Predictors of Neuropsychological Test Performance After Pediatric Traumatic Brain Injury

Transcription

1 ASSESSMENT / Donders, Nesbit-Greene / DEMOGRAPHIC VARIABLES Predictors of Neuropsychological Test Performance After Pediatric Traumatic Brain Injury Jacobus Donders Mary Free Bed Rehabilitation Hospital Kelly Nesbit-Greene Mount Washington Pediatric Hospital The influence of neurological and demographic variables on neuropsychological test performance was examined in to 16-year-old children with traumatic brain injury (TBI). Regression analyses were conducted to determine the relative contributions of coma, neuroimaging findings, ethnicity, socioeconomic status, and gender to variance in performance on the Wechsler Intelligence Scale for Children Third Edition (WISC III), California Verbal Learning Test Children s Version (CVLT C), and thechildren s Category Test. Both neurological and demographic variables contributed to performance on various WISC III factor index scores as well as the CVLT C. No evidence for a moderating effect of demographic variables was found, but speed of information processing mediated the effect of neurological and demographic variables on CLVT C performance. It is concluded that demographic variables have an incremental effect on the neuropsychological test performance of children with TBI above and beyond the influence of injury severity. Keywords: demographic variables; neuropsychological test performance Traumatic brain injury (TBI) is a common pediatric health problem affecting, on average, 180 per 100,000 children each year (Kraus, 1995). It has been well established that the risk of resulting cognitive deficits increases with greater injury severity such as prolonged length of coma or presence of intracranial lesions (for reviews, see Donders & Kuldanek, 1998; Yeates, 2000). However, little is known about the relative effect of demographic variables on neuropsychological test performance after pediatric TBI. There is ample evidence from the adult literature that variables such as education, ethnicity, and gender can influence psychometric test results in both normal (Heaton, Taylor, & Manly, 2003) and diverse clinical samples (Moses, Pritchard, & Adams, 1999; Vanderploeg, Axelrod, Sherer, Scott, & Adams, 1997) and that this is also the case, specifically, in adults with TBI (Donders, Tulsky, & Zhu, 2001; Sherrill-Pattison, Donders, & Thompson, 2000). The purpose of the current investigation was to determine the degree to which demographic as well as neurological variables explain variance on neuropsychological test performance after TBI in children. One demographic variable that has been studied extensively with regard to its relationship to outcome after pediatric TBI is age. There is no doubt that severe TBI that is sustained earlier in life, particularly during infancy and the preschool period, is associated with a greater risk for longterm neurobehavioral deficits probably because such injuries interfere with vulnerable skills that are still in a phase of emerging or rapid development (Anderson et al., 1997; Chapman & McKinnon, 2000; Ewing-Cobbs et al., 1997; Taylor & Alden, 1997; Woodward et al., 1999). In the current investigation, we used psychometric tests that were This research was supported by a grant from the Mary Free Bed Guild Fund (#60). Correspondence concerning this article should be addressed to Jacobus Donders, Psychology Service, Mary Free Bed Rehabilitation Hospital, 235 Wealthy S. E., Grand Rapids, Michigan jdonders@mfbrc.com Assessment, Volume 11, No. 4, December DOI: / Sage Publications

2 276 ASSESSMENT normed and standardized on large samples with adequate age stratification, including the Wechsler Intelligence Scale for Children Third Edition (WISC III; Wechsler, 1991), the California Verbal Learning Test Children s Version (CVLT C; Delis, Kramer, Kaplan, & Ober, 1994), and the Children s Category Test Level 2 (CCT; Boll, 1993). The CVLT C and CCT were also age-normed on the same sample, which facilitates comparison of test results in clinical practice. Thus, the focus of this investigation was on the influence of demographic variables other than age on performance on these tests after pediatric TBI when considered in concert with injury severity. The WISC III is a test of psychometric intelligence that has been used extensively in the evaluation of sequelae of childhood TBI. Several studies have indicated that the perceptual organization and processing speed indexes are relatively more sensitive to the effect of TBI than the verbal comprehension and freedom from distractibility indexes because of the emphasis on fast and efficient performance on the subtests comprising the former variables (Donders & Warschausky, 1997; Tremont, Mittenberg, & Miller, 1999). The CVLT C is a test of verbal learning and memory