Am J Psychiatry 160:9, September 2003
Anatomic Brain Abnormalities in Monozygotic Twins
Discordant for Attention Deficit Hyperactivity Disorder
F. Xavier Castellanos, M.D.
Wendy S. Sharp, M.S.W.
Rebecca F. Gottesman, M.D.
Deanna K. Greenstein, Ph.D.
Jay N. Giedd, M.D.
Judith L. Rapoport, M.D.
Objective: To examine brain-behavior relationships in atten-
tion deficit hyperactivity disorder (ADHD), the authors obtained
magnetic resonance imaging (MRI) scans of monozygotic twins
discordant for ADHD.
Method: National recruitment was followed by in-person as-
sessment. MRI scans were measured algorithmically for nine
pairs of monozygotic twins discordant for ADHD.
Results: The affected twins had significantly smaller caudate
volumes (mean difference=–0.56 ml, CI=–0.92 to –0.21) than
their unaffected co-twins.
Conclusions: These results provide further support for striatal
models of ADHD pathophysiology.
(Am J Psychiatry 2003; 160:1693–1696)
Beyond informing estimates of heritability, studies of
monozygotic twins discordant for a given disorder can
illuminate relationships between brain structures and
functions and can identify effects of environmental fac-
tors and interactions, since genetic factors are controlled.
Following the approach of Hyde et al. (1), we recruited
monozygotic twins discordant for attention deficit hyper-
activity disorder (ADHD) for an anatomic brain magnetic
resonance imaging (MRI) study. Since monozygotic twins
are genetically identical, we anticipated that this study
would highlight brain regions linked to ADHD that are
particularly susceptible to environmental factors.
Twin pairs discordant for ADHD, as defined by DSM-IV, were
recruited nationally as described elsewhere in detail (2). Exclu-
Am J Psychiatry 160:9, September 2003
sion criteria were a full-scale IQ less than 80, medical or neurolog-
ical disorders determined from examination or history, and other
primary axis I psychiatric disorders. The study was conducted at
the Child Psychiatry Branch of the National Institute of Mental
Health (NIMH) in Bethesda, Md., between 1996 and 2001. The
NIMH institutional review board approved the research protocol,
and written informed consent and assent were obtained from
parents and children, respectively.
All subjects were studied on the same 1.5-T GE Signa scanner
(GE Medical Systems, Milwaukee). T1-weighted images with con-
tiguous 1.5-mm axial and 2.0-mm coronal slices were obtained by
using three-dimensional spoiled gradient recalled echo in the
steady state with echo time=5 msec, repetition time=24 msec, flip
angle=45°, acquisition matrix=256×192, number of excitations=1,
and field of view=24 cm. Head placement was standardized as
previously described (3). T2-weighted images were obtained for
evaluation by a clinical neuroradiologist. Quantification of MRI
images was performed by means of a highly reliable fully auto-
mated process that determines gray and white matter volumes
for the frontal, temporal, parietal, and occipital lobes as well as
caudate and cerebellum volumes, as described elsewhere (4). The
minimum and maximum Talairach (x, y, z) coordinates that de-
fined the left caudate were –20, –28, –12 and –3, 23, 27, respec-
tively; the corresponding values for the right caudate were 5, –24,
–10 and 20, 23, 26. Although the algorithm provided volumes for
the right and left caudate, these did not accord well with values
determined by hand tracing; the intraclass correlation coefficient
(ICC) was <0.40. By contrast, the total caudate volumes were
highly concordant with values derived by hand tracing of the cau-
date head plus body (N=17, ICC=0.91).
Paired t tests contrasting the affected and unaffected co-twins
and Pearson correlations representing twin-twin differences in
brain volumes and symptom severity ratings were performed by
using SPSS 10.0 for Windows (5). Two-tailed significance levels
were used, and the criterion for significance was p≤0.05.
The study began with 25 twin pairs, who completed 3-
day in-person assessments that included anatomic MRI
scans. Eleven pairs were excluded because the putatively
unaffected co-twin had six or more ADHD symptoms of in-
attention or hyperactivity/impulsivity (without necessarily
meeting the impairment criteria); four pairs were excluded
for comorbid conditions, including Tourette’s disorder, ob-
sessive-compulsive disorder, and bipolar disorder; and one
pair was excluded because of motion artifact. The resulting
nine discordant twin pairs included one set of girls. The av-
erage age was 11.0 years (SD=3.2, range=5.6–15.6). All of
the affected male twins had been previously treated with
stimulants, and one of the unaffected twins had received a
1-month empiric trial of methylphenidate; both of the girls
were untreated. The duration of stimulant treatment
ranged from 1 to 36 months (mean=20.8, SD=10.7).
One affected subject was found to have an unsuspected
focal abnormality in the left caudate, left putamen, and
adjacent white matter, consistent with a cerebrovascular
FIGURE 1. Coronal T1-Weighted MRI Brain Images of a 15-Year-Old Boy With ADHD and a Brain Lesion and His Unaffected
aThe focal abnormality was in the left caudate, putamen, and adjacent white matter (see arrow).
