- Email: [email protected]
Evidence of Brain Dysfunction in Attention Deficit-Hyperactivity Disorder: a Controlled Study with Proton Magnetic Resonance Spectroscopy
Evidence of Brain Dysfunction in Attention DeficitHyperactivity Disorder: a Controlled Study with Proton Magnetic Resonance Spectroscopy1 Nicolás Fayed, Pedro J. Modrego, Julio Castillo, Jorge Dávila
Rationale and Objectives: Attention deficit-hyperactivity disorder (ADHD) is a socially disabling condition whose pathophysiology is mostly unknown. Previous magnetic resonance imaging (MRI)-based reports have shown structural abnormalities in the prefrontal region and the striatum, but with inconsistencies across the studies with regard to right/left specificity of changes. Our study is aimed at finding evidence of dysfunction with more refined MRI techniques such as diffusion-weighted MRI and spectroscopy. Materials and Methods: We enrolled 22 ADHD children (mean age 9; SD 2.91) and 8 healthy children (mean age 7.5; SD 3). All of them underwent diffusion-weighted MRI in several areas of the brain bilaterally: prefrontal, lentiform nucleus, posterior cingulate, and centrum semiovale; and single-voxel proton magnetic resonance spectroscopy in the left centrum semiovale and right prefrontal region. Results: We did not see either apparent structural abnormalities of the brain in conventional MRI or differences in the apparent-diffusion coefficients in any of the areas studied. However, we observed significant differences in the N-acetylaspartate/creatine ratios in relation to controls in the right prefrontal corticosubcortical region: 1.58 (SD 0.09) versus 1.47 (0.08), P ⫽ .01); and in the left centrum semiovale: 2.02 (0.13) versus 1.79 (0.13), P ⫽ .0003. This finding is consistent with a published report on eight ADHD children in whom N-acetyl-aspartate/creatine ratios were also elevated. Conclusions: Given these results, we hypothesize that a biochemical dysfunction might underlie in the brain of ADHD children. The N-acetyl-aspartate/creatine ratio may be regarded as a potential marker of the disease. Key words: Attention deficit-hyperactivity disorder; magnetic resonance spectroscope. ©
AUR, 2007
INTRODUCTION Attention deficit-hyperactivity disorder (ADHD) is a common developmental disorder afflicting 3–7% of school-age children. It is characterized by overactivity, impulsivity, and
Acad Radiol 2007; 14:1029 –1035 1
From the Magnetic Resonance Unit, Clinica Quirón (N.F., J.C., J.D.), and Department of Neurology, Hospital Universitario Miguel Servet, Avda Isabel la Católica 1-3 (P.J.M.), 50009 Zaragoza, Spain. Received March 24, 2007; accepted May 27, 2007. Address correspondence to: PJM. E-mail: [email protected]
© AUR, 2007 doi:10.1016/j.acra.2007.05.017
inattentiveness, which lead to negative academic and social consequences (1). In the past, these children were considered to have minimal brain dysfunction, but so far the etiology and pathophysiology are mostly unknown. Magnetic resonance (MR)-based techniques have provided useful information and clues about the areas of the brain that are supposedly involved in this disorder. The majority of studies agree that the reduced volumes underlay in areas involved in attentional and executive functions (prefrontal-striatal areas). However, there are inconsistencies across the studies, especially in relation to right/left predominance of decreased brain volumes and other changes.
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Smaller volumes than normal were found in the right frontal lobe as well as cingulate and striatal regions (2). The mean right caudate volume was significantly smaller in 50 ADHD boys in comparison to controls, suggesting abnormalities of frontal-striatal circuits (3). Another work with 15 ADHD children found no differences in hemispheric volumes, but smaller left caudate volumes in relation to controls (4). Slightly smaller total volumes were found but no differences in asymmetries in 50 ADHD girls (5), but in 50 boys, the same authors found smaller right globus pallidus and right anterior frontal region (6). Several studies with volumetry have also disclosed interesting findings in frontal lobes. Reduction in gray-matter volumes was more marked in the right prefrontal area but white matter volume reduction was specific to the left hemisphere (7). In a group of 30 ADHD children, there were volume reductions in the right prefrontal gray matter (8). However, in a study with 25 ADHD children, the smaller volumes were specific for left cortical areas (motor, premotor, whole left cingulate, temporal, and parahippocampal) with no differences in the white matter (9). In 28 ADHD boys, the smaller volumes in comparison to controls were those of right frontal-pallidal-parietal gray matter as well as white matter bilaterally, with cerebellar and striatal volume deficits in boys with comorbidities (10). Prefrontal gray-matter volumes were also reduced in ADHD adults (11). Publications with radiologic functional studies are scarce. Positron emission tomography (PET) offers information on glucose metabolism in the whole brain and in selected areas. Global and regional (prefrontal and premotor cortex) glucose metabolism was reduced in 25 adults with ADHD in relation to normal adults (12). Two other PET studies with small samples of adolescents did not see global differences in the glucose metabolism (13,14). Proton magnetic resonance spectroscopy may be useful because provides information on living tissues. A previous report with 23 ADHD children did not find significant differences in neurometabolites of the dorsolateral frontal region (15). There are two works reporting higher N-acetyl-aspartate/creatine ratios in eight ADHD children than in controls in the right frontal area (16) and the left centrum semiovale (17), respectively. The aim of the present study is to search for and find possible structural and functional changes in a larger sample of ADHD children by means of more refined MR techniques: diffusion-weighted MR imaging and MR spectroscopy of the brain. Based on the afore-
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mentioned reports, we hypothesize that a marker of the disease could be found in prefrontal region or centrum semiovale.
