Social behaviour and task orientation in early-treated PKU

Response variability in Attention-Deficit/Hyperactivity Disorder: a neuronal and glial energetics hypothesis

BACKGROUND

Current concepts of Attention-Deficit/Hyperactivity Disorder (ADHD) emphasize the role of higher-order cognitive functions and reinforcement processes attributed to structural and biochemical anomalies in cortical and limbic neural networks innervated by the monoamines, dopamine, noradrenaline and serotonin. However, these explanations do not account for the ubiquitous findings in ADHD of intra-individual performance variability, particularly on tasks that require continual responses to rapid, externally-paced stimuli. Nor do they consider attention as a temporal process dependent upon a continuous energy supply for efficient and consistent function. A consideration of this feature of intra-individual response variability, which is not unique to ADHD but is also found in other disorders, leads to a new perspective on the causes and potential remedies of specific aspects of ADHD.

THE HYPOTHESIS

We propose that in ADHD, astrocyte function is insufficient, particularly in terms of its formation and supply of lactate. This insufficiency has implications both for performance and development: H1) In rapidly firing neurons there is deficient ATP production, slow restoration of ionic gradients across neuronal membranes and delayed neuronal firing; H2) In oligodendrocytes insufficient lactate supply impairs fatty acid synthesis and myelination of axons during development. These effects occur over vastly different time scales: those due to deficient ATP (H1) occur over milliseconds, whereas those due to deficient myelination (H2) occur over months and years. Collectively the neural outcomes of impaired astrocytic release of lactate manifest behaviourally as inefficient and inconsistent performance (variable response times across the lifespan, especially during activities that require sustained speeded responses and complex information processing).

TESTING THE HYPOTHESIS

Multi-level and multi-method approaches are required. These include: 1) Use of dynamic strategies to evaluate cognitive performance under conditions that vary in duration, complexity, speed, and reinforcement; 2) Use of sensitive neuroimaging techniques such as diffusion tensor imaging, magnetic resonance spectroscopy, electroencephalography or magnetoencephalopathy to quantify developmental changes in myelination in ADHD as a potential basis for the delayed maturation of brain function and coordination, and 3) Investigation of the prevalence of genetic markers for factors that regulate energy metabolism (lactate, glutamate, glucose transporters, glycogen synthase, glycogen phosphorylase, glycolytic enzymes), release of glutamate from synaptic terminals and glutamate-stimulated lactate production (SNAP25, glutamate receptors, adenosine receptors, neurexins, intracellular Ca2+), as well as astrocyte function (alpha1, alpha2 and beta-adrenoceptors, dopamine D1 receptors) and myelin synthesis (lactate transporter, Lingo-1, Quaking homolog, leukemia inhibitory factor, and Transferrin).

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis extends existing theories of ADHD by proposing a physiological basis for specific aspects of the ADHD phenotype - namely frequent, transient and impairing fluctuations in functioning, particularly during performance of speeded, effortful tasks. The immediate effects of deficient ATP production and slow restoration of ionic gradients across membranes of rapidly firing neurons have implications for daily functioning: For individuals with ADHD, performance efficacy would be enhanced if repetitive and lengthy effortful tasks were segmented to reduce concurrent demands for speed and accuracy of response (introduction of breaks into lengthy/effortful activities such as examinations, motorway driving, assembly-line production). Also, variations in task or modality and the use of self- rather than system-paced schedules would be helpful. This would enable energetic demands to be distributed to alternate neural resources, and energy reserves to be re-established. Longer-term effects may manifest as reduction in regional brain volumes since brain areas with the highest energy demand will be most affected by a restricted energy supply and may be reduced in size. Novel forms of therapeutic agent and delivery system could be based on factors that regulate energy production and myelin synthesis. Since the phenomena and our proposed basis for it are not unique to ADHD but also manifests in other disorders, the implications of our hypotheses may be relevant to understanding and remediating these other conditions as well.

State regulation and response inhibition in children with ADHD and children with early- and continuously treated phenylketonuria: an event-related potential comparison

BACKGROUND

The presentation rate of stimuli plays an important role in explaining the performance inefficiency in children with ADHD. In general, children with ADHD have been found to perform more poorly in conditions of relatively slow event rates as compared with fast and moderate event rates. The state regulation hypothesis states that these children have problems in correcting their energetic state necessary to counteract a performance decrement, which requires extra effort allocation. In this study, we investigated state regulation in children with ADHD and used children with early- and continuously treated phenylketonuria (PKU) as a clinical contrast group.

METHOD

We measured the parietal P3 during a Go/No-Go task that incorporated a condition with a fast and a slow presentation rate.

RESULTS

We were able to show that children with ADHD, relative to controls, responded more slowly and more variably in the slow condition only, which was accompanied by a smaller P3, suggesting less effort allocation. In contrast, the children with PKU did not show a state regulation deficit. The PKU group showed prolonged stimulus evaluation processing, as indexed by P3 latency, compared to controls and children with ADHD. In addition, they made more errors of commission than the controls and the ADHD group.

CONCLUSIONS

Our electrophysiological data support the state regulation hypothesis of ADHD. Only the children with PKU had more problems in inhibiting pre-potent responding than controls, which is in accord with the prefrontal dysfunction hypothesis of PKU.

