The Italian version of the Brief Assessment of Cognition in Affective Disorders: performance of patients with bipolar disorder and healthy controls.

OBJECTIVES: Cognitive deficits in Bipolar Disorder (BD) are significant enough to have an impact on daily functioning. Therefore, appropriate tools must be used to improve our understanding of the nature and severity of cognitive deficits in BD. In this study, we aimed to compare the cognitive profiles of patients with BD and healthy controls (HC) applying the Italian version of the Brief Assessment of Cognition in Affective Disorders (BAC-A). METHODS: This cross-sectional study included 127 patients with BD and 134 HC. The participants' cognitive profiles were evaluated using the Italian version of the BAC-A, which assesses verbal memory, working memory, motor speed, verbal fluency, attention & processing speed, executive functions, and two new measures of affective processing. The BAC-A raw scores were corrected using the normative data for the Italian population. In addition, we explored whether intelligence quotient (IQ) and specific clinical variables would predict the BAC-A affective, non-affective, and total composite scores of patients with BD and HC. RESULTS: HC performed better than patients with BD in all BAC-A subtests (all p < .001), except for subtests of the Affective Interference Test. (p ≥ .05). The effect sizes varied in magnitude and ranged between d = 0.02 and d = 1.27. In patients with BD, lower BAC-A composite scores were predicted by a higher number of hospitalizations. There was a significant association between IQ and BAC-A composite scores in both bipolar patients and HC. CONCLUSIONS: The Italian BAC-A is sensitive to the cognitive impairments of patients with BD in both affective and non-affective cognitive domains.

Correction to: Psychiatric disorders and SLC6A4 gene variants: possible effects on alcohol dependence and alzheimer's disease.

The following authors have double affiliations: Stefania Boccia, Marco Di Nicola and they should read.

Psychiatric disorders and SLC6A4 gene variants: possible effects on alcohol dependence and alzheimer's disease.

Serotoninergic system is one of the most important neurotransmission systems investigated in the field of psychiatry. Extensive evidence reveals how alterations of this system, and especially of the SLC6A4 gene, may be associated with psychiatric disorders. In this study we aimed to evaluate the pleiotropic nature of SLC6A4 alterations and their association with the overall risk of brain diseases rather than disorder-specific. SLC6A4 variants, namely 5HTTLPR, STin2, rs2066713, rs25531, rs4251417, rs6354 and rs7224199 were investigated in 4 independent cohorts of subjects with specific psychiatric disorders, including Alcohol dependence disorder (ALC), Alzheimer disease (ALZ), Schizophrenia (SCZ) and Bipolar disorder (BPD). Other variables (biochemical parameters and Psychiatric scales scores) were also tested for association. SLC6A4 polymorphisms are not associated with the risk of developing major psychiatric disorders (SCZ and BPD); however some signals were detected in ALC (HTTLPR pd = 9.25 × 10-03, pr = 7.24 × 10-03; rs2066713 pd = 6.35 × 10-08; rs25531 pd = 2.95 × 10-02; rs4251417 pd = 2.46 × 10-03), and ALZ (rs6354 pr = 1.22 × 10-02; rs7224199 pd = 1.00 × 10-08, pr = 2.65 × 10-02) cohorts. Some associations were also observed on exploratory analyses. Our findings did not reveal any major influence on SCZ and BPD development; On the other hand, some alteration of the SLC6A4 sequence were associated with an increased risk of ALC and ALZ disorders, suggesting common pathways. The results of this study should be carefully interpreted since it suffers of some inherent limitations (e.g. cohort size, slight ethnic heterogeneity). Further analyses may provide better detail on the molecular processes behind SLC6A4 alterations.

Opinion paper: poor response to treatment of depression in people in high occupational levels.

The working environment may have a significant effect on response to treatment of depression and this issue has not yet been sufficiently addressed in the scientific literature. There is evidence showing that being engaged in high-level positions can be an obstacle to the success of treatment. This article discusses the few evidence in the literature and some of the possible mechanisms involved. Specific personality attributes and difficulties in adapting to depression may delay access to care and may also reduce treatment compliance. The presence of stress in jobs that require high cognitive function and lack of social support may be elements that hinder the recovery process. Residual symptoms that impact on cognitive functions may undermine adherence to treatment and adversely affect the response. The implications of these issues are potentially relevant for clinical practice in the treatment of depression and for future research.