PATIENTS AND METHODS In this cross-sectional study, we consecutively enrolled 22 adolescent medication-naive patients fulfilling the DSM-IV-TR criteria of ADHD (1) with neither learning disabilities nor comorbidities. Written consent was obtained from the children’ parents. These children were referred by pediatricians and neurologists for radiologic evaluation. The mean age was 9 years (SD 2.91; range 6 –16), with 18 boys and 4 girls. We also recruited eight healthy children as the control group with a mean age of 7.5 years (SD 3; range 4 –12); four boys and four girls. The difference of age between patients and controls was not statistically significant (P ⫽ .2). All of them underwent T1- and T2-weighted conventional MR imaging of the brain on a 1.5 T clinical scanner (Signa Horizon; General Electric Medical Systems; Milwaukee, WI). Transverse single-shot echoplanar diffusion-weighted MR imaging was carried out according to the following parameters: echo time 97 milliseconds; repetition time 10,000 milliseconds; flip angle 90°; slice thickness 5 mm; interslice thickness 2.5 mm; field of view 24 ⫻ 24; matrix size: 128 ⫻ 128; number of excitations 1; diffusion sensitive gradient (b value) 1,000 seconds/mm2. The following areas of the brain (regions of interest) were studied bilaterally: prefrontal, lentiform nucleus, posterior cingulate, and centrum semiovale (Fig 1). The whole brain was imaged with 20 sections, and we acquired four images per section. On three images per section the apparent diffusion coefficient (ADC) was calculated and gray scale encoded with a diffusion-sensitizing gradient in x, y, and z directions. On a fourth image, the directionally averaged ADC was calculated and gray scale encoded as the average of the three directional coefficients. The ADC values were measured within every region of interest. The examined areas were chosen on the basis of the previous substantive publications concerning ADHD and pointing to either structural abnormalities or dysfunction therein. MR spectroscopy was performed as follows: sagittal T1-weighted topogram and T2 axial localizing series were used to place volume (2 ⫻ 2 ⫻ 2 cm) in the left centrum semiovale (Fig 2) and the right prefrontal corticosubcortical area just before the genu of the corpus
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Figure 3. T2-weighted magnetic resonance imaging with voxel placement in the right prefrontal region. Figure 1. Echo-planar diffusion-weighted magnetic resonance imaging showing the regions of interest explored bilaterally: prefrontal, lentiform nucleus, centrum semiovale, and posterior cingulate.
Figure 2. T2-weighted magnetic resonance imaging with voxel placement in the left centrum semiovale.
callosum (Fig 3). Both areas were chosen because of they are free of partial volume effect and contamination from cerebrospinal fluid, and because previous works found abnormalities therein. Single-voxel 1HMRS was carried out by means of an echo time of 30 milliseconds and a repetition time of 2,500 milliseconds with spin-echo technique that uses selective excitation with gradient spoiling for water suppression. The mode of spectral acquisition was probe-p (PRESS technique). The pure metabolite signal was spoiled, zerofilled, and Fourier transformed to produce a spectrum, scaled, drawn onto a 512 ⫻ 512 image, and stored as an image in the system database. Every spectrum was automatically fitted to four peaks corresponding to levels of N-acetyl-aspartate (NAA), 2.02 ppm; total creatine (Cr), 3.03 ppm; choline-containing compounds (Ch), 3.23 ppm; and myo-Inositol (mI), 3.56 ppm. We also obtained the peak amplitude of the metabolites relative to Cr. For this purpose, we used the algorithms provided by the manufacturer with the following steps: 1. Setting a global frequency fit parameter. 2. Performing line width and line shape enhancement by appropriate apodization of the time domain signal.
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Table 1 Mean Apparent Diffusion Coefficients (ADC) by Every Area Area Examined Right prefrontal Left prefrontal Right striatum Left striatum Right cingulate Left cingulate Right centrum semiovale Left centrum semiovale
Mean ADC of patients (n ⫽ 24) 6.6993 6.8423 5.8381 5.8195 7.3568 7.1395 6.1618 5.8192
E-10 E-10 E-10 E-10 E-10 E-10 E-10 E-10
Mean ADC of controls (n ⫽ 8) 6.9925 6.7462 5.7475 5.79 7.2112 7.1287 6.1925 5.8685
E-10 E-10 E-10 E-10 E-10 E-10 E-10 E-10
NS NS NS NS NS NS NS NS
Table 2 Mean and SD Values of the Metabolites Relative to Cr Left Centrum Semiovale Metabolite Ratio to Cr NAA/Cr Ch/Cr mI/Cr Right Prefrontal Region Metabolite Ratio to Cr NAA/Cr Ch/Cr mI/Cr
ADHD Children (n ⫽ 24)
Controls (n ⫽ 8)
2.02 (SD 0.13) 1.15 (SD 0.12) 0.67 (SD 0.09)
1.79 (SD 0.13) 1.13 (SD 0.2) 0.67 (SD 0.07)
ADHD Children (n ⫽ 24) 1.58 (SD 0.09) 0.87 (SD 0.13) 0.73 (SD 0.06)
Controls (n ⫽ 8) 1.47 (SD 0.08) 0.84 (SD 0.07) 0.72 (SD 0.07)
Significance P ⫽ .0003 P ⫽ .6 P ⫽ .8 Significance P ⫽ .01 P ⫽ .07 P ⫽ .6
Note.—Cr ⫽ creatine; ADHD ⫽ attention deficit hyperactivity disorder; NAA ⫽ N-acetyl-aspartate/ creatine; Ch ⫽ choline; mI ⫽ myo-Inositol
3. Fourier transformation of the signal to the appropriate frequency resolution and number of points. 4. Calculation of a baseline correction from the frequency domain signal 5. Curve fitting the desired regions of the frequency domain signal. The test-retest reliability of the NAA/Cr ratios in our scanner was previously assessed in 15 healthy people with an intraclass correlation coefficient of 0.92 (18). Statistical analysis was based on t-test for mean comparisons of the metabolite ratios of patients and controls. Owing to the small sample size of controls we also used a nonparametric test (Mann-Whitney U test) so as to confirm significance of differences.
