Objective:

To investigate patient reporting of stimulation-induced sensory side effects that occur during monopolar review in globus pallidus internus (GPi) deep brain stimulation (DBS).

Background:

The upper limit of the therapeutic window for a DBS lead contact is defined by the stimulation amplitude that elicits persistent side effects. Motor side effects, including muscle contractions and dysarthria, are most common with GPi DBS due to current spread to the internal capsule medially or posteriorly. Patients occasionally report “sensory” symptoms, but this is poorly described in the literature and does not have a clear anatomical basis. 

Design/Methods:

Monopolar review testing data were collected for 167 GPi DBS leads in 113 unique patients (39 female, 74 male) at a single academic institution between January 1, 2021, and July 1, 2023, for Parkinson’s disease (142 leads), dystonia (23 leads), and Huntington’s disease (2 leads). Monopolar review was performed for all 4 contacts on each lead using a pulse width of 90 μs and a frequency of 135 Hz. 

Results:

Motor side effects were elicited at 507/668 contacts (75.9%). Sensory side effects were elicited at 84/668 contacts (12.6%). Sensory side effects were most common at the deepest contact (18.6% of leads) but were also reported for more superficial contacts (8.4-13.2% of leads). When sensory side effects occurred, 41.6% of these contacts had concurrent motor side effects.  

Conclusions:

During threshold testing for GPi DBS, sensory side effects are the second most common after motor side effects. In our experience, increasing stimulation amplitude when sensory side effects occur causes sensory symptoms to manifest as visible muscle contractions. Sensory side effects may represent sub-threshold motor activity due to internal capsule stimulation, which can be further investigated using seed-based tractography. Recognizing sensory side effects as a pre-motor phenomenon may result in more accurate therapeutic windows for GPi DBS programming.