that has also demonstrated considerable sensitivity to severity of pediatric TBI (Levin, Song, et al., 2000; Roman et al., 1998; Yeates, Blumenstein, Patterson, & Delis, 1995). Research with the CCT, a task requiring problem-solving with visual materials, suggests that it too may be affected to some extent by severity of pediatric TBI, although less so than the CVLT C or the WISC III processing speed index (Hoffman, Donders, & Thompson, 2000). There has been some research concerning the effect of various demographic variables in the standardization samples of the WISC III, CVLT C, and CCT. Higher socioeconomic status as reflected in parental education or occupation has been shown to be associated with overall higher levels of performance in the normative samples of the WISC III (Donders, 1996; Glutting, McDermott, & Konold, 1997) as well as the CVLT C (Donders, 1999) and CCT (Donders, 1998). Socioeconomic status has been reported to moderate long-term behavioral adjustment and academic achievement after pediatric TBI (Taylor et al., 2002), but this effect appeared to be less robust for cognitive outcomes as assessed by WISC III and CVLT C variables (Yeates et al., 2002). It has also been well established that members of some ethnic minority groups tend to have slightly lower overall levels of performance on general ability tests like the WISC III (Neisser et al., 1996), although there is considerable debate about the reasons for this, with opinions ranging from Eurocentric bias (Kwate, 2001) to environmental and socioeconomic factors such as poverty and barriers to educational opportunities (Prifitera, Weiss, & Saklofske, 1998; Suzuki & Valencia, 1997). In the combined standardization sample of the CVLT C and CCT, gender effects that favor girls have been found for the former (Kramer, Delis, Kaplan, O Donnell, & Prifitera, 1997) but not latter instrument (Donders, 1998). In one prior investigation in our laboratory (Donders & Hoffman, 2002), gender was found to explain a modest amount of incremental variance in performance on the CVLT C after pediatric TBI above and beyond the effects of injury severity, but that investigation did not address the effect of demographic variables on other tests such as the WISC III or CCT. The fact that demographic variables affect psychometric test performance in healthy children does not necessarily implicate that such effects are present to the same degree in children with TBI. In fact, some researchers have suggested that the effect of brain damage essentially obviates the normal influence of demographic variables on pediatric neuropsychological test performance (Reitan & Wolfson, 1995). On the other hand, if demographic variables do have an effect on psychometric test performance, even after accounting for the effect of neurological injury, this might help to identify those children most at risk for complicated outcome. Thus, the goal of this exploratory investigation was to determine the influence of ethnicity, gender, and socioeconomic status on the performance on age-normed tests of intelligence, learning and memory, and problem solving when considered in concert with neurological injury variables. METHOD Participants The participants for this investigation were selected from a consecutive series of inpatient and outpatient referrals to a regional Midwestern rehabilitation hospital. Patient selection continued for approximately 7½ years until there were 100 children who met all of the following criteria: (a) history of TBI through an external force to the head with associated alteration of consciousness; (b) age between 9 and 16 years at the time of psychometric assessment (to ensure applicability of age-based norms for all of the tests used); (c) absence of a premorbid neurological, psychiatric, personal abuse, or special education history; and (d) assessment with the WISC III, CVLT C, and CCT within 1 year after injury. During the time period that these data were collected, these three tests were administered routinely to all children who were referred for neuropsychological evaluation following TBI at the organization where this study was completed except in cases

