Abstracts

RATIONALE: Many patients with medically intractable epilepsy are not candidates for traditional resective or transective surgeries. The anterior thalamus projects to medial frontal areas and may be involved in propagation of seizures. This thalamic involvement led to the use of DBS in the anterior thalamus in an attempt to abolish seizure propagation. Some patients had improved seizure control with anterior thalamic DBS. Antiepileptic drugs are known to change excitatory and inhibitory circuits in the motor cortex. The effects of anterior thalamic stimulation on motor cortex excitability is not known. The aim of the study is to examine motor cortex excitability in patients who have bilateral deep brain stimulation (DBS) electrodes in the anterior thalamus for treatment of intractable epilepsy.
METHODS: Three patients (33 yrs(F), 48 yrs(F), 43yrs(M)) with bilateral anterior thalamus DBS were studied. Three conditions were tested: stimulator switched off (Off state), continuous stimulation (Continuous state) and cycling stimulation (1 min on, 5 mins off)(Cycling state). Each state was tested in separate days in random order. The stimulator frequency was 100 Hz and pulse width was 90[mu]s. Transcranial magnetic stimulation (TMS) was applied over the hand area of left motor cortex and electromyography (EMG) was recorded from the first dorsal interosseous muscle. The muscle was either at rest (Rest state) or active to 10% of its maximum (Active state). The Rest and Active motor thresholds were determined. Silent Period (SP) (duration of EMG suppression after TMS) was determined using suprathreshold single pulses during the Active state. Paired pulse paradigms used to measure cortical facilitation and inhibition were short interval intracortical inhibition (SICI), intracortical facilitation (ICF) and long interval intracortical inhibition (LICI). For SICI, a subthreshold conditioning pulse preceded the test pulse by 2 msec. For ICF, an interstimulus interval (ISI) of 10 msec was used. For LICI, suprathreshold conditioning and test pulses at ISIs of 50 to 200 msec were used.
RESULTS: Rest and Active motor thresholds, and SP duration were significantly higher in patients than in controls for all three stimulator settings. Rest SICI was significantly reduced in the Continuous state compared to normal controls. Active SICI was significantly reduced in the Continuous and Cycling states compared to controls. Patients had reduced Rest ICF. Rest LICI at ISI of 50 msec was significantly reduced in patients. Patients had significantly increased Rest LICI (ISI 200) compared to normal controls. There were no differences among the three stimulator settings for motor threshold, SP, ICF and LICI.
CONCLUSIONS: These preliminary results suggest that anterior thalamic DBS may change some motor cortical circuits. Other changes in motor cortex excitability may be a result of the underlying disease or antiepileptic medications since DBS settings had no effect.
Objective: At the end of this activity the participants should be able to discuss the effects of anterior thalamic stimulation on cortical excitability in patients with epilepsy.
[Supported by: Canadian Institutes of Health Research (GFM, RC), Canada Foundation for Innovation, Ontario Innovation Trust (RC), University Health Network Krembil Family Chair in Neurology (RC).]; (Disclosure: Consulting - Drs. Lozano and Wennberg are consultants for Medtronic, Inc.)