RESULTS T1- and T2-weighted MR images were evaluated by two radiologists without finding structural differences
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between patients and controls. With regard to diffusion-weighted images in Table 1 are reported the mean ADC values for every area studied. We did not detect significant differences in any of the areas. The acquisition of the MR spectra was successful in all of the children. The absolute values of NAA tended to be higher in patients than in controls but without reaching statistical significance. Table 2 reports the metabolite values relative to Cr disclosed by the MR spectroscopy in patients and controls. Figures 4 and 5 are examples of spectrum for the left centrum semiovale and the right prefrontal region, respectively. Whereas no significant differences were found for Ch/Cr and mI/Cr ratios, these were significant for NAA. In the left centrum semiovale, the mean NAA/Cr ratio was 2.02 (SD 0.13) for ADHD children versus 1.79 (SD 0.13) for controls; t ⫽ 4.08, P ⫽ .0003. In the right prefrontal area, the mean NAA/Cr value for ADHD children was 1.58 (SD 0.09) in comparison to 1.47 (SD: 0.08) for controls; t ⫽ 2.74, P ⫽ .01. This
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Figure 6. Box plot representing the ordinal values for the N-acetyl-aspartate/creatine ratios obtained in the left centrum semiovale of patients and controls.
Figure 4. Example of spectrum of an attention deficit hyperactivity disorder boy in the left centrum semiovale. NAA ⫽ N-acetyl aspartate.
Figure 7. Box plot representing the ordinal values for the N-acetyl-aspartate/creatine ratios obtained in the right prefrontal region of patients and controls.
DISCUSSION
Figure 5. Example of spectrum in the right prefrontal region of a healthy boy.
significance was also seen in the nonparametric test: P ⫽ .001, and P ⫽ .02 for the NAA/Cr ratios in the centrum semiovale and prefrontal region, respectively. In Fig 6 and 7 are box plots with the ordinal values of the NAA/Cr ratios for semiovale and prefrontal area, respectively.
In spite of the large amount of publications on ADHD, the pathophysiology is unknown. In addition, clinicopathologic correlates are lacking and the neuroradiologic studies show discordant results, although most of them point to the right prefrontal region as the area with the supposed underlying dysfunction. The prefrontal cortex regulates attentional control, organization, and planning (19). The enigma of what the specific dysfunction is has not been untangled. That the patients improve with stimulants blocking catecholamine reuptake (eg, methylphenidate) has led to the hypothesis of catecholamine deficiency and high levels of dopamine transporters (20). However, a
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PET study published recently did not corroborate increases in dopamine transporters that were even reduced in some patients (21). An alternate hypothesis of hyperdopaminergic and hypernoradrenergic state in the striatum has emerged giving rise to bewilderment. The latter hypothesis is based on two facts: 18F-DOPA hyperactivity in the striatum and nucleus accumbens assessed by PET in ADHD children (22), and the correlation between the cerebrospinal fluid concentration of homovalinic acid, a metabolite of dopamine, and the severity of the disease (23). In our study, we were not able to demonstrate structural abnormalities in ADHD children even with diffusion MR imaging. We recognize that diffusion-weighted imaging is not sensitive enough to detect changes in this disorder. Alterations in the white matter were seen in a study with diffusion tensor imaging in the right premotor and striatal regions of 15 ADHD patients in which fractionated anisotropy was decreased in comparison to controls (24). This technique would have been preferable, but it was not available until recently in our hospital facilities. The concentration of some metabolites assessed by MR spectroscopy offers important information on neuronal and axonal functioning. NAA is one of the most abundant amino acids in the central nervous system. Although it is labeled as a surrogate marker on neuronal and axonal density, its function is not completely understood (25). NAA levels have been also correlated with mitochondrial energy metabolism (26). The postnatal membrane turnover has been reported to be high with increasing brain concentrations of NAA and Cr, which return to adult levels by adolescence years (27,28). In this study, the mean NAA/Cr ratio obtained in the centrum semiovale was 2.02 in ADHD children compared with 1.79 in controls. In our previous study with 8 ADHD children and 12 controls (17), the mean NAA/Cr ratios were 2.2 and 1.9, respectively. The subjects of both studies underwent MR spectroscopy on the same clinical scanner set at the same parameters, and in both studies the differences are significant. Based on one of the hypotheses mentioned previously, we could hypothesize that in ADHD a hypercatecholaminergic state might increase mitochondrial metabolism and NAA/Cr ratios. A failure to downregulate the NAA levels was also reported to occur in the prefrontal lobe of 14 patients with Asperger syndrome in which those levels correlated with obsessive behavior (29). However, a reduction in the absolute concentration of NAA was reported in a small sample of five ADHD adult patients in dorsolateral prefrontal cortex (30) and a reduc-
