In Reply (2011)

We appreciate and share Flick and Warner's concern that multiple surgical procedures early in life can be associated with learning disabilities later in life, as suggested in their cohort study of 593 children younger than 4 yr.1Of those, 100 (16.9%) children underwent two episodes of anesthesia and 44 (7.4%) underwent three or more such episodes and experienced significant comorbidities known to be associated with learning disabilities. Thus, the extent to which anesthesia may or may not contribute to these remains to be elucidated.2,3Our research is based on more than 45,000 Danish children who underwent surgery before the age of 1 yr during the period 1977–1990.In our first study, we found no evidence for a general effect of anesthesia on academic performance.4For several reasons, we chose to focus on children born between 1986 and 1990 who underwent surgery in the first year of life for inguinal hernia (n = 2,689) compared with a 5% random sample of the whole population (n = 14,575). Those reasons are: (1) Such a study will have more public health relevance because a much larger proportion of infants undergo only one episode of general anesthesia (surgery) in infancy, (2) children undergoing multiple episodes of anesthesia (surgeries) are likely to have more severe underlying conditions that may be the reason for later learning disabilities, and (3) our approach is likely to be unbiased or overestimate the effect of anesthesia on later learning abilities unless children with inguinal hernias are less likely to have other anesthetics (i.e. , children with hernia are otherwise healthier than the background population, which would be difficult to argue for biologically).5For clarity, as requested by Flick and Warner, we included all children in Denmark born from 1986 to 1990 who underwent surgery for inguinal hernia before the age of 1 yr (n = 2,689); of those had 2,445 (90.9%) had one hernia operation, 221 (8.2%) had two hernia operations, 20 (0.7%) had three hernia operations, 2 (0.1%) had four hernia operations, and 1 (0.0%) had five hernia operations. Our 5% randomly selected background population consisted of 14,575 individuals of the same cohort, excluding children who underwent hernia repair. Children who underwent additional surgeries were not excluded from the exposure or the control group.We are studying the effects of multiple episodes of anesthesia (and surgery) among other children (including neonates) operated on during the first year of life. We expect this group to have bigger learning problems later in life than the background population, but it will be difficult to disentangle the effect of the more severe underlying disease(s) that prompted several episodes of anesthesia from the potential effect of multiple episodes of anesthesia. Thus, we reported on all children with hernia in our first report because they represent a group for whom the underlying morbidities are unlikely directly to affect later learning disabilities.Finally, Flick and Warner question whether we may have missed cases in our cohort because the Danish National Hospital Register did not include outpatients before 1995. In Denmark during the period 1986–1990, all infants and young children who underwent anesthesia and surgery (including inguinal hernia repair) were inpatients.