Alzheimer's Disease and Neurotransmission Gene Variants: Focus on Their Effects on Psychiatric Comorbidities and Inflammatory Parameters.

BACKGROUND: Alzheimer's disease (AD) is a neurodegenerative disorder accounting for 60-70% of dementia cases. Genetic origin accounts for 49-79% of disease risk. This paper aims to investigate the association of 17 polymorphisms within 7 genes involved in neurotransmission (COMT, HTR2A, PPP3CC, RORA, SIGMAR1, SIRT1, and SORBS3) and AD. METHODS: A Greek and an Italian sample were investigated, for a total of 156 AD subjects and 301 healthy controls. Exploratory analyses on psychosis and depression comorbidities were performed, as well as on other available clinical and serological parameters. RESULTS: AD was associated with rs4680 within the COMT gene in the total sample. Trends of association were found in the 2 subsamples. Some nominal associations were found for the depressive phenotype. rs10997871 and rs10997875 within SIRT1 were nominally associated with depression in the total sample and in the Greek subsample. rs174696 within COMT was associated with depression comorbidity in the Italian subsample. DISCUSSION: Our data support the role of COMT, and particularly of rs4680, in the pathogenesis of AD. Furthermore, the SIRT1 gene seems to modulate depressive symptomatology in the AD population.

Major Depression and the Degree of Suicidality: Results of the European Group for the Study of Resistant Depression (GSRD).

Background: This European multicenter study aimed to elucidate suicidality in major depressive disorder. Previous surveys suggest a prevalence of suicidality in major depressive disorder of ≥50%, but little is known about the association of different degrees of suicidality with socio-demographic, psychosocial, and clinical characteristics. Methods: We stratified 1410 major depressive disorder patients into 3 categories of suicidality based on the Hamilton Rating Scale for Depression item 3 (suicidality) ratings (0=no suicidality; 1-2=mild/moderate suicidality; 3-4=severe suicidality). Chi-squared tests, analyses of covariance, and Spearman correlation analyses were applied for the data analyses. Results: The prevalence rate of suicidality in major depressive disorder amounted to 46.67% (Hamilton Rating Scale for Depression item 3 score ≥1). 53.33% were allocated into the no, 38.44% into the mild/moderate, and 8.23% into the severe suicidality patient group. Due to the stratification of our major depressive disorder patient sample according to different levels of suicidality, we identified some socio-demographic, psychosocial, and clinical variables differentiating from the patient group without suicidality already in presence of mild/moderate suicidality (depressive symptom severity, treatment resistance, psychotic features, add-on medications in general), whereas others separated only when severe suicidality was manifest (inpatient treatment, augmentation with antipsychotics and benzodiazepines, melancholic features, somatic comorbidities). Conclusions: As even mild/moderate suicidality is associated with a failure of achieving treatment response, adequate recognition of this condition should be ensured in the clinical practice.

Temperament and Character Inventory in Bipolar Disorder versus Healthy Controls and Modulatory Effects of 3 Key Functional Gene Variants.

BACKGROUND: Bipolar disorder (BD) has been associated with temperamental and personality traits, although the relationship is still to be fully elucidated. Several studies investigated the genetic basis of temperament and character, identifying catechol-O-methyltransferase (COMT), brain derived neurotrophic factor (BDNF), and serotonin transporter (5-HTT) gene variants as strong candidates. METHODS: In the GECO-BIP study, 125 BD patients and 173 HC were recruited. Subjects underwent to a detailed assessment and the temperament and character inventory 125 items (TCI) was administrated. Three functional genetic variants within key candidate genes (COMT rs4680, BDNF rs6265, and the serotonin-transporter-linked polymorphic region (5-HTTLPR)) were genotyped. Univariate and multivariate analyses were performed. RESULTS: Compared to HC, BD patients showed higher scores in novelty seeking (NS; p = 0.001), harm avoidance (HA; p < 0.001), and self transcendence (St; p < 0.001), and lower scores in self directness (p < 0.001) and cooperativeness (p < 0.001) TCI dimensions. Concerning the genetic analyses, COMT rs4680 was associated with NS in the total sample (p = 0.007) and in the male subsample (p = 0.022). When performing the analysis in the HC and BD samples, the association was confirmed only in HC (p = 0.012), and in the HC male subgroup in particular (p = 0.004). BDNF rs6265 was associated with St in the BD group (p = 0.017). CONCLUSION: COMT rs4680 may modulate NS in males in the general population. This effect was not detected in BD patients, probably because BD alters the neurobiological basis of some TCI dimensions. BDNF rs6265 seems to modulate St TCI dimension only in BD patients, possibly modulating the previously reported association between rs6265 and BD treatment response. Further studies are needed to confirm our findings.

