Executive function in treated phenylketonuria as measured by the one-back and two-back versions of the continuous performance test

Neuropsychological Profile of Children with Early and Continuously Treated Phenylketonuria: Systematic Review and Future Approaches

OBJECTIVE

To provide a comprehensive systematic review of the literature by examining studies published on all cognitive aspects of children with early and continuously treated phenylketonuria (ECT-PKU) included in the databases Medline, PsycINFO, and PsycARTICLE.

METHOD

In addition to a classical approach, we summarized methodology and results of each study in order to discuss current theoretical and methodological issues. We also examined recent advances in biochemical markers and treatments of PKU, with implications for future research on metabolic control and its role as a determinant of neuropsychological outcome.

RESULTS

Consistent with previous reviews, the hypothesis of a specific and central executive impairment in children with ECT-PKU was suggested. However, findings are inconclusive regarding the nature of executive impairments as well as their specificity, impact on everyday life, persistence over time, and etiology.

CONCLUSION

Given the current state of the science, we suggest future directions for research that utilizes a developmental and integrative approach to examine the effects of recent advances in biochemical markers and treatment of PKU. (JINS, 2019, 25, 624-643).

Efficacy and safety of sapropterin dihydrochloride in patients with phenylketonuria: A meta-analysis of randomized controlled trials

AIMS

The aim of the present meta-analysis was to evaluate the efficacy and safety of sapropterin dihydrochloride in phenylketonuria (PKU) patients.

METHODS

The following databases were searched for randomized controlled trials (RCT) regarding PKU patients treated with sapropterin dihydrochloride: PubMed, Embase, Cochrane Library and clinicaltrials. Two authors independently selected studies, assessed the risk of bias and extracted data. The meta-analysis was performed in RevMan 5.3 provided by the Cochrane Collaboration.

RESULTS

Four studies met the inclusion criteria. In PKU patients with low blood phenylalanine (Phe) concentration, no significant difference was indicated for the decrease of Phe level (weighted mean difference (WMD) = -7.75 μmol L-1 ; 95% confidence intervals (CI): -82.63 to 67.13, P = 0.84, I2  = 0%), however, the dietary Phe tolerance was significantly improved in the sapropterin group (WMD = 19.89 mg kg-1  d-1 ; 95% CI: 10.26 to 29.52, P < 0.0001, I2  = 0%). In PKU patients with high blood Phe level, sapropterin showed a significant lowering in blood Phe concentration (WMD = -225.31 μmol L-1 ; 95% CI: -312.28 to -138.34, P < 0.00001, I2  = 0%). There was no significant difference for adverse events.

CONCLUSIONS

Sapropterin could bring benefit for PKU patients with high or low Phe level, due to Phe reduction in a short time or dietary Phe tolerance improvement respectively. Sapropterin has an acceptable safety profile.

Executive dysfunction in treated phenylketonuric patients

OBJECTIVES

Executive function deficits have been described in early and continuously treated patients with phenylketonuria (PKU). The aim of this study was to examine performance on executive function tasks of treated patients with PKU diagnosed by 2 years of age.

PATIENTS AND METHODS

Ten patients with PKU and normal intelligence score who were diagnosed before the age of 2 years and subsequently treated continuously, were compared with 15 typically developing control children on a battery of neuropsychological tests, including the tower of London (TOL), continuous performance test (CPT), and Stroop test.

RESULTS

PKU cases showed significantly poorer performance on the TOL task compared to the control group with the difference being significant in the first three levels of the test. With the CPT, PKU cases had significantly more omission errors than control subjects. On the Stroop test there was no statistically significant difference between the groups. No significant correlation was found between the concurrent serum phenylalanine (Phe) level and results of the executive tests in PKU patients.

CONCLUSION

This study identified executive dysfunction in early-treated PKU patients with normal IQ, particularly in the planning and attention domains. Further studies are required to compare the results with those from other neurodevelopmental disorders such as ADHD and autism, to establish whether the pattern of findings is specific to PKU.