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tion in the NAA/Cr ratio in the lentiform nuclei (31). In these two studies the levels were especially lower in the subgroup with hyperactivity. Hopefully, other hypothesis could be taken into account to explain the symptoms of the disorder. Significant bilateral decreases in the pallidal NAA/Cr ratios were observed in 12 ADHD boys. Treatment with methylphenidate did not change these ratios, so neuronal loss or severe striatal dysfunction was suggested as causative factor (32). Changes in other metabolites are seldom reported and its significance is poorly understood. mI/Cr ratios were reported to be elevated in relation to controls in both frontal regions (16). The involvement of the glutamatergic system was suggested as glutamate-glutamine/Cr ratios were reduced in the right anterior cingulate cortex (33) and in the striatum (34). More recently, another study revealed greater glutamate/glutamine and Cr in the striatum of ADHD children in comparison to controls but not in prefrontal cortex (35). The small size of the samples is the main limitation to interpret the results of the studies with MRS. We understand that it is not easy to recruit larger samples of children with ADHD, but this problem also occurs with PET studies. Our study confirms dysfunction in the right prefrontal region found in previous studies, and dysfunction in the white matter found by us in a small sample of eight ADHD children. Although the ADHD patients are slightly older than controls, this fact is unlikely to explain the differences in the NAA/Cr ratios because the maximum levels of NAA are reached at age 4 years (36). We conclude that the NAA/Cr ratio could be a marker of the disease but, given the some degree of overlapping of NAA/Cr values seen between patients and controls, further studies are warranted before recommending MR spectroscopy to ADHD children as systematic ancillary exploration. REFERENCES 1. The American Psychiatric Association. Diagnostical and statistical manual of mental disorders, ed. 4 Arlington, VA: The American Psychiatric Association, 2000. 2. Hynd GW, Semrud-Clikeman M, Lorys AR, et al. Brain morphology in developmental dyslexia and attention deficit disorder/hyperactivity. Arch Neurol 1990; 47:919 –926. 3. Castellanos FX, Giedd JN, Eckburg P, et al. Quantitative morphology of the caudate nucleus in attention deficit hyperactivity disorder. Am J Psychiatry 1994; 151:1791–1796. 4. Filipek PA, Semrud-Clikeman M, Steingard RJ, et al. Volumetric MRI analysis comparing subjects having attention-deficit hyperactivity disorder with normal controls. Neurology 1997; 48: 89 – 601.
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5. Castellanos FX, Giedd JN, Berquin PC, et al. Quantitative brain magnetic resonance imaging in girls with attention-deficit/hyperactivity disorder. Arch Gen Psychiatry 2001; 58:289 –295. 6. Castellanos FX, Giedd JN, Marsh WL, et al. Quantitative brain magnetic resonance imaging in attention-deficit hyperactivity disorder. Arch Gen Psychiatry 1996; 53 607– 616. 7. Mostofsky SH, Cooper KL, Kates WR, et al. Smaller prefrontal and premotor volumes in boys with attention-deficit/hyperactivity disorder. Biol Psychiatry 2002; 52:785–794. 8. Durston S, Hulshoff Pol HE, Schnack HG, et al. Magnetic resonance imaging of boys with attention-deficit/hyperactivity disorder and their unaffected siblings. J Am Acad Child Adolesc Psychiatry 2004; 43:332–340. 9. Carmona S, Villarroya O, Bielsa A, et al. Global and regional gray matter reductions in ADHD: a voxel-based morphometric study. Neurosci Lett 2005; 389:88 –93. 10. Mc Alonan GM, Cheung V, Chua SE, et al. Mapping brain structure in attention deficit-hyperactivity disorder: A voxel-based MRI study of regional grey and white matter volume. Psychiatry Res 2007; 154:171– 180. 11. Seidman LJ, Valera EM, Makris N, et al. Dorsolateral prefrontal and anterior cingulate cortex volumetric abnormalities in adults with attentiondeficit/hyperactivity disorder identified by magnetic resonance imaging. Biol Psychiatry 2006; 60:1071–1080. 12. Zametkin AJ, Nordahl TE, Gross M, et al. Cerebral glucose metabolism in adults with hyperactivity of childhood onset. N Engl J Med 1990; 323:1361–1366. 13. Zametkin AJ, Liebenauer LL, Fitzgerald GA, et al. Brain metabolism in teenagers with attention-deficit hyperactivity disorder. Arch Gen Psychiatry 1993; 50: 33–340. 14. Ernst M, Cohen RM, Liebenauer LL, et al. Cerebral glucose metabolism in adolescent girls with attention-deficit/hyperactivity disorder. J Am Acad Child Adolesc Psychiatry 1997; 36:1399 –1406. 15. Yeo RA, Hill DE, Campbell RA, et al. Proton magnetic resonance spectroscopy investigation of the right frontal lobe in children with attention-deficit/hyperactivity disorder. J Am Acad Child Adolesc Psychiatry 2003; 42:303–310. 16. Courvoisie H, Hooper SR, Fine C, et al. Neurometabolic functioning and neuropsychological correlates in children with ADHD: preliminary findings. Neuropsychiatry Clin Neurosci 2004; 16:63– 69. 17. Fayed N, Modrego PJ. Comparative study of the cerebral white matter in autism and attention-deficit/hyperactivity disorder by means of magnetic resonance spectroscopy. Acad Radiol 2005; 12:566 –569. 18. Modrego PJ, Pina MA, Fayed N, et al. Changes in metabolite ratios after treatment with rivastigmine in Alzheimer’s disease. A non-randomised controlled trial with Magnetic Resonance Spectroscopy. CNS Drugs 2006; 20:867– 877. 19. Solanto MV. Dopamine dysfunction in AD/HD: integrating clinical and basic neuroscience research. Behav Brain Res 2002; 130:65–71. 20. Madras BK, Miller GM, Fischman AJ. The dopamine transporter and attention-deficit/hyperactivity disorder. Biol Psychiatry 2005; 57:1397– 1409.