Catechol-O-Methyltransferase Val158Met Polymorphism and Clinical Response to Antipsychotic Treatment in Schizophrenia and Schizo-Affective Disorder Patients: a Meta-Analysis.

BACKGROUND: The catechol-O-methyltransferase (COMT) enzyme plays a crucial role in dopamine degradation, and the COMT Val158Met polymorphism (rs4680) is associated with significant differences in enzymatic activity and consequently dopamine concentrations in the prefrontal cortex. Multiple studies have analyzed the COMT Val158Met variant in relation to antipsychotic response. Here, we conducted a meta-analysis examining the relationship between COMT Val158Met and antipsychotic response. METHODS: Searches using PubMed, Web of Science, and PsycInfo databases (03/01/2015) yielded 23 studies investigating COMT Val158Met variation and antipsychotic response in schizophrenia and schizo-affective disorder. Responders/nonresponders were defined using each study's original criteria. If no binary response definition was used, authors were asked to define response according to at least 30% Positive and Negative Syndrome Scale score reduction (or equivalent in other scales). Analysis was conducted under a fixed-effects model. RESULTS: Ten studies met inclusion criteria for the meta-analysis. Five additional antipsychotic-treated samples were analyzed for Val158Met and response and included in the meta-analysis (ntotal=1416). Met/Met individuals were significantly more likely to respond than Val-carriers (P=.039, ORMet/Met=1.37, 95% CI: 1.02-1.85). Met/Met patients also experienced significantly greater improvement in positive symptoms relative to Val-carriers (P=.030, SMD=0.24, 95% CI: 0.024-0.46). Posthoc analyses on patients treated with atypical antipsychotics (n=1207) showed that Met/Met patients were significantly more likely to respond relative to Val-carriers (P=.0098, ORMet/Met=1.54, 95% CI: 1.11-2.14), while no difference was observed for typical-antipsychotic-treated patients (n=155) (P=.65). CONCLUSIONS: Our findings suggest that the COMT Val158Met polymorphism is associated with response to antipsychotics in schizophrenia and schizo-affective disorder patients. This effect may be more pronounced for atypical antipsychotics.

Association between Sirtuin 1 Gene rs10997870 Polymorphism and Suicide Behaviors in Bipolar Disorder.

BACKGROUND/AIMS: Suicidal behavior (SB) in bipolar disorder (BD) is a complex multifactorial event resulting from an interaction of genetic, neurobiological and psychosocial factors. Recent studies identified new possible mechanisms, suggesting a role for sirtuins (SIRTs 1-7), a family of nicotinamide adenine dinucleotide-dependent enzymes with a multifaceted role in the central nervous system. The aims of the present study were: (1) to investigate the effects of the rs10997870 SIRT1 gene variant on SB in BD; (2) to explore the effects of the same gene variant on specific depressive symptoms at the severest episode. METHODS: One hundred and eighty DSM-IV bipolar outpatients were enrolled in a naturalistic cohort study. The rs10997870 polymorphism within the SIRT1 gene was analyzed. RESULTS: An association between the GG genotype and SB was detected (lifetime: p = 0.015). Compared to other genotypes, GG carriers presented more frequently psychomotor agitation (p = 0.009) and a higher Hamilton Depression Rating Scale total score (p = 0.014) at the severest depressive episode. SB and psychomotor agitation were found to be associated with GG carriers and G allele in a multivariate analysis as well. CONCLUSION: Our findings suggest a role of the rs10997870 SIRT1 gene variant in SB amongst BD patients and its association with specific depressive symptoms. Despite a number of limitations of this exploratory study, our results may provide new insight into the mechanisms underlying SB in BD.

PDE7B, NMBR and EPM2A Variants and Schizophrenia: A Case-Control and Pharmacogenetics Study.