3 Donders, Nesbit-Greene / DEMOGRAPHIC VARIABLES 277 where there were confounding factors (e.g., non-englishlanguage background, orthopedic injury to the dominant forearm, and severe, uncorrected visual impairment) that could have invalidated the child s performance. In the event that more than one evaluation was completed, only the first one was used in this investigation. The vast majority of this sample had been included previously in a different investigation concerning the factor structure of the CCT (Nesbit-Greene & Donders, 2002), but that study did not address the questions of the current investigation. In addition, slightly more than half of this sample had previously been included in an investigation that specifically addressed gender differences in performance on the CVLT C (Donders & Hoffman, 2002), but that investigation also included younger children (6 8 years), and it did not explore similar effects on WISC III or CCT variables. The final sample included 57 boys and 43 girls. Ethnicity was determined on the basis of the primary caregiving parent s designation. The majority of the sample was Caucasian (n = 78) with 10 Latino, 6 African American, and 6 Asian American children. Two biracial children were classified as, respectively, African American and Asian American based on parental designation. Measures of parental education were not available for a minority of the sample, but parent-reported occupation was available for all participants, and this variable was used as a proxy for socioeconomic status. Parental occupations were coded according to the primary wage earner in the household and included professional (n = 19), clerical (n = 28), skilled labor (n = 40), and unskilled labor or homemaker (n = 13). On average, children were seen for neuropsychological assessment at days after injury (SD = 60.79, Mdn = 61.50, Range = ) at an age of years (SD = 2.32, Mdn = 14.69, Range = ). Most of the participants had been injured in motor vehicle accidents as passengers (n = 45), pedestrians (n = 15), cyclists (n = 7), or drivers (n = 10). Other injury circumstances included falls (n = 11), recreational accidents (n = 8), and other (n = 4). In light of the fact that Glasgow Coma Scale (Teasdale & Jennett, 1974) scores were quite variable within the first 24 hr after injury in some cases and because measures of posttraumatic amnesia were not reliable because of a need for retrospective estimation in many cases, injury severity was defined on the basis of length of coma (i.e., time until there was a meaningful response to verbal commands) and results from neuroimaging of the brain in acute care. Children with mild TBI (n = 35) had length of coma less than 24 hr and no evidence for an acute intracranial lesion on CT or MRI scan. Children with moderate-severe TBI (n = 65) had either coma of at least 24 hr (n = 32) or evidence for an intracranial lesion on neuroimaging (n = 63) or both. In the latter group, mean length of coma was 2.90 days (SD = 5.18, Mdn = 0.50, Range = 0 25). Lesion localization was coded by the first author based on neuroimaging reports with subsequent consultation with the original radiologist or attending physician in case of lack of clarity. There were 23 children with diffuse lesions such as edema or axonal shearing. Fifty-five children had focal lesions such as cortical contusion or intracerebral hemorrhage with 12 of these children also having a diffuse lesion. Focal lesion localization was classified along four dimensions that were not mutually exclusive (to account for lesions that overlapped regions): left hemisphere (n = 31), right hemisphere (n = 34), frontal (n = 33), and extrafrontal (n = 24). Because many children had multifocal or bilateral lesions and the focus of this investigation was not primarily on matching lesion localization to outcome, we decided to use for the statistical analyses only two neuroimaging variables: presence or absence of a diffuse intracranial lesion and presence or absence of a focal intracranial lesion. Materials The WISC-III is a widely used, comprehensive measure of psychometric intelligence that yields four factor index scores (M = 100, SD = 15). In the complete sample, average performance on these indexes was as follows: verbal comprehension = (SD = 14.19), perceptual organization = (SD = 16.17), freedom from distractibility = (SD = 14.12), and processing speed (PS) = (SD = 17.74). Higher scores reflect better performance on all of these variables. The CVLT-C is an individually administered test of a child s ability to learn and remember verbally presented information, specifically a shopping list containing five items from each of three semantic categories. Although numerous indexes can be obtained from the CVLT C, the main variable of interest for the current investigation was the summary T score (M = 50, SD = 10), reflecting a global index of immediate free recall during successive trials with higher scores reflecting better performance. In the complete sample, the mean composite T score on the CVLT C was (SD = 12.82). The CCT is an individually administered test of a child s ability to solve problems on the basis of corrective feedback ( right / wrong ) about each attempt at identifying the conceptual rule underlying visually presented test items. The total number of errors on this task can be converted into a T score (M = 50, SD = 10) with higher scores reflecting better performance. In the complete sample, the mean composite T score on the CCT was (SD = 9.75).

