Children With Comorbid Attention-Deficit- Hyperactivity Disorder and Tic Disorder: Evidence for Additive Inhibitory Deficits Within the Motor System Gunther H.Moll,MD, 1 Hartmut Heinrich, PhD, 1 Go¨tz-Erik Trott, MD, 2 Sigrun Wirth, MD, 2 Nathalie Bock, MD, 1 and Aribert Rothenberger, MD 1 For children with attention-deficit-hyperactivity disorder (ADHD) or tic disorder (TD), we recently reported defi- cient inhibitory mechanisms within the motor system by using transcranial magnetic stimulation. These deficits— stated as reduced intracortical inhibition in ADHD and shortened cortical silent period in TD—could be seen as neurophysiological correlates of motor hyperactivity and tics,respectively. To investigate neurophysiological as- pectsof comorbidity, we measured motor system excit- ability for the first time also in children with combined ADHD and TD. The findings of a reduced intracortical inhibition as well as a shortened cortical silent period in these comorbid children provide evidence for additive ef- fects at the level of motor system excitability. Ann Neurol 2001;49:393–396 In attention-deficit-hyperactivity disorder(ADHD), motoric hyperactivity is usually the striking abnormal- ity to parents and physicians, typically seen in restless- ness,fidgeting, and generally unnecessary gross body movements. 1 In tic disorder (TD), fluctuating motor and phonic tics can becharacterized as involuntary, sudden, abrupt, repetitive movements, gestures, or ut- terances thatmay be seen as poorly modulated frag- ments of normal sensorimotor behavior. 2 As a possible neurophysiological correlateof thesehypermotoric symptoms in children with ADHD or TD, we recently tested the hypothesis of deficient inhibitory motor con- trolthroughout the sensorimotor circuit by investigat- ing motorsystem excitability using the technique of focaltranscranial magnetic stimulation (TMS) in a single- and paired-stimulus paradigm. 3,4 Whereas chil- dren with ADHD showed a reduced intracortical inhi- bition, 5 children with TD had a shortened cortical si- lent period compared to healthy children. 6 Because these two hypermotoric disorders coexist in up to 50% in clinical samples, 7 we investigated neuro- physiological correlates of motor system excitability in a comorbid group (ADHD 1 TD) relative to the two single-disorder groups(ADHD only orTD only) to testwhether a distinctdysfunctional pattern for this common comorbidity exists. Patients and Methods Patients The studywasperformed on 16 children with ADHD (ADHD only),16 children with TD (chronic motor tic dis- order/Tourette’s disorder; TD only), 16 children with co- morbid ADHD and TD (ADHD 1 TD), and 16 healthy children (controls). The demographic and clinical data are summarized in Table 1. All children (60 boys, 4 girls) were right-handed and of at least normal intelligence (IQ . 80). The fourgroupsdid notdiffersignificantly in mean age and sex. All disordered children had to fulfill the diagnostic criteria for ADHD and/or chronic motor tic disorder/Tourette’s dis- order according to the APA’s DSM-IV. None of the ADHD only,TD only,or ADHD 1 TD children fulfilled the cri- teria for any other psychiatric disorder as stated in a struc- tured clinical interview (DISC). In all of the children with an ADHD diagnosis, the score of the abbreviated (10 item) Connersscale was $15. 8 In addition, none ofthe ADHD only children had any actual tic symptoms nor any history of tics.For the TD-only group, a Conners score $15 was used as the exclusionary criterion. Children with an ADHD diagnosis either had never taken any psychostimulant medication or had been drug-free for at least 48 hours before TMS investigation (the only drug used wasmethylphenidate in a standard formulation). Some of the children with a TD diagnosis received a neuroleptic med- ication (Table 1). The healthy children were drug-free and devoid of any child psychiatric disorder. The subjects of all four groups lacked gross neurological or other organic disorders. After a complete description of the study to the children and their parents, assent was obtained from the children and written informed consent from their parents. The study was conducted according to the declaration of Helsinkiand was approved by the local ethics committee. TMS FocalTMS was applied to the hand area of the leftmotor cortex (figure-eight magneticoil with a diameter of one wing of 70 mm, Magstim 200 HP magnetic stimulator; Magstim, Whiteland, United Kingdom), and surface electro- myography wasrecorded from the rightabductordigiti minimimuscle. The exact equipment and protocol are re- ported in detail by Ziemann et al. 4 Resting and active motor threshold were expressed as a percentage of the maximum stimulator output to elicit a mo- From the 1 Child and Adolescent Psychiatry Department, University of Go¨ttingen, Go¨ttingen; and 2 Practice of Child and Adolescent Psychiatry, Aschaffenburg, Germany Received Sep 1, 2000,and in revised form Oct 13. Accepted for publication Oct 18, 2000. Address correspondence to Prof Rothenberger, von-Siebold-Str. 5, D-37075 Go¨ttingen, Germany. E-mail:arothen@gwdg.de BRIEF COMMUNICATIONS © 2001 Wiley-Liss, Inc. 393