BACKGROUND: We investigated phosphodiesterase 7B (PDE7B), neuromedin B receptor (NMBR) and epilepsy progressive myoclonus type 2A (EPM2A) genes in schizophrenia (SCZ). To the best of our knowledge, these genes have been poorly investigated in studies of SCZ. METHODS: Five hundred and seventy-three SCZ inpatients of Korean ethnicity and 560 healthy controls were genotyped for 2 PDE7B, 3 NMBR and 3 EPM2A polymorphisms. Differences in the allelic and genetic frequencies among healthy subjects and patients were calculated using the x03C7;2 statistics. Repeated-measure ANOVA was used to test possible influences of single-nucleotide polymorphisms on treatment efficacy. In case of positive findings, clinical and demographic variables were added as covariates, in order to investigate possible stratixFB01;cation bias. RESULTS: The rs2717 and rs6926279 within the NMBR gene and rs702304 and rs2235481 within the EPM2A gene were associated with SCZ liability. rs1415744 was also associated with Positive and Negative Symptom Scale negative clinical improvement. The results remained the same after inclusion of the covariates and were partially confirmed in the allelic and haplotype analyses. CONCLUSION: Our preliminary findings suggest a possible role of NMBR and EPM2A genes in SCZ susceptibility and, for the second one, also in antipsychotic pharmacogenetics. Nonetheless, further research is needed to conxFB01;rm our findings.

Clinical factors related to schizophrenia relapse.

OBJECTIVE: Relapses represent one of the main problems of schizophrenia management. This article reviews the clinical factors associated with schizophrenia relapse. METHODS: A research of the last 22 years of literature data was performed. Two-hundred nineteen studies have been included. RESULTS: Three main groups of factors are related to relapse: factors associated with pharmacological treatment, add-on psychotherapeutic treatments and general risk factors. Overall, the absence of a maintenance therapy and treatment with first generation antipsychotics has been associated with higher risk of relapse. Further, psychotherapy add-on, particularly with cognitive behaviour therapy and psycho-education for both patients and relatives, has shown a good efficacy for reducing the relapse rate. Among general risk factors, some could be modified, such as the duration of untreated psychosis or the substance misuse, while others could not be modified as male gender or low pre-morbid level of functioning. CONCLUSION: Several classes of risk factors have been proved to be relevant in the risk of relapse. Thus, a careful assessment of the risk factors here identified should be performed in daily clinical practice in order to individualise the relapse risk for each patient and to provide a targeted treatment in high-risk subjects.

Genetic variability in CYP2E1 and catalase gene among currently and formerly alcohol-dependent male subjects.

AIMS: The present study explored whether specific single-nucleotide polymorphisms in alcohol metabolic pathway are associated with alcohol dependence or alcohol-related psychopathological symptoms. METHODS: Three groups of male unrelated subjects were included: 101 currently alcohol-dependent patients, 100 formerly alcohol-dependent subjects and 97 healthy controls. The following questionnaires were implemented: AUDIT, Zung Depression and Anxiety scale, Brief Social Phobia Scale, Yale-Brown Obsessive Compulsive Scale, Obsessive Compulsive Drinking Scale and Buss-Durkee Hostility Inventory. All the subjects were genotyped for CYP2E1 c.-1053C>T and CAT c.-262C>T. RESULTS: Statistically significant differences in the distribution of genotypes and alleles for CAT c.-262C>T polymorphism were observed among the three investigated groups. We observed a higher frequency of CAT -262T allele in alcohol-dependent subjects (OR = 1.74, 95% CI = 1.164-2.610). Among currently dependent patients CAT -262T allele carriers had higher AUDIT scores (P = 0.023), while CYP2E1-1053T allele carriers had significantly higher YBOCS-obsession subscale scores (P = 0.005) and Zung Anxiety Scale scores (P = 0.011). CONCLUSIONS: Our findings suggest that the CAT c.-262C>T genetic polymorphism influences the susceptibility to alcohol dependence and severity of alcohol dependence, while CYP2E1 c.-1053C>T polymorphism influences the expression of obsessive-compulsive and anxiety symptoms.

The influence of AHI1 variants on the diagnosis and treatment outcome in schizophrenia.