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21. Volkow ND, Wang GJ, Newcorn J, et al. Brain dopamine transporter levels in treatment and drug naive adults with ADHD. Neuroimage 2007; 34:1182–1190. 22. Ernst M, Zametkin AJ, Matochilk JA, et al. High midbrain 18F-Dopa accumulation in children with ADHD. Am J Psychiatry 1999; 156:1209 – 1215. 23. Castellanos FX, Elia J, Kruesi MJ, et al. Cerebrospinal fluid monoamine metabolites in boys with attention-deficit/hyperactivity disorder. Neuropsychopharmacology 1996; 14:125–137. 24. Ashtari M, Kumra S, Bhaskar SL, et al. Attention-deficit/hyperactivity disorder: a preliminary diffusion tensor imaging study. Biol Psychiatry 2005; 57:448 – 455. 25. Barker PB. N-acetyl-aspartate: a neuronal marker? Ann Neurol 2001; 49:423– 424. 26. Clark JB. N-acetyl-aspartate: a marker for neuronal loss or mitochondrial dysfunction. Dev Neurosci 1998; 20:271–276. 27. Toft PB, Leth H, Lou HC, et al. Metabolite concentrations in the developing brain estimated with proton MR spectroscopy. J Magn Reson Imaging 1994; 4:674 – 680. 28. Kato T, Nishina M, Matshushita K, et al. Neuronal maturation and Nacetyl-L-aspartic acid development in human fetal and child brains. Brain Dev 1997; 19:131–133. 29. Murphy DG, Critchley HD, Schmitz N, et al. Asperger syndrome: a proton magnetic resonance spectroscopy study of the brain. Arch Gen Psychiatry 2002: 59:885– 891. 30. Hesslinger B, Thiel T, Tebartz van Elst L, et al. Attention-deficit disorder in adults with or without hyperactivity; where is the difference? A study in humans using short echo (1)H-magnetic resonance spectroscopy. Neurosci Lett 2001; 304:117–119. 31. Sun L, Jin Z, Zang YF, et al. Differences between attention-deficit disorder with and without hyperactivity: a 1H-magnetic resonance spectroscopy study. Brain Dev 2005; 27:340 –344. 32. Jin Z, Zang YF, Zeng YW, et al. Striatal neuronal loss or dysfunction and choline rise in children with attention-deficit hyperactivity disorder a 1H-magnetic resonance spectroscopy study. Neurosci Lett 2001; 315:45– 48. 33. Perlov E, Philipsen A, Hesslinger B, et al. Reduced cingulate glutamate/glutamine-to-creatine ratios in adult patients with attention deficit/ hyperactivity disorder. A magnetic resonance spectroscopy study. Psychiatry Res 2007;epub ahead of print. 34. Carrey N, MacMaster FP, Fogel J, et al. Metabolite changes resulting from treatment in children with ADHD: a 1H-MRS study. Clin Neuropharmacol 2003; 26:218 –221. 35. Carrey NJ, MacMaster FP, Gaudet L, et al. Striatal creatine and glutamate/glutamine in attention-deficit/hyperactivity disorder. J Child Adolesc Psychopharmacol 2007; 17:11–17. 36. Kreis R, Ernst T, Ross BD. Development of the human brain: in vivo quantification of metabolite and water content with proton magnetic resonance spectroscopy. Magn Reson Med 1993; 30:424 – 437.
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Rationale and Objectives: Attention deficit-hyperactivity disorder (ADHD) is a socially disabling condition whose pathophysiology is mostly unknown. Previous magnetic resonance imaging (MRI)-based reports have shown structural abnormalities in the prefrontal region and the striatum, but with inconsistencies across the studies with regard to right/left specificity of changes. Our study is aimed at finding evidence of dysfunction with more refined MRI techniques such as diffusion-weighted MRI and spectroscopy. Materials and Methods: We enrolled 22 ADHD children (mean age 9; SD 2.91) and 8 healthy children (mean age 7.5; SD 3). All of them underwent diffusion-weighted MRI in several areas of the brain bilaterally: prefrontal, lentiform nucleus, posterior cingulate, and centrum semiovale; and single-voxel proton magnetic resonance spectroscopy in the left centrum semiovale and right prefrontal region. Results: We did not see either apparent structural abnormalities of the brain in conventional MRI or differences in the apparent-diffusion coefficients in any of the areas studied. However, we observed significant differences in the N-acetylaspartate/creatine ratios in relation to controls in the right prefrontal corticosubcortical region: 1.58 (SD 0.09) versus 1.47 (0.08), P ⫽ .01); and in the left centrum semiovale: 2.02 (0.13) versus 1.79 (0.13), P ⫽ .0003. This finding is consistent with a published report on eight ADHD children in whom N-acetyl-aspartate/creatine ratios were also elevated. Conclusions: Given these results, we hypothesize that a biochemical dysfunction might underlie in the brain of ADHD children. The N-acetyl-aspartate/creatine ratio may be regarded as a potential marker of the disease. Key words: Attention deficit-hyperactivity disorder; magnetic resonance spectroscope. ©
AUR, 2007
INTRODUCTION Attention deficit-hyperactivity disorder (ADHD) is a common developmental disorder afflicting 3–7% of school-age children. It is characterized by overactivity, impulsivity, and
Acad Radiol 2007; 14:1029 –1035 1
From the Magnetic Resonance Unit, Clinica Quirón (N.F., J.C., J.D.), and Department of Neurology, Hospital Universitario Miguel Servet, Avda Isabel la Católica 1-3 (P.J.M.), 50009 Zaragoza, Spain. Received March 24, 2007; accepted May 27, 2007. Address correspondence to: PJM. E-mail: [email protected]
© AUR, 2007 doi:10.1016/j.acra.2007.05.017
inattentiveness, which lead to negative academic and social consequences (1). In the past, these children were considered to have minimal brain dysfunction, but so far the etiology and pathophysiology are mostly unknown. Magnetic resonance (MR)-based techniques have provided useful information and clues about the areas of the brain that are supposedly involved in this disorder. The majority of studies agree that the reduced volumes underlay in areas involved in attentional and executive functions (prefrontal-striatal areas). However, there are inconsistencies across the studies, especially in relation to right/left predominance of decreased brain volumes and other changes.
