(18) Comparison of DBS Motor Evoked Potentials During Activation and Rest

Introduction: Deep brain stimulation (DBS) is a well-established treatment for Parkinson’s Disease (PD), but it can induce muscle spasms if electrical field spreads out to pyramidal tract. In current practice, side effect thresholds are determined by clinician observation or patient report while at rest. Motor evoked potentials (mEP) - stimulation-induced muscle activation measured using electromyography – could provide an objective assessment of motor side effect thresholds. A previous mEP study found that resting motor thresholds (RMT) were higher than active motor thresholds (AMT) [1]. Here, we investigated the variability of motor thresholds and mEP amplitudes between resting and active states across different muscles.

Methods: We used EMG to record DBS induced mEP from multiple face and limb muscles (12-15 muscles per patient). DBS frequency was 5 or 6 Hz to allow detection of mEP during interpulse interval. Stimulation intensities ranged from 0.5 mA to 7 mA in 0.5 mA steps. Patients were asked to relax in a chair for rest recordings, or squeeze a ball for hand activation, or purse lips for mouth activation. EMG signals were epoched and averaged (~70 pulses) and mEP presence and amplitude were visually determined. The mEP threshold was defined as the lowest stimulation amplitude at which mEP was detected. Paired t-test was used for statistical comparison.

Results: We report results from 4 PD patients (3 STN, 1 GPi). Compared to rest, facial muscles had lower mEP thresholds during mouth activation (5.9±1.5mA vs 5.1±1.7mA; paired t-test p=0.005), but comparable thresholds during hand activation (5.9±1.5mA vs 6.3±1.5mA). Compared to rest, arm muscles had lower thresholds during hand activation (5.3±1.9mA vs 3.9±1.2mA; p=0.0006), but comparable thresholds during mouth activation (5.3±1.9mA vs 5.2±1.6mA). MEP amplitude for facial muscles was higher during mouth activation compared to rest (14.4±47.94uV vs 4.4±17.46uV; p< 0.0001), and unchanged during hand activation (4.4±17.46uV vs 4.9±21.44uV). MEP amplitude for arm muscles was higher during hand activation compared to rest (89.2±246.75uV vs 39.0±167.3uV; p< 0.0001), but lower during face activation compared to rest (15.1±45.29uV vs 39±167.3uV; p=0.018).

Conclusion: We demonstrate that DBS can activate muscles at lower stimulation amplitudes during voluntary action in the same body region compared to rest. The mEP amplitudes were also higher during same body region activation. This suggests that clinical side effect thresholds may be overestimated since they are determined at rest, and which could lead to overstimulation and insidious side effects such as speech or gait disturbances during chronic stimulation.