Pupillometric and behavioral markers of a developmental shift in the temporal dynamics of cognitive control

The capacity to anticipate and prepare for future events is thought to be critical for cognitive control. Dominant accounts of cognitive control treat the developing system as merely a weaker version of the adult system, progressively strengthening over time. Using the AX Continuous Performance Task (AX-CPT) in combination with high-resolution pupillometry, we find that whereas 8-year-old children resemble adults in their proactive use of cognitive control, 3.5-year-old children exhibit a qualitatively different, reactive form of cognitive control, responding to events only as they unfold and retrieving information from memory as needed in the moment. These results demonstrate the need to reconsider the origins of cognitive control and the basis for children's behaviors across domains.

Twenty-five years of research on neurocognitive outcomes in early-treated phenylketonuria: intelligence and executive function

Studies published from 1980 through 2004, which explored intelligence and executive function outcomes in early-treated PKU, were included in the present meta-analysis. The objective of this study was to examine the degree of difference between early-treated PKU and unaffected control groups (primarily non-familial controls), known as effect size, for various components of measured intelligence (i.e., full-scale, verbal, performance, and "other") and executive function (i.e., planning, working memory, inhibition, flexibility, and "other"). Secondly, in the case of heterogeneity among observed effect sizes, the moderator variables of "years since publication" and "measurement tools" were examined for their potential contribution to this heterogeneity. Thirty-three primary studies, with a total 200 outcomes meeting our inclusionary and exclusionary criteria, were meta-analyzed using Hedges g for effect size to correct for small samples in primary studies. The effect sizes for intelligence outcomes were "small" to "moderate" falling in the range of .20 to .42, with no significant heterogeneity among outcomes. Effect sizes for executive function and its various component processes were in the "moderate" and "large range" falling between .591 (planning) and 1.15 (flexibility). Significant heterogeneity among the executive function effect sizes was identified, and both "years since publication" and "measurement tools" were found to be significant moderators. Studies published more recently exhibited larger effect sizes, and particular executive function tasks demonstrated larger effect sizes than other tests. The results are discussed in terms of the contributions this meta-analysis makes to our understanding of neurocognitive outcomes in early-treated PKU, as well as suggestions for the conduct and reporting of future research in this area.

Efficacy of sapropterin dihydrochloride (tetrahydrobiopterin, 6R-BH4) for reduction of phenylalanine concentration in patients with phenylketonuria: a phase III randomised placebo-controlled study

BACKGROUND

Early and strict dietary management of phenylketonuria is the only option to prevent mental retardation. We aimed to test the efficacy of sapropterin, a synthetic form of tetrahydrobiopterin (BH4), for reduction of blood phenylalanine concentration.

METHODS

We enrolled 89 patients with phenylketonuria in a Phase III, multicentre, randomised, double-blind, placebo-controlled trial. We randomly assigned 42 patients to receive oral doses of sapropterin (10 mg/kg) and 47 patients to receive placebo, once daily for 6 weeks. The primary endpoint was mean change from baseline in concentration of phenylalanine in blood after 6 weeks. Analysis was on an intention-to-treat basis. The study is registered with ClinicalTrials.gov, number NCT00104247.

FINDINGS

88 of 89 enrolled patients received at least one dose of study drug, and 87 attended the week 6 visit. Mean age was 20 (SD 9.7) years. At baseline, mean concentration of phenylalanine in blood was 843 (300) micromol/L in patients assigned to receive sapropterin, and 888 (323) micromol/L in controls. After 6 weeks of treatment, patients given sapropterin had a decrease in mean blood phenylalanine of 236 (257) micromol/L, compared with a 3 (240) micromol/L increase in the placebo group (p<0.0001). After 6 weeks, 18/41 (44%) patients (95% CI 28-60) in the sapropterin group and 4/47 (9%) controls (95% CI 2-20) had a reduction in blood phenylalanine concentration of 30% or greater from baseline. Blood phenylalanine concentrations fell by about 200 micromol/L after 1 week in the sapropterin group and this reduction persisted for the remaining 5 weeks of the study (p<0.0001). 11/47 (23%) patients in the sapropterin group and 8/41 (20%) in the placebo group experienced adverse events that might have been drug-related (p=0.80). Upper respiratory tract infections were the most common disorder.