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Smaller volumes than normal were found in the right frontal lobe as well as cingulate and striatal regions (2). The mean right caudate volume was significantly smaller in 50 ADHD boys in comparison to controls, suggesting abnormalities of frontal-striatal circuits (3). Another work with 15 ADHD children found no differences in hemispheric volumes, but smaller left caudate volumes in relation to controls (4). Slightly smaller total volumes were found but no differences in asymmetries in 50 ADHD girls (5), but in 50 boys, the same authors found smaller right globus pallidus and right anterior frontal region (6). Several studies with volumetry have also disclosed interesting findings in frontal lobes. Reduction in gray-matter volumes was more marked in the right prefrontal area but white matter volume reduction was specific to the left hemisphere (7). In a group of 30 ADHD children, there were volume reductions in the right prefrontal gray matter (8). However, in a study with 25 ADHD children, the smaller volumes were specific for left cortical areas (motor, premotor, whole left cingulate, temporal, and parahippocampal) with no differences in the white matter (9). In 28 ADHD boys, the smaller volumes in comparison to controls were those of right frontal-pallidal-parietal gray matter as well as white matter bilaterally, with cerebellar and striatal volume deficits in boys with comorbidities (10). Prefrontal gray-matter volumes were also reduced in ADHD adults (11). Publications with radiologic functional studies are scarce. Positron emission tomography (PET) offers information on glucose metabolism in the whole brain and in selected areas. Global and regional (prefrontal and premotor cortex) glucose metabolism was reduced in 25 adults with ADHD in relation to normal adults (12). Two other PET studies with small samples of adolescents did not see global differences in the glucose metabolism (13,14). Proton magnetic resonance spectroscopy may be useful because provides information on living tissues. A previous report with 23 ADHD children did not find significant differences in neurometabolites of the dorsolateral frontal region (15). There are two works reporting higher N-acetyl-aspartate/creatine ratios in eight ADHD children than in controls in the right frontal area (16) and the left centrum semiovale (17), respectively. The aim of the present study is to search for and find possible structural and functional changes in a larger sample of ADHD children by means of more refined MR techniques: diffusion-weighted MR imaging and MR spectroscopy of the brain. Based on the afore-
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mentioned reports, we hypothesize that a marker of the disease could be found in prefrontal region or centrum semiovale.
PATIENTS AND METHODS In this cross-sectional study, we consecutively enrolled 22 adolescent medication-naive patients fulfilling the DSM-IV-TR criteria of ADHD (1) with neither learning disabilities nor comorbidities. Written consent was obtained from the children’ parents. These children were referred by pediatricians and neurologists for radiologic evaluation. The mean age was 9 years (SD 2.91; range 6 –16), with 18 boys and 4 girls. We also recruited eight healthy children as the control group with a mean age of 7.5 years (SD 3; range 4 –12); four boys and four girls. The difference of age between patients and controls was not statistically significant (P ⫽ .2). All of them underwent T1- and T2-weighted conventional MR imaging of the brain on a 1.5 T clinical scanner (Signa Horizon; General Electric Medical Systems; Milwaukee, WI). Transverse single-shot echoplanar diffusion-weighted MR imaging was carried out according to the following parameters: echo time 97 milliseconds; repetition time 10,000 milliseconds; flip angle 90°; slice thickness 5 mm; interslice thickness 2.5 mm; field of view 24 ⫻ 24; matrix size: 128 ⫻ 128; number of excitations 1; diffusion sensitive gradient (b value) 1,000 seconds/mm2. The following areas of the brain (regions of interest) were studied bilaterally: prefrontal, lentiform nucleus, posterior cingulate, and centrum semiovale (Fig 1). The whole brain was imaged with 20 sections, and we acquired four images per section. On three images per section the apparent diffusion coefficient (ADC) was calculated and gray scale encoded with a diffusion-sensitizing gradient in x, y, and z directions. On a fourth image, the directionally averaged ADC was calculated and gray scale encoded as the average of the three directional coefficients. The ADC values were measured within every region of interest. The examined areas were chosen on the basis of the previous substantive publications concerning ADHD and pointing to either structural abnormalities or dysfunction therein. MR spectroscopy was performed as follows: sagittal T1-weighted topogram and T2 axial localizing series were used to place volume (2 ⫻ 2 ⫻ 2 cm) in the left centrum semiovale (Fig 2) and the right prefrontal corticosubcortical area just before the genu of the corpus
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Figure 3. T2-weighted magnetic resonance imaging with voxel placement in the right prefrontal region. Figure 1. Echo-planar diffusion-weighted magnetic resonance imaging showing the regions of interest explored bilaterally: prefrontal, lentiform nucleus, centrum semiovale, and posterior cingulate.
