The purpose of this meta-analytic project was to conduct a quantitative review of research conducted between 1980 and 1999 examining treatment outcomes for the genetic disorder, PKU. The charge set forth by the
NIH for the Consensus Development Conference on PKU was to focus this literature review on the following four broad aims:
What is the effect of treatment on cognitive and behavior outcomes?
What is known about age at diet discontinuation on outcome?
What is known about the reversibility of clinical symptoms upon reinstitution of treatment different ages?
What is known about the relationship of blood levels of phenylalanine and tyrosine, and cognition and behavior outcomes at different age levels?
Before discussing the range of meta-analytic results and their interpretation, two caveats are in order. First, with the exception of intelligence, there is little clear consensus in the field of cognitive psychology regarding how to define constructs such as attention, executive function, and speed of processing. For this project, it was important to evaluate treatment effectiveness for different cognitive and behavioral domains separately, as the past several decades of empirical research in the area of PKU suggest that there is a profile of weaknesses and relative strengths across a range of cognitive skills and behaviors. Study outcomes were classified into five overall categories (IQ, Executive Function, Attention, Behavior, and Other Cognitive and Motor Skills), each comprised of several sub-categories. For the most part, classification of the results from a particular study was guided by the authors’ interpretation of the cognitive or behavioral domain measured by the task used. For example, if the authors referred to their experimental task as a measure of "sustained attention," then the outcome was classified as such. When the authors referred to their measure as tapping more than one cognitive function or behavioral process, the outcome was classified under the single category and sub-category that was most consistent with the interpretation of the measure in the scientific literature. However, many, if not most, experimental cognitive and neuropsychological tests lack clear construct validity, and our classification of the outcome (based on the authors’ interpretations) would not be without debate. In particular, there is probably substantial overlap among tests of sustained and selective attention and those assessing the executive functions of inhibition, flexibility and working memory. Classifying the various cognitive and behavioral results into "independent" categories is a first step to exploring the profile of impairments in PKU and those processes linked to blood phenylalanine levels; however, one must be cautious in the interpretation of the findings across these categories.
Second, a clinically important question is the effect of developmental level on the effectiveness of treatment and the association of Phe level with cognitive and behavioral outcomes. However, given the incidence of PKU, one in 10 - 20,000 live births, it is rare to find studies that narrow their focus to a particular age or small age range. The unit of analysis in a meta-analysis is the average or "overall" data from each study, rather than the raw subject data. Therefore, when the vast majority of studies in an area collapse over a large age range, one cannot easily analyze for age effects.
Both of these concerns regarding cognitive and behavioral constructs and developmental level will be discussed at greater length in the last section of the discussion, "Directions for Future Research."
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The analyses relevant to
Aim One address the effect of treatment on cognitive and behavioral outcomes in individuals diagnosed with PKU. This broad question essentially asks to what degree the low Phe diet has been effective in averting the various negative cognitive and behavioral sequelae of this metabolic disorder. One approach to addressing this question is to examine studies of PKU individuals who were treated in early infancy with the restricted dietary regimen (the majority of whom are still on diet) on a range of cognitive and behavioral outcomes. In these studies, the PKU children and adults were compared to age-matched, unaffected control participants who were unrelated to the PKU participants. The effect sizes for all the outcomes were positive, indicating poorer performance or greater behavioral problems in the PKU group; however, these mean effect sizes ranged from .074 to 1.344. That is, there appears to be a profile of relative strengths and weaknesses in groups of early-treated, primarily on-diet, PKU participants relative to the unaffected peers.
The smallest mean effect sizes were found in the Behavior category, where effect sizes of 0.074 (behavior disorders) and 0.16 (general behavior ratings and self-concept) indicate that the mean scores of the PKU group were virtually indistinguishable from those of the control group. Therefore, although individual studies of general behavior (eg., Schor, 1983, 1984) and of psychopathology (eg., Fisch, Sines, & Chang, 1981) have suggested a greater degree of behavioral disturbance in early-treated PKU children, this meta-analysis indicates that the difference between the clinical and control groups is negligible. It is important to note that behavioral outcomes were combined across studies with samples that were heterogeneous with regard to age (ie., from young childhood to early adulthood). It is possible that given enough studies of participants within a smaller age range (eg., toddlers, adolescents, etc.), a future meta-analysis would identify an age range during which PKU individuals exhibit greater behavioral problems, as compared to normal controls.
