Sometimes chronic pain stems from a known cause, like arthritis. With arthritis, persistent joint inflammation triggers repetitive nerve impulses that signal injury. Other examples of known causes of chronic pain include cancer, multiple sclerosis and AIDS.

Often, however, no ongoing disease or injury can be linked directly to chronic pain. Research suggests that many chronic pain conditions arise from a process called sensitization. In this process, the brain and other parts of the nervous system overreact to sensory signals and those signals are perceived as pain.

Sensitization can be compared to the volume control on a radio. Sensitization “turns up the volume” of messages sent from your senses, making them stronger and, at times, distorted.

The biology of sensitization is complex, but the basic idea behind it is fairly simple:

1. As signals travel through the nervous system, they may turn on, or “activate,” circuits. The brain uses these signals to warn you that something is happening that might be dangerous for you.
2. Pain results from the brain’s attempt to warn you of danger. The more the circuits turn on, the easier it is for them to turn on again and again.
3. Repeated warning signals can cause changes in the brain. The brain may start to use more areas to process danger signals. In time, even one danger signal may activate many regions of the brain, triggering pain.

It is normal for the brain to change in response to sensory input. This can actually be helpful. For example, a concert pianist likely plays the piano many hours each day. With repeated practice, more areas of the brain help to move the fingers. In time, finger movements become “hard wired” in the brain. Then the person can play almost without thinking.

Unfortunately, the same thing can happen when the sensory input is danger. As more of the brain processes danger, the resulting perception of pain becomes the response. This can lead to:

• Pain that hurts more or lasts longer than is usual for an injury or illness.
• Pain from something that typically does not cause physical pain, such as moderate activity, a light touch or a hug.
• Pain that spreads to other parts of the body.
• Pain with no known trigger.
• Burning or tingling sensations without underlying nerve damage.

Why sensitization happens to some people and not to others is still not known. Factors that may play a role include:

• Injury or infection.
• Family history.
• Use of opioid pain medicines for more than 30 days.
• Poor sleep.
• Low mood.
• Smoking.
• Trauma.
• Prolonged physical, emotional or social stress.
• Hormonal factors.
• Chronic pain is more common in women than men.

Chronic pain can make it hard to participate in your life. It can take control of your mood, job, relationships and productivity.

Common chronic pain symptoms include:

• Pain that interferes with your daily life, including taking care of yourself, working or leisure activities
• Nausea
• Vomiting
• Fatigue
• Motion sickness
• Dizziness
• Depression or anxiety
• Trouble sleeping
• Sensitivity to sensory stimuli, like loud noises, smells and touch

It’s hard to predict who will get chronic pain. Even people if two people had the same history of illness and injury, they could have different outcomes. However, people who are more at risk for chronic pain include:

• Women
• Non-Hispanic white adults
• Military veterans
• People who live in rural areas
• People ages 65 and older
• People with depression and anxiety
• Those who have an existing chronic inflammatory condition, such as rheumatoid arthritis
• Those who have a nerve-related condition, such as diabetes