Table 1.
Demographic characterisation of the neonatal population.
Figure 1.
Summary of the experimental groups, stimulation type and EMG recording set-up used in the study.
Experimental groups are grouped by stimulation type (A, noxious stimulation – clinically required heel lance; B, tactile stimulation; C, punctate stimulation) and EMG recording set-up (ipsilateral or paired ipsi-contralateral). Schematic of the EMG recording set-up shown on left side of figure panel: ipsilateral (top), or paired ipsi¬contralateral (bottom); grey circles represent surface EMG electrode location over the infant limb.
Figure 2.
Nociceptive flexion withdrawal reflex activity in preterm and term infants.
A: Experiment set-up, surface EMG leads were placed over the ipsilateral biceps femoris to record flexion withdrawal reflex EMG activity evoked by a heel lance, example trace from term and preterm infant below; arrow indicates time of noxious stimulus. B –comparison of flexion reflex biceps femoris EMG activity evoked by a heel lance in term (black, n = 20) and preterm infants (grey, n = 19). Data is expressed as fold increase in EMG activity measured in 250 ms time epochs for 4 seconds post-stimulus. The two groups are significantly different [two-way ANOVA, *p = 0.04 age group, ***p = 0.001 time]. Significantly greater responses were recorded in the 1000–1500 ms post-stimulus epochs in preterm infants [Student's unpaired t-test, 1000–1250 ms, *p = 0.04, 1250–1500 ms, *p = 0.04]. C: A graph showing correlation between EMG amplitude in the 1000–1250 ms epoch after heel lance and gestational age at study (R2 = 0.1; P = 0.025, n = 39).
Figure 3.
Noxious stimulation evokes ipsilateral and contralateral flexion activity in preterm and term infants.
A: Evoked activity on the two sides are not significantly different in A, term infants (n = 13) and B: preterm infants (n = 11). Inset: Experiment set-up, surface EMG leads were placed over the ipsilateral (filled lines) and contralateral (dashed lines) biceps femoris to record flexion reflex EMG activity evoked by a heel lance. Example trace from term and preterm infant below, arrow indicates time of noxious stimulus.
Figure 4.
Flexion withdrawal reflex biceps femoris EMG activity evoked by noxious and tactile stimulation.
Only data from those infants that displayed a paired tactile and lance response is shown here. A. term (n = 7) and B. preterm infants (n = 5). Data is expressed as fold increase in EMG activity measured in 250 ms time epochs for 4 secs post stimulus. While in term infants (A) the flexor reflex response to noxious lance stimulation is significantly greater than to tactile stimulation, in preterm infants, the two responses are not significantly different. [Term infants two-way ANOVA, **p = 0.006 time, ****p<0.0001 stimulus. Student's t-test, at 250 ms, *p = 0.014, at 500 ms, ***p = 0.0005, and at 750 ms, **p = 0.006].
Figure 5.
Single and repeated punctate stimulation evokes flexion activity that is gestational age dependent.
A, Cutaneous sensory threshold to punctate stimulation significantly increases with gestational age; B, threshold force required to evoke leg withdrawal and example flexion reflex EMG traces from term and preterm infants are shown. C, Threshold flexion reflex EMG responses in preterm infants are significantly greater than in term (two-way ANOVA, ***p<0.0001 age group, **p = <0.0001 time; Fisher's unpaired t-test, 250 ms, *p = 0.02; 500 ms, ***p<0.0001, 750 ms, ***p<0.0001). Note- two EMG recordings excluded from the analysis due to technical artefact. D, Brief, low frequency repeated punctate stimulation sensitised infant flexion reflex EMG activity as shown by a significantly larger flexion response to a threshold hair after 10 seconds of repeated stimulation, compared to the responses to the same threshold hair when tested 10 s before the train (two-way ANOVA, *p = 0.04 treatment,***p = 0.002, time; Fisher's paired t-test, 250 ms, *p = 0,002, n = 11, pooled term and preterm.