Proton magnetic resonance spectroscopy in youth with severe mood dysregulation

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Psychiatry Research: Neuroimaging 163 (2008) 30 – 39 www.elsevier.com/locate/psychresns

Proton magnetic resonance spectroscopy in youth with severe mood dysregulation Daniel P. Dicksteina,d,e,⁎, Jan Willem van der Veenb , Lisa Knopf a,c , Kenneth E. Towbina,c , Daniel S. Pinec , Ellen Leibenluft a,c a

b

Unit on Bipolar Spectrum Disorders, Division of Intramural Research Programs, National Institute of Mental Health, Bethesda, MD, USA Magnetic Resonance Spectroscopy Core, Division of Intramural Research Programs, National Institute of Mental Health, Bethesda, MD, USA c Emotion and Development Branch, Division of Intramural Research Programs, National Institute of Mental Health, Bethesda, MD, USA d E.P. Bradley Hospital, East Providence, RI, USA e Warren Alpert Medical School of Brown University, Providence, RI, USA Received 22 March 2007; received in revised form 11 September 2007; accepted 19 November 2007

Abstract Increasing numbers of youth are presenting for psychiatric evaluation with markedly irritable mood plus “hyperarousal” symptoms. Diagnostically homeless in current nosology, the syndrome (as well as its underlying neurobiology) is little understood. To address this problem, we conducted an exploratory proton magnetic resonance spectroscopy (MRS) study in a large sample of youth with chronic, functionally disabling irritability accompanied by hyperarousal, a clinical syndrome known as “severe mood dysregulation” (SMD), which may represent a broad phenotype of pediatric bipolar disorder. Medication-free SMD youth (N = 36) and controls (N = 48) underwent 1.5 Tesla MRS in four regions of interest. The following three neurometabolites, relative to creatine (Cr), were quantified with LCModel Software: (a) myo-inositol (mI), a marker of intra-cellular second messengers linked to the neurobiology of bipolar disorder; (b) glutamate/glutamine (GLX), a marker of the major excitatory neurotransmitter glutamate; and (c) N-acetyl aspartate (NAA), a marker of neuronal energetics. SMD subjects had significantly lower temporal mI/ Cr versus controls. However, this difference did not survive correction for multiple comparisons. Given studies implicating mI in lithium's action in BD adults and youth, further work is necessary to determine potential therapeutic implications of our present finding and how SMD youth differ pathophysiologically from those with strictly defined BD. Published by Elsevier Ireland Ltd. Keywords: Magnetic resonance spectroscopy; Irritable mood; Child; Adolescent

1. Introduction

⁎ Corresponding author. Bradley/Hasbro Children's Research Center, 1 Hoppin Street, Coro West Rm 2.056, Providence RI 02903, USA. Tel.: +1 401 793 8028; fax: +401 444 8742. E-mail address: [email protected] (D.P. Dickstein). 0925-4927/$ - see front matter. Published by Elsevier Ireland Ltd. doi:10.1016/j.pscychresns.2007.11.006

In psychiatry, increasing attention is being focused on youth presenting with markedly irritable mood plus “hyperarousal” symptoms similar to those seen in attention deficit hyperactivity disorder (ADHD) (Carlson, 1998; Pogge et al., 2001). Indeed, a recent epidemiologic study suggested that as many as 3.3% of children in the

