Pain experience in infants

“Babies feel more pain than doctors realise,” reported The Daily Telegraph . The newspaper said that a study has found that the pain experienced by babies is underestimated because “some show no outward signs of discomfort”. The Daily Mail also covered the story and said that brain scans had recorded far higher pain levels than the standard tests of “changing heart rate, grimacing, flared nostrils and narrowed eyes”. It added that scans sometimes showed that babies were in pain even though they did not grimace or cry.

The reports are based on a study which suggests that the traditional signs doctors rely on as pain indicators may be insufficient. It found that babies who show no facial expressions in response to pain, such as the heel prick test, still appear to have a positive brain response to the painful stimulus. The study, though small, has highlighted that there is a lot to learn about the assessment of pain in infants. More research in this area could lead to improved procedures or increase the confidence that common procedures cause minimal discomfort.

Where did the story come from?

Dr Rebeccah Slater and colleagues from University College London, the Elizabeth Garrett Anderson and Obstetric Hospital and the Institute of Child Health carried out the study. Their research was funded by the Wellcome Trust, the Medical Research Council and SPARKS. The study was published in the peer-reviewed medical journal PLoS Medicine.

What kind of scientific study was this?

This study looked at the association between a clinical pain assessment tool for infants and brain scans showing activity in the region of their brains involved in pain detection. This was a small cohort (group) study of 12 newborns (age from conception estimated at 25 to 43 weeks). During the study, the infants underwent a total 33 heel lances as part of their normal treatment. Heel lances are a standard procedure in which the skin of the heel is punctured to allow blood collection, and are part of routine investigations into possible clinical diagnoses.

While the infants were having their heels pricked, the activity in their brain was measured using a technique called near-infrared spectroscopy (NIRS). This can examine brain function by detecting changes in concentrations of oxygenated and deoxygenated blood haemoglobin. NIRS is an indicator of neural activity, and the assumption of this study was that the frequency of neural activity and the number of activated neurones (nerve cells) in the somatosensory cortex (an area involved in the detection of sensations such as touch, temperature, and pain) reflects the intensity of pain being experienced.

The standard procedure for heel pricks is to clean the heel, lance it and squeeze the puncture to collect some blood. In this study, for a period of 30 seconds after the puncture, the heel was not squeezed. This was to ensure that any neural activity recorded was in response to the heel prick itself and not to the squeezing of the heel.

During the heel prick, a standard technique to assess infant pain was also used. Facial expressions were recorded using a handheld camcorder, and these were taken away for analysis using a commonly used tool called the premature infant pain profile (PIPP). This scores particular facial expressions (eye squeeze, brow bulge and nasolabial furrow) and physiological measures (heart rate and blood oxygen saturation) to arrive at an overall figure that represents the infant’s pain experience. It is often used to determine how much pain infants are in, and to decide how to manage this pain. The researchers then examined the correlation between the results of the PIPP (from two independent raters) and the neural activity evidenced through the NIRS.

What were the results of the study?

Overall, the score on the PIPP was linked with evidence of increased brain activity. However, when the behavioural and the physiological components of PIPP were looked at separately, it was found that neural activity was strongly linked to behavioural scores on PIPP, but not so strongly linked to physiological responses (heart rate and blood oxygen). The researchers also found that in 13 of the 33 heel lances, there was no observed change in facial expression. Despite this, 10 of these 13 cases showed evidence of a brain response to the procedure.

What interpretations did the researchers draw from these results?

The researchers say that this study is the first to simultaneously measure brain activity to ‘noxious stimulation’ and score the response using a validated pain assessment tool. Although overall these two measures were ‘well correlated’ (i.e. linked), in some cases it was possible to record brain activity with no associated changes in behaviour.

What does the NHS Knowledge Service make of this study?

This small cohort study raises the important and contentious issue of how to measure pain in young infants. There are several points to bear in mind when interpreting the results:

• It is not known if the increased neural activity in response to stimuli actually represents the real pain experience of the infant. The researchers themselves say that ‘how much this cortical response contributes to pain awareness or pain experience is not known’. In essence, this small study has shown that some increased neural activity in response to a procedure may not be reflected by behaviour change.
• It is possible that behavioural tools alone may underestimate pain, but this can only be concluded from this study by assuming that the increased brain activity was reflected real pain being experienced by the infants.
• There is limited evidence that pain experienced in response to routine clinical investigations – such as heel pricks – has any lasting negative effects on child development.
• This research was only carried out in 12 infants. The results from this small sample may not be applicable to all newborns, and replication of the results in larger studies will increase confidence in the results. 

Blood samples taken from newborns are important in screening for metabolic disorders. The results from this important study highlight that there is more to learn about assessment, and subsequent management, of pain in infants.

Sir Muir Gray adds...

Nothing new but very, very relevant and important.