Saccadometry: A novel clinical tool for quantification of the motor effects of subthalamic nucleus stimulation in Parkinson's disease

High frequency stimulation of the subthalamic nucleus (STN HFS) is the therapy of choice in the surgical management of patients suffering from advanced Parkinson's disease (PD). Worldwide, more than 30 000 patients have undergone this procedure. At present, there is a need for a rapid method of assessing its therapeutic effect that is ideally also target-specific, objective, automated, quantitative, and with high overall reliability. Saccadic latency, that reflects the operation of central decision-making mechanisms, is increasingly being used as a way of obtaining quantitative, objective information about cerebral performance in general. In this study we investigated the possible application of saccadic eye movements as a tool for assessing the therapeutic motor effects of STN HFS in patients with advanced PD. Patients received bilateral STN HFS and were evaluated in stimulation and medication on and off conditions, together with conventional motor assessments under the same conditions. We found that saccadometry can provide a sensitive and objective measure of the effects of STN HFS in PD patients that correlates well with conventional, subjective, evaluation of motor impairment. The effect appears to be specific to the site of stimulation, and in terms of the LATER model appears to be due to an increase in the mean rate of rise of the underlying decision signal, combined with a degree of suppression of the mechanism responsible for the early component. A possible interpretation of our findings is that electrical stimulation of the STN, which is known to have a powerful influence on substantia nigra pars reticulata, enhances both the descending facilitation that passes from the cortex to the colliculus via the basal ganglia, thus increasing the mean rate of rise of the decision signal, and also the tonic background inhibition that normally suppresses unwanted early responses.