Cortical motor excitability in adults with ADHD: Results of a TMS-study

Introduction: The Attention Deficit-/Hyperactivity Disorder (ADHD) persists into adulthood in up to 60% of the cases (1). During transition into adulthood, motor hyperactive behaviour is changed as well as quantitatively as also qualitatively (2, 3). Pathophysiologically, disturbances of cortical motor excitability have been discussed. Motor cortical excitability can investigate non-invasively with the help of transcranial magnetic stimulation (TMS). Studies in ADHD children have shown impairments of inter-hemispheric inhibition, intra-cortical inhibition (short interval cortical inhibition, SICI) and intra-cortical facilitation (ICF). These dysfunctions were normalized on a treatment with methylphenidate (MPH) (4, 5). For the background of the declined core symptom motor hyperactivity in ADHD adults, we postulate a reduction of motor cortical dysfunctions in adults with ADHD.

Methods: 21 patients with ADHD (9 ♀; mean age: 28.9±9.2 years) were investigated with the TMS and compared with an age- and gender-matched healthy control group. We measured the „cortical silent period“ (CSP), „ipsilateral silent period“ (ISP), SICI, ICF and long interval cortical inhibition (LICI). Clinical symptoms were assessed by the use of the Conner's Adult ADHD Rating Scale (CAARS). 16 of these patients were investigated again on a MPH-treatment.

Results: Until a significant shortened duration of the ISP in ADHD patients (ADHD: 17.1±4.0ms; control-group: 22.8±6.6ms; p=0.002) was found, no further significant differences between patients and normal controls could be detected, e.g. the CSP, the latency of the ISP, SICI, ICF und LICI were undisturbed. In addition, we revealed a significantly negative correlation between the duration of the ISP and clinical symptoms showing remaining hyperactive behaviour (CAARS-subscore-B), whereas the correlation between the duration of the ISP and inattentive symptoms (CAARS-subscore-E) were not significantly. The group of 16 medicated patients (MPH – mean dosage: 0.44±0.22mg/kg/d) showed a significant prolongation of the duration of ISP compared with their premedication state (22.2±6.5ms//17.4±4.5ms; p=0.007).

Discussion: Our study in adults with ADHD is one of the first using the same TMS protocol, which allowed us to detect disturbances of the inter- and intra-hemispheric inhibition in ADHD children before. Contrary to that and with exception of the ISP, we did not find impairments of cortical motor excitability in our adult ADHD patients compared to age- and gender-matched normal control subjects. Interestingly, exclusively the duration of the ISP was shortened similar to the results in children, correlated with remaining hyperactive symptoms and was normalized on the partial dopamine agonist MPH (6). Pathophysiologically, a partly normalization of a dopamine mediated deficit of cortical motor inhibition through synaptic matu-ration processes could be assumed as one possible reason, whereas the still disturbed inter-hemispheric inhibition in our patients could explain the remaining hyperactive behaviour in ADHD adults.

References:

[1] Barkley R. A. et al. 2002, J. Abnorm. Psychol.;

[2] Biederman J. et al. 2000, Am. J. Psychiatry;

[3] Lahey B. B. et al. 1994, Biol. Psychiatry;

[4] Buchmann J. et al. 2007, Biol. Psychiatry;

[5] Buchmann J. et al. 2003, Clin. Neurophysiol;

[6] Höppner J. et al. 2008, J. Neural Transm