Figure 2. T2-weighted magnetic resonance imaging with voxel placement in the left centrum semiovale.
callosum (Fig 3). Both areas were chosen because of they are free of partial volume effect and contamination from cerebrospinal fluid, and because previous works found abnormalities therein. Single-voxel 1HMRS was carried out by means of an echo time of 30 milliseconds and a repetition time of 2,500 milliseconds with spin-echo technique that uses selective excitation with gradient spoiling for water suppression. The mode of spectral acquisition was probe-p (PRESS technique). The pure metabolite signal was spoiled, zerofilled, and Fourier transformed to produce a spectrum, scaled, drawn onto a 512 ⫻ 512 image, and stored as an image in the system database. Every spectrum was automatically fitted to four peaks corresponding to levels of N-acetyl-aspartate (NAA), 2.02 ppm; total creatine (Cr), 3.03 ppm; choline-containing compounds (Ch), 3.23 ppm; and myo-Inositol (mI), 3.56 ppm. We also obtained the peak amplitude of the metabolites relative to Cr. For this purpose, we used the algorithms provided by the manufacturer with the following steps: 1. Setting a global frequency fit parameter. 2. Performing line width and line shape enhancement by appropriate apodization of the time domain signal.
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Table 1 Mean Apparent Diffusion Coefficients (ADC) by Every Area Area Examined Right prefrontal Left prefrontal Right striatum Left striatum Right cingulate Left cingulate Right centrum semiovale Left centrum semiovale
Mean ADC of patients (n ⫽ 24) 6.6993 6.8423 5.8381 5.8195 7.3568 7.1395 6.1618 5.8192
E-10 E-10 E-10 E-10 E-10 E-10 E-10 E-10
Mean ADC of controls (n ⫽ 8) 6.9925 6.7462 5.7475 5.79 7.2112 7.1287 6.1925 5.8685
E-10 E-10 E-10 E-10 E-10 E-10 E-10 E-10
NS NS NS NS NS NS NS NS
Table 2 Mean and SD Values of the Metabolites Relative to Cr Left Centrum Semiovale Metabolite Ratio to Cr NAA/Cr Ch/Cr mI/Cr Right Prefrontal Region Metabolite Ratio to Cr NAA/Cr Ch/Cr mI/Cr
ADHD Children (n ⫽ 24)
Controls (n ⫽ 8)
2.02 (SD 0.13) 1.15 (SD 0.12) 0.67 (SD 0.09)
1.79 (SD 0.13) 1.13 (SD 0.2) 0.67 (SD 0.07)
ADHD Children (n ⫽ 24) 1.58 (SD 0.09) 0.87 (SD 0.13) 0.73 (SD 0.06)
Controls (n ⫽ 8) 1.47 (SD 0.08) 0.84 (SD 0.07) 0.72 (SD 0.07)
Significance P ⫽ .0003 P ⫽ .6 P ⫽ .8 Significance P ⫽ .01 P ⫽ .07 P ⫽ .6
Note.—Cr ⫽ creatine; ADHD ⫽ attention deficit hyperactivity disorder; NAA ⫽ N-acetyl-aspartate/ creatine; Ch ⫽ choline; mI ⫽ myo-Inositol
3. Fourier transformation of the signal to the appropriate frequency resolution and number of points. 4. Calculation of a baseline correction from the frequency domain signal 5. Curve fitting the desired regions of the frequency domain signal. The test-retest reliability of the NAA/Cr ratios in our scanner was previously assessed in 15 healthy people with an intraclass correlation coefficient of 0.92 (18). Statistical analysis was based on t-test for mean comparisons of the metabolite ratios of patients and controls. Owing to the small sample size of controls we also used a nonparametric test (Mann-Whitney U test) so as to confirm significance of differences.
RESULTS T1- and T2-weighted MR images were evaluated by two radiologists without finding structural differences
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between patients and controls. With regard to diffusion-weighted images in Table 1 are reported the mean ADC values for every area studied. We did not detect significant differences in any of the areas. The acquisition of the MR spectra was successful in all of the children. The absolute values of NAA tended to be higher in patients than in controls but without reaching statistical significance. Table 2 reports the metabolite values relative to Cr disclosed by the MR spectroscopy in patients and controls. Figures 4 and 5 are examples of spectrum for the left centrum semiovale and the right prefrontal region, respectively. Whereas no significant differences were found for Ch/Cr and mI/Cr ratios, these were significant for NAA. In the left centrum semiovale, the mean NAA/Cr ratio was 2.02 (SD 0.13) for ADHD children versus 1.79 (SD 0.13) for controls; t ⫽ 4.08, P ⫽ .0003. In the right prefrontal area, the mean NAA/Cr value for ADHD children was 1.58 (SD 0.09) in comparison to 1.47 (SD: 0.08) for controls; t ⫽ 2.74, P ⫽ .01. This
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EVIDENCE OF BRAIN DYSFUNCTION IN ADHD
Figure 6. Box plot representing the ordinal values for the N-acetyl-aspartate/creatine ratios obtained in the left centrum semiovale of patients and controls.
Figure 4. Example of spectrum of an attention deficit hyperactivity disorder boy in the left centrum semiovale. NAA ⫽ N-acetyl aspartate.
Figure 7. Box plot representing the ordinal values for the N-acetyl-aspartate/creatine ratios obtained in the right prefrontal region of patients and controls.
