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Early babies 'feel more pain'

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Premature babies face a “lifetime of pain sensitivity”, according to The Times. It says a new study has shown that premature babies become oversensitive to pain due to the intensive-care treatments, such as injections, that they receive after birth.

Behind this story lies a small observational study in which the researchers compared the brain activity of small groups of premature and term babies. They tested exposure to both medically necessary heel lances used for taking blood, as well as responses to a non-harmful stimulus, by tapping their feet with a rubber bung. The results showed that the premature babies had a greater brain response to lancing only.

Pain is a complex and subjective experience that is difficult to measure, particularly in children who cannot communicate their experience verbally. On this basis, the assumption that a greater brain response equates to experiencing more pain is not validated, a shortcoming that the researchers themselves highlight. As such, newspaper claims of children going on to experience a “lifetime of pain” seem unsupported by the research, which is an early foray into this important subject.

Where did the story come from?

The study was carried out by researchers from University College London and was funded by the Medical Research Council, the British Pain Society and the UCL/UCLH Comprehensive Biomedical Research Centre. The study was published in the peer-reviewed medical journal NeuroImage.

What kind of research was this?

This was an observational study that compared the neurological response of eight infants born at term (i.e. not premature) during heel lances with the responses of seven prematurely born infants. The heel lances were all clinically essential, and were performed in order to take a small blood sample. To make these analyses of pain response comparable, they were carried out when the infants were the same ‘postmenstrual age’, a measure of age which takes into account the degree of prematurity.

What did the research involve?

The researchers say that previous research has established that older children born pre-term have greater pain sensitivity than their full-term peers. In this research, they attempted to investigate the links between noxious (physically harmful) stimuli and brain activity in both premature and term babies.

The researchers undertook their study in a hospital setting in a special care baby unit. The eight term babies were ‘normal term infants’ aged less than seven days. The seven preterm babies were studied when they reached the equivalent post-menstrual age (which ranged from 40 to 116 days after birth).

The researchers compared the responses of the two groups of babies to both noxious and non-noxious stimuli, i.e. to heel-lance and to ‘innocuous’ tapping of a rubber bung against the babies’ heels. They also had a no-stimulation period as a control. Responses to the stimuli were assessed using EEG to measure brain activity. The EEG has 17 electrodes placed on the scalp and body, although activity at two particular sites (CPz and Cz) were considered the most important.

The analysis methods used to compare the study data appear to be quite complex, but seem appropriate for this study. The researchers used a technique called ‘principle component analysis’ to analyse the brain activity at two electrode sites.

What were the basic results?

The study found that the response to heel lance was dependent on age, while the response to non-noxious stimulus was not. The authors say this suggests that a larger population of cortical neurons is activated in the premature infants than in controls when they experience the same stimulus.

How did the researchers interpret the results?

The researchers say that their study has shown that prematurely born infants who have experienced at least 40 days of intensive or special care “have an increased neuronal response to noxious stimuli compared to healthy newborns at the same corrected age”.

Conclusion

This small observational study has some shortcomings. The researchers highlight the main limitation with this study – the assumption that the amplitude of response, in terms of brain waves at particular sites, directly reflects the magnitude of perceived pain. They say that ‘while generally true’, this may not always be the case. The study did not use other well-known methods to assess whether the neonates were actually experiencing pain, such as validated pain scales that assess facial expressions or crying.

Additionally, premature babies are highly likely to have had more heel lances than term infants, so the study may just be measuring the heightened sensitivity of the feet to such stimuli. If this is the case, it is not correct to say that this study demonstrates that premature babies feel more pain than term ones. It is also unclear whether the researchers adjusted for factors such as the baby’s weight, which may affect the amplitude of EEG readings.

Overall, this study of an important subject area is an early one. It provides some evidence that children who are born prematurely process noxious stimuli (by way of heel lance) differently to babies born full term. However, it is not yet clear whether this means they experience more pain per se nor why the neurological responses may be different. More research is needed to explore the factors.

Links to the headlines

Premature babies face lifetime of pain sensitivity. The Times, May 10 2010

Premature babies ‘feel more pain in later life’. Daily Mail, May 10 2010

Links to the science

Slater R, Fabrizi L, Worley A et al. Premature infants display increased noxious-evoked neuronal activity in the brain compared to healthy age-matched term-born infants. NeuroImage [Article in Press, Uncorrected Proof]

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