Facial behaviour not a reliable measure of pain in infants with high background stress
Stress appears to disrupt the behavioural response of an infant to pain, suggests a new study, which shows that while stress amplifies the brain response to pain, infants may not react to the stimulus behaviourally, such as through facial expressions.
“The data presented here show that higher levels of background stress in infants are associated with greater noxious evoked activity in the brain,” researchers said, adding that this could have long-term effects on the nervous system over time.
The research team measured background physiological stress in 56 healthy, term-born infants (mean age 3.9±2.4 days; males n=29) through salivary cortisol levels and high-frequency heart rate variability. The noxious test was performed using a heel lance.
Electroencephalography was performed to measure the nociceptive event-related potential (nERP), which was used to represent brain activity in response to the pain stimulus. Video recordings were used to record nociceptive behaviours.
The heel lance test evoked a clear brain response, with EEGs from 49 infants showing the characteristic nERP N3P3 waveform. Characteristic nociceptive facial behaviours were also observed in 51 percent (n=23) of participants, all of whom yielded a median score of 3. [Current Biology 2017;27:1-6]
Using the results from the behavioural and physiological measures in 38 infants, researchers calculated the premature infant pain profile (PIPP) scores and found that 63 percent (n=24) exhibited mild to no pain in response to the lance. Moderate and severe pain responses were observed in 26 (n=10) and 11 (n=4) percent, respectively.
In individual infants, the amplitude of the cortical nERP was significantly associated with the PIPP score (p=0.033) and modestly associated with facial behaviour (p=0.068).
However, when infants were analysed according to background stress levels, linear regression found a significant association between facial behaviour and nERP amplitude only in those with low cortisol concentrations (p=0.023). In contrast, this relationship was insignificant in infants with high cortisol levels (p=0.630).
“[O]ur data suggest that increase in pain reactivity by background stress in human infants is reflected only in brain activity, possibly through thalamocortical pathways, and not reflected in their motor behaviour,” researchers said.
“The fact that stress-related brain activity is not accompanied by changes in infant pain behaviour means that the influence of stress may escape the of caregivers,” they added.
Conversely, activities that aim to soothe pain but rely on behavioural responses as measures of efficacy may not, in actuality, be reducing physiological responses or preventing potential long-term impacts, researchers said.