INTERPRETATION

In some patients with phenylketonuria who are responsive to BH4, sapropterin treatment to reduce blood phenylalanine could be used as an adjunct to a restrictive low-phenylalanine diet, and might even replace the diet in some instances.

Executive functioning in children and adolescents with phenylketonuria

This study addresses the effects of dietary adherence, phenylalanine (phe) levels, and age on performance of executive functioning (EF) tasks in children and adolescents with phenylketonuria (PKU). We herein collate formerly discrete findings to understand the relationship among actual clinical parameters and EF in PKU. Fifteen subjects (age range 8-20 years) with PKU were compared with the normative sample on the Delis-Kaplan EF Battery and on the Wechsler Abbreviated Scales of Intelligence to examine the relationship between EF skills, phe levels, age, and dietary adherence. At the time of the assessment, the mean age of participants was 14.8 years, mean lifetime phe levels ranged from 216 to 1200 microM (mean 594 microM); and concurrent phe levels ranged from 222 to 1730 microM (mean 660 microM). Children and adolescents with PKU showed lower performance in several EF skills: initiation of problem solving, concept formation, and reasoning. Performance on EF tasks requiring inhibitory control, cognitive flexibility and set shifting decreased at higher phe levels. Phe levels were positively correlated to age and inversely related to dietary adherence. We conclude that dynamic clinical parameters appear to govern EF in patients with PKU. We suggest that when adolescents decrease dietary compliance, changes in EF skills occur. Therefore, there is a need to specifically monitor EF skills in patients with PKU during the transition to, and during, adolescence.

Effects of dietary management of phenylketonuria on long-term cognitive outcome

BACKGROUND

Phenylketonuria (PKU) is associated with dopaminergic depletion in the dorsolateral prefrontal cortex and abnormalities of myelination. Both mechanisms may lead to deficits in cognitive functioning. Studies of cognitive outcome in children treated with PKU at an early stage have suggested that there are benefits in remaining on diet into adolescence.

AIM

To assess the nature and extent of any cognitive deficits in adults treated at an early stage with PKU who had discontinued their diets in adolescence.

METHOD

25 patients (aged 18-38 years) who were diagnosed early and had discontinued their diets in adolescence were compared with 25 adults (aged 18-38 years) with PKU on continuous diet, and with a healthy control group (n = 45).

RESULTS

The groups differed significantly on accuracy (p = 0.007) and speed (p = 0.001) of performance on an n-back working memory task and on speed of performance (p = 0.001) on a flanker inhibitory task, but not on flanker accuracy, object alternation learning or perceptual judgement tasks (all p>0.05). The off-diet group performed significantly below the on-diet group on n-back accuracy (p = 0.007) and flanker speed (p = 0.05), and significantly below the control group on n-back speed (p = 0.002) and flanker speed (p = 0.001).

CONCLUSION

The findings suggest that although discontinuing diet in adolescence appears disadvantageous compared with remaining on continuous diet, any deficits are relatively subtle.

Inhibitory control in children with phenylketonuria

Past studies have reported impairments in children with early-treated phenylketonuria (PKU) in executive abilities such as strategic processing and working memory. Findings have been inconsistent in terms of the integrity of inhibitory control, another executive ability. This study administered 4 inhibitory tasks (flanker, Stroop, go/no-go, antisaccade) to 26 children with PKU and 25 typically developing control children. Children with PKU performed more poorly than typically developing children on the 2 inhibitory tasks with the strongest experimental manipulations (go/no-go and antisaccade) between control and inhibitory conditions. Findings suggest that the inhibitory deficit associated with PKU is subtle and that inconsistent findings in past studies may be largely due to the insensitivity of experimental manipulations in some tasks.

