Patients’ Eye Color a Clue to Pain Tolerance

TAMPA -- Caucasian women with light-colored eyes -- blue or green -- appear to tolerate pain better than Caucasian women with brown or hazel eyes, researchers reported here.

In a study involving 58 pregnant women -- 24 with dark-colored eyes and 34 with light-colored eyes -- those with lighter eyes achieved greater reductions in postpartum anxiety (P=0.02), depression (P=0.08), and catastrophizing/rumination (P=o.15), said Inna Belfer, MD, of the University of Pittsburgh School of Medicine.

"This is just a pilot study with small number of women," Belfer told MedPage Today at her poster presentation at the annual scientific meeting of the American Pain Society.

Cindy Teng, BA, a medical student at University of Pittsburgh and the lead author of the study, said that another indication that dark-eyed women had greater pain was their increased pain reduction when they were given epidural analgesia. They experienced about a 60% reduction in pain at rest compared with about a 45% reduction in pain at rest among the women with light-colored eyes (P=0.22). There was about a 55% reduction in pain during movement among the dark-eyed women, compared with a 40% reduction in pain among the light-eyed patients (P=0.28).

"These figures in pain reduction after the epidural show a trend, but they are not statistically significant," Teng told MedPage Today. She suggested that there is more of a reduction in pain with the epidural because those with dark eyes appear to have more sensitivity to pain and therefore may get a stronger impact from pain relief treatment.

Belfer said that differences in hair color have been linked to resistance to anesthesia, and eye color has been associated with behavior and possibly neural transmission. "However, there has been limited research examining the relationship between eye color and the human pain experience," she said. "Our research is too early to hypothesize why there should be link between eye color and pain. I suspect there is a genetic component."

She said that the researchers for the pilot study sought to have as homogeneous population as possible, so they selected pregnant Caucasian women. Belfer said further studies would include nonpregnant women and would also look at the relationship between eye color and pain among men.

The women in the current study were recruited from the University of Pittsburgh Magee Women's Hospital. The researchers administered validated surveys that addressed pain, mood, sleep, and coping behavior both antepartum and postpartum. Physical pain thresholds were assessed through heat stimulation on the skin.

"Research in pain phenotypes and more readily identifiable features like eye color could enhance clinical care and treatment effectiveness, which influence patients' physical and psychosocial well-being," Belfer said.

She said the researchers were sparked to consider the subject by a water-cooler discussion about eye color and pain. One of the residents told Belfer, "See, that woman has brown eyes ... she's gonna be trouble."

Gregory Terman, MD, PhD, president-elect of the American Pain Society and director of pain medicine research at the University of Washington in Seattle, told MedPage Today that researchers have been trying to determine why there is such a heterogeneous reaction to pain. "I don't know why there would be a relationship between eye color and pain unless it has to do with an genetic link that somehow includes eye color. It is an interesting subject to pursue."

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