4 278 ASSESSMENT TABLE 1 Zero-Order Correlations Between All Independent and Dependent Variables Days of coma 0.31* *** 0.33** 0.53*** 0.51*** Diffuse lesion *** 0.33** Focal lesion ** Parental occupation ** * Ethnicity * Gender ** 0.29* VC 0.63*** 0.54*** 0.40*** 0.39*** PO 0.49*** 0.65*** 0.54*** 0.42*** 9. FD 0.45*** 0.39*** 0.26* 10. PS 0.63*** 0.36** 11. CVLT C 0.28* 12. CCT NOTE: VC = verbal comprehension; PO = perceptual organization; FD = freedom from distractibility; PS = processing speed; CVLT C = California Verbal Learning Test Children s Version; CCT = Children s Category Test. *p <.01. **p <.001. ***p < Procedure The WISC III, CVLT C, and CCT were administered in a standardized manner as part of neuropsychological evaluations that were requested in the context of rehabilitation, most often (84%) on an outpatient basis. Evaluations were performed only when the children were medically stable and could recall meaningful information from day to day. Data Analyses A series of regression analyses was conducted to examine the relative contributions of neurological variables (coma, diffuse lesion, and focal lesion) and demographic variables (parental occupational status, ethnicity, and gender) to each of the six dependent variables: the four WISC III factor indexes and the T scores on the CVLT C and the CCT. The variable ethnicity was dichotomized to have sufficient numbers in each cell. Specifically, ethnicity was treated as Caucasian versus other. All independent variables were entered into the models as a block. Consistent with previously offered recommendations (Harrell, 2001; Millis, 2003), it was decided to leave both statistically significant and nonsignificant variables in the resulting models to allow the reader to determine independently the relative contributions of various neurological and demographic factors. Because we had deliberately chosen a restricted age range for this study to have exactly the same psychometric tests for all children, we did not anticipate an effect of age-related variables and therefore did not include them in the regression analyses. Table 1 presents the zero-order correlations between all the independent and dependent variables. Because of the multitude of independent computations, it was decided a priori that only those correlations that were statistically significant at p <.01 would be considered meaningful. Inspection of Table 1 suggests that among the independent variables there was only one correlation that was statistically significant. As would be expected, children with diffuse lesions on neuroimaging tended to have longer lengths of coma. The absence of other strong correlations among the independent variables was considered to be desirable to avoid multicollinearity. However, for all of the regression models, variance inflation factors were also considered to rule out collinearity effects because those cannot always be ascertained from bivariate correlations only (Fox, 1991). To interpret proportions of variance explained by the regression models we used the criteria suggested by Murphy and Myors (2004), who suggested that R 2 values less than.10 can be classified as small, R 2 values between.10 and.25 as medium, and R 2 values greater than.25 as large. Possible moderating and mediating effects were evaluated post hoc by exploring, respectively, interaction terms and the degree of attenuation of the relationships between independent and dependent variables consistent with previous research (Baron & Kenny, 1986; Coscia et al., 2001). RESULTS In light of the fact that sample averages can obscure the proportions of participants with clinically significant deficits, we first calculated for each psychometric test the numbers of children who obtained standardized scores below the 10th percentile rank (i.e., WISC III factor index scores at or below 80; CCT or CVLT C T scores at or below 37). These findings are presented in Figure 1. Inspec-

5 Donders, Nesbit-Greene / DEMOGRAPHIC VARIABLES 279 FIGURE 1 Proportions of Children Performing Below the 10th Percentile for Age on Psychometric Tests % VC PO FD PS CVLT-C CCT Test variable TABLE 3 Regression Model for WISC III Perceptual Organization Days of coma <.001 Diffuse lesion >.10 Focal lesion >.10 Parental occupational status >.10 Ethnicity <.05 Gender >.10 NOTE: WISC III = Wechsler Intelligence Scale for Children Third Edition; SRC = standardized regression coefficient. Statistically significant NOTE: VC = verbal comprehension; PO = perceptual organization; FD = freedom from distractibility; PS = processing speed; CVLT C = California Verbal Learning Test Children s Version; CCT = Children s Category Test. TABLE 2 Regression Model for WISC III Verbal Comprehension Days of coma >.10 Diffuse lesion >.10 Focal lesion >.10 Parental occupational status <.01 Ethnicity <.05 Gender >.10 NOTE: WISC III = Wechsler Intelligence Scale for Children Third Edition; SRC = standardized regression coefficient. Statistically significant tion of Figure 1 suggests that scores in the clinically impaired range were particularly common for WISC III processing speed and for the CVLT C and relatively uncommon for WISC III freedom from distractibility. The regression model for the WISC III verbal comprehension index is presented in Table 2. This model was statistically significant, F(6, 93) = 3.98, p <.01, and explained a medium amount of the variance (adjusted R 2 =.15). Higher parental occupational status and Caucasian ethnicity were associated with better performance on verbal comprehension. None of the neurological injury variables were statistically significant predictors of performance on this WISC III index. Table 3 presents the regression model for the WISC III perceptual organization index. This model was statistically significant, F(6, 93) = 4.87, p <.001, with a medium amount of the variance accounted for (adjusted R 2 =.19). Longer length of coma was associated with worse performance on perceptual organization whereas Caucasian ethnicity was associated with better performance on it. TABLE 4 Regression Model for WISC III Freedom From Distractibility Days of coma <.01 Diffuse lesion >.10 Focal lesion >.10 Parental occupational status <.01 Ethnicity >.10 Gender >.10 NOTE: WISC III = Wechsler Intelligence Scale for Children Third Edition; SRC = standardized regression coefficient. Statistically significant The regression model for the WISC III freedom from distractibility index is presented in Table 4. This model was statistically significant, F(6, 93) = 4.04, p <.001, with a medium amount of the variance accounted for (adjusted R 2 =.16). Longer length of coma and lower parental occupational status were associated with worse performance on freedom from distractibility. Table 5 presents the regression model for the WISC III processing speed index. This model was highly statistically significant, F(6, 93) = 13.34, p <.0001, and the amount of the variance accounted for was large (adjusted R 2 =.43). Longer length of coma, presence of a diffuse lesion on neuroimaging, and lower parental occupational status were all associated with worse performance on processing speed. At the same time, female gender was associated with better performance on this index. The regression model for the CVLT C is presented in Table 6. This model was also highly statistically significant, F(6, 93) = 9.77, p <.0001, and the amount of the variance accounted for was also large (adjusted R 2 = 35). Longer length of coma was associated with worse performance on this test of verbal learning and memory, whereas female gender was associated with better performance on it. Additional trends for the presence of diffuse and focal

6 280 ASSESSMENT TABLE 5 Regression Model for WISC III Processing Speed Days of coma <.0001 Diffuse lesion <.05 Focal lesion >.10 Parental occupational status <.05 Ethnicity >.10 Gender <.0001 NOTE: WISC III = Wechsler Intelligence Scale for Children Third Edition; SRC = standardized regression coefficient. Statistically significant TABLE 6 Regression Model for the California Verbal Learning Test Children s Version Days of coma <.0001 Diffuse lesion <.10 Focal lesion <.10 Parental occupational status >.10 Ethnicity >.10 Gender <.01 NOTE: SRC = standardized regression coefficient. Statistically significant intracranial lesions to have a detrimental effect on the CVLT C fell short of statistical significance. Table 7 presents the regression model for the CCT. The model for this test of problem solving with visual materials failed to reach statistical significance, F(6, 93) = 1.84, p >.10, and the amount of the variance accounted for was small (adjusted R 2 =.05). There was only a statistically nonsignificant trend for the presence of a diffuse lesion to have a negative effect on performance on the CCT. Because of the fact that several of the regression models included statistically significant effects for both neurological and demographic variables we wanted to investigate post hoc the possibility that demographic characteristics might moderate the relationship between injury severity and neuropsychological test performance. A moderator variable specifies the conditions under which a given predictor is related to an outcome (Holmbeck, 2002). In other words, the association between the predictor (e.g., length of coma) and an outcome variable (e.g., results on the WISC III or CVLT C) might depend on a moderating demographic variable (e.g., gender, parental occupational status). In light of previous research with a different memory test in a completely independent sample that suggested a potential moderating role of gender (Donders & Woodward, 