Infants process pain in their brains in different ways based on age

Reliance on behavioral indicators, such as crying, to assess pain in infants underestimates how much pain babies actually feel when they undergo stressful medical procedures. This has long-term implications on brain development and impacts future adult pain sensitivity, according to a British neurobiologist speaking at the American Pain Society Scientific Summit today.

"Infants respond to noxious stimulation from birth, but the neural pathways underlying pain behavior and pain perception change through infancy, childhood and adolescence," said Maria Fitzgerald, FMedSci FRS, professor, Department of Neuroscience, University College London. "Understanding how pain is processed at each stage has considerable impact on the clinical assessment and treatment of pain in children."

Fitzgerald explained the reason for slow progress in infant pain management is lack of an agreed method for reliably assessing babies' pain perception. Dr. Fitzgerald's research has moved away from using observations of behavior and physiological responses to measure pain in infants.

"We began by using electromyography to record reflex responses to clinically required painful procedures. We learned that premature infants are more sensitive to sensory stimulation than older infants and responded to both painful stimulation and non-painful touch with whole body movements," Fitzgerald explained. "Older babies, however, display a more specific response and only withdrew the stimulated limb to a painful stimulus.

"We realized that as the perception of pain is generated by the central nervous system, our research needed to move toward measuring neuronal activity responsible for the sensation of pain," said Fitzgerald.

To observe this activity, Fitzgerald and colleagues used brain imaging techniques, electroencephalography (EEG) and near infra-red spectroscopy (NIRS), following a painful event. The tests showed that infants process pain in their brains but in different ways at based on age.

"We learned that premature infants are more sensitive to sensory stimulation than older infants and responded to both painful and non-painful touch with whole body movements," Fitzgerald explained. "Older babies, however, simply withdrew the stimulated limb, which indicated a more specific response to pain rather than touch.

"Premature infants in EEG tests exhibited large bursts of brain activity not specific to pain but toward normal term they were more likely to show a clear brain-specific brainwave similar to adults," Fitzgerald stated.

Since hospitalized babies often undergo several painful procedures during their stays, they can become stressed. Fitzgerald simultaneously measured brain activity, behavioral responses and hormonal stress responses in 56 hospitalized newborns following a blood test. She showed that babies who are more stressed exhibit a stronger brain response following a painful procedure, but this is not accompanied by changes in behavior. "As a result," Fitzgerald noted, "stressed babies may not seem to respond to pain although their brains are still processing it. This means that doctors and nurses may underestimate their pain."

Based on her research, Fitzgerald recommends that clinicians treating hospitalized newborns should assume they are experiencing pain and use all methods available to minimize it.​​​