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community may have such a syndrome (Brotman et al., 2006). These children are “diagnostically homeless” because their symptoms do not fit clearly into Diagnostic and Statistical Manual of Mental Disorders (DSM-IVTR) criteria for ADHD, given their significant impairment from mood symptoms (American Psychiatric Association, 2000). Nor do their symptoms fit clearly into mania and bipolar disorder (BD), since their mood disturbance does not involve distinct episodes. Also, these children do not fit clearly into oppositional defiant disorder (ODD) because their symptoms are not merely rule defiance. To address this problem, Leibenluft et al. (2003) proposed criteria for a clinical syndrome called “severe mood dysregulation” (SMD; Table 1), which was intended to capture these “irritable ADHD” youth. Importantly, these children are often diagnosed with BD in clinical settings despite, as noted above, the fact that they do not experience distinct episodes of mania (Pogge et al., 2001; Leibenluft et al., 2003; Reich et al., 2005). Unlike patients with ADHD alone, SMD youth have a mood disturbance. Unlike patients with ODD, SMD youth have an irritable mood not restricted to rule defiance and oppositionality. Furthermore, DSM-IV-TR does not operationalize or define irritability, although a number of DSM-IV-TR diagnoses involve irritability as either an explicit criterion (i.e., manic episodes of BD, depressive episodes of major depressive disorder Table 1 Severe mood dysregulation (SMD) criteria ( Leibenluft et al., 2003) Inclusion: (1) age 7–18 years; (2) markedly increased reactivity to negative emotional stimuli manifest verbally or behaviorally at least three times weekly; (3) abnormal mood (anger or sadness), present at least half of the day most days; (4) hyperarousal (≥3 of insomnia, agitation, distractibility, racing thoughts/flight of ideas, pressured speech, intrusiveness); (5) symptoms cause severe impairment in at least one setting (home, school, or peers) and at least mild impairment in a second setting; (6) SMD symptom onset must occur before age 12; (7) SMD symptoms must be currently present for at least 12 months without symptom-free periods greater than 2 months. Exclusion: (1) presence of cardinal bipolar symptoms including elevated/expansive mood, grandiosity/inflated self-esteem, or episodically decreased need for sleep (Geller et al., 1998); (2) distinct episodes greater than 1 day; (3) autistic or Asperger's disorder (4) psychosis; (5) substance abuse within 3 months; (6) medical illness that is unstable or could cause SMD symptoms; (7) IQ ≤ 70; (8) pregnancy.

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(MDD)) or an associated symptom (i.e., ADHD, ODD, intermittent explosive disorder, or pervasive developmental disorder (PDD)). To address this issue, SMD criteria precisely define irritability as (1) markedly increased reactivity to negative emotional stimuli manifest verbally or behaviorally at least three times weekly and (2) abnormal mood (anger or sadness), present at least half of the day most days. It remains unclear if SMD youth suffer from a “broad phenotype” of pediatric BD or if they have another form of psychiatric illness (NIMH, 2001). Using these SMD criteria, we conducted an exploratory magnetic resonance spectroscopy (MRS) study of SMD youth versus controls. Since ours is the first MRS study of SMD youth, we were guided by prior MRS studies of youth with related forms of psychopathology, including ADHD, BD, and MDD. As with those studies, we focused on three neurometabolites: (1) myo-inositol (mI), a marker of intra-cellular second messengers linked most closely to the pathophysiology of BD (Gould et al., 2004); (2) combined glutamate and glutamine (GLX), a marker of the major excitatory neurotransmitter glutamate whose resonances cannot be fully resolved in magnetic fields below 4 Tesla (Zarate et al., 2002; Stork and Renshaw, 2005); (3) and N-acetyl aspartate (NAA), a putative indicator of energetics within neuronal mitochondria (Tsai and Coyle, 1995; Stork and Renshaw, 2005). With regard to mI, manic or depressed adults with BD appear to have increased mI (Gould et al., 2004; Silverstone et al., 2005). Moreover, the anti-manic action of lithium is suggested to result from depletion of mI via non-competitive inhibition of inositol monophosphate phosphatase, thus causing inhibition of downstream serotonergic, glutamatergic, and cholinergic neurotransmission (Berridge and Irvine, 1989). At therapeutically relevant concentrations, lithium administration in BD adults is associated with significantly decreased frontal mI, with a similar trend in the hippocampus (Moore et al., 1999). In BD youth, lithium administration also results in significantly decreased frontal mI (Davanzo et al., 2001), although a recent report in depressed BD youth failed to corroborate this finding (Patel et al., 2006). In contrast, healthy controls do not have such mI alterations in response to lithium administration (Silverstone et al., 1996). However, studies examining the specificity of frontal mI alterations in BD youth compared with those with other psychiatric disorders have not demonstrated consistent deficits in BD subjects, possibly because these studies included some subjects taking psychotropic medications. Specifically, Davanzo et al. found increased frontal mI in BD youth versus both controls and those

