Confirmation of an inhibitory control deficit in attention-deficit/hyperactivity disorder.

The objective of this study was to determine whether deficient inhibitory control distinguishes children with a diagnosis of attention-deficit/hyperactivity (ADHD) disorder, conduct disorder (CD), and comorbid ADHD + CD from normally developing children. Participants were rigorously diagnosed children (age 7 to 12 years) with ADHD (N = 72), CD (N = 13) or ADHD + CD (N = 47) and 33 control children (NC). We studied inhibitory control using the stop-signal paradigm, a laboratory task that assessed the ability to inhibit an ongoing action. The ADHD group had significantly impaired inhibitory control compared to NC, CD, and ADHD + CD children. These results indicate that children with ADHD have deficient inhibition as measured in the stop-signal paradigm and that ADHD occurring in the presence of ADHD + CD may represent a phenocopy of CD rather than a variant of ADHD.

Neuropsychological function in mild hyperphenylalaninemia.

Whether specific cognitive deficits related to frontal-lobe dysfunction that have been reported in individuals with phenylketonuria (PKU) are also characteristic of mild hyperphenylalaninemia (MHP) was investigated. Tests of executive function and control tasks not assessing executive function were administered to a group of individuals with MHP and a group without MHP, similar in age, gender, and IQ. Tests of academic skills and behavior-rating questionnaires were also administered to the group with MHP. No group differences were found for any measure, suggesting that the mild elevations of phenylalanine in individuals with MHP are not sufficient to produce behavioral and cognitive impairments characteristic of PKU.

Randomised controlled trial of tyrosine supplementation on neuropsychological performance in phenylketonuria.

OBJECTIVE: To test the efficacy of tyrosine supplementation, as an adjunct to dietary treatment, on neuropsychological test performance in individuals with phenylketonuria. DESIGN: A randomised controlled trial of tyrosine supplementation using a double blind crossover procedure with three four week phases. SETTING: The Hospital for Sick Children, Toronto. PARTICIPANTS: 21 individuals with phenylketonuria (ages 6 to 28 years, mean 11.3). INTERVENTION: Participants were given 100 mg/kg body weight/d of L-tyrosine or L-alanine (placebo). RESULTS: At baseline, performance on several of the neuropsychological test measures was correlated with tyrosine levels. Dietary supplements of tyrosine increased plasma tyrosine concentrations; however, no change in test performance was found across the tyrosine and placebo phases of the study. CONCLUSIONS: Tyrosine supplementation of this type does not appear to alter neuropsychological performance in individuals with phenylketonuria.