The internal and external validity of sluggish cognitive tempo and its relation with DSM-IV ADHD.

Studies of subtypes of DSM-IV attention-deficit/hyperactivity disorder (ADHD) have provided inconsistent support for the discriminant validity of the combined type (ADHD-C) and predominantly inattentive type (ADHD-I). A large sample of children and adolescents with ADHD (N = 410) and a comparison group without ADHD (N = 311) were used to test the internal and external validity of sluggish cognitive tempo (SCT), a dimension characterized by low energy and sleepy and sluggish behavior. SCT scores were then incorporated in analyses of ADHD subtypes to test whether the discriminant validity of ADHD-C and ADHD-I could be improved by including SCT symptoms as part of the criteria for ADHD-I. Factor analyses of parent and teacher ratings indicated that six SCT items loaded on a factor separate from symptoms of ADHD and other psychopathology, providing important support for the internal validity of SCT. The external validity of SCT was supported by significant associations between SCT and measures of functional impairment and neuropsychological functioning when symptoms of ADHD and other psychopathology were controlled. However, contrary to initial predictions, high levels of SCT did not identify a subgroup of ADHD-I that was clearly distinct from ADHD-C. Instead, the current results suggest that DSM-IV inattention and SCT are separate but correlated symptom dimensions that are each independently associated with important aspects of functional impairment and neuropsychological functioning.

Colorado Learning Difficulties Questionnaire: validation of a parent-report screening measure.

This study evaluated the internal structure and convergent and discriminant evidence for the Colorado Learning Difficulties Questionnaire (CLDQ), a 20-item parent-report rating scale that was developed to provide a brief screening measure for learning difficulties. CLDQ ratings were obtained from parents of children in 2 large community samples and 2 samples from clinics that specialize in the assessment of learning disabilities and related disorders (total N = 8,004). Exploratory and confirmatory factor analyses revealed 5 correlated but separable dimensions that were labeled reading, math, social cognition, social anxiety, and spatial difficulties. Results revealed strong convergent and discriminant evidence for the CLDQ Reading scale, suggesting that this scale may provide a useful method to screen for reading difficulties in both research studies and clinical settings. Results are also promising for the other 4 CLDQ scales, but additional research is needed to refine each of these measures.

Etiology and neuropsychology of comorbidity between RD and ADHD: the case for multiple-deficit models.

INTRODUCTION: Attention-deficit/hyperactivity disorder (ADHD) and reading disability (RD) are complex childhood disorders that frequently co-occur, but the etiology of this comorbidity remains unknown. METHOD: Participants were 457 twin pairs from the Colorado Learning Disabilities Research Center (CLDRC) twin study, an ongoing study of the etiology of RD, ADHD, and related disorders. Phenotypic analyses compared groups with and without RD and ADHD on composite measures of six cognitive domains. Twin analyses were then used to test the etiology of the relations between the disorders and any cognitive weaknesses. RESULTS: Phenotypic analyses supported the hypothesis that both RD and ADHD arise from multiple cognitive deficits rather than a single primary cognitive deficit. RD was associated independently with weaknesses on measures of phoneme awareness, verbal reasoning, and working memory, whereas ADHD was independently associated with a heritable weakness in inhibitory control. RD and ADHD share a common cognitive deficit in processing speed, and twin analyses indicated that this shared weakness is primarily due to common genetic influences that increase susceptibility to both disorders. CONCLUSIONS: Individual differences in processing speed are influenced by genes that also increase risk for RD, ADHD, and their comorbidity. These results suggest that processing speed measures may be useful for future molecular genetic studies of the etiology of comorbidity between RD and ADHD.

Neuropsychological analyses of comorbidity between reading disability and attention deficit hyperactivity disorder: in search of the common deficit.

Measures of component reading and language skills, executive functions, and processing speed were administered to groups of children with attention deficit hyperactivity disorder (ADHD; n = 113), reading disability (RD; n = 109), both RD and ADHD (n = 64), and neither RD nor ADHD (n = 151). Groups with RD exhibited pronounced deficits on all measures of component reading and language skills, as well as significant weaknesses on measures of verbal working memory, processing speed, and response inhibition. Groups with ADHD exhibited weaknesses on all response-inhibition and processing speed tasks and were impaired on some measures of component reading skills and verbal working memory. The group with comorbid RD and ADHD exhibited the combination of the deficits in the RD-only and ADHD-only groups, providing evidence against the phenocopy and cognitive subtype hypotheses as explanations for the co-occurrence of RD and ADHD. Slow and variable processing speed was characteristic of all 3 clinical groups, suggesting that measures of this domain may be useful for future studies that search for the common genes that increase susceptibility to RD and ADHD.

Etiology of the comorbidity between RD and ADHD: exploration of the non-random mating hypothesis.

The present research sought to test the validity of the non-random mating hypothesis in a community sample in order to determine whether it contributes significantly to the comorbidity between attention-deficit/hyperactivity disorder (ADHD) and reading disability (RD) and to test whether the two disorders segregate independently. Data were gathered from 394 twin participants of the Colorado Learning Disabilities Research Center (CLDRC) and their biological parents. Parents were asked to complete retrospective questionnaires regarding their own ADHD and RD symptomatology as children, as well as rate their children on DSM-IV ADHD symptomatology before age 12. Twin RD was assessed by a composite score consisting of several reading measures. Chi-square and correlational analyses assessed the presence of non-random mating, as well as whether the two disorders were transmitted independently. Non-random mating between those with ADHD and those with RD did not significantly contribute to the comorbidity of the two disorders. Furthermore, there was some evidence that these two disorders did not segregate independently in this sample. These results are consistent with recent evidence that ADHD and RD are often comorbid because of significant genetic overlap. There was little evidence to support that non-random mating also contributes to this comorbidity in the present sample.