2003), we specifically considered a possible moderating influence of gender on length of coma with regard to the effect of the latter variable on performance on either the WISC III processing speed index or the CVLT C. Independent, previous research has yielded conflicting findings about a possible moderating effect of socioeconomic status on outcome after pediatric TBI (Taylor et al., 2002; Yeates et al., 2002). In the current evaluation, parental occupational status was a statistically significant predictor only in the model for WISC III processing speed and not in that for the CVLT C. For these reasons, we wanted to consider a possible moderating influence of parental occupational status on length of coma with regard to the effect of the latter variable on WISC III processing speed. Prior to reanalyzing the regression data, we determined that TABLE 7 Regression Model for the Children s Category Test Days of coma >.10 Diffuse lesion <.10 Focal lesion >.10 Parental occupational status >.10 Ethnicity >.10 Gender >.10 NOTE: SRC = standardized regression coefficient. Statistically significant there were no systematic differences between boys and girls in length of coma, F(1, 98) = 0.61, p >.10. There was a slight gender difference in parental occupational status, χ 2 (3, N = 100) = 8.30, p <.05, associated with a small effect size (ϕ 2 =.08). Boys were more likely than girls to have parents with clerical occupations, whereas the distributions in the other three occupational categories were relatively more balanced. Consistent with the criteria suggested by Baron and Kenny (1986), we specified a priori that to demonstrate that gender or parental occupational status acts as a moderator variable, the interaction between either of these two variables and coma should account for a significant, unique proportion of the variance in processing speed or CVLT C scores after controlling for the effect of length of coma and gender. When either of these interaction terms was added to the model for processing speed presented in Table 5 or to the model for CVLT C presented in Table 6, there were no increases in the respective values of adjusted R 2 (within 0.01 of the original values), the interaction terms were not statistically significant predictors (p >.10) and problems with collinearity occurred as reflected in variance inflation factors greater than four (Fox, 1991). Problems with collinearity remained after centering all continuous predictor variables in the model as suggested by Holmbeck (2002). Thus, gender and parental

7 Donders, Nesbit-Greene / DEMOGRAPHIC VARIABLES 281 occupational status did not appear to meet the criteria for being a moderating variable. Finally, we also wanted to explore post hoc the possibility of mediating variables. According to Holmbeck (2002), a mediator is a variable that explains the process or the mechanism by which a predictor variable affects an outcome variable such that the predictor is associated with the mediator, which in turn affects the outcome measure. In one previous study in our laboratory, gender differences in performance on a composite index of verbal and visual memory were reduced to a statistically nonsignificant trend after taking into account performance on the WISC III processing speed index (Donders & Woodward, 2003). In light of the fact that the models with the largest amount of variance explained in the current investigation (i.e., WISC III processing speed and CVLT C) shared some of the same predictor variables (i.e., coma and gender) and also considering the fact that there was a considerable correlation in this sample between processing speed and CVLT C scores (see Table 1), it appeared plausible that performance on the CVLT C might be mediated by speed of information processing as reflected in the processing speed index. Consistent with the criteria suggested by Baron and Kenny (1986), we specified a priori that to demonstrate that processing speed acts as a mediator with regard to the effect of coma or gender on CVLT C the relationship between either of these latter two predictors and CVLT C should be attenuated significantly after controlling for the effect of processing speed. Adding processing speed to a model for CVLT C was not associated with collinearity (e.g., variance inflation factors < 2). The results are presented in Table 8. Inspection of this table suggests that the effect of gender was no longer statistically significant after controlling for the effect of processing speed. The influence of coma on CVLT C, although still statistically significant, was grossly attenuated after controlling for the effect of processing speed. The associated drop in variance accounted for by coma was statistically significant (z = 4.40, p <.01). These findings suggest that the effects of both gender and coma on