DISCUSSION
Figure 5. Example of spectrum in the right prefrontal region of a healthy boy.
significance was also seen in the nonparametric test: P ⫽ .001, and P ⫽ .02 for the NAA/Cr ratios in the centrum semiovale and prefrontal region, respectively. In Fig 6 and 7 are box plots with the ordinal values of the NAA/Cr ratios for semiovale and prefrontal area, respectively.
In spite of the large amount of publications on ADHD, the pathophysiology is unknown. In addition, clinicopathologic correlates are lacking and the neuroradiologic studies show discordant results, although most of them point to the right prefrontal region as the area with the supposed underlying dysfunction. The prefrontal cortex regulates attentional control, organization, and planning (19). The enigma of what the specific dysfunction is has not been untangled. That the patients improve with stimulants blocking catecholamine reuptake (eg, methylphenidate) has led to the hypothesis of catecholamine deficiency and high levels of dopamine transporters (20). However, a
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PET study published recently did not corroborate increases in dopamine transporters that were even reduced in some patients (21). An alternate hypothesis of hyperdopaminergic and hypernoradrenergic state in the striatum has emerged giving rise to bewilderment. The latter hypothesis is based on two facts: 18F-DOPA hyperactivity in the striatum and nucleus accumbens assessed by PET in ADHD children (22), and the correlation between the cerebrospinal fluid concentration of homovalinic acid, a metabolite of dopamine, and the severity of the disease (23). In our study, we were not able to demonstrate structural abnormalities in ADHD children even with diffusion MR imaging. We recognize that diffusion-weighted imaging is not sensitive enough to detect changes in this disorder. Alterations in the white matter were seen in a study with diffusion tensor imaging in the right premotor and striatal regions of 15 ADHD patients in which fractionated anisotropy was decreased in comparison to controls (24). This technique would have been preferable, but it was not available until recently in our hospital facilities. The concentration of some metabolites assessed by MR spectroscopy offers important information on neuronal and axonal functioning. NAA is one of the most abundant amino acids in the central nervous system. Although it is labeled as a surrogate marker on neuronal and axonal density, its function is not completely understood (25). NAA levels have been also correlated with mitochondrial energy metabolism (26). The postnatal membrane turnover has been reported to be high with increasing brain concentrations of NAA and Cr, which return to adult levels by adolescence years (27,28). In this study, the mean NAA/Cr ratio obtained in the centrum semiovale was 2.02 in ADHD children compared with 1.79 in controls. In our previous study with 8 ADHD children and 12 controls (17), the mean NAA/Cr ratios were 2.2 and 1.9, respectively. The subjects of both studies underwent MR spectroscopy on the same clinical scanner set at the same parameters, and in both studies the differences are significant. Based on one of the hypotheses mentioned previously, we could hypothesize that in ADHD a hypercatecholaminergic state might increase mitochondrial metabolism and NAA/Cr ratios. A failure to downregulate the NAA levels was also reported to occur in the prefrontal lobe of 14 patients with Asperger syndrome in which those levels correlated with obsessive behavior (29). However, a reduction in the absolute concentration of NAA was reported in a small sample of five ADHD adult patients in dorsolateral prefrontal cortex (30) and a reduc-
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tion in the NAA/Cr ratio in the lentiform nuclei (31). In these two studies the levels were especially lower in the subgroup with hyperactivity. Hopefully, other hypothesis could be taken into account to explain the symptoms of the disorder. Significant bilateral decreases in the pallidal NAA/Cr ratios were observed in 12 ADHD boys. Treatment with methylphenidate did not change these ratios, so neuronal loss or severe striatal dysfunction was suggested as causative factor (32). Changes in other metabolites are seldom reported and its significance is poorly understood. mI/Cr ratios were reported to be elevated in relation to controls in both frontal regions (16). The involvement of the glutamatergic system was suggested as glutamate-glutamine/Cr ratios were reduced in the right anterior cingulate cortex (33) and in the striatum (34). More recently, another study revealed greater glutamate/glutamine and Cr in the striatum of ADHD children in comparison to controls but not in prefrontal cortex (35). The small size of the samples is the main limitation to interpret the results of the studies with MRS. We understand that it is not easy to recruit larger samples of children with ADHD, but this problem also occurs with PET studies. Our study confirms dysfunction in the right prefrontal region found in previous studies, and dysfunction in the white matter found by us in a small sample of eight ADHD children. Although the ADHD patients are slightly older than controls, this fact is unlikely to explain the differences in the NAA/Cr ratios because the maximum levels of NAA are reached at age 4 years (36). We conclude that the NAA/Cr ratio could be a marker of the disease but, given the some degree of overlapping of NAA/Cr values seen between patients and controls, further studies are warranted before recommending MR spectroscopy to ADHD children as systematic ancillary exploration. REFERENCES 1. The American Psychiatric Association. Diagnostical and statistical manual of mental disorders, ed. 4 Arlington, VA: The American Psychiatric Association, 2000. 2. Hynd GW, Semrud-Clikeman M, Lorys AR, et al. Brain morphology in developmental dyslexia and attention deficit disorder/hyperactivity. Arch Neurol 1990; 47:919 –926. 3. Castellanos FX, Giedd JN, Eckburg P, et al. Quantitative morphology of the caudate nucleus in attention deficit hyperactivity disorder. Am J Psychiatry 1994; 151:1791–1796. 4. Filipek PA, Semrud-Clikeman M, Steingard RJ, et al. Volumetric MRI analysis comparing subjects having attention-deficit hyperactivity disorder with normal controls. Neurology 1997; 48: 89 – 601.
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