The analysis of IQ overall, and the sub-categories of full-scale, verbal, and performance IQ, revealed small mean effect sizes ranging from 0.323 (verbal IQ) to 0.440 (performance IQ). Effect sizes of this magnitude indicate that the average difference between the PKU and control means on these measures is about one-third of the pooled standard deviation. These results are consistent with the general consensus among researchers and clinicians that the intellectual level of early-treated PKU individuals is within the normal range (eg., Williamson, Koch, Azen, & Chang, 1981).
The analyses of both Executive Function and Attention, two cognitive categories that are probably very closely related, revealed mean effect sizes of 0.468 and 0.658, respectively. Such effect size magnitude would be classified as "moderate" by Cohen (1992), and reflects about a one-half standard deviation difference between the means of the PKU and control groups. With regard to Executive Function, the effect size for measures of inhibition and flexibility was 0.342; however, there were too few outcomes to analyze the effect size for planning or working memory processes, specifically. The popularity of a "prefrontal dysfunction model" for PKU in which the inability to metabolize Phe into dopamine should compromise prefrontal function (eg., Welsh, 1996) has been bolstered by the evidence of recent studies that have found specific executive function deficits in this population (eg., Diamond et al., 1997; Griffiths et al., 1998; Welsh et al., 1990). However, to effectively test this model via meta-analysis, a greater number of studies of this kind need to be conducted. The analyses of Attention overall, and sustained and selective attention specifically, yielded mean effect sizes that were somewhat larger than those for Executive Function (
d range from 0.635 to 0.658). Interestingly, the results for both Executive Function and Attention converge with the evidence of a moderate mean effect size (
d = 0.50) for the Other Cognitive and Motor Skills category. This effect size appears to be driven by one moderate effect size for speed of processing measures (
d = 0.529) and one very large effect size for performance on visual-spatial tasks (
d = 1.34). The results with regard to visual-spatial performance substantiates a long-standing assumption that individuals with PKU have specific deficits in "visual analysis and synthesis" (eg., Mims, McIntyre, & Murray, 1981, 1883). One reasonable explanation for the similar mean effect sizes seen across the cognitive categories of executive function, attention, speed of processing, and visual-spatial skills is that performance on these tasks are mediated by overlapping cognitive processes. For example, one easily can see how a measure of selective attention would also require executive processes of working memory and inhibition, not to mention visual-spatial and speed of processing components. Again, the somewhat artificial demarcation among these cognitive and behavioral categories must be viewed cautiously.
A more conservative approach to examining the differences between early-treated PKU individuals and unaffected control participants across a range of behavioral domains, is to compare the clinically-diagnosed individuals to familial controls. Comparing PKU participants to their own siblings or parents will control, to some extent, the contribution of other genetic and environmental factors to performance on the cognitive and behavioral measures. Because the PKU individuals are more similar to the sibling or parental controls in terms of genetic endowment and environmental influences, one would expect them to be more similar to these control participants than they would be to non-familial controls. The results concerning the Behavior category were quite similar to the earlier analysis of non-familial controls, revealing a negligible effect size of about 0.140. Interestingly, the analysis of overall IQ, as well as full-scale, verbal and performance IQ, found somewhat larger effect sizes (
Md range from 0.597 -0 .787) than in the earlier analysis. This relatively large discrepancy between the intellectual level of PKU individuals as compared to that of their unaffected family members, is consistent with the notion that, although IQ is in the normal range, it does not achieve the levels predicted by familial IQ (Koch, Azen, Firedman, & Williamson, 1984). However, the small number of outcomes on which some of these analyses of IQ were based suggest that this may be an unreliable finding. Only one other cognitive category could be analyzed, and PKU individuals also exhibited a relatively large deficit in visual-spatial processing (
d = -0.966), as compared to siblings and/or parents.
Another way to explore the overall effectiveness of the dietary treatment regimen for PKU is to examine the degree to aspects of this treatment are related to cognitive and behavioral outcomes. For example, the age at which the dietary treatment began was inversely related to performance on standardized IQ tests, with correlations in the -0.30 range. Given that this analysis targeted studies published in the past twenty years, the vast majority of PKU participants were considered "early-treated." The age at diet initiation typically ranged from the neonatal period to sometime within the first year of life, and only a few studies included those treated after the first year. However, even with a relatively restricted range in the age at diet initiation, the expected inverse relationship between this variable and cognitive and behavioral performance was revealed. A second method for examining the effectiveness of treatment is to ask whether the quality of control while on diet influences these cognitive and behavior outcomes in predictable ways. The results of this meta-analysis indicate that individuals with early-treated PKU whose dietary control is judged to be of lower quality will exhibit deficits in IQ relative to early-treated individuals who have maintained a higher quality of dietary control. Similarly, the quality of dietary control is related to performance on measures of sustained and selective attention. Unfortunately, due to a insufficient number of effect sizes, this important treatment variable could not be explored with respect to executive functions and other cognitive and behavioral outcomes.
