Unlike normal or "acute" pain - the sensation triggered by the nervous system to alert the body to possible injury - chronic pain is persistent, with pain signals continuing to fire in the nervous system for weeks, months or years. Chronic pain can come from an ongoing ailment, such as arthritis, cancer or an infection; from a one-off injury such as a sprained back; or it can even occur in people who have suffered no specific injury or illness.
People who suffer from chronic pain may get combinations of headaches, low back pain or nerve pain, among other symptoms. People with chronic pain also might have chronic fatigue syndrome, endometriosis, fibromyalgia, inflammatory bowel disease, interstitial cystitis, tempromandibular joint dysfunction.
Doctors may take a variety of approaches in treating chronic pain. Drugs, acupuncture, local electrical stimulation, brain stimulation and even surgery may be used, depending on the case. Some doctors also report success in treating patients with placebos. Less invasive treatments include psychotherapy, relaxation therapy, biofeedback and behavior modification.
Previous research into chronic pain has found that patients with this condition often have low levels of endorphins in their spinal fluid, so some treatments are aimed at stimulating endorphin levels.
As part of the new study, researchers asked 2,721 patients with chronic pain to rate the intensity of their pain from 0 to 10. All of the patients were taking prescribed opioid pain medications.
The researchers divided the participants into three groups. Those who scored their pain 1-3 were classed as having "low pain perception," people who scored 4-6 had "moderate pain perception," while "high pain perception" was defined by scoring 7-10.
The majority of the people in the study (46%) had moderate pain perception, closely followed by high pain perception (45%). Only 9% of the participants had low pain perception, and anyone giving their pain a rating of 0 was disqualified from the study.
The research team found that a gene variant, DRD1, was 33% more common in the low pain group than in the high pain group. The people in the moderate pain group were more likely to have another two variants - COMT, which was 25% more common in this group than in the high pain group, and OPRK, which was 19% more common. The high pain group, meanwhile, were 25% more likely to have the variant DRD2 than the people in the moderate group.
This study is quite significant because it provides an objective way to understand pain and why different individuals have different pain tolerance levels. Identifying whether a person has any of these four genes could help doctors better understand their patients' pain perception.