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with intermittent explosive disorder (Davanzo et al., 2003), while Moore et al. did not find significant differences in frontal mI in comparisons between youth with primary ADHD and those with BD plus ADHD or controls, although the ratio of GLX/mI did differentiate ADHD subjects from the other two groups (Moore et al., 2006). In contrast, no differences in frontal mI have been shown in medication-free youth with either ADHD (Courvoisie et al., 2004) or MDD (Mirza et al., 2004) compared with controls. Alterations of GLX have been reported in ADHD, BD, and MDD. Of these, the most robust data are in ADHD, where four studies have demonstrated increased frontal and striatal GLX in both medicated and medication-free subjects with ADHD compared to healthy controls (MacMaster et al., 2003; Courvoisie et al., 2004; Moore et al., 2006; Carrey et al., 2007). For example, Moore et al. (2006) found that ADHD subjects had significantly greater anterior cingulate cortex GLX/mI than either healthy controls or those with BD and comorbid ADHD, although some ADHD and ADHD + BD subjects were taking psychotropic medications. A study of medication-free BD youth found greater GLX in both medial frontal cortex and basal ganglia compared with controls (Castillo et al., 2000). In MDD, two studies have found decreased frontal GLX in medication-free depressed youth (Mirza et al., 2004; Rosenberg et al., 2005). Thus, while there is evidence of perturbations in GLX levels in several childhood psychopathologies, the precise nature of these abnormalities remains unclear. NAA was our third metabolite of interest. Three of four studies of BD youth, including both medicated and medication-free subjects, have found decreased frontal cortex NAA (Castillo et al., 2000; Cecil et al., 2003; Chang et al., 2003; Sassi et al., 2005). In contrast, compared with controls, ADHD youth abstaining from stimulant medication for 24 h prior to scan had increased frontal NAA (Courvoisie et al., 2004), whereas medication-naïve depressed youth did not have altered frontal NAA (Farchione et al., 2002). No studies have examined the specificity of these findings by comparing youth with BD, ADHD, or MDD to one another. Based on the prior research outlined above, we conducted an exploratory study examining neurometabolite differences between SMD and healthy control youth. We focused on mI, GLX, and NAA quantified in four regions of interest (ROIs), but did not test specific hypotheses regarding the direction (increased or decreased) of the expected findings because of the limited data available regarding the pathophysiology of SMD.

2. Methods 2.1. Subjects SMD and healthy control subjects were enrolled in an IRB-approved protocol at the intramural NIMH. After the study was explained and prior to participation, parents gave written informed consent and children gave written assent. Subjects were recruited through advertisements placed on support groups' websites and distributed to psychiatrists nationwide. All MRS scans were conducted while SMD subjects were free of psychotropic medications for a minimum of four drug half-lives. This was done in the context of a double-blind randomized placebo-controlled study of lithium in SMD youth while subjects were admitted to the NIMH's pediatric psychiatry inpatient unit. Controls were naïve to psychotropic medication and had a negative lifetime psychiatric history. All MRS scans were conducted without sedation while subjects were at rest. SMD (N = 36) inclusion and exclusion criteria are listed in Table 1 (Leibenluft et al., 2003). Following a telephone interview to screen for relevant inclusion/ exclusion criteria, record review, and consultation with the child's treating clinician, potential SMD subjects were invited to the NIMH. On-site screening included the Child Schedule for Affective Disorders Present and Lifetime Version (K-SADS-PL) (Kaufman et al., 1997) with an additional SMD supplement, designed in collaboration with Joan Kaufman, Ph.D., to ascertain whether children met criteria for this syndrome. All diagnostic measures were administered to parent and child individually by trained graduate level clinicians with established interrater reliability (kappa N 0.9, including distinguishing SMD subjects from those satisfying the research criteria of Leibenluft et al. for narrow phenotype bipolar disorder—i.e. those with clear-cut episodes of elevated, expansive mood plus three DSM-IV-TR “B” symptoms) (Kaufman et al., 1997; Leibenluft et al., 2003). Diagnoses were based on best-estimate procedures (Leckman et al., 1982) generated in a consensus conference led by two psychiatrists with extensive experience evaluating children with bipolar-spectrum illness. Besides K-SADS-PL diagnostic interviews, clinicians also assessed SMD children using the Young Mania Rating Scale (YMRS) (Young et al., 1978), Children's Depression Rating Scale (CDRS) (Emslie et al., 1990), and Clinician's Global Assessment of Severity (CGAS) (Shaffer et al., 1983). Of note, SMD subjects' YMRS scores should not be interpreted as a measure of mania severity per se, but rather as a measure of the severity of the criteria “B” symptoms characterizing their illness