Executive functioning and speed of processing in phenylketonuria

Treated phenylketonuria (PKU) has been linked to dopaminergic depletion in the dorsolateral prefrontal cortex, potentially leading to selective executive impairment. White matter abnormalities may lead to generalized slowing of information processing. These 2 hypotheses were evaluated in adults with PKU on a lifelong diet. Those with PKU were significantly slower than the control group regardless of working memory load on an n-back task and marginally slower regardless of trial type (inhibitory or noninhibitory) on a flanker task. There were no significant differences in speed on object alternation learning or perceptual judgment tasks. There were no group differences in accuracy on any task. These findings do not appear consistent with the selective executive hypothesis. A cognitive slowing account may prove more informative in adults with PKU, but more evidence is needed. The findings suggest that continuous dietary management is a fairly successful strategy in terms of cognitive outcome for adults.

Executive functioning, memory, and learning in phenylketonuria

The executive deficit hypothesis of treated phenylketonuria (PKU) suggests that dopaminergic depletion in the lateral prefrontal cortex leads to selective executive impairment. This was examined by comparing adults with PKU on a lifelong diet with a matched healthy control group. Those with PKU were impaired on selective and sustained attention, working memory (Self-Ordered Pointing), and letter fluency. However, they failed to show differential sensitivity to increased cognitive load on the attentional and working memory tasks, and they did not differ significantly on the remaining executive tasks (rule finding, inhibition, and multitasking). Nor did they differ significantly on recall or recognition memory. Overall, the findings provided little support for the executive deficit hypothesis. A possible explanation in terms of slowed information processing speed is explored.

Subtle brain dysfunction in treated 6-pyruvoyl-tetrahydropterin synthase deficiency: relationship to motor tasks and neurophysiological tests

6-Pyruvoyl-tetrahydropterin synthase (6PTPS) deficiency is a major cause of biopterin deficiency. 6PTPS patients usually have an elevated serum phenylalanine level, a deficiency of neurotransmitters (serotonin and dopamine), and neurological symptoms, if without treatment. We herein investigated the possibility of neurological dysfunction in early-treated patients. In the study, 12 early-treated 6PTPS patients were studied. Their auditory simple reaction time, movement rhythm variation (MRV), somatosensory evoked potentials to median nerve stimulation, and hand muscle responses to transcranial magnetic stimulation, were measured. MRV is a test of repetitive voluntary movements, and was used with and without auditory cues at 0.3 Hz. The 6PTPS patients had an increased motor threshold but normal motor and sensory central conduction times. They performed very well in simple reactions (6PTPS 208.4+/-16.7 ms, control 200.3+/-11.7 ms, p=0.18), but not in continuous tasks. The continuous performance tests showed that MRV had increased in the 6PTPS patients (with cues: 6PTPS 7.35+/-0.94, control 5.47+/-0.80, p<0.0001; without cues: 6PTPS 9.87+/-1.44, control 6.59+/-0.68, p<0.0001). Without cues, MRV had increased in both the 6PTPS and control groups, but more significantly in the 6PTPS patients (6PTPS 2.51+/-0.97, control 1.25+/-0.42; p=0.0001). Our findings indicate that early-treated 6PTPS patients have subtle neurological dysfunctions. They may not maintain movement rhythm as well as normal subjects, even with external cues. Hence, MRV is a good method to assess motor control.