Finally, a set of analyses explored the nature of age differences (cross-sectionally) and age changes (longitudinally) for individuals who were currently on the phenylalanine-restricted diet. If the diet is effective, one might expect improvements with age on cognitive task performance as would be observed in their unaffected age-mates. Only a small amount of improvement was observed for IQ, as indicated by effect sizes in the 0.12 range. However, because standard scores are generally reported for IQ, these are already "age corrected" and obscure the potential improvement with age that could be seen in the raw scores. The fact that the PKU children appear to be "keeping up" with the peers in terms of their standardized score, suggests that very normal developmental improvement in intelligence was occurring. A much larger improvement in Executive Function (
d = 1.52) and specifically inhibition and flexibility skills (
d = 1.89) was observed with age; however, the small number of outcomes on which these analyses were based (less than 10), suggests that these results are of suspect reliability.
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The second aim of the project was to examine the influence that diet discontinuation has on a diversity of cognitive and behavioral outcomes. Again, this broad aim was explored from a few different perspectives. First, a direct comparison of on-diet performance to off-diet performance was conducted, both by looking within-subjects (ie., the same individuals on- and off-diet) and between-subjects (ie., different individuals in the on- and off-diet groups). Somewhat surprisingly, the differences between on-diet and off-diet performance was not large for IQ (
d = 0.301), Behavior (
d = 0.241) or Other Cognitive and Motor Skills (
d = 0.212). There was a larger effect size for measures of sustained and selective attention (
d = 0.772); however, this result was again based on a relatively small sample of outcomes (N=7). The smaller than expected differences between on-diet and off-diet performance may not be surprising, in light of the fact that the off-diet performances are measured on PKU individuals who are early- and continuously-treated, with some individuals maintaining treatment well into adolescence. Consistent with this explanation, the IQ performance of PKU individuals who are off diet at the time of testing compares favorable to the IQ performance of unaffected control participants (
d = 0.112). However, there is a relatively greater difference between PKU diet terminators and unaffected controls on overall Attention (
d = 0.90) and speed of processing tasks (
d = 0.688). This contrast between the results on tests of intelligence and performance on other cognitive tasks, confirms the notion expressed by many researchers (eg., Welsh et al., 1990) that standardized intelligence tests, and particularly "crystallized intelligence," may not be as sensitive to the core impairments characterizing PKU as are tests of "fluid" intelligence and information processing. A third set of analyses converge with this perspective on diet termination and cognitive sequelae: greater longitudinal changes in PKU individuals post-diet termination were seen on measures of Other Cognitive and Motor Skills (
d = 0.723), than were seen on measures of intelligence (
d = 0.137).
A clinically important issue regarding diet termination and outcome is the age at which individuals with PKU can "safely" terminate diet. Currently, clinics across the country and around the world have widely varying policies with regard to the age at which diet termination is acceptable. Ideally for a meta-analysis, one would be able to collapse across a large set of outcomes for
each age or
small age range during which diet discontinuation occurred. However, the reality of the empirical literature exploring PKU is that few studies have homogeneous samples with regard to diet termination, and instead, there exists a range of ages of diet discontinuation within a given sample. For the purposes of this meta-analysis, outcomes were grouped on the basis of "early diet termination" (ie., before age 8) and "late diet termination" (ie., 8 years or later). These analyses revealed moderate mean effect sizes for overall IQ (
d = 0.603) and the three sub-categories of IQ: full-scale (
d = 0.571), verbal (
d = 0.677), and performance (
d = 0.592). These positive effect sizes indicate that those individuals terminating diet early exhibited lower IQ scores than those individuals terminating diet later. These results are consistent with the finding of Holtzman, Kronmal, van Doorninck, Azen, and Koch (1986), in which diet discontinuation prior to age 6 resulted in the largest IQ discrepancies as compared to parental IQ.