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Fig. 1. Voxel placement for each region of interest (ROI): (A) Frontal: Placed in the right orbitofrontal cortex (OFC). (B) Temporal: Placed in the left hippocampus. (C) Central Parieto-Occipital: Placed in vicinity of precuneus. (D) Parietal: Placed in white matter of corona radiata.

because SMD subjects do not fulfill DSM-IV-TR mania criteria. Our inpatient unit teachers completed the 39-item Conners teacher report of ADHD symptoms (Werry et al., 1975). Finally, we administered the Wechsler Abbreviated Scale of Intelligence (WASI) to determine full-scale intelligence quotient (FSIQ) in SMD subjects and controls. Typically developing child controls (N = 43) were also evaluated with an initial telephone screen followed by on-site K-SADS-PL administered by graduate level clinicians. Control inclusion criteria were: (1) age 7– 18 years; (2) negative past or present psychiatric history, including substance abuse; (3) negative past or present history of mood or anxiety disorders in control's firstdegree relatives; (4) identified primary care physician. Exclusion criteria were: (1) ongoing medical illness; (2) regular medication use; (3) history of abuse; (4) IQ ≤ 70; and (5) pregnancy. 2.2. MRS protocol MRS scans were performed on a 1.5 Tesla General Electric Signa scanner (GE, Milwaukee, WI) using the standard GE Point Resolved SpectroScopy (PRESS) sequence with the following parameters: echo time 30 ms, repetition time 2000 ms, number of excitations (NEX) = 8, water suppression on, and total acquisition time 5 min. Based upon prior MRS studies in BD adults, spectra were acquired in four 8-ml ROIs (Moore et al., 1999, 2000). MRI technologists, who were blind to participant group, placed each ROI to maximize gray matter content according to landmarks in: (1) the right frontal cortex (orbitofrontal cortex [OFC]; lateral to the falx cerebri, anterior to the frontal horn of the lateral ventricle, and superior to the orbits); (2) left temporal cortex centered on the left hippocampus (slice inferior to frontal voxel slice that maximally captures the hippocampus); (3) central parieto-occipital lobe (centered on posterior falx cerebri, posterior to cingulate gyrus, and including midline precuneus), and (4) left parietal lobe

predominantly white matter (same slice as parietooccipital voxel centered on left corona radiata and lateral to posterior horn of the lateral ventricle) (Fig. 1). 2.3. Quantitative MRS analysis All MRS scans were analyzed with the Linear Combinations Model (LCModel, v6.1-0, Steven Provencher, Ontario, Canada). As described elsewhere (Provencher, 1993), the LCModel is an automated program for in vivo MRS analysis that fits acquired MRS data based on standard reference information supplied by the manufacturer. While the full reference set was used for spectral fitting, our data analysis focused on the MRS intensities of: (1) glutamate/glutamine (GLX, which cannot be fully resolved at 1.5 Tesla) at a chemical shift of 2.1–2.5 parts per million (ppm), (2) NAA at 2.02 ppm, and (3) mI at 3.5 ppm (see Fig. 2 Sample spectra). Metabolites were normalized to the creatine/ phosphocreatine (Cr) resonance at 3.02 ppm in order to avoid partial volume effects associated with the alternative approach—i.e., using the water signal as a reference (Komoroski et al., 2004; Hancu et al., 2005; Gasparovic et al., 2006). To ensure data quality, we used Cramer-Rao bounds of 20% or less as a cutoff for each metabolite. 2.4. Data analysis Given that this is the first spectroscopy study in a large sample of medication-free SMD youth, our goal was to conduct an exploratory study that would reduce the risk of Type II errors and generate findings to be pursued in future studies. Therefore, we used a statistical threshold of α = 0.05, without correction for the number of statistical tests, and sample size calculated to provide 90% power to detect between-group differences having an effect size of 0.8. We conducted independent sample t-tests and chi-squared analyses for categorical variables implemented in the Statistical Package for Social

