The evaluation of motor cortex excitability measures and the association of these measures with the clinical course in patients with obsessive - compulsive disorder

Introduction: Until today, there have been several biological hypotheses concerning the pathophysiology of Obsessive ? Compulsive Disorder (OCD) and many drug, brain imaging and electrophysiological investigations supporting these hypotheses. The number of electrophysiological studies in OCD are quite insufficient and the data concerning with the cortical excitability seem considerably controversial. Additionally, sample sizes of many studies are fairly small.Aim: The aim of the present study is to investigate whether there would be differences in the motor thresholds and other excitability measures between OCD patients and healthy controls in a large sample, without excluding comorbidity and medication use.Method: A total of 32 OCD patients and 21 healthy volunteers between 18 ? 65 years of age were included in the study between May 2008 and February 2009. The axis I disorders of all subjects were evaluated structured interview with MINI (Mini International Neuropsychiatric Interview). Any volunteers with axis I disorder and/or alcohol and substance use disorders were excluded from the study. All the subjects were evaluated by Y-BOCS, HAM-D, HAM-A, CGI, BDI and VAS (Visual Analog Scale) scales simultaneously with the electrophysiological procedures (± 1 week). For transcranial stimulation of the subjects ?MagProX100 incl. MagOption? (Medtronic A/S, Denmark) TMS device and figure of 8 coil with diameter of 70 mm in each ring (MCF-B65) were used. EMG measures were recorded from Abductor Pollicis Brevis (APB) muscle of the right hands of the subjects with surface electrodes. Eventually, RMT, SICI, ICF and LICI parameters of cortical excitability were determined by stimulating the left motor cortex.Findings: The main findings of the present study were that RMT measures were not significantly different between the healthy volunteers and OCD patients, but were significantly increased in the patient subgroups with comorbid major depression and with severe illness. Concerning SICI measures, there was not any significant difference either between healthy volunteers and patients or between the patient subgroups. ICF measures were not significantly different between healthy volunteers and the patients either, however were significantly higher in the patient subgroups with combination treatment and with longer duration of illness (> 10 years). As for LICI measures, there were not any significant difference between the healthy volunteers and patients, however were detected significantly to be higher in the patient subgroup with early disease onset.Conclusion: This study is the one with the largest sample size among the excitability studies concerning with OCD. In our study, no significant difference was detected in the electrophysiological measures of cortical excitability between the healthy subjects and OCD patients. This finding may either be due to a bias in the selection of the healthy subjects or the potential methodological differences between our study and the previous ones. Also, in may result from comorbid disorders that the most of our patients had or from the medications that they were on. On the other hand, the findings of the previous studies concerning excitability measures in OCD might also be affected from small sample sizes, bias in selecting the control groups and the patients with TS / tic disorder that most of them included. The electrophysiological measures of OCD patients with major depression in our study were especially concordant with some other excitability studies on depression. Therefore, it is important selecting the electrophysiological methodology as much similar as the previous studies, more homogeneous patient groups in terms of medication and considering the potential effects of comorbidity for the future studies aiming to evaluate the cortical excitability in OCD.Key Words: Obsessive ? Compulsive Disorder (OCD), Cortical Excitability, Motor Threshold, Transcranial Magnetic Stimulation (TMS).