Bodily Progression of Motor Responses to Increasing Mechanical Force Stimulation in the Newborn Infant and the Effect of Heel Prick

Abstract

Background: The newborn infant’s response to stimulation with von Frey filaments has previously been examined only at the spinal level as the flexion withdrawal response or abdominal reflex. The threshold for the spinal responses has been shown to be lower following skin damage and visceral pathology. Higher forces of mechanical stimulation elicit other body responses, which are likely to arise from higher levels in the nervous system: these have not been investigated before. Objective: To investigate the newborn infants’ responses to increasing forces of mechanical (von Frey filament) stimulation and whether their progression is affected by repeated heel prick. Subjects and Methods: The study was performed in 31 full term and 77 preterm infants. Graded mechanical forces (using von Frey filaments) were applied to the heels and the abdominal skin. The thresholds for the flexion withdrawal reflex or abdominal reflex and other body responses were recorded. Results: The thresholds for the flexion withdrawal reflex and other body movements to graded mechanical force on the heel were significantly lower in preterm infants compared to full term infants. The threshold for the abdominal reflex and other body re- sponses from graded mechanical stimulation of the abdomen was also significantly lower in preterm infants. In all infants thresholds were further reduced following sensitisation from previous heel pricks. The progression of the responses is independent of gestation or prior sensitisation. Conclusion: Both preterm and full term newborn infants respond to graded mechanical stimuli by a series of body responses, which progress in response to increasing force from a simple spinal withdrawal reflex to more complex responses involving higher levels of the central nervous system.