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Fig. 2. Sample LC Model Processed Spectra. Black line represents acquired spectra; red line represents fitted spectra from the same subject. Horizontal axis is chemical shift in parts per million (ppm). LC Model: version 6.1-0, S.W. Provencher, Ontario Canada. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Sciences (Chicago, IL, v14.0). Within the SMD sample, we also conducted Pearson correlations between GLX/ Cr, mI/Cr, NAA/Cr, and creatine concentrations in each

voxel and ratings of mood (CDRS, YMRS), ADHD symptoms (Conners 39-item teacher rating), FSIQ, and function (CGAS). Lastly, given emerging data about

Table 2 Sample demographics Group

Age Full-scale IQ a Sex

Severe mood dysregulation (SMD; N = 36)

Normal controls (NC; N = 43)

Mean

S.D.

Mean

S.D.

12.2 101.6 Male: 25 Female: 11

2.1 14.6

12.9 107.1 Male: 22 Female: 21

2.3 13.9

K-SADS-PL diagnoses: number (%) ADHD Oppositional Defiant Disorder Major Depressive Disorder Generalized Anxiety Disorder Separation Anxiety Disorder Simple Phobia Social Phobia

– – – – – – –

32 (88.9%) 32 (88.9%) 8 (22.2%) 13 (36.1%) 9 (25%) 6 (16.7%) 6 (16.7%)

Mood/functional ratings in SMD subjects YMRS CDRS Conners Teacher Form (ADHD) CGAS a

12.1 28.2 37.5 49.3

FSIQ not obtained in 2 SMD and 2 NC subjects.

6.7 6.3 17.2 8.3

– – – –

P = 0.14; df = 77 P = 0.11; df = 73 χ2 (1,79) = 2.72 P = 0.10

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gender effects on brain development, we conducted a post-hoc examination of potential gender effects by dividing our sample by gender and repeating the analyses (Lenroot et al., 2007).

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3.3. MRS results We conducted a total of 16 statistical tests (betweengroup contrasts for 4 voxels and 4 metabolites), only one of which was statistically significant. Compared with controls, medication-free SMD subjects had significantly lower mI/Cr in the temporal lobe (P = 0.03) (Table 3). There were no other significant betweengroup differences in mI/Cr, NAA/Cr, or GLX/Cr in any of the other ROIs. Furthermore, there were no significant correlations in the SMD sample between decreased mI/Cr and mood/functional measures—i.e. YMRS, CDRS, Conner's teacher form, CGAS, or FSIQ.

3. Results 3.1. Subject characteristics SMD subjects did not differ from controls (NC) in age (SMD mean 12.2 ± 2.1 years, NC mean 12.9 ± 2.3 years), FSIQ (SMD mean 101.6 ± 14.6, NC mean 107.1 ± 13.9), or sex (SMD 11 females and 25 males; NC 21 females and 22 males). The most common DSM-IV-TR diagnoses in the SMD subjects were ADHD (N = 32, 88.9%) and oppositional defiant disorder (ODD; N = 32, 88.9%). SMD subjects had moderate functional impairment (CGAS mean 49.3 ± 8.3). SMD subjects were not depressed (CDRS mean 28.2 ± 6.3) nor did they have elevated YMRS scores at the time of the scan (YMRS mean 12.1 ± 6.7) (Table 2).

