bone scan and other types of radionuclide imaging. Introduce radioactive tracers into the body that emit rays detected by special devices.
  • Arthography. Injects a contrast medium into a joint to enhance an x-ray.
  • Discography. Injects a contrast agent into an intervertebral disc, followed by an x-ray.

Electrodiagnostic testing

Tests that measure the electrical activity of nerves and muscles include:
  • Nerve conduction studies (NCS). Tests that evaluate for muscle or nerve damage.
  • Electromyography (EMG). Tests that identify muscles and nerves affected by weakness or pain.

Quantitative sensory testing

These tests assess damage or changes to nerve endings.

Treatment of pain

There are a number of ways to treat acute pain and chronic pain. A physician may recommend lifestyle improvements such as exercise, quitting smoking, stress management, reduced intake of alcohol, weight loss or diet. Other categories of interventions:


These tests assess damage or changes to nerve endings.
  • Analgesics (painkillers), including non-narcotics such as acetaminophen and opioids such as morphine
  • Anti-inflammatory drugs, including nonsteroidal anti-inflammatory drugs (NSAIDs, such as aspirin, ibuprofen, naproxen and COX-2 inhibitors) and corticosteroids
  • Disease-modifying antirheumatic drugs (DMARDs), to reduce joint damage caused by conditions such as rheumatoid arthritis and lupus
  • Antidepressants, sometimes prescribed for certain types of chronic pain
  • Anesthetics and interventional injections, which includes nerve blocks, epidural steroid injections and facet joint injections


Several health professions offer interventions to help deal with pain, including:
  • Physical therapy, to increase strength, range of motion, coordination and balance
  • Occupational therapy, to maximize function in daily activities such as dressing and cooking
  • Manipulation therapy (e.g., chiropractic, massage therapy), to improve bodily movements
  • Cognitive behavioral therapy, to help patients replace negative thought patterns with positive ones

Modalities (physical agents)

Examples of modalities that can relieve pain and increase function include:
  • Thermotherapy (heat), such as ultrasound therapy
  • Cryotherapy (cold)
  • Hydrotherapy (water)
  • Electrical therapy, such as TENS

Complementary and alternative methods

It is best to consult a physician before trying treatments such as:
  • Acupuncture, a Chinese practice inserting needles into trigger points
  • Acupressure, a needle-free variation on acupuncture
  • Biofeedback, which trains the mind to control bodily functions
  • Glucosamine and chondroitin, supplements that may reduce arthritic pain


If noninvasive measures are inappropriate (such as with appendicitis) or do not succeed, an operation may be necessary. Types include:
  • Arthroscopy, involving small incisions
  • Arthroplasty, such as a joint replacement
  • Carpal tunnel release, to treat carpal tunnel syndrome
  • Spine surgery, such as laminectomy or fusion

Issues in pain relief

According to the American Pain Foundation, more than 50 million people in the United States experience chronic pain that interferes with daily life. But most pain is untreated, undertreated or improperly treated.

Many patients, particularly men and elders, underreport pain for a variety of reasons. Some patients fear complaints will not be taken seriously, whereas others believe relief is not available. Some physicians do not treat pain sufficiently because they fear that patients will become addicted to pain medications.

There are a number of ways that patients can address this issue, including getting a correct diagnosis, being an advocate for pain needs, making a commitment to pain reduction and carrying out lifestyle improvements that may help, such as exercise and weight loss.

Questions for your doctor regarding pain

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about pain:
  • What are the most common conditions that may cause me to experience acute pain?
  • What are the most common conditions that may cause me to experience chronic pain?
  • How can I tell the difference between acute and chronic pain? At what point would my acute condition be considered chronic if it is not resolved?
  • What is a pain diagram? When might I use one?
  • What other types of pain assessment might be used with me?
  • How might my condition be diagnosed?
  • Does my medical history reveal anything significant about my pain or risk of certain complications?
  • What is the difference between an x-ray, CAT scan, MRI and other imaging tests? Why is one test recommended for me over another?
  • How can I tell when I can treat my pain myself and when I need medical attention?
  • Which medications can treat my pain?
  • Should I take acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID) to treat my pain at home?
  • Are there ways to treat my pain that do not involve medications? Do you recommend physical therapy, occupational therapy or other therapy for me?
  • Are complementary and alternative methods such as acupuncture or biofeedback effective ways to treat my pain?