CVLT C were mediated by processing speed. DISCUSSION TABLE 8 Mediator Model for the Effect of Processing Speed on the CVLT C Semipartial R 2 Step 1 Days of coma < Gender < Step 2 Days of coma < Gender > Processing speed < NOTE: CVLT C = California Verbal Learning Test Children s Version; SRC = standardized regression coefficient. The purpose of this investigation was to determine the relative contribution of neurological and demographic variables to performance on three widely used psychometric tests in children with TBI. The results suggest that none of the dependent variables were predicted by neurological injury parameters only, one was predicted by demographic variables only, and the majority of the regression models included both neurological and demographic variables. Thus, demographic variables appeared to explain incremental variance in performance on a wide range of psychometric measures after pediatric TBI even after accounting for injury severity characteristics. However, no evidence was found for a moderating effect of demographic variables with regard to the effect of TBI on neuropsychological test performance in children in the 9 to 16 years age range. At the same time, speed of information processing appeared to mediate the association between demographic and neurological variables on one hand and performance on a test of verbal learning and memory on the other hand. The regression models for the two psychometric indexes with the relatively greatest amount of the variance accounted for, WISC III processing speed and CVLT C, included both neurological and demographic variables. In both cases, longer length of coma was associated with worse performance whereas at the same time gender explained incremental variance in outcome with girls doing better than boys. Among the WISC III variables, processing speed has been reported in the literature as being the relatively most sensitive to injury characteristics (Donders & Warschausky, 1997; Tremont et al., 1999). Furthermore, the CVLT C has been shown to have considerable predictive validity with regard to educational outcome at 2 years postpediatric TBI (Miller & Donders, 2003). Thus, of all the psychometric indexes used in this investigation, these two appear to be relatively most relevant in the evaluation of sequelae of pediatric TBI, and it is precisely with these variables that demographic characteristics have an effect beyond the effect of injury severity. Whether the influence of gender in this regard is related to differences in cerebral organization (McGlone, 1980), speed of recovery (Raz et al., 1995), hormonal influences (Geschwind & Galaburda, 1985; Stein, 2001) or other factors needs to be determined in future research. We did not find evidence to suggest that gender moderated the effect of neurological variables in this investigation but rather that the effect of gender was additive and not interactive. In previous research (Donders & Woodward, 2003) we did find evi-

8 282 ASSESSMENT dence for such a moderating role, but that study also involved younger children (6 8 years) and a different memory test. Whether a potentially moderating effect of gender occurs only at younger ages or whether it is related to neurological variables that could not be evaluated in the current investigation (e.g., cerebral perfusion pressure) needs to be determined in future research. At this time, the current findings simply suggest that demographic factors should not be ignored when evaluating children with TBI and that boys may be at relatively increased risk for deficits in speed of information processing and verbal learning. Ethnicity and parental occupational status affected performance on some of the psychometric indexes in this investigation. Most notably, WISC III verbal comprehension did not demonstrate any significant sensitivity to neurological injury, but both Caucasian ethnicity and higher parental occupational status were associated with better performance on this index. This may suggest that the influence of ethnicity cannot be explained completely on the basis of socioeconomic factors. Moreover, although premorbid ethnic or socioeconomic disadvantage tended to suppress psychometric test performance after pediatric TBI, this influence did not occur primarily on those variables that were relatively most affected by injury severity and for which the greatest amount of the variance could be accounted for (e.g., CVLT C). It is important to realize that this investigation used a sample with a restricted age range (i.e., 9 16 years). Most studies have found that TBI is of relatively greatest concern with children who are much younger (Anderson et al., 1997; Chapman & McKinnon, 2000; Ewing-Cobbs et al., 1997; Taylor & Alden, 1997; Woodward et al., 1999). The current findings suggest that with older