Neurocognitive Functioning Mediates the Prospective Association of Birth Weight With Youth ADHD Symptoms

Abstract

Although birth weight is a potential causal risk factor for attention-deficit/hyperactivity disorder (ADHD) symptoms, both the specificity of this association and its mediating pathways are largely unknown. We carefully assessed youth with and without ADHD (i.e., Wave 1), and followed them prospectively for 2 years (i.e., Wave 2). We (a) tested the association of birth weight with Wave 2 ADHD symptoms, and (b) evaluated biologically plausible neurocognitive functions from Wave 1 as temporally ordered mediators of birth weight and Wave 2 ADHD symptoms in a multiple mediation framework. At Wave 1, 222 ethnically diverse youth (30% female; ages 5–10) completed the Digit Span, Vocabulary, Symbol Search, and Arithmetic subtests of the Wechsler Intelligence Scale for Children–IV. At both Wave 1 and Wave 2 (ages 7–13), multiple informants (i.e., parents, teachers) rated youth ADHD symptoms and co-occurring psychopathology using multiple methods (i.e., structured interview, rating scale). Controlling for demographic factors, gestational age, and co-occurring externalizing and internalizing psychopathology, birth weight inversely predicted Wave 2 ADHD symptoms across multiple methods and informants. Additionally, controlling for Wave 1 ADHD symptoms and relevant covariates, Wave 1 Arithmetic uniquely mediated the association of birth weight with multi-method/informant Wave 2 ADHD symptoms. These findings suggest that birth weight is a relatively specific risk factor for youth ADHD symptoms and they implicate individual differences in fluid reasoning as a preliminary causal mediator of this association. We discuss implications for future research evaluating causal mechanisms underlying risk factors for ADHD.

Fluid Reasoning Mediates the Association of Birth Weight With ADHD Symptoms in Youth From Multiplex Families With ADHD

“…In particular, the previous study investigated WISC mediators at ages 5 to 10 and ADHD at ages 7 to 13 in unrelated youth with and without ADHD as well as controlled for co-occurring internalizing and externalizing symptoms (Morgan et al, 2016). In contrast, the present study extended hypotheses to a larger sample consisting of siblings from a broad age range with high genetic load for ADHD (although age was controlled in all analyses), and controlled for IQ.…”

“…To our knowledge, however, only one study evaluated temporally ordered mediators of birth weight and ADHD symptoms, which is necessary to infer causal mediation (Kraemer, Stice, Kazdin, Offord, & Kupfer, 2001). Specifically, baseline scores on the Arithmetic subtest of the Wechsler Intelligence Scale for Children (WISC) at ages 5 to 10 uniquely mediated the association of birth weight with multi-method/informant ADHD symptoms at a 2-year follow-up (ages 7-13), controlling for demographic factors, co-occurring psychopathology, and baseline ADHD symptoms (Morgan et al, 2016). Indirect effects of other WISC subtests (e.g., Digit Span, Vocabulary) were not significant.…”

Pathways from Birth Weight to ADHD Symptoms through Fluid Reasoning in Youth with or without Intellectual Disability

“…Moreover, the observed indirect effect through Arithmetic replicates evidence from separate case-control and sibling studies of ADHD, including similar effect sizes for the indirect effect; this is especially notable given important differences across all studies. Specifically, two previous studies discovered an indirect pathway from birth weight to dimensional measures of ADHD symptoms through Arithmetic, but in ADHD-oriented samples of youth with cognitive development in the typical range: (1) a case-control ADHD study (youth ages 5–10 at baseline and 7–13 at follow-up) that statistically controlled for co-occurring internalizing and externalizing symptoms (Morgan, Loo et al, 2016), and (2) a study of siblings from a broad age range with high genetic load for ADHD that controlled for youth IQ (Morgan, Lee et al, 2016). Importantly, the present study extended hypotheses to a prospective longitudinal sample of youth with or without ID, suggesting that ADHD symptoms have similar etiologies from birth weight in youth with ID and youth with typical cognitive abilities.…”

“…Although the CBCL and TRF are not diagnostic measures of ADHD, they are (1) highly correlated with DSM-based symptom measures, (2) extensively validated with excellent reliability and validity (e.g., test-retest reliability of .92 and 95 for the CBCL and TRF Attention Problems scale in a normative sample, ability to discriminate between referred and non-referred youth), and (3) can be easily combined as a single multi-informant measure to conservatively reduce the number of tests (Achenbach & Rescorla, 2001). Thus, similar to prior studies on mediation of birth weight and ADHD symptoms (Morgan, Loo et al, 2016), we used a mean composite of parent and teacher reported T scores from the Attention Problems scale (age 9 r s = .65, p < .001; age 13 r s = .45, p < .001), which includes inattention and hyperactivity/impulsivity items. Parent ratings were used exclusively when teacher data were missing (age 9 n = 38; age 13 n = 52), given that youth with teacher data were similar to youth without teacher data with respect to demographic factors, ID status, birth weight, fluid reasoning, and ADHD symptoms (χ 2 < 7.76 for all tests with categorical variables, Z < 1.42 for all tests with continuous variables, p > .10 for all tests with either continuous or categorical variables, suggesting that teacher data were missing at random).…”

“…Beyond its biological plausibility, fluid reasoning uniquely mediated (i.e., beyond other neurocognitive constructs) predictions of dimensionally-measured ADHD symptoms from birth weight in two prior studies, although both studies focused on youth with IQs above 70. First, the Arithmetic subtest of the Wechsler Intelligence Scale for Children (WISC; Wechsler, 2003), a measure of fluid reasoning (Keith, Fine, Taub, Reynolds, & Kranzler, 2006; Weiss, Keith, Zhu, & Chen, 2013), mediated birth weight and multi-method/informant ADHD symptoms in a prospective longitudinal study of 222 youth with and without ADHD (ages 5–10 years at baseline, 7–13 years at follow-up; M IQ = 106.88, SD = 14.55, range = 73–144), controlling for demographic factors and co-occurring internalizing and externalizing psychopathology (Morgan, Loo, & Lee, 2016). Indirect effects of other WISC subtests tested alongside Arithmetic (i.e., Digit Span, Vocabulary, Symbol Search) were not significant.…”