Another issue with substantial clinical and policy implications is the degree to which diet reinstitution in treated PKU individuals who have previously terminated diet restrictions, has a positive impact on behavior and cognition. This question is particularly important given the concerns surrounding maternal PKU, and the teratogenic effect that high levels of circulating Phe has on fetal development (eg., Waisbren, Hamilton, St. James, Shiloh, S. & Levy, H. L.,1995). For this reason, young adult women with PKU who have discontinued diet are frequently encouraged to return to the regimen. However, evidence supporting other positive cognitive and behavioral effects of diet reinstitution would suggest that this course of action may be in the best interest of
both female and male PKU adults. The focus of this Aim was to investigate the impact of diet reinstitution on cognitive and behavioral processes; however, the nature and degree of this impact is most likely moderated by a variety of factors: the age at which diet was terminated, the quality of control while on the newly reinstituted diet, as well as the quality of control while on the dietary regimen previously. Unfortunately, there were relatively few studies in the
NLM bibliography on diet reinstitution, making it difficult to address the general question, and nearly impossible to adequately explore the effect of moderator variables.
The effects of diet reinstitution are typically explored in a within-subjects design, examining cognitive performance or behavior both pre- and post- the return to dietary restrictions. In the attempt to explore effects of treatment within specific behavioral domains, there were too few outcomes to analyze, with the exception of the Behavior sub-category of psychological disorders. This analysis revealed essentially no effect of diet reinstitution (
d = 0.025); that is, diet reinstitution did not have a substantial positive impact on ratings of psychopathology (eg., depression, anxiety). Again, one needs to be wary of the small sample size of outcomes (N=8) in drawing conclusions from this result. Another concern regarding this finding is whether the particular rating scales used to determine characteristics of psychological disorders are sensitive to small, possibly transitory changes, that might occur with diet reinstitution.
The objective of the dietary restrictions prescribed for all early-identified PKU individuals is to restrict the intake of Phe, an amino acid that cannot be metabolized due to decreased levels of phenylalanine hydroxylase. The particular genetic mutation characteristic of classical PKU, and resulting high circulating levels of Phe, disrupts catecholamine biosynthesis (McKean, 1972), as well as the neuronal uptake of critical neurochemical precursors in the central nervous system. Consistent with these ideas, Krause et al. (1985) documented the inverse association between Phe and dopamine. Thus, levels of circulating Phe at various points in development may be expected to impact brain development and function, resulting in a distinctive profile of behavioral and cognitive sequelae.
This reasoning is the rationale for examining the association between four measures of Phe level and the various cognitive and behavioral outcomes targeted in this project. First, the correlations between the Phe level concurrent with the testing session and the scores on the cognitive or behavioral measures were examined. Concurrent Phe level did exhibit the expected inverse association with overall IQ (
r = -0.643) and full-scale IQ (
r = -0.725). There was a moderate relationship between concurrent Phe and overall Attention (
r = -0.538), but a nonsignificant association with the Behavior sub-category of general behavior ratings (
r = -0.143). Somewhat surprisingly, there was a relatively large association between concurrent Phe and ratings of psychopathology (
r = -0.749); however, this result is based on a small number of outcomes (N=9). An unexpectedly strong negative correlation was found between concurrent Phe level and memory (
r = -0.93); however, the small number of outcomes (N=6) for this analysis warrants caution in the interpretation of the finding. The prefrontal dysfunction hypothesis (eg., Welsh, 1996) predicts that concurrent Phe level should be strongly associated with the behaviors subsumed within the Executive Function category; however, with the exception of the sub-category of planning (
r = -0.663), the mean effect sizes in this category are quite small. Again, this important analysis linking concurrent Phe level to executive function processes is hampered by a small sample size of relevant outcomes that were derived from only a few studies (eg., Griffiths et al., 1998; Welsh et al., 1990).
Second, the association between the highest diagnostic level of Phe prior to diet initiation, referred to as infant Phe level, and cognitive and behavioral sequelae also was explored. Relatively fewer studies report infant Phe level; thus, the number of outcomes within most specific behavior categories was insufficient to conduct this analysis. The association between infant Phe level and overall IQ was analyzed and only a weak relationship was revealed (
r = -0.215). The question as to whether high central levels of Phe during a rapid period of brain development has negative implications for cognition and behavior is an important one, but unfortunately too few published accounts of studies report this particular Phe level.