children and appropriately age-normed instruments, demographic variables other than age still contribute to the variance in outcome of neuropsychological test performance after pediatric TBI. The model for one of the dependent variables, CCT, did not include any statistically significant predictors. Part of the problem with this test may be the fact that its summary T score is based on six different subtests that differ widely in their sensitivity to injury severity (Nesbit-Greene & Donders, 2002). In addition, the CCT does not make any demands on speed of performance while at the same time providing frequent corrective feedback to the child. It is quite possible that this kind of structure may artificially compensate for any executive difficulties that children with TBI may have, which raises concerns about ecological validity (Silver, 2000). The results from this investigation suggest that caution is needed with utilization of this test in the evaluation of cognitive sequelae after pediatric TBI. Finally, we found evidence for a role of speed of information processing as a mediator variable. In particular, the influences of both coma and gender on performance on the CVLT C were statistically significantly attenuated when WISC III processing speed was added to the model. This mediating effect of processing speed makes sense from a theoretical as well as a practical point of view. There is considerable research that has demonstrated that increases in speed of information processing are associated with greater accuracy in recall (Bjorklund & Douglas, 1997). On the CVLT C in particular, the stimuli are presented at a fairly rapid pace (about one word per second), and children who are slow at processing the information may simply not be able to encode the words elaborately or deep enough to facilitate later retrieval. Potential limitations of this investigation must also be considered. This sample was comprised of older children, and the current findings cannot be generalized to children under the age of 9 years. There were not enough children in various ethnic minority groups to allow us to analyze them separately. Thus, we cannot rule out the possibility of a moderating effect of specific ethnic minority background. We used a referred convenience sample that included relatively more participants with severe injuries than would likely have been found with consecutive emergency room admissions. However, this also guarded against restriction of range effects on the dependent variables. A more significant limitation is that we had to rely on radiologists reports for the neuroimaging data, and we did not have the opportunity to do more specific morphometric or volumetric lesion analyses, which have shown some promise in recent research (Bigler, 1999; Levin, Benavidez, et al., 2000; Verger et al., 2001). The vast majority (82%) of the neuroimaging reports also pertained to CT instead of MRI scans, and the latter would likely have shown more anatomical detail. Thus, although length of coma and, to a lesser extent, diffuse lesions clearly appeared to be relevant factors in the current investigation, a potentially relevant contribution of specific focal lesions cannot be ruled out on the basis of these findings. We also did not include variables of family characteristics such as burden or coping style, which have recently been shown to have a moderating effect with regard to outcome after pediatric TBI (Taylor et al., 2002; Yeates et al., 2002). Thus, future research should evaluate the relative contribution of family stress and family dynamics in concert with basic demographic (e.g., ethnicity, gender, socioeconomic status) and neurological (e.g., coma, focal and diffuse intracranial lesions) variables on neuropsychological test performance after TBI. With these reservations in mind, the current findings suggest that performance on a wide range of psychometric variables after pediatric TBI is affected by both neurologi-

9 Donders, Nesbit-Greene / DEMOGRAPHIC VARIABLES 283 cal and demographic variables. Gender in particular appears to have an incremental effect on those measures (CVLT C, WISC III processing speed) that are most sensitive to injury severity and that also tap into abilities that are important for future school success such as new learning and fast and efficient performance. Moreover, speed of information processing appears to function as a variable that mediates the effect of both neurological and demographic variables on new learning after pediatric TBI. Although our findings may not necessitate that each and every single test be stratified by ethnicity, gender, and socioeconomic status, they do suggest that demographic factors need to be considered in the long-term follow-up of these children. 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