Early Exposure to Anesthesia and Learning Disabilities in a Population-based Birth Cohort

Abstract

EXPOSURE to alcohol and some anesthetic and sedative drugs cause histopathologic changes in the developing brains of animals.1–3Implicated drugs include N -methyl-d-aspartate glutamate receptor antagonists (e.g. , ketamine, nitrous oxide) and agents with γ-aminobutyric acid A mimetic properties (e.g. , pentobarbital, diazepam, isoflurane, halothane, propofol). Some studies suggest that even relatively brief single exposures trigger changes, especially when combinations of agents are used.1–6In one report, histologic neurodegeneration was associated with a diminished capacity to retain learned behaviors.7It is not known whether findings in rodent models can be extrapolated to humans, but emerging histologic data in nonhuman primates8tend to confirm findings in rodents. These findings have engendered considerable concern among the U.S. Food and Drug Administration and others.5Except for case series reporting developmental outcomes of critically ill neonates and children undergoing repair of congenital heart disease,9–12which have multiple limitations, there are no data that can yield insight into whether exposure to anesthesia and surgery during human development causes clinically relevant impairment in neural development.One challenge to determining whether exposure to anesthesia and surgery in early life impairs neural development is defining relevant outcomes. Learning disabilities (LD) may be an appropriate outcome measure. Children with LD experience problems with one or more of the basic psychological processes involved in understanding or in using spoken or written language, which may manifest itself in an imperfect ability to listen, think, speak, read, write, spell, or perform mathematical calculations. Because learning disabilities are routinely sought on the basis of standardized individualized testing in educational settings and because of the relatively high incidence rate of LD,13this outcome is available in large populations of children.The purpose of the current study was to determine whether there was an association between exposure to anesthesia during the first 4 yr of life and the development of any learning disability in a birth cohort of children in Olmsted County, Minnesota.The Mayo Clinic and Olmsted Medical Center Institutional Review Boards (both in Rochester, Minnesota) approved this study. A birth cohort of children born in Rochester, Minnesota, identified in previous work by the authors13–16formed the basis of the current study. All children (n = 8,548) born between January 1, 1976, and December 31, 1982, to mothers residing at the time of delivery in the five Olmsted County, Minnesota, townships (comprising Minnesota Independent School District No. 535, the Rochester public and private school system) were identified through computerized birth certificate information obtained from the Minnesota Department of Health, Division of Vital Statistics. To ascertain vital status (still living in Rochester, moved, or deceased), for each member of the birth cohort during the 1995–1996 school year, resources available from the Rochester Epidemiology Project,17Minnesota Independent School District No. 535, the Reading Center/Dyslexia Institute of Minnesota, and the Minnesota Department of Health were used. Children who left Olmstead County before age 5 yr (i.e. , moved or died; n = 2,830) were not included in the final study cohort.14Through the Rochester Epidemiology Project, all diagnoses and surgical procedures recorded at all Rochester medical facilities are indexed for automated retrieval. This diagnostic index expedites retrieval of the unit (or dossier) medical record, which includes the history of all encounters in the hospital, community and ambulatory medical and social services, emergency department, outpatient clinics, and home visits as well as laboratory and psychological test results from birth until patients no longer reside in the community. The evaluation and instructional resources of Minnesota Independent School District No. 535, the school system for the city of Rochester, are of high quality, and the district has a long tradition of excellent care and management of children with all types of handicapping conditions, including LD. Through a contractual research agreement, all public (19 primary, 3 junior high, 3 high schools) and nonpublic (12 primary, 10 junior high, 4 high schools) schools gave permission to access their richly documented cumulative educational records for every child from this birth cohort. Under a second research agreement, permission was obtained to access the resources of the privately owned Reading Center/Dyslexia Institute of Minnesota, the only private tutoring agency in the community during the years relevant to this study. The Reading Center/Dyslexia Institute of Minnesota files included a pool of some 3,000 evaluations and outcomes of tutorial instruction that spanned nearly 50 yr. All of these records, including the results of all individually administered intelligence quotient (IQ; primarily age-appropriate Wechsler scales) and achievement (primarily Woodcock–Johnson tests) tests and medical, educational, and socioeconomic information were abstracted by trained personnel, using detailed data abstraction protocols.Learning disabilities (including reading, written language, and math disabilities) were diagnosed using research criteria based on three formulas. In each of the following formulas, X is equal to the study subject's IQ score, and Y represents the predicted standard score from the achievement test. The regression formula–Minnesota, Y < 