Intramuscular injections in newborns: analgesic treatment and sex-linked response

To compare the analgesic effect of three treatments to relieve the pain produced by intramuscular injections (IMI) in term newborns, and to assess sex-linked differences in their response to pain.

We studied 62 babies. Each baby received antibiotic IMIs for clinical aims. During each IMI, one of the following analgesic treatments was utilized: oral 33% glucose (OG), sensorial saturation (SS), or topic anesthetic cream (TAC). SS is a validated analgesic method, based on the combination of three stimulations (tactile, acoustic and gustative). During the IMI, pain level was assessed with the use of the DAN scale, a validated neonatal pain scale. All babies who received three distinct analgesic procedures for three distinct IMIs were enrolled. Mean pain scores of the three analgesic treatment groups were compared. We then compared mean pain scores of females vs males in the whole cohort and within each treatment group.

The 95% Confidence Intervals of pain scores were 5.6-6.5 for TAC, 1.4-2.3 for OG and 0.6-1.2 for SS: when treated with TAC, babies' pain scores were significantly higher than with OG or SS (p <0.0001); when treated with OG, babies' pain scores were higher than SS (p = 0.002). Females' mean pain score was significantly higher than males' mean pain score: (95% CI: 2.9-4.1 vs 2.0-3.1; p = 0.01). OG and SS produced significantly higher mean DAN scores in females than in males. Also in the TAC group females' mean DAN scores were higher than males, though this last difference was not statistically significant.

This is the first study to show the effectiveness of nonpharmacologic analgesia in relieving IMI pain. It is also the first study to clearly show that the sex differences in pain perception are present since birth.