FIGURE 2. Total Caudate Volumes of Monozygotic Twins
Discordant for ADHDa
aBoth of the female twins were unmedicated.
Boy with brain lesion
Total Caudate Volume (ml)
Am J Psychiatry 160:9, September 2003
accident at some unknown time (Figure 1). We evaluated
this subject and his twin brother at age 15. Symptoms sug-
gestive of ADHD were first noted at age 3. Structured psy-
chiatric interviews, psychoeducational testing, and parent
and teacher ratings confirmed combined-type ADHD, and
he had a full-scale WISC-III score of 87. The same assess-
ment of his co-twin revealed no psychiatric or neurologic
diagnoses and a full-scale IQ of 95. Both twins were born at
38–39 weeks by uneventful cesarean section. Both twins re-
ceived physical therapy and speech therapy from ages 3 to
5 years. Their developmental milestones were mildly de-
layed and included walking at 17 months and use of only
simple words until age 3. Their family history revealed pos-
sible ADHD in their father and maternal grandfather but
no history of treated or formally diagnosed cases.
Overall, the affected monozygotic twins differed signif-
icantly from their unaffected co-twins in total caudate
volume (mean difference=–0.56 ml, 95% CI=–0.92 to –0.21)
(t=3.67, df=8, p=0.006) even when the previously described
twin pair was excluded from the analysis (t=3.50, df=7, p=
0.01) (Figure 2). Twin-twin differences in caudate volume
did not correlate significantly with differences in the teacher
or parent Conners rating of hyperactivity severity, with
differences in birth weight, or with lifetime duration of stim-
ulant exposure. None of the other twin-twin brain differ-
ences (frontal, parietal, temporal, occipital gray and white
matter, cerebellum) approached significance (p>0.14).
Discordant monozygotic twins provide optimal subjects
for elucidating environmental influences on brain devel-
opment in patients with neuropsychiatric conditions (6).
Our findings provide additional support for current mod-
els of the pathophysiology of ADHD, which implicate pre-
frontal-striatal circuitry. Unilateral caudate infarcts have
been associated with agitation, hyperactivity, disinhibi-
tion, and inattention (7, 8). The size and location of the le-
sion in one boy with ADHD and the absence of ADHD
symptoms in his co-twin suggest that this abnormality
contributed to his ADHD. These data are consistent with
findings that lesions of the caudate and putamen after
closed-head injury are statistically associated with the
emergence of secondary symptoms of ADHD (9) and that
focal stroke lesions of the putamen in childhood are asso-
ciated at a trend level (p=0.10) with ADHD symptoms (10).
Since most of the twins referred to us turned out to be
concordant for ADHD and were thus excluded from this
study (2), we were unable to recruit a group of discordant
twins who had not been previously treated with medica-
tions. Previously we failed to detect significant effects of
medication on regional brain volumes in 49 previously un-
medicated singletons with ADHD compared with 103
medicated patients and 139 comparison subjects (4).
Although we cannot rule out medication effects in the
present study group, we conclude that the association be-
tween smaller caudate volume and ADHD is most consis-
tent with a selective vulnerability of the striatum (11, 12) to
adverse prenatal environmental factors, such as hypoxia.
However, our small group limits our ability to make infer-
ences regarding specific causative factors or to attach
much weight to negative findings. Elsewhere (2) we re-
ported that fathers of twins discordant for ADHD rated
themselves as significantly lower on the Wender Utah Rat-
ing Scale of childhood ADHD symptoms than did fathers
of singletons affected with ADHD and that the rate of
breech presentation was significantly higher in affected
twins than in affected singletons. Those data suggest that
discordant twins represent an enriched sample of nonge-
netic phenocopies that can also inform our understanding
of the various causal pathways that can lead to ADHD. Fi-
nally, these cases suggest that in the unlikely event of
monozygotic twins discordant for ADHD or related condi-
tions in which environmental influences are anticipated,
imaging with MRI may yield clinically relevant and scien-
tifically useful information. In the vast majority of ADHD
cases, however, neuroimaging is not clinically indicated.
Received Aug. 6, 2002; revision received March 11, 2003; accepted
March 19, 2003. From the Child Psychiatry Branch, NIMH, Bethesda,
Md. Address reprint requests to Dr. Castellanos, Child Study Center,
New York University School of Medicine, Rm. 204, 577 First Ave., New
York, NY 10016; firstname.lastname@example.org (e-mail).
The authors thank Children and Adults With Attention-Deficit/Hy-
peractivity Disorder, the National Mothers of Twins Clubs, and Judy
Silberg, Ph.D., from the Virginia Twin-Family Study of Adolescent Be-
havioral Development for help in recruitment for this study, and they
thank the children and families who participated.
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