17.40 + 0.62X, is issued by the Minnesota Department of Education.18Children classified as having LD by this formula had standard scores in academic achievement that were more than 1.75 SDs below their predicted standard score from an individually administered measure of cognitive ability (IQ). The value 0.62 represents the correlation between IQ and achievement used in the formula from the state of Minnesota. The discrepancy nonregression method was used in Minnesota Independent School District No. 535 before 1989 and included the school years of the children in our birth cohort. By using this approach, differences between standard scores on measures of intelligence and aptitude and measures of test achievement that were believed to be important varied by grade as follows: (1) kindergarten through 3rd grade, 15 or more standard score points difference, with achievement lower; (2) 4th through 6th grade, 19 or more points difference, achievement lower; and (3) 7th through 12th grade, 23 or more points difference. Finally, the low-achievement method (X ≥ 80 [aptitude] and Y ≤ 90 [achievement]) represents a recent concept in identifying LD independent of measured cognitive ability, assuming that cognitive ability is at least in the low average range.19Children meeting the criteria before age 19 yr for at least one of the three LDs (reading, written language, and math disabilities) using IQ and achievement scores obtained within the same calendar year were identified as LD cases regardless of presence or absence of any comorbid conditions.13We identified all members of the birth cohort who underwent any type of surgery or diagnostic procedure necessitating general anesthesia before their fourth birthday, using the Mayo Clinic Surgical Information Retrieval System and a similar resource for procedures performed at Olmsted County Medical Center. The choice of age range was based on the analogous developmental stage of animal models in which anesthetic effects on neurodevelopment have been shown.1,20Procedures such as neonatal circumcision performed without anesthesia were excluded from review.For the children undergoing anesthesia, the following information was abstracted: American Society of Anesthesiologists physical status (ASA PS) classification, type of surgery or procedure and urgency, total duration of anesthesia, number of anesthetic exposures, age(s) at which exposure occurred, anesthetic agents (inhalational, intravenous, sedatives), and comorbidities (including syndromes that can be associated with mental retardation, congenital heart disease, and neurologic diseases). For all children in the cohort (including those who did not receive anesthesia), sex, gestational age at birth, birth weight, Apgar scores at 1 and 5 min (Apgar scores were available for children born 1980–1982), complications of pregnancy, complications of labor and delivery, number of births (multiple or single), need for induced labor, and the mother's and father's age and level of education (< 12 yr, 12 yr, > 12 yr) were available from birth certificates.The primary outcome for the current analysis was LD based on individually administered IQ and academic achievement test scores using any of the three standard formulas for determining the presence of reading written language or math LD. The primary risk factor of interest for this investigation was exposure to general anesthesia before age 4 yr. Preliminary analyses were performed to compare demographic, pregnancy and delivery, and parental characteristics between those who were exposed versus unexposed to general anesthesia before age 4 yr using the two-sample t test (or rank sum test) for continuous variables and the chi-square test (or Fisher exact test) for categorical variables.Individuals were followed up from birth until the date they first met the LD criteria using any of the three standard formulas for determining the presence of reading, written language, and math LD. Cumulative incidence rates of LD were calculated according to the method of Kaplan and Meier with data censored at the initial occurrence of emigration, death, last follow-up date, or age 19 yr. Proportional hazards regression was used to assess whether anesthetic exposure was a risk factor for LD. For these analyses, anesthetic exposure was quantified as any exposure to general anesthesia before age 4 yr (yes vs. no), the number of exposures to general anesthesia before age 4 yr (none, one, two, three or more), and the cumulative duration of exposure (treated as a continuous variable and also categorically using 30-min intervals). Both unadjusted and adjusted analyses were performed. In all cases, separate models were used to evaluate the different anesthesia exposure variables. The covariates that were included in the adjusted analyses include gestational age, sex, and birth weight. In the adjusted analysis, only those individuals for whom complete covariate information was available were included. Results were summarized using hazard ratio estimates and corresponding 95% confidence intervals (CIs). In all cases, two-tailed P values less than 0.05 were considered to be statistically significant. Analyses were performed using SAS statistical software (version 9.1; SAS Institute, Inc., Cary, NC).Between 1976 and 1982, there were 8,548 children born in the five Olmsted County, Minnesota, townships comprising Minnesota Independent School District No. 535, and 5,718 of these children still resided in the community at age 5 yr. Of these, 19 individuals were diagnosed with severe mental retardation and were excluded, as were 342 patients who denied research authorization for the use of their medical records. Therefore, 