Introduction The olfactory system is well structured at 32-34 weeks of gestation, but it has not been described if this system is operational earlier and/or if it undergoes progressive maturation with increasing postconceptional age.
Objective The aim of this study was to assess the maturational process of the NIRS cortical response to olfactory stimuli in the premature infant. Our hypothesis is that the premature infant will have progressive changes in the cortical response directly related to postconceptional gestational age (PCA).
Methods We included infants between 27 and 31 weeks of gestation who were extubated and without evidence of intraventricular bleed or periventricular leukomalacia. The first NIRS measurement was done at the time of recruitment and thereafter every 2 weeks if extubated and clinically stable, until the infant reached 33 weeks PCA. NIRS monitoring (NIRO-300 Hamamatsu) was done by placing two optodes abovethe left anterior orbitofrontal gyri (LAOFG). Each recording consists of baseline (BL) for 120 sec followed by 60 sec of a smell stimulus (vanilla) subsequently followed by negative control (NC) with 60 sec of distilled water. This was followed by exposure to a different smell stimulus (benzoin tincture) for 60 sec followed by another NC. We measured the relative changes of [Hb O2],[Hb H] y [Cyt aa3] every 5 seconds. The last measurement was done in a random cortical area to assess if the response was limited to the olfactory area. ANOVA was used to analyze the variance between groups. Associated probability between PCA and the magnitude of the response to stimuli was done by linear regression and correlation coefficient. Significance, p < .05. Approval from the university IRB was obtained as well as informed consent.
Results Seventeen experiments have been done so far in 7 infants. Gestational age at birth was 28 6 1.9 and PCA at time of study ranged from 27 to 32 weeks. During exposure to both olfactory stimuli, there were no changes in the areas not related to the olfactory region. However, in all infants both olfactory stimuli produced a significant increase in [Hb O2] and a significant decrease of [Hb H] as compared with BL (p < .001) in the LAOFG. No change was observed in [Cyt.aa3]. The magnitude of the changes in [Hb O2] following olfactory stimuli is related to PCA in this ongoing study; r = .676 (p = .006).
Conclusions There are changes in regional cerebral blood volume associated with olfactory stimuli in the preterm infant < 32 weeks that can be measured by NIRS. These changes can be detected as early as 27 weeks PCA. Finally, the magnitude of the response seems to be directly correlated to PCA, suggesting a progressive postnatal maturation of the olfactory system between 27 and 32 weeks.
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