Proceedings of The Physiological Society

Cardiff University (2009) Proc Physiol Soc 17, PC09

Poster Communications

Development of human infant pain behaviour: Nociceptive flexion withdrawal reflex EMG activity and facial motor responses in preterm and term infants

L. Cornelissen1, R. Slater1, S. Boyd2, M. Fitzgerald1

1. Department of Neuroscience, Physiology and Pharmacology, University College London, London, United Kingdom. 2. Clinical Neurophysiology, Great Ormond Street Hospital for Children, London, United Kingdom.

INTRODUCTION: Premature infants in intensive-care are exposed to multiple painful procedures as part of necessary clinical care. Nociceptive flexion reflex withdrawal and facial motor activity are used as measures of spinal nociceptive processing and are important components of pain behaviour. Here we compare the pattern of nociceptive flexion reflex activity in preterm and term infants following clinically required heel lances. For each infant we also compare the flexion reflex with facial motor behaviour. METHOD: The noxious stimulus was a time-locked clinical heel lance. A non-noxious sham was applied by rotating a heel lancet so that the blade did not contact the heel. Flexion withdrawal reflex activity was recorded from the ipsilateral biceps femoris using surface EMG in 12 preterm(32-36.9wk postmenstrual age) and 18 term(37-43wk) infants. Activity from the contralateral biceps femoris was recorded on 17 occasions (7 preterm,10 term). The evoked activity was quantified by calculating the root mean square (RMS) of the EMG signal in 250 ms bins. Facial motor activity was simultaneously video-recorded and infants were classified according to whether or not they exhibited facial motor behaviour (12 preterm,10 term). RESULTS: Noxious stimulation evoked EMG activity in the ipsilateral biceps femoris in all infants. EMG activity was maintained for at least 1.5s and was significantly greater than the sham in all infants (p<0.01, 2-way ANOVA). Preterm and term infants showed different patterns of noxious-evoked EMG activity; preterm infants exhibited a sustained response (mean+sem; RMS250mspost-onset= 28.23+4.31µV, RMS1spost-onset= 26.24+8.04µV); term infants exhibited an initial phase of increased muscle activity and a late phase of reduced but sustained activity(RMS250ms= 50.77+6.64µV, RMS1s= 19.55+3.64µV). The pattern of response in both preterm and term infants was synchronous in the ipsilateral and contralateral leg with no significant differences in the post-onset activity. Flexor EMG activity was always evoked by noxious heel lance whereas changes in facial expression were not observed in 18% (4/22) of infants. CONCLUSION: The pattern of flexor motoneurone activity evoked by a noxious stimulus differs in preterm and term infants. Late-phase reduced activity seen in term infants may reflect the influence of supraspinal inhibitory input. Coincident activation of both muscles in all age groups indicates an immature balance of spinal excitation and inhibition, with focused limb withdrawal occurring at a later age in development. EMG recordings of flexion reflex withdrawal are more sensitive measures of noxious-evoked activity in infants than observed changes in facial behaviour.

Where applicable, experiments conform with Society ethical requirements