Targeting the hot spot in a patient with essential tremor and Parkinson's disease: Tractography matters

Thalamic ventralis intermedius deep brain stimulation (VIM-DBS) is generally effective in treating refractory tremor in Parkinson's disease (PD) and in essential tremor (ET), but some patients do not respond well due to side effects or from loss of the effect of stimulation over time. The caudal zona incerta (ZI) has emerged as a promising target in ET, and the effects of ZI-DBS on PD tremor are less studied. Here, we describe a rare situation in which both ET and PD coexist in a 72-year-old male referred for ZI-DBS due to refractory tremor. The aim of this study was to evaluate whether there was a difference in the area stimulated to improve each type of tremor and whether tractography could improve and predict motor outcome.

Two months after the surgery, in order to define which of the cathodes was the most effective towards improving the tremor and parkinsonian symptoms, a double-blinded, monopolar evaluation was conducted on both hemispheres separately. Once the best contact and parameters were defined, the volume of tissue activation (VTA) was represented spatially for each type of tremor and, finally, the image model was fused with the tractography.

For both types of tremor, the hot spot stimulated region achieved the dentato-rubro-thalamic tract (DRTT) at the higher fiber density region. The DRTT fibers were asymmetrical between the right and left hemispheres.

Regardless of the type of tremor, DRTT can be the most effective region for stimulation. Tractography should be considered when planning the surgical target since the DRTT is not always symmetrical, and the reconstruction of the VTA together with the tractography can greatly improve the DBS programming, and, probably, the patient's outcome to the stimulation.