Wound sensitivity as a measure of analgesic effects following surgery in human neonates and infants

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Abstract
Postoperative wound sensitivity following abdominal surgery in neonates and infants has been little investigated, and is not well understood. Our aim was to quantify this sensitivity using the abdominal skin reflex (ASR), and to test the hypothesis that the threshold of this reflex would decrease following surgery, and would increase with the administration of analgesia. We measured ASR thresholds in infants under 1 year receiving unilateral abdominal surgery and in a small comparison group of non-surgical infants. The reflex was elicited by applying calibrated von Frey hairs to both sides of the abdomen at the same segmental level. In addition to threshold, the degree of reflex radiation as denoted by hip flexion was measured. The reflex threshold was significantly lowered at the wound site by up to 78% following surgery, and subsequently increased to varying degrees based on the type of analgesia used. The post-surgical drop in threshold was accompanied by an increase in reflex radiation. Thresholds remained below pre-operative values 24 h after surgery on the operated side. In addition, it was observed that infants in whom the indication for surgery was a chronic condition, displayed lower threshold values on the affected side prior to surgery. An objective and quantitative measure has been developed of wound sensitivity and level of analgesia in infants following surgery. The reflex threshold also provides a means of quantifying human central sensory processing, and may be used to detect referred visceral hypersensitivity in this age group.