gms | German Medical Science

Objective: A 69-year-old patient with successful reduction of his disabling upper extremity essential tremor experienced an almost complete but during stimulation cessation reversible ageusia under bilateral thalamic ventral intermediate nucleus (VIM) deep brain stimulation (DBS). At time of surgery, the patient had a history of nearly total anosmia for several years. An evaluation of diffusion tensor (DTI) neuroimaging studies was performed in order to elucidate the to date incompletely defined gustatory pathways and to develop treatment options for this therapeutic dilemma.

Methods: Using preoperative DTI (Intera 3T, Philips, Best, Netherlands, acquisition with 32 gradient directions) of the patient and the postoperative helical cranial computed tomography (CT), effective electrode contact positions (EC) were determined in relation to the medial lemnisci (ML) and the dentate-rubro-thalamic tracts (DRT). Rhinomanometry, subjective gustometry und olfactometry were conducted with deactivated stimulation, unilateral right and left stimulation, respectively, as well as bilateral stimulation.

Results: The patient improved on the ETRS score from 67 to 19, showing a 72% improvement. Stimulation settings were: left, 2.8 V, 180 Hz, 90 µs, C+, 1-; right, 2.7 V, 180 Hz, 90 µs, C+, 9- (ACTIVA PC, Medtronic, USA). Unilateral left and bilateral stimulation resulted in almost complete bilateral ageusia, while unilateral right stimulation produced an almost complete unilateral right ageusia but only minor taste reductions on the left. Both EC are situated within the posterior part of the DRT, which enters the thalamus close to VIM with the left electrode being closer to the ipsilateral ML than the right, probably allowing the effective electric field to also modulate this structure.

Conclusions: Tremor reduction is explained through electrode location in the DRT. The posterior position with proximity to the ML, especially on the left, might explain this unusual sensory side effect of stimulation-induced ageusia by current spread, which is necessary to cover DRT fibers for tremor reduction into more posterior structures (ML). A therapeutic option may be a patient-selectable stimulation program. This case report stresses the importance of fiber tracts for DBS surgery. It furthermore exemplifies how preoperative DTI studies together with postoperative helical CT may be utilized to identify neuro(patho)physiological processes in the human brain.