A third measure of Phe level is the mean (or median) level from the point of diet initiation to the time of testing; this level is referred to as the mean lifetime Phe level. This Phe level could be an average taken across 4 years or 24 years, depending on the age of the PKU individual tested. It reasonable to suspect that these mean lifetime Phe levels have different implications depending on the developmental period represented. With the heterogeneity
within most study samples and the variability
across studies with respect to age, conducting these analyses collapsing across age can make interpretation difficult. There were very weak, nonsignificant negative correlations between mean lifetime Phe level and overall IQ (
r = -0.057) and Executive Function (
r = -0.158); however, the correlation with the overall Other Cognitive and Motor Skills category was surprisingly strong (
r = -0.976). This latter correlation was based on only five outcomes and, again, may be an unreliable result.
The fourth measure of Phe level represents a combination of various measures that do not fit in the aforementioned three Phe level categories. The "other" Phe level category includes such measures as the most recent Phe level, and the mean Phe level taken during the year of the testing. A wide range of the behavior and cognitive outcomes could be analyzed with respect to the association with this "other" Phe level. This measure of Phe level was moderately correlated with overall IQ (
r = -0.424 to -0.573), but only weakly correlated with Attention (
r = -.123), Executive Function (
r = -.022), Behavior (
r = -0.129), and Other Cognitive and Motor Skills (r = -0.057).
The effects of experimental manipulations of phenylalanine, as well as other amino acids and neurochemical precursors were examined with regard to the range of behavioral and cognitive outcomes of interest. The effects of experimentally increasing Phe levels ("phe loading") was seen for overall Attention (
r =-0.385), sustained and selective attention (
r = -0.428), and motor skills (
r = -0.396). In all cases, an increase in Phe level was associated with a decrease in performance. Due to an insufficient number of outcomes from other behavioral categories, such as Executive Function, it is not known whether Phe manipulations affect other cognitive processes as well. Other studies have been designed to provide supplementation with tyrosine, L-dopa, and other amino acids to boost catecholamine production (especially, dopamine), presumed to be lacking in individuals with PKU. Although there are relatively few studies describing such supplementation procedures, there was a sufficient number of outcomes to analyze a few behavioral domains. Supplementation appeared to be positively related to better performance for Executive Function overall (
r = 0.726), and inhibition/flexibility skills specifically (
r = 0.777). Supplementation also was moderately correlated with increased scores on Attention tasks, overall (
r = 0.457).
The typical meta-analytic project involves five steps: (1) formulation of the questions to be answered or specific aims, (2) execution of a complete literature search, (3) collection, classification and coding of studies meeting inclusion criteria, (4) application of statistical techniques for pooling and analyzing the compiled data, (5) evaluation and interpretation of the results in the form of a written report. While the broad aims for the project were provided by the
NIH, the researchers were free to generate more specific analytic questions to address these aims.
The first limitation of this project may have been the requirement to include only studies in the
NLM bibliography. While the search culminating in this bibliography was comprehensive, there are likely to be articles, dissertations, conference presentations, etc. that are missing from this bibliography that may have provided relevant data. In meta-analytic work, restricting one’s literature search to published studies creates what is known as the "file drawer problem" wherein unpublished studies (often with nonsignificant results) fail to be included in the analyses. Obviously, one outcome of this bias is that certain effect sizes may appear larger than they really are, as they have not been "balanced" by the nonsignificant findings. This is one of the many sources of publication bias that could not be examined in this project.
A second limitation is reflected in the fact that only about half of the coded outcomes could be analyzed in this meta-analysis. The most common reason for eliminating research outcomes was that insufficient information was provided in the published report to calculate an effect size estimate. For example, mean data often were presented in figures without an accompanying table providing the necessary standard deviation data. Given more time, the authors of these studies could be contacted for the necessary data, and these studies could be coded and included in the analyses. Moreover, of the 98 studies and over 1,500 outcomes for which effect size estimates could be calculated, many research questions could not be analyzed due to a small number of outcomes relevant to that question. Compounding this problem is that the ratio of number of outcomes to studies is higher than is desired for meta-analytic work. This "multiple outcome bias" reflects the concern that if a relative few studies are contributing the vast majority of outcomes to a particular analysis, then any problems with the internal or external validity of these studies (eg., recruitment of participants, methodology, etc) will complicate the interpretation of this analysis, as well.