5,357 children are included in the current report. Of those, 593 underwent procedures requiring general anesthesia before age 4 yr. In comparison to unexposed children, those exposed to anesthesia before age 4 yr had lower birth weight (P < 0.001), lower gestational age (P = 0.001), and were more likely to be male (P < 0.001) (table 1and Supplemental Digital Content 1 [see table, which illustrates birth, maternal, and paternal characteristics, https://links.lww.com/A825]). All of these factors were subsequently used as adjustors in multivariate analysis. Children exposed to anesthesia also had mothers with higher levels of maternal education (P = 0.039); however, this factor was not included as an adjustor in subsequent analysis because these data were missing for approximately 10% of children. Apgar scores were not different between two groups for those in whom data were available. The peripartum complications of those exposed or not exposed to anesthesia were similar (table 2and Supplemental Digital Content 2 [see table, which illustrates peripartum complications from birth certificates, https://links.lww.com/A826]), with the exception that the mothers of those exposed to anesthesia experienced a slightly higher rate of peripartum hemorrhage and prolonged labor.The 593 children exposed to anesthesia underwent 875 procedures, with 449 (75.7%) having a single procedure (table 3). Of the children exposed to anesthesia, 438 (74%) were classified as ASA PS I (table 3). Types of surgeries are shown in table 4. Most anesthetics included halothane (88%) and nitrous oxide (91%); 9% included ketamine (table 5).A total of 932 children developed an LD before age 19 yr as assessed by individually administered IQ and achievement tests scored using any of the three standard formulas (estimated cumulative incidence 20.5%; 95% CI, 19.3–21.7%). For both unadjusted and adjusted (for sex, birth weight, and gestational age) analyses, the risk for the development of LD (compared with children not exposed to anesthesia) increased (P < 0.001) with the number of exposures to anesthesia before age 4 yr (table 6). The risk was not increased for the 449 children exposed to a single anesthetic (adjusted hazard ratio = 1.00; 95% CI, 0.79–1.27; table 6). However, the risk was significantly increased for children exposed to two or more anesthetics (table 6and fig. 1). The estimated incidence of LD by age 19 yr was 20.0% (95% CI, 18.8–21.3%) in those with no exposure to anesthesia, 20.4% (95% CI, 16.3–24.3%) in those with single exposure, and 35.1% (95% CI, 26.2–42.9%) in those with multiple exposures. The risk for LD was also increased with longer cumulative duration of anesthesia (P = 0.016 and P = 0.027 from adjusted analysis assessing cumulative duration of anesthesia as a continuous and as a categorical variable, respectively; table 6).Children with multiple exposures to anesthesia were also more likely to be assigned a higher ASA PS compared with those with a single exposure, indicative of more severe comorbidity in the judgment of the anesthesia provider (fig. 2). However, when the analyses assessing the relation between number of exposures and cumulative duration of exposures with development of LD were repeated after eliminating surgical patients with ASA PS of III or greater, the findings were similar: anesthesia was a significant risk factor for the development of LD in children receiving multiple, but not single, anesthetics (data not shown). Detailed information regarding each of the 144 children who received multiple anesthetic exposures before age 4 yr, including their medical diagnoses before and after age 4 yr, are provided in Supplemental Digital Content 3 (see table, which illustrates diagnoses from the medical record in the 144 children who received multiple anesthetics before age 4 yr, https://links.lww.com/A827).In this population-based birth cohort, exposure to anesthesia before age 4 yr was a risk factor for the development of LD in children receiving multiple, but not single, anesthetics. The cumulative incidence of LD diagnosed by age 19 yr among those with repeated anesthetic exposures was almost twice as high (35.1%) compared with children not exposed to anesthesia (20.0%).Late prenatal and early postnatal neural development is vulnerable to pharmacologic and environmental influences.20–22Exposure of immature animals to compounds with γ-aminobutyric acid mimetic receptor agonist or N -methyl-d-aspartate receptor antagonist properties induces apoptotic degeneration of neurons in various brain regions.1–4,7,23In particular, drugs with sedative and anesthetic properties (including isoflurane, nitrous ketamine, halothane, and neurodegeneration when administered at and of some animal these histologic changes have been associated with learning and assessed by and these data have and research on the of the Food and Drug Administration and is not known whether exposure to anesthetics or in A recent neurologic after surgery and anesthesia in children. Some studies of developmental outcomes in patients undergoing repair for congenital heart suggest neurologic studies outcomes of neonates with and or the of these was not to the effects of characteristics, and surgery from the effects of anesthesia, and the for is a birth cohort to study the incidence of learning disabilities in a cohort provided All of these children resided in the same any of public private and received care at one of two facilities Clinic and Olmsted County Medical to all available medical and educational records. These records, with of to perform a of a clinically significant outcome that the learning in an animal complete data available from birth records to