Neuropsychological outcome of subjects participating in the PKU adult collaborative study: a preliminary review

Adult subjects with classical phenylketonuria (PKU) who were diagnosed and treated neonatally participated in this long-term follow-up study. Twenty-four subjects received neuropsychological (NP) assessment and a subset received magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) to identify: (1) pattern of cognitive dysfunction; (2) effect of high blood phenylalanine (Phe) level at time of cognitive testing; and (3) treatment variables that may be associated with cognitive difficulties in adulthood. All subjects had average IQ except one subject in the borderline range. Diet was initiated by the 15th day of life. All subjects except one were on diet until age 6 years (mean years of treatment = 15). Blood Phe levels at cognitive testing ranged from 157 to 1713 micromol/L (mean = 1038); 11 subjects had levels < 1000 micromol/L and 13 subjects had levels >1000 micromol/L. Results suggest that adults with early-treated PKU demonstrate specific cognitive deficits, a number of which are associated with the frontal and temporal area of the brain. Deficits were noted in several domains including executive functioning, attention, verbal memory, expressive naming and verbal fluency. Self-report measures of depression and anxiety were generally in the normal/mild range. The group with a Phe level > 1000 micromol/L scored lower than the group with Phe level < 1000 micromol/L on measures of focused attention, verbal fluency, reaction time, verbal recognition memory, visual memory and naming. Tests of cognitive functioning were often correlated with measures of treatment during childhood rather than with Phe level at the time of cognitive testing. Subjects with abnormal MRI scored significantly lower on two cognitive tests (Trails A and CVLT Recognition Memory). We found no significant correlation between current brain Phe level obtained through MRS (n = 10) and neuropsychological functioning. Future longitudinal investigation with a larger sample size will assist in clarifying the aetiology of neuropsychological deficits and association with treatment history.

Phenylketonuria: no specific frontal lobe-dependent neuropsychological deficits of early-treated patients in comparison with diabetics

Neuropsychologic studies have shown that even phenylketonuric patients treated early suffer from phenylalanine-related deficits in all age periods, from childhood to adulthood. This study was performed to determine whether phenylketonuric children show specific frontal lobe-dependent deficits when compared with diabetic patients. The comparative study included 42 phenylketonuric patients, 10 to 18 y of age [mean 14.7 (years, months), SD 2.9], and 42 diabetic patients matched for sex, age, and socioeconomic status. Patients were assessed for intelligence quotient (Culture Fair Intelligence Test), information processing (Wisconsin Card Sorting Test, Trail-Making Test), and selective (Stroop task) as well as sustained attention (Test d-2). Phenylketonuric patients had significantly poorer results than the diabetic patients. Within all tests, however, this was due to reduced performance speed, not to deficits in specific functions. Patients did not show deficits in insight and learning. The selection abilities and the sustained attention of the phenylketonuric patients were not impaired. Performance speed and blood phenylalanine levels were negatively correlated. Elevated phenylalanine levels may cause an imbalance in neurotransmitter metabolism. However, this seems to refer to a global neurotoxic effect rather than to specific effects on the dopaminergic system, which would affect specifically the activation of the frontal lobes.

Regression of neuropsychological deficits in early-treated phenylketonurics during adolescence

Even early-treated phenylketonuric patients suffer from phenylalanine-associated (mild) neuropsychological impairment. To date it is still unclear whether patients' deficits show a progression on ageing. This unsolved question seems to be an important aspect in the still ongoing debate about how long and how strictly the patients should be maintained on diet. Twenty early-treated (20 +/- 10, 9-30 days) adolescent phenylketonurics (10 boys, 10 girls) and 20 healthy controls, matched for age, sex and IQ, were investigated twice at a mean ages of 11 and 14 years for their IQ (Culture Fair Intelligence Test-Scale 2; CFT-20), fine motor abilities (Motor Performance Task), sustained (Test d2) and selective attention (Stroop-Task). At the first test, examinations revealed significant blood phenylalanine-correlated neuropsychological deficits in PKU patients. In spite of raised blood phenylalanine concentrations during the following 3 years and significantly elevated concurrent blood phenylalanine concentrations, the repeated measurements revealed a significant decrease of patients' deficits compared to controls. Clinical-neurological status of patients and controls was normal at both test times. The results indicate a decreased vulnerability of PKU-patients with respect to their neuropsychological functioning against elevated blood phenylalanine levels on ageing.