Due to the timeframe for this project, which was extremely short by typical meta-analytic standards, the data set suffers from the following weaknesses (some of which are described above):
In general, a meta-analytic project of this magnitude; that is, one that deals with complex questions, a variety of research designs, and a myriad of operational definitions for the behavioral constructs; would demand extensive pilot work and a longer time frame for proper execution (e.g., 2 to 3 years).
Although there has been a great deal of excellent empirical work exploring the cognitive and behavioral sequelae of PKU, further advances in this field are needed. One objective of conducting meta-analyses on data in a research domain, is to identify gaps in our knowledge base and fruitful directions for future research. The current meta-analytic project on treatment outcomes for the genetic disorder, PKU, has accomplished this goal. Although this meta-analysis was inherently limited in its scope due to the externally-imposed time constraints, gaps and a lack of clarity in the empirical literature still can be identified. Three general recommendations for future research in this area are derived from what was learned in the course of this meta-analysis of treatment effectiveness in PKU.
First, it is clear that critical questions surrounding the clinical care and management of PKU concern the issues of developmental level and age. In particular, definitive answers are sought to questions such as "
at what age can the dietary restrictions be discontinued or relaxed?," "what are the implications of returning a PKU individual to diet
at various ages?" and "what is the association between blood Phe level and cognition or behavior
at different ages?" Unfortunately, it is the rare study in the PKU literature that selects and tests a sample of PKU individuals that are homogeneous with respect to age. Given the relatively low incidence of PKU in the population, it can take years to collect data within a restricted age band (eg., Diamond et al., 1997; Welsh et al., 1990); however, to address these very important developmental questions, this research must be conducted. In a sense, a quantitative review of the literature is only "as good" as the studies it combines for the meta-analyses, and if the individual studies do not narrow the focus to specific ages or age groups, the meta-analysis certainly cannot address these issues either. When individual studies report the correlation between the concurrent Phe level and executive function skills, for example, and their sample includes a wide age range, the manner in which age moderates this correlation is obscured. Therefore, the first recommendation that follows from this meta-analysis is that researchers in the area of PKU seriously consider how to better account for age and developmental level in their future studies.
The second recommendation for more effective research on PKU treatment outcome, is not specific to this research area, but applies to all research on cognitive and neuropsychological development in typical and atypical individuals. In light of the fact that there appears to be a profile of relative strengths and weaknesses in the cognitive skills and behavior of early-treated PKU individuals, researchers must move toward greater consensus regarding how to define and measure the key cognitive constructs comprising this profile. This meta-analysis suggested that the relative weaknesses in the cognitive profile of early-treated individuals, even while on diet, are somewhat specific to tasks of sustained and selective attention, visual-spatial processes, speed of processing, and executive functions. At this point it is a matter of debate whether particular experimental paradigms (eg., a continuous performance test) presumed to measure a specific cognitive process (eg., sustained and selective attention) actually measures that process. Before we can ascribe a characteristic pattern of cognitive strengths and weaknesses to a particular clinical condition, the construct validity of the tests typically used to measure cognitive and behavioral processes must be examined and established. In the current meta-analysis, findings were grouped by cognitive and behavioral process on the basis of the researchers’ presumptions about what processes mediated performance on their psychological measures. It is beyond the purview of meta-analysis to make judgments regarding whether the profile of weaknesses identified across studies indicates deficits in several different (albeit, related) cognitive processes or whether one common cognitive mechanism is responsible for the cluster of impairments. Our understanding of the nature and scope of cognitive constructs such as intelligence, executive function, and attention, as well as the most psychometrically-sound and conceptually-clear methods of measurement, remains one of the biggest challenges for the fields of cognitive psychology and neuropsychology.
Finally, this meta-analysis exposed several areas that are "under-researched" with regard to clinical and cognitive outcomes in early-treated PKU. These areas are challenging and difficult to explore for many of the general reasons discussed above, as well as for many reasons unique to each research question. In addition to the call for more research focused on specific age groups already mentioned, the following areas should be a priority for PKU researchers in the near future:
A quantitative review of the literature accomplished via meta-analytic techniques is an important companion to the more traditional qualitative review. In the case of this meta-analysis of treatment outcome research in PKU, many of the findings are consistent with interpretations set forth in recent qualitative reviews of this empirical literature. However, this initial analysis of the data set (and more analyses will follow) also has revealed some surprising results that suggest future areas of scientific investigation.
Some tentative conclusions that can be drawn from this meta-analysis project are as follows:
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