3.3.1. Post-hoc examination of gender effects To evaluate the effect of gender, we divided the sample by gender and repeated the above analyses. We did not find any significant differences for any metabolite in any ROI, comparing female patients with SMD to female controls and comparing male patients with SMD to male controls. However, we found the following non-significant trends in female patients with SMD vs. female controls: (a) decreased temporal mI/Cr (female SMD 0.93 ± 0.27; female NC 1.28 ± 0.56; t = 1.99, df = 28, P = 0.07), (b) increased temporal Cr (female SMD 5.38 ± 2.04; female NC 4.41 ± 0.98; t = 1.79, df = 28, P = 0.08), and (c) increased parietal GLX/Cr (female SMD 2.51 ± 0.92; female NC 1.85 ± 0.29; t = 1.99, df = 7.8, P = 0.08). Since we found decreased temporal mI/Cr in patients vs. controls in the overall sample, we also compared temporal mI/Cr in the control sample stratified by gender and found that female controls had significantly increased temporal mI/Cr vs. male controls (female NC 1.28 ± 0.56; male NC

3.2. MRS quality Over 85% percent of acquired mI/Cr, NAA/Cr, and Cr spectra met our spectral quality standard of 20% CramerRao lower bounds. For these metabolites, there was no significant between-group difference in percentage of acquired spectra meeting this standard. With regard to GLX/Cr, fewer spectra met this standard. Furthermore, there was a significant between-group difference in frontal GLX/Cr, with fewer controls meeting this standard (SMD 86% included, NC 67% included; χ2 = 3.74, P = 0.05).

Table 3 MRS results in youth with severe mood dysregulation (SMD) versus normal controls (NC) ROI

Frontal Temporal Parietal Parietooccipital

Group

SMD NC SMD NC SMD NC SMD NC

mI/Cr

NAA/Cr

GLX/Cr

Cr

Mean

S.D.

N

Mean

S.D.

N

Mean

S.D.

N

Mean

S.D.

N

0.90 0.93 0.96⁎ 1.13⁎ 0.90 0.87 0.82 0.85

0.23 0.34 0.18 0.44 0.31 0.24 0.11 0.16

34 (94%) 38 (88%) 34 (100%) 41 (100%) 34 (97%) 42 (98%) 34 (100%) 39 (95%)

1.37 1.39 1.16 1.17 1.50 1.51 1.36 1.36

0.21 0.38 0.18 0.19 0.23 0.16 0.15 0.18

34 (94%) 43 (100%) 33 (97%) 41 (100%) 34 (97%) 42 (98%) 34 (100%) 41 (100%)

2.25 2.37 2.17 2.18 2.08 1.93 2.29 2.13

0.57 0.93 0.36 0.50 0.62 0.32 0.37 0.41

31 (86%) 29 (67%) 32 (94%) 33 (80%) 28 (80%) 31 (72%) 31 (80%) 33 (72%)

4.59 4.72 4.81 4.63 4.52 4.63 5.15 5.15

0.87 1.50 1.25 0.87 0.78 0.55 0.49 0.61

36 (100%) 43 (100%) 34 (100%) 41 (100%) 34 (97%) 43 (100%) 34 (100%) 41 (100%)

⁎Temporal mI/Cr SMD N NC P = 0.03 t = − 2.26. MRS data were obtained in the following numbers of SMD and NC subject, by ROI: (i) Frontal SMD N = 36, NC N = 43; (ii) Temporal SMD N = 34, NC N = 41; (iii) Parietal SMD N = 35, NC N = 43 (iv) Parieto-occipital SMD N = 34, NC N = 41. Then, those data meeting Cramer-Rao lower bounds of 20% or less were included for analysis. N (%) = Number and (percent) of total spectra acquired that were included in each analysis after meeting the above Cramer-Rao cutoff for data quality.