The present study aimed to explore whether four single nucleotide polymorphisms (SNPs) within the AHI1 gene could be associated with schizophrenia (SCZ) and whether they could predict the clinical outcomes in SCZ patients treated with antipsychotics. Four hundred twenty-six (426) in-patients with SCZ and 345 controls were genotyped for four AHI1 SNPs (rs11154801, rs7750586, rs9647635 and rs9321501). Baseline and clinical measures for SCZ patients were assessed through the Positive and Negative Syndrome Scale (PANSS). Allelic and genotypic frequencies in SCZ subjects were compared with those of controls using the χ2 statistics. The repeated-measure ANOVA was used for the assessment of treatment outcomes measured by PANSS changes. The case-control analysis did not show any difference in the genotypic distribution of the SNPs, while in the allelic analysis, a weak association was found between the rs9647635 A allele and SCZ. Furthermore, in the haplotype analysis, three haplotypes resulted in being associated with SCZ. On the other hand, two SNPs (rs7750586 and rs9647635) were associated with clinical improvement of negative symptoms in the allelic analysis, although in the genotypic analysis, only trends of association were found for the same SNPs. Our findings suggest a possible influence of AHI1 variants on SCZ susceptibility and antipsychotic response, particularly concerning negative symptomatology. Subsequent well-designed studies would be mandatory to confirm our results due to the methodological shortcomings of the present study.

Quetiapine extended release: preliminary evidence of a rapid onset of the antidepressant effect in bipolar depression.

BACKGROUND AND AIMS: Quetiapine (QTP) has been shown to be effective as an acute treatment in patients with bipolar depression. Nonetheless, the time at onset of QTP antidepressant action has not been clarified. We aimed to evaluate the onset of the antidepressant effect of QTP extended release (XR) in bipolar depression. We also compared the different efficacy and adverse effect profile of 300- and 600-mg/d dosages. METHODS: Twenty-one acutely bipolar depressed patients were recruited; 13 were treated with QTP XR 300 and 8 with 600 mg/d. Assessment was performed with Hamilton Depression Rating Scale (also considering clusters "core," "somatic anxiety," "psychic anxiety," "activity," and "delusion"), Hamilton Anxiety Rating Scale, Dosage Record and Treatment Emergent Symptom Scale. RESULTS: Quetiapine XR was effective since the first 3 days of treatment in reducing all the efficacy measures except for somatic anxiety. The comparison of 300- and 600-mg dosages was limited by the small sample size. However, the analysis did not show any significant difference in terms of efficacy, although with a trend in favor of 600 mg. The incidence of hypotension was significantly higher in patients taking QTP 600 mg (P = 0.004). DISCUSSION AND CONCLUSION: Quetiapine seems to be effective in bipolar depression within the first days of treatment. There may be not a significant advantage for the 600-mg dose in comparison with the 300-mg one. The clinical effect seems to be not associated with sedation, suggesting that it may be due to the molecular drug effect. Further studies focusing on the first days of treatment are needed to confirm our findings.

Genes involved in neuroplasticity and stressful life events act on the short-term response to antidepressant treatment: a complex interplay between genetics and environment.

OBJECTIVE: During the last few years, an increasing number of studies have focused on the association between neuroplasticity and affective disorders. The aim of the study is to evaluate the interactive effect of stressful life events (SLEs) and genes involved in neuroplasticity (BDNF and ST8SIA) on the short-term response to antidepressant treatment. METHODS: A total of 114 patients affected by mood or anxiety disorders under antidepressant treatment were enrolled in the study. We evaluated the interactive effects of three BDNF single-nucleotide polymorphisms (SNPs) and five ST8SIA SNPs and SLEs at different time points (childhood SLEs, SLEs at illness onset, and SLEs reported over the last preceding treatment) over 1-month of antidepressant treatment. RESULTS: Carriers of some genetic variants in both BDNF and ST8SIA had a slower response to antidepressants if non-exposed to the onset SLEs, whereas they had a similar trend compared with the carriers of the opposite variant if exposed (allelic analysis: BDNF p = 0.00003, p = 0.00609; ST8SIA p = 0.04, p = 0.033). The BDNF haplotype analysis confirmed this trend (p = 0.00016). CONCLUSIONS: Though our results are limited by the small sample size, variants in BDNF and ST8SIA may slow down the early response to antidepressants in subjects non-exposed to stressors at the illness onset, with a remarkable gene-environment interaction.

Association between Sirtuin 2 gene rs10410544 polymorphism and depression in Alzheimer's disease in two independent European samples.

