UNIVERSITY COLLEGE LONDON THE DEVELOPMENT OF THE CUTANEOUS FLEXION REFLEX IN HUMAN INFANTS: THE EFFECTS OF NOXIOUS STIMULI AND TISSUE DAMAGE IN THE NEWBORN. This thesis is submitted in part fulfilment of the requirements for the degree of Doctor of Philosophy, in Neuroscience. Department of Anatomy and Developmental Biology, University College London. KATHARINE ANN ANDREWS May 1997 SUPERVISOR Professor Maria Fitzgerald ProQuest Number: 10106776 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10106776 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Owr lives begin to end the day we become silent about things that matter^* Dr Martin Luther King Jr. ABSTRACT OF THESIS The aim was to investigate the development of spinal sensory processing in the human neonate using the cutaneous flexion reflex, and to measure changes in the reflex resulting from repeated tissue damage. A further aim was to quantify flexion reflex responses elicited by different intensities of mechanical and electrical stimuli using EMG recordings. Experiments were performed with ethical approval and informed parental consent on a group o f preterm and full-term infants aged between 27 and 42 weeks postconceptional age. Mechanical stimuli were applied to the sole of the foot using calibrated von Frey hairs mounted on a piezo transducer which triggered the sweep o f an EMG unit. These were delivered at threshold intensities, and at 2.8, 7.6, and 21 x threshold. Heel lance stimuli were delivered with a modified ‘Autolet’ device which also triggered the EMG sweep. Electrical stimuli were applied to the skin overlying the sural nerve using a neonatal bipolar stimulator at threshold intensities, and in steps of 0.5mA to 1.85 x threshold. Single surface EMG responses were recorded ipsilaterally from lower limb flexor muscles, and the latency, amplitude, duration and area of the response were recorded. There was variability between neonates and between successive responses in all reflex parameters. Nevertheless, a clear correlation was found between stimulus intensity and the latency, amplitude, duration and area of the mechanically-evoked reflex. The latency of the response decreased with increasing stimulus intensity from threshold to heel lance. The amplitude, duration and area of the response increased with stronger stimuli across all PC ages. Repeated suprathreshold mechanical stimulation clearly affected the duration and area of the response. Onset latencies of electrically-evoked 3 reflexes were more constant, but the relationship between stimulus intensity and response parameters was less clear. Finally, the flexion reflex threshold was clearly lowered by tissue damage. TABLE OF CONTENTS PAGE LIST OF TABLES 9 LIST OF FIGURES II LIST OF PLATES 15 ACKNOWLEDGEMENTS 16 CHAPTER 1 : INTRODUCTION AND LITERATURE REVIEW 19 LIA review of pain response studies in human neonates 21 1.1.1 Physiological indices 21 1.1.2 Behavioural indices 26 1.1.3 The cutaneous flexion/withdrawal reflex as an indicator of neonatal responses to tactile and painful stimuli 29 1.2 The development o f neural pathways underlying responses to sensory stimuli in neonates. 32 1.2.1 Development o f peripheral primary sensory terminals 34 1.2.1.1 Anatomical development 34 1.2.1.2 Physiological development 39 1.2.2 Development of dorsal root ganglion sensory neurons 41 1.2.2.1 Anatomical development 41 1.2.2.2 Physiological development 44 1.2.3 Development o f central primary afferent terminals and their cormections 45 1.2.3.1 Anatomical development 45 1.2.3.2 Physiological development 49 1.2.4 Development o f the dorsal horn 51 1.2.4.1 Anatomical development 51 1.2.4.2 Physiological development 56 1.2.5 Development of motoneurons and their cormections 58 1.2.5.1 Anatomical development 58 1.2.5.2 Physiological development 63 1.2.6 Development of descending cormections in the spinal cord 67 1.2.6.1 Anatomical development 67 1.2.6.1.1 Propriospinal cormections 67 1.2.6.1.2 Descending inhibitory cormections 72 1.2.6.2 Physiological development 72 1.2.6.2.1 Propriospinal cormections 72 1.2.6.2.2 Descending inhibitory connections 74 1.3 Development of reflexes 74 1.3.1 In the human 74 1.3.2 In the rat and other species 78 1.4 Statement o f hypotheses 83 CHAPTER 2: METHODS 86 2.1 The Sample 86 2.2 Ethical permission and consent sought, and information given 89 5 2.3 When the infants were tested 91 2.4 Recording electromyograph (EMG) activity 92 2.5 Skin preparation and electrode placement for EMG recording 94 2.6 Types of stimulation 96 2.6.1 Mechanical stimulation 96 2.6.1.1 Innocuous mechanical stimulation 96 2.6.1.2 Noxious mechanical stimulation 98 2.6.2 Electrical stimulation 99 2.7 Experiments performed 100 2.7.1 Mechanical stimulation 100 2.7.1.1 