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0.99 ± 0.20; t = 2.18, df = 23.7, P = 0.04), but there was no significant difference in temporal mI/Cr in female SMD patients vs. male SMD patients (female SMD 0.93 ± 0.27; male SMD 0.98 ± 0.14; t = 0.56, df = 11, P = 0.59). 4. Discussion We sought to address potential confounding variables present in prior MRS studies. First, given that psychotropic medications, including lithium, valproate, and psychostimulants, have been shown to alter neurometabolite concentrations (Silverstone et al., 2002a,b; Carrey et al., 2003; O'Donnell et al., 2003; Silverstone et al., 2003), we only scanned medication-free SMD subjects. Second, our SMD and control samples were substantially larger than prior pediatric MRS studies, which frequently involve groups of 12 or fewer subjects. Third, although SMD is not a DSM-IV-TR diagnosis, our patient sample was rigorously evaluated by experienced graduate-level clinicians with high inter-rater reliability. These clinicians used both the K-SADS-PL and a module operationalizing the SMD criteria, thus reducing heterogeneity in the SMD group. Limitations of our study include the fact that we used a liberal statistical threshold. Since this is the first neuroimaging study of SMD youth, we wished to avoid type II errors and generate preliminary findings. We found that SMD subjects had decreased mI/Cr in our temporal ROI. Moreover, post-hoc analyses suggest that gender may play a role, with female controls having greater temporal mI/Cr than either female SMD subjects or male controls. Yet, these findings would not remain statistically significant after correction for multiple comparisons. However, further research is warranted to evaluate the neurobiology of SMD, including gender and developmental effects. Since SMD may represent a developmental subtype of BD, and since prior studies provide some evidence of increased frontotemporal mI/Cr in youth with BD, one might have expected SMD youth to exhibit a similar abnormality. However, our study indicates medicationfree SMD youth have decreased, as opposed to increased, temporal mI/Cr. Unfortunately, methodological differences, including different ROI locations and magnetic field strength, limit direct comparisons with prior MRS studies in BD youth. Of two such studies of BD youth, one showed that increased mI/Cr differentiated BD youth from controls as well as from youth with intermittent explosive disorder (Davanzo et al., 2003), while a second found no difference between youth with BD and controls (Chang et al., 2003). Importantly, both prior pediatric BD studies included some subjects who were taking psycho-

tropic medications, which have been shown to alter mI (Silverstone et al., 2002a,b), and both included fewer than 15 subjects per group. Thus, it is difficult to determine if our present finding of decreased, rather than increased, temporal cortex mI/Cr indicates that the molecular underpinnings of the SMD syndrome (which may or may not be on the “bipolar spectrum”) differ from those of BD, or if such differences from prior MRS studies of BD youth result from medication effects or small sample sizes. Additionally, while prior studies have not shown mI/Cr differences in either ADHD or MDD youth, such studies have not evaluated the same temporal ROI as our present study (Courvoisie et al., 2004; Mirza et al., 2004). Further work, including a direct comparison of neurometabolites in medication-free BD vs. SMD youth, is needed to determine whether, and how, the pathophysiology underlying these two clinical presentations differs. It is possible that our largely negative findings are due to the fact that the SMD criteria identify a heterogeneous sample. However, data suggest that the SMD syndrome has construct validity, in that the criteria identify a group of pre-teens at high risk for depressive illness in early adulthood (Brotman et al., 2006). Furthermore, recent pathophysiological studies differentiate SMD youth from controls and, in some instances, from patients with BD. These findings include deficits in early attentional processes and reward-related processes, including reversal learning (Rich et al., 2007; Dickstein et al., 2007). These studies support the position that SMD criteria may define a distinct, clinically important group of children and adolescents who do not fit into current DSM-IV-TR nosology. Unfortunately, it is not possible to parse out the extent to which our present findings are due to either ADHD or ODD symptoms in our SMD subjects because of multicollinearity due to the overlap in symptoms. Such ADHD and ODD symptoms should not be considered comorbid to SMD, since SMD is not a DSM-IV-TR diagnosis. While many SMD subjects have ADHD and/or ODD symptoms, the reverse would not be true, as suggested by the fact that SMD subjects have considerably more functional impairment, i.e., high rates of suicidality, psychiatric hospitalization, and treatment with two or more psychotropic medications (Dickstein et al., 2005). Determining how the neurobiology of SMD differs from that of ADHD and/or ODD requires studies directly comparing these three subject groups on the same brain/behavioral measures. Another potential limitation is that SMD subjects were medication-free but not medication-naïve. While it is known that psychotropic medications influence concentrations of MRS neurometabolites (Silverstone et al., 2002a,b; Carrey et al., 2003; O'Donnell et al., 2003;