Among the several genes associated with late-onset Alzheimer's disease (LOAD), recently, Sirtuin genes have roused a growing interest because of their involvement in metabolic homeostasis and in brain aging. Particularly SIRT2 gene has been associated with Alzheimer's disease (AD) as well as with mood disorders. The aim of this study is to investigate the possible associations between Sirtuin 2 gene (SIRT2) rs10410544 polymorphism and AD as well as depression in AD. In addition, we performed some exploratory analyses to investigate possible associations between the rs10410544 genotype and clinical features. We investigated these associations in two independent samples: the first one was composed of 275 Greek inhabitants and 117 patients; the second sample counted 181 Italian people and 43 patients. All patients were affected by LOAD. We failed to find any association between rs10410544 genotype and AD in the two samples. On the other hand, we found an association between the single nucleotide polymorphism (SNP) and depressive symptomatology (in the total sample p = 0.002), which was modulated by the tumor necrosis factor (TNF) values. Particularly, TT genotype seems to be protective versus depression. Finally, in the exploratory analyses, we found that the TT genotype was associated with earlier AD onset and a longer duration of the illness. In conclusion, we confirmed the association between SIRT2 gene and mood disturbances, although in AD patients. Further, we provided evidence that the TT genotype may be protective versus depressive symptoms, allowing an easier and thus earlier diagnosis of AD. This awareness may lead to a more detailed approach to these patients concerning diagnosis and therapy.

Influence of ANKK1 and DRD2 polymorphisms in response to haloperidol.

The present study explores whether ankyrin repeat and kinase domain containing 1 (ANKK1) and dopamine receptor D2 (DRD2) variants could predict efficacy and tolerability of haloperidol in the treatment of psychotic patients. We also attempted to replicate findings in a group of schizophrenic patients from the Clinical Antipsychotic Trials in Intervention Effectiveness (CATIE) study. Eighty-eight acutely psychotic patients were genotyped for 9 ANKK1 and 27 DRD2 SNPs. Treatment efficacy and tolerability were assessed using the Positive and Negative Symptoms Scale and the Udvalg for Kliniske Undersogelser side effects rating scales, respectively. Multivariate analyses were employed to test possible influences of single-nucleotide polymorphisms on clinical and safety variables. Analysis of haplotypes was also performed. Outcomes in the replication sample were response versus nonresponse and the presence versus absence of motor side effects at 1 month of treatment. rs2242592 within ANKK1 gene and rs1124493 within DRD2 gene were associated with clinical improvement (p = 0.008 and p = 0.001, respectively). Results were confirmed in the allelic analysis. Three haplotype blocks, one among ANKK1 and two among DRD2 gene were associated with better clinical improvement. Our results were not replicated in the CATIE sample, although rs11604671, which is in strong linkage disequilibrium with rs2242592, was associated with response in the replication sample. Our findings support a possible role of ANKK1 and DRD2 variability on haloperidol efficacy. However, due to the discrepancies between the results in the two samples, our results need further validation.

Influence of BDNF variants on diagnosis and response to treatment in patients with major depression, bipolar disorder and schizophrenia.

AIM: The present study aimed to explore whether some single nucleotide polymorphisms (SNPs) within the BDNF gene could be associated with major depression (MD), bipolar disorder (BD) and schizophrenia, and whether they could predict clinical outcomes in Korean inpatients treated with antidepressants, mood stabilizers and antipsychotics, respectively. METHODS: One hundred and forty-five patients with MD, 132 patients with BD, 221 patients with schizophrenia and 170 psychiatrically healthy controls were genotyped for 5 BDNF SNPs (rs2030324, rs7103873, rs10835210, rs11030101 and rs6265). Baseline and final clinical measures--including the Montgomery-Asberg Depression Rating Scale, Young Mania Rating Scale and Positive and Negative Symptoms Scale for patients with MD, BD and schizophrenia, respectively--were recorded. RESULTS: rs10835210 CA and rs11030101 AT genotype frequencies were higher in BD and schizophrenia patients than in healthy and MD subjects. No significant association was found with clinical improvement. DISCUSSION: Our findings provide evidence of an association between BDNF and BD and schizophrenia. However, taking into account the several limitations of our study, including the moderately small sample size, further research is needed to draw more definitive conclusions.