Initial threshold 100 2.7.1.2 Repeated stimulation 101 2.7.1.3 Suprathreshold stimulation with single stimuli 101 2.7.1.4 Dummy heel lance and heel lance 102 2.7.2 Electrical stimulation 102 2.7.2.1 Initial threshold 102 2.7.2.2 Repeated stimulation 103 2.7.2.3 Suprathreshold stimulation with single trains of stimuli 103 2.8 Statistical analysis 104 2.9 Difficulties encountered in the execution of the methods 105 2.9.1 Subject recruitment 105 2 .9.2 Recording and analysis of EMG activity 106 2.9.3 Definition of flexion reflex responses, and their distinction from spontaneous leg movements 108 CHAPTER 3: METHODS DEVELOPMENT 132 3.1 Introduction 132 3.2 The Piezo Transducer Unit 134 3.2.1 Design 134 3.2.2 Accuracy 135 3.2.3 Clinical use 137 3.2.4 Elimination of ‘cross-talk’ with the ‘Sapphire’ EMG Unit 138 3.3 The ‘Autolet’ Trigger 139 3.3.1 Design 139 3.3.2 Accuracy 140 3.3.3 Elimination of ‘cross-talk’ with the ‘Sapphire’ EMG Unit 140 3.4 The neonatal bipolar electrical stimulator 140 3.4.1 Design 140 3.4.2 Efficiency of operation 141 3.5 The train stimulator box 141 3.5.1 Design 141 3.5.2 Efficiency of operation 142 3.6 Exclusion of movement artefact in EMG recordings 142 3 .6.1 Construction of the movement sensor 143 3.6.2 Experiments conducted 144 3.6.3 Findings 144 CHAPTER 4; RESULTS 162 4.1 The Sample 162 4.2 Effect of postconceptional age upon initial flexion reflex threshold 163 4.2.1 Mechanical stimulation 164 6 4.2.2 Electrical stimulation 166 4.3 Stimulus-response characteristics of the flexion reflex 166 4.3.1 Mechanical stimulation 166 4.3.1.1 Latency 169 4.3.1.2 Amplitude 169 4.3.1.3 Duration 170 4.3.1.4 Area 170 4.3.1.5 EMG Recordings 171 4.3.2 Electrical stimulation 171 4.4 Effect of repeated stimulation on the flexion reflex response 173 4.4.1 Flexion reflex thresholds before and after repeated stimulation 173 4.4.1.1 Mechanical stimulation at 2.8 x threshold 173 4.4.1.2 Mechanical stimulation at 7.6 x threshold 174 4.4.1.3 Electrical stimulation at 1.85 x threshold 174 4.4.2 The number of responses on repeated stimulation versus PCA 175 4.4.2.1 Mechanical stimulation at 2.8 x threshold 175 4.4.2.2 Mechanical stimulation at 7.6 x threshold 176 4.4.2.3 Electrical stimulation at 1.85 x threshold 177 4.4.3 The effect o f repeated stimulation on EMG response parameters: latency, amplitude, duration, and area 177 4.4.3.1 Mechanical stimulation at 2.8 x threshold 177 4.4.3.1.1 Duration 178 4.4.3.1.2 Area 178 4.4.3.2 Mechanical stimulation at 7.6 x threshold 179 4.4.3.3 Electrical stimulation at 1.85 x threshold 180 4.5 A preliminary comparison of flexion reflex responses in biceps femoris and tibialis anterior 180 4.5.1 Mechanical stimulation 180 4.5.1.1 EMG parameters in biceps and tibialis 181 4.5.1.1.1 Latency 181 4.5.1.1.2 Amplitude 182 4.5.1.1.3 Duration 182 4.5.1.1.4 Area 183 4.5.2 Electrical stimulation 183 4.5.2.1 EMG parameters in biceps and tibialis 183 4.5.2.1.1 Latency 184 4.5.2.1.2 Amplitude 184 4.5.2.1.3 Duration 184 4.5.2.1.4 Area 184 4.6 Repeated experiments on the same day in the same infant 185 4.6.1 Initial threshold for the flexion reflex 185 4.6.2 Comparison o f stimulus intensity and EMG parameters 185 4.6.3 The effect of repeated stimulation on flexion reflex threshold 186 4.6.4 The effect of repeated stimulation on the number of responses 186 4.7 Interesting features of EMG recordings 187 4.7.1 Periods of inhibition following the initial reflex response 187 4.7.2 Responses at 2 different latencies 188 CHAPTER 5: DISCUSSION 216 5.1 Observations on specific results 216 5.1.1 Effect of postconceptional age on initial flexion reflex threshold 216 7 5.1.2 Stimulus-response characteristics of the flexion reflex 219 5.1.2.1 Afferent input of the flexion reflex 223 5.1.2.2 Mechanical versus electrical stimulation 224 5.1.2.3 Change of reflex properties with age 227 5.1.3 Effect of repeated stimulation on the flexion reflex response 230 5.1.3.1 Habituation versus sensitization 235 5.1.4 A preliminary comparison of flexion reflex responses in biceps femoris and tibialis anterior 239 5.1.5 Repeated experiments on the same day in the same infant (mechanical stimulation only) 242 5.1.6 Other features of neonatal EMG recordings 246 5.2 General observations on variability 251 5.2.1 Intrinsic factors 251 5.2.2 Extrinsic factors 255 5.3 Strategy for validating EMG data prior to publication 257 5.4 Improvements in the design and methods which would ensure more reliable data collection in future 258 5.5 Conclusions and implications for clinical practice 261 APPENDIX 1 : Plots of values from individual infants of the stimulus- response characteristics of the mechanically-evoked flexion reflex, relative to PCA, at stimulus intensities from threshold to heel lance.