This study is the first to examine relationships of cumulative early procedural pain and morphine exposure, with subsequent cortisol, behavioral, and cardiac responses in preterm infants in the NICU who were free from postnatal exogenous steroid exposure. Among infants born at extremely low gestational ages (≤ 28 weeks), greater cumulative exposure to painful (skin breaking) procedures since birth was associated with lower cortisol response to a stressor (clustered nursing procedures). This relationship was independent of illness severity on day 1 after birth, and of cumulative exposure to intravenous morphine since birth. Importantly, this down-regulation of cortisol response to the subsequent stressor was observed while the infants were still in the NICU. In contrast, in a previous study at 8 months of age corrected for prematurity, cortisol levels were elevated in infants exposed to higher numbers of skin breaking procedures since birth (Grunau et al., 2004
), which is consistent with data from animal studies indicating sensitization following early exposure to stress (Anisman et al., 1998
; Ladd et al., 2000
; Pryce and Feldon, 2003
). Thus, it appears that following higher levels of exposure to neonatal pain in physiologically immature infants, there may be a shift in HPA responsiveness over time, such that cortisol responses are dampened while infants are still in the hospital (and thereby exposed to ongoing environmental stress), but then increased later in infancy. Similarly, in a different study at age 8 months (adjusted for prematurity), cumulative neonatal pain was associated with elevated basal HR (Grunau et al., 2001b
). Thus early pain may lead to altered developmental patterns in stress arousal systems.
Long-term effects of early stress in animal models appear to differ depending on the nature of the stressor. Our findings of elevated cortisol responses to stressors in ELGA preterm infants at 8 months who had the most exposure to painful procedures since birth are consistent with those found in animal models of maternal separation stress, which reported sensitization
of the HPA axis to subsequent stressors (e.g. Plotsky and Meaney, 1993
). In contrast, in animal studies of neonatal pain exposure
, alterations in HPA reactivity in adulthood are typically not observed (Anand et al., 1999
; Walker et al., 2003
). However Walker et al. (2003
) found that maternal care is increased when rat pups have been exposed to pain. They propose that the lack of potentiation of the HPA axis in neonatal rats exposed to early repeated physical pain may be due to moderating effects of this increase in maternal care. Maternal behavior is precluded for long periods of time in the maternal separation paradigm, which may explain these differential results.
The dampened cortisol response in the ELGA infants in the NICU may have occurred, at least in part, because preterm infants have varying degrees of immaturity in HPA axis function during the neonatal period (Bolt et al., 2000
; Korte et al., 1996
). In particular, infants born at lower gestational ages who are sickest commonly show low basal cortisol and adrenal insufficiency (e.g. Hanna et al., 1997
; Kari et al., 1996
; Ng et al., 2001
; Scott and Watterberg, 1995
). In addition, preterm infants recovering from chronic lung disease may show decreased cortisol in response to ACTH 3 weeks after birth (Watterberg et al., 2001
). In our study it is unlikely that the diminished cortisol response was caused by relative insensitivity of the adrenal cortex, as we examined stress reactivity at 32 weeks PCA; after approximately 30–32 weeks, adrenal sensitivity appears to be comparable to that in term infants (Winter, 1998
Another major focus of this study was the relationships between prior cumulative procedural pain and subsequent facial and cardiac reactivity to pain during blood collection. Previously we found that higher exposure to neonatal dexamethasone contributed to dampened biobehavioral response to subsequent pain in the NICU at 32 weeks PCA (Grunau et al., 2001a
). In the present study, in infants without exposure to postnatal dexamethasone, we replicated our earlier finding that dampened facial reactivity to pain during blood collection at 32 weeks in the NICU was significantly related to higher number of pain procedures since birth. However, it must be noted that the magnitude and direction of effects of both cumulative prior pain (r
= −0.44) and cumulative morphine exposure (r
=–0.42) in relation to facial pain response was the same (i.e. both dampened facial pain response). Thus morphine did not appear to ameliorate long-term effects of prior pain on subsequent behavioral reactivity to pain. It is noteworthy that the association between cumulative prior pain and both facial reactivity and cortisol response showed a downward direction. This suggests that both behavioral and cortisol reactivity were reflecting alterations to underlying stress response systems. However, unlike facial response where separate effects of prior pain and morphine were not distinguishable, cortisol was specifically associated with prior pain. In contrast, we found no relationships between pain or morphine exposure and HR or HRV in these infants who were free of exposure to postnatal corticosteroids. In our earlier NICU study (Grunau et al., 2001a
), higher dexamethasone exposure since birth was associated with dampened biobehavioral response, and morphine appeared to ameliorate this association using a measure which primarily tapped sympathetic reactivity. However, given the relatively high doses of dexamethasone used at that time in medical care of extremely preterm infants, and the actions of corticosteroids on multiple physiologic systems, it appears likely that the differences between the present and prior studies was the exclusion of infants exposed to postnatal steroids in the present study.
Another important question is whether a ‘cutoff’ can be identified in the number of procedures at which point the relationship between prior pain and subsequent dampened reactivity ‘kicks-in’. In our earlier paper, preterm infants who received less that 20 procedures showed vigorous responses to blood collection, whereas more than 20 procedures was associated with dampened response (Grunau et al., 2001a
). In the present sample we were unable to identify a ‘cutoff’ as shown in Figs. and , where considerable variability was evident.
Advances in knowledge of the developmental neurobiology of pain and stress systems (Fitzgerald, 1993
; Fitzgerald and Beggs, 2001
) have increased attention on the long-term effects of pain on preterm infants in the NICU (Fitzgerald, 2004
; Walker et al., 2001
). However, recently there is evidence that early morphine in mechanically ventilated infants may not be particularly efficacious in ameliorating pain in these infants (Simons et al., 2003
), and furthermore, may not decrease major adverse sequelae (Anand et al., 2004
). In the present study we did not find evidence that cumulative morphine ameliorated relationships between early pain and later dampened facial or cortisol responses.
In clinical studies it is difficult to ‘unpack’ specific effects of cumulative pain exposure versus biological immaturity, which are highly intercorrelated. We attempted to address this by examining associations of cumulative procedural pain separately for infants born extremely low (≤ 28 weeks) and very low (29–32 weeks) gestational age. Time in the NICU and postconceptional age are identical, and inversely related to gestational age, all indicating degree of biological immaturity. Repeated pain exposure likely acts synergistically together with neonatal physiological immaturity to drive altered pain and stress responses, both in the NICU and later after hospital discharge.
While experimental randomized studies are the only way to address causality, there are limits to these as well. With human infants, the ethical issues presented by needing to provide pain relief in clinical care lead to a high crossover rate in randomized pharmacologic trials (e.g. Anand et al., 2004
). In studies using animal models, the intervention itself can induce alterations in maternal care (Walker et al., 2003
). Our results are consistent with recent findings that reduction of environmental sources of stress in the NICU promotes improved outcomes (Als et al., 2004
There are a number of limitations in this study. First, total skin breaking procedures reflect not only pain, but also cumulative exposure to stress since birth. Secondly, we did not evaluate effects of different types of procedures which vary in intensity. Lastly, we could not separate biological immaturity from prior pain/stress exposure, however, this limitation affects this field in general.