for factors known to the of LD gestational age, and birth anesthesia records were available for all procedures, and anesthetic was The of surgical procedures the population-based of the cohort, is not patients undergoing more procedures, as is the case with studies at academic exposure to anesthesia significantly there be a relation between exposure and a relevant for such a relation between anesthetic exposure and LD in two risk was increased for children requiring multiple exposure (adjusted hazard ratio of for two exposures and for three or more table but not for single exposures to anesthesia (adjusted hazard ratio of risk was increased for longer of anesthesia, statistical for cumulative duration of min or (table 6). anesthesia exposure was as a variable exposure before age 4 exposure was a significant risk factor for LD in unadjusted ratio of but not adjusted ratio of analysis. The likely the that the of children received only one exposure, which was not associated with increased study has between effects of anesthesia itself and factors associated with anesthesia, such as the to surgical children requiring anesthesia may in important from those who and such differences may risk for LD. Therefore, that requiring multiple anesthetics is a for that LD risk and that exposure to anesthetic drugs is not adjusted for known factors (for which data were to LD risk that between the groups with the exception of maternal because data were missing in a significant number of children. However, when analysis was repeated children with missing data and including maternal education as a the results were the same (data not shown). Children requiring repeated procedures may have a higher of which may risk for LD. For and children requiring repair of congenital heart may more children requiring multiple procedures were by their to have more severe disease, as by higher ASA did not the medical records of the children not requiring anesthesia, not use medical diagnoses as covariates in the ASA PS is not available in these children. However, among the 144 children receiving multiple LD was not more in children with higher ASA and among all children receiving anesthesia, LD did not with ASA PS in analysis (data not shown). the association of LD with repeated anesthetic exposure was still when the analysis was repeated after eliminating surgical patients with ASA PS of III or These findings suggest that ASA PS was not associated with LD risk in our cohort, and that the increased of LD in children receiving multiple anesthetics be primarily to those children with multiple medical of anesthetic is an but of the of anesthetics than nitrous cause of children received ketamine to perform a separate analysis for this animal there is a time of to the effects of anesthetic exposure (e.g. , approximately after birth in to to a of the to study the effects of anesthetic exposure may be In humans, the of has been considered to through age 3 was the basis for our choice of the fourth as the age to anesthetic However, the between of human and animal neurodevelopment is and suggest that the corresponding to the time of risk in animal models (e.g. , approximately before birth until 2 after birth in is in of and first after repeated our analysis anesthetic exposure before age 2 yr than age 4 yr) on the risk of LD and similar results (see Supplemental Digital Content which illustrates the cumulative of learning disabilities by the age at exposure, and table, Supplemental Digital Content which illustrates the effects of anesthetic exposure before age 2 yr on risk for developing learning did not have of cases to a more age range (e.g. , is to from the birth cohort of 8,548 children. cohort studies can be as a of from the community. For the and of care available in Rochester, children with a higher level of medical need may not to and be in the cohort. This to the surgical children with more severe However, a comparison of children who left the community before age 5 yr and those who after age 5 yr age of school that the children included in the study are of the birth to the of this cohort is that in these years Rochester was a which may the of these results to to be whether LD is a relevant outcome measure for any effects of anesthesia in humans, that and socioeconomic factors may also LD. on the animal studies an association between the neurodegeneration by exposure to anesthetics and learning that LD is a relevant in also to be whether the in the of LD among children with multiple anesthetic exposures is to one type of LD (i.e. , written language, and reading For of this analysis, a of LD to the number of children with LD and the ability to by type of LD be of but be by the between types (i.e. , some children have more than one type of and the need for increased of to of the in the current study. are a of outcomes that be but testing that is to in large population-based in this population-based birth cohort, exposure to anesthesia before age 4 yr was a significant risk factor for the development of LD in children receiving multiple, but not single, anesthetics. These data whether exposure to anesthesia itself may to the of LD or whether the need for anesthesia is a for factors that to LD. However, these results suggest that the of effects of repeated anesthetic exposures on human neurodevelopment be the Mayo Rochester, for in the Rochester Epidemiology Project, and of Mayo for and experience in the of learning The Mayo and members of the Learning for data Independent School District No. and the Reading Center/Dyslexia Institute of Minnesota for their and The also Mayo and in for in medical records from Olmsted County Medical Rochester, Minnesota.