Pain, especially chronic pain, is an emotional condition as well as a physical sensation. It is a complex experience that affects thought, mood, and behavior and can lead to isolation, immobility, and drug dependence.

In those ways, it resembles depression, and the relationship is intimate. Pain is depressing, and depression causes and intensifies pain. People with chronic pain have three times the average risk of developing psychiatric symptoms — usually mood or anxiety disorders — and depressed patients have three times the average risk of developing chronic pain.

The association of depression with migraine headaches, which affect more than 10% of Americans, is especially close. One study found that over a two-year period, a person with a history of major depression was three times more likely than average to have a first migraine attack, and a person with a history of migraine was five times more likely than average to have a first episode of depression.

In somatoform disorders, including hypochondria, according to one theory, depression and anxiety are converted into physical symptoms. But often, when low energy, insomnia, and hopelessness resulting from depression and anxiety perpetuate and aggravate physical pain, it becomes almost impossible to tell which came first or where one leaves off and the other begins. In a statement by the International Association for the Study of Pain, pain is defined as “an unpleasant sensory or emotional experience associated with actual or potential tissue damage or described in terms of such damage.”

Brain pathways

The convergence of depression and pain is reflected in the circuitry of the nervous system. In the experience of pain, communication between body and brain goes both ways. Normally, the brain diverts signals of physical discomfort so that we can concentrate on the external world. When this shutoff mechanism is impaired, physical sensations, including pain, are more likely to become the center of attention. Brain pathways that handle the reception of pain signals, including the seat of emotions in the limbic region, use some of the same neurotransmitters involved in the regulation of mood, especially serotonin and norepinephrine. When regulation fails, pain is intensified along with sadness, hopelessness, and anxiety. And chronic pain, like chronic depression, can alter the functioning of the nervous system and perpetuate itself.

The mysterious disorder known as fibromyalgia may illustrate these biological links between pain and depression. Its symptoms include widespread muscle pain and tenderness at certain pressure points, with no evidence of tissue damage. Brain scans of people with fibromyalgia show highly active pain centers, and the disorder is more closely associated with depression than most other medical conditions. Fibromyalgia could be caused by a brain malfunction that heightens sensitivity to both physical discomfort and mood changes.

Depression, disability, and pain

Depression contributes greatly to the disability caused by headaches, backaches, or arthritis. People in pain who are also depressed become extremely heavy consumers of medical services, even if they have no severe underlying illness. But that doesn’t mean they receive better treatment; studies show that they actually use fewer mental health services than other patients with mood disorders. According to some estimates, more than 50% of depressed patients who visit general practitioners complain only of physical symptoms, and in most cases the symptoms include pain. Some studies suggest that if physicians tested all pain patients for depression, they might discover 60% of currently undetected depression.

Pain slows recovery from depression, and depression makes pain more difficult to treat; for example, it may cause patients to drop out of pain rehabilitation programs. Worse, both pain and depression feed on themselves, by changing both brain function and behavior. Depression leads to isolation and isolation leads to further depression; pain causes fear of movement, and immobility creates the conditions for further pain. When depression is treated, pain often fades into the background, and when pain goes away, so does much of the suffering that causes depression.

Treating pain and depression in combination

In pain rehabilitation centers, specialists treat both problems together, often with the same techniques, including progressive muscle relaxation, hypnosis, and meditation. Physicians prescribe standard analgesics — acetaminophen, aspirin and other nonsteroidal anti-inflammatory drugs, and in severe cases, opiates — along with a variety of psychiatric drugs (see “Medicating pain and depression” box above).

Physical therapists provide exercises not only to break the vicious cycle of pain and immobility but also to help relieve depression. Cognitive and behavioral therapies teach pain patients how to avoid fearful anticipation, banish discouraging thoughts, and adjust everyday routines to ward off physical and emotional suffering. Psychotherapy helps demoralized patients and their families tell their stories and describe the experience of pain in its relation to other problems in their lives.

Pain specialists can improve their practice by learning more about the interactions among psychological, neurological, and hormonal influences that link pain and depression. Why do some people recover from injuries without pain while others develop chronic symptoms, and how is that process related to depression and anxiety? How do psychotherapy and antidepressant drugs affect brain function in depressed people with chronic pain? What kinds of psychotherapy are helpful for them, and how long should psychotherapy continue? In investigating these questions, and in all treatment of both pain and depression, the goal is not just comfort or the absence of symptoms but restoring the capacity to lead a productive life.