Prevalence of stimulant use for attentional dysfunction in children with phenylketonuria

Recent data suggest that children with phenylketonuria (PKU) and poor metabolic control may have an increased prevalence of attentional dysfunction. However, few formal studies have addressed this topic in detail. We reviewed the medical records of 38 school-aged children with early and continuously treated PKU to determine the prevalence of stimulant use for attentional dysfunction, and to determine the relationship between metabolic control and attentional symptoms. Twenty-six per cent of the PKU children used a stimulant medication for attentional dysfunction. This is significantly higher than in an age- and sex-matched control group consisting of children with type I diabetes mellitus (6.5%, p <0.006), and also considerably higher than population norms for attention deficit hyperactivity disorder (ADHD) (5%). We also found a significant relationship between phenylalanine levels and stimulant use or attentional symptoms. Mean plasma phenylalanine concentration was 486 micromol/L in the non-stimulant-using group and 792 micromol/L in the stimulant-using group (p <0.02). Mean phenylalanine concentration was 462 micromol/L in the group not reporting attentional symptoms, and was 702 micromol/L in the symptomatic group (p <0.05). Parents of the stimulant-using children felt that the stimulants were efficacious in treating their child's attentional symptoms. Stimulant use and parent reports of attentional dysfunction are quite common in our PKU patients and appear to be strongly related to higher phenylalanine concentrations.

Behaviour and school achievement in patients with early and continuously treated phenylketonuria

Thirty patients with early and continuously treated phenylketonuria (PKU) between 8 and 20 years of age were compared with 30 controls, matched individually for age, sex, and educational level of both parents, on behaviour rating scales for parents and teachers as well as a school achievement scale. PKU patients, as a group, demonstrated more problems in task-oriented behaviour and average academic performance than did matched controls. Interestingly, whereas male PKU patients were rated significantly lower on introversion by their teachers, female patients were rated significantly higher on introversion and lower on extraversion than matched controls. This sex difference was also reflected in the relationship between measures of dietary control and the behaviour clusters, suggesting that male and female patients respond differently to elevated Phe levels or the stress associated with PKU. The teacher rating on average academic performance of the PKU patients was associated with recent level of dietary control, which suggests that it might be improved by more strict adherence to the diet. In addition, academic performance correlated negatively with the behaviour cluster negative task orientation. Further studies are recommended to obtain a more complete evaluation of this relationship and to replicate the current findings on larger samples. Over the years a number of studies have examined behaviour and school achievement in patients with early treated phenylketonuria (PKU; McKusick 261600). In general, these studies have found that despite early treatment with a phenylalanine (Phe)-restricted diet, PKU patients demonstrate more behavioural and school problems than do healthy controls. The behaviour problems include both internalizing symptoms (e.g. solitary, unresponsive, anxious, depressed mood: Pietz et al 1997; Smith et al 1988; Weglage et al 1992) and externalizing symptoms (e.g. hyperactive, talkative, impulsive, restless: Hendrikx et al 1994; Kalverboer et al 1994; Realmuto et al 1986; Smith et al 1988), but not antisocial or socially negative symptoms (e.g. lying, teasing, disobedience: Kalverboer et al 1994; Pietz et al 1997; Smith et al 1988). With respect to school achievement, studies have shown that patients with early treated PKU more often repeat classes or need special tutoring (Berry et al 1979; Brunner et al 1983; Koch et al 1987; Rey et al 1996; Verkerk 1995), have to work harder than healthy controls to achieve the same results (Weglage et al 1993), or have specific deficits in arithmetic achievement scores (Azen et al 1991; Berry et al 1979; Fishler et al 1987; Koch et al 1987; Weglage et al 1993). Nevertheless, many questions regarding the behavioural and school problems of patients with early treated PKU remain unanswered. For instance, the relationship between behavioural and school problems on the one hand and levels of dietary control on the other is still relatively unclear. The few studies that examined this relationship, have focused primarily on children in primary school (Azen et al 1991; Koch et al 1987; Smith et al 1988). Furthermore, although several psychological studies have shown that the pattern of behavioural problems varies by sex (see Prior et al 1999a for a discussion), so far very few studies have examined this issue in PKU patients and results are contradictory (Kalverboer et al 1994; Pietz et al 1997; Smith et al 1988; Weglage et al 1992). In addition, so far no study has actually examined whether there is a relationship between the behavioural problems and school difficulties of PKU patients, even though this relationship has been well documented in the psychological literature (Prior et al 1999b; Richards et al 1995). The aim of the present study is therefore to examine these issues in patients with early and continuously treated PKU over a wide age range and in relation to dietary control. More specifically, school achievement as well as social and task-oriented behaviour (at home

Information processing in patients with early and continuously-treated phenylketonuria

A total of 33 patients with early and continuously-treated phenylketonuria (PKU) between 7 and 16 years of age and 33 matched controls participated in a study examining perceptual, central, and response-related mechanisms of information processing. The specific mechanisms studied were: perceptual filtering, memory search, response selection, response execution, and motor presetting. In addition, groups were compared on mean intelligence level and task oriented behaviour. The performance of the PKU patients practically matched that of the controls on all three tasks, suggesting that PKU patients who are continuously maintained on a well-controlled phenylalanine-restricted diet are not impaired in the elementary mechanisms of information processing. Furthermore, groups did not differ in mean IQ or task-oriented behaviour.

CONCLUSION

These results underline the importance of continued, well-controlled dietary treatment. Further studies are recommended to obtain a more complete evaluation of the potential of PKU patients under these stricter dietary treatment conditions.