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Silverstone et al., 2003), it remains unknown if such medications have a residual effect after they have been discontinued. To minimize this impact in our present study, SMD subjects underwent drug washout for four drug half-lives on our inpatient psychiatric research unit. Further study of pediatric patients before and after exposure to psychotropic medications, including antimanic agents, are required to fully determine the impact of these medications on neurochemistry (DelBello et al., 2006). Beyond patient factors, our present study is limited by the MRS methodology. Our study evaluated neurometabolites in four ROIs, but different ROI locations or a whole-brain approach might result in different findings. Another potential limitation is the use of Cr ratios since some have reported between-group differences in studies of BD or MDD (Cecil et al., 2003; Mirza et al., 2004). Cr is commonly used as a reference signal in MRS studies, including those of youth with BD and ADHD (Davanzo et al., 2001; Chang et al., 2003; Moore et al., 2006), because it emanates from the same tissue compartment (gray or white matter) as other metabolites (mI, NAA, GLX). Alternatively, if absolute concentrations are reported, the water signal used as a reference may come from gray matter, white matter, or CSF. There could be a substantial contribution of the water signal from CSF, but CSF does not contain any metabolites, resulting in artificially altered concentrations depending on the amount of CSF in the voxel. Thus, absolute concentrations referenced to the water signal may be more susceptible to variability due to between-subject differences in voxel placement causing partial-volume effects—i.e., having some white matter or CSF present in a primarily gray matter voxel—or to minor MR scanner fluctuations (Komoroski et al., 2004; Hancu et al., 2005). Segmentation is not a perfect solution to this problem because different segmentation routines may lead to considerably different MRS results, especially for voxels centered in gray matter, such as ours (Gasparovic et al., 2006). Clearly, there is a need for further study of such MRS methodological issues in children and adolescents. Lastly, our GLX/Cr findings should be interpreted with caution since magnetic fields weaker than 4 Tesla cannot fully resolve this peak into its primary components of glutamate and glutamine (Zarate et al., 2002; Stork and Renshaw, 2005). Such heterogeneity of the GLX peak is illustrated in our present 1.5 Tesla study by the fact that fewer GLX/Cr spectra met our Cramer-Rao MRS quality cutoff than those of the other metabolites of interest. Nevertheless, our study is a first step towards understanding the neural basis of functionally disabling irri-

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tability and mood dysregulation in children and adolescents. Further MRS research is necessary to explore potential neurobiological differences between SMD youth and those with other forms of psychopathology involving irritability, including those meeting DSMIV-TR criteria for BD, ADHD, MDD, and ODD. Where possible, such studies should include direct comparisons of neurometabolites in two or more patient groups, as well as in healthy controls. Also, such studies should attempt to evaluate medication-free subjects in order to avoid potential medication effects, although research requirements must be secondary to the patients' clinical needs. Moreover, studies pairing neuroimaging techniques, including MRS, with treatment are needed to elucidate the brain/behavior effects of such treatments, both psychopharmacologic and psychotherapeutic, on children and adolescents suffering from SMD and other forms of psychopathology (DelBello et al., 2006). 5. Conclusion In conclusion, psychiatrists are increasingly confronted with children and adolescents suffering from ADHD symptoms plus functionally disabling nonepisodic irritability who are diagnostically homeless in current nosology. Using well-operationalized criteria to identify these SMD youth, our present study is a first attempt to explore the neurobiology of this syndrome. We found that medication-free SMD youth have decreased temporal mI/Cr compared with findings in typically developing controls. Given prior studies implicating mI in lithium's action in BD adults and youth, further work is necessary to determine potential therapeutic implications of our present finding, as well as to determine how SMD youth are pathophysiologically different from those with strictly defined BD. Acknowledgement Funding for this study was provided by the NIMH DIRP. Dr. Dickstein is also supported by NIMH K22 MH74945 and a NARSAD Young Investigator Award. The authors of the present manuscript do not have any current financial ties with any for-profit enterprises, including industry research funding, consulting relationships, or industry speaker's bureaus. References American Psychiatric Association, 2000. Diagnostic and Statistical Manual of Mental Disorders 4th Edition Text Revision (DSM-IVTR). APA, Washington, D.C.

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