Stressed infants have increased pain brain activity, but this stress was not linked with their behavior, researchers found.

Infants with high levels of stress exhibited larger “amplitude cortical nociceptor responses”, or responses to pain, but it was not correlated with “nociceptive behavior,” or behavior exhibiting pain, as measured by brain activity and facial expressions, reported Laura Jones, MD, of University College London in the U.K., and colleagues.

But there was a relationship between pain brain activity and behavior in infants with lower amounts of stress, suggesting that stress “disrupts” the relationship between the brain activity and behavior, the authors wrote in Current Biology.

In a statement, another of the co-authors, Judith Meeks, MD, of University College London Hospitals, called the results of the study important for people who care for newborn babies, because behavior such as crying or facial expressions are used to measure a baby’s pain in the form of a pain score. “While these methods are very useful, our findings suggest that they may not be appropriate for babies who are already stressed. We need to explore better ways to monitor pain, reduce stress, and tailor our interventions accordingly.”

Jones and colleagues added that little is known about the effect of background levels of stress on an infant’s pain response, as many infants are raised in a psychologically stressful environment. In adults, the researchers explained, acute psychological stress can cause hyperalgesia, or increased sensitivity to pain, and pain increases with background stress.

The study included 56 infants ages 36 to 42 weeks, with the team looking at nociceptive behavior, using measures such as salivary cortisol levels, heart rate variability, EEG event-related potentials, and facial expressions following a “clinically required” heel stick. An average of these measurements pre- and post-heel lance were used as a measure of stress.

The overall pain score indicated that 24 infants exhibited mild to no pain response to the heel stick, while 10 were in moderate pain and four in severe pain, the authors said. In addition, two-thirds of infants had both brain waves and behavior that showed typical responses to pain. The authors found that the measure of nociceptive event-related potential as measured by EEG is larger, suggesting that babies who were stressed felt more pain, even if their behavior did not indicate it.

“These data indicate the importance of understanding stress levels when measuring the effects of noxious stimulation in non-verbal subjects, as behavior alone will not indicate the extent of brain activation,” Jones et al wrote.

They argued that because stress-related brain activity is not accompanied by changes in infant pain behavior, the influence of stress may escape the attention of caregivers.

In addition, Jones said in a statement, “We know that repeated painful and stressful experiences in early life can negatively impact on the development of the central nervous system, and our results suggest that controlling the stress levels of [hospitalized] infants may not only reduce their pain but also contribute to their healthy development.”