gms | German Medical Science

Objective: Patients with chronic hemodynamic ischemia have a submaximally reduced cerebral bloodflow and an impaired cerebrovascular reserve capacity (CVRC) accompanied by transitory ischemic attacks and/or fixed minor neurological deficits. Extracranial-intracranial (EC-IC) bypass surgery may be used to revert hemodynamic compromise. From clinical experience with patients improving after cerebral revascularization we hypothesize a reversible loss of cortical function. Objective of the current study is to examine neurophysiological parameters of excitability and plasticity in the motor system in patients scheduled for EC-IC bypass surgery by navigated transcranial magnetic stimulation (TMS).

Method: We prospectively examined patients with unilateral carotid artery occlusion or stenosis fulfilling the criteria for EC-IC bypass surgery. Electrophysiological parameters were obtained preoperatively and three months after surgery. Motorcortical excitability was determined by navigated TMS via identification of the resting motor threshold (RMT) for both hemispheres. RMT ist the minimum intensity to elicit a motor evoked potential. In addition short- interval cortical inhibition (SICI) and intracortical facilitation (ICF) were determined by paired pulse transcranial magnetic stimulation with a conditioning stimulus of 80% and a test stimulus of 120%. Interstimulus intervals for inhibition were 3 ms and for facilitation 5 and 17 ms.

Results: 35 consecutive patients with a mean age of 58.1 ± 1.7 years were included and underwent STA-MCA bypass. Preoperative RMT was higher in the symptomatic hemisphere compared to the contralateral asymptomatic side (45.7 ± 2.4 → 38.8 ± 1.5, n=35, p = < .05) and normalized after revascularization (53.1 ± 2.3 → 45.3 ± 2.1, n=18, p = < .05). In paired pulse paradigms there was a significant motor disinhibition displayed by an increase in SICI with ISI 2 (p < .05) in the operated hemisphere. Responses of the asymptomatic hemisphere remained nearly unchanged.

Conclusions: Our data demonstrate a reversibly impaired motorcortical function in the chronic ischemic brain. Cerebral revascularization leads to a significant motor disinhibition, a known factor for cerebral reorganization. In conclusion TMS is an objective tool for the assessment of the functional efficacy of EC-IC bypass surgery.