: While gamma-aminobutyric acid (GABA)-edited MRS has become popular for non-invasive measurements of brain GABA levels in a variety of neurological and psychiatric diseases¹, little is known about the effects of medication on the measured GABA levels. For example, it is not known whether current use of the medication primidone affects brain GABA concentrations. This is an important potential confounder in studies of the pathophysiology of essential tremor (ET), one of the most common neurological diseases. Studies have found a 30-40% reduction of Purkinje cells (PC)² in ET, which release γ-aminobutyric acid (GABA) into the cerebellar dentate nucleus³. Thus, there is an interest in investigating the possibility of cerebellar dentate GABA level serving as an in vivo marker of PC number. However, it is estimated that approximately 50% of all ET patients haven taken primidone⁴, one of the two main medications for ET treatment^5,6. Primidone is known to bind to GABAA receptor, potentiating GABA-ergic neurotransmission. Thus, this study aims at investigating whether current use of primidone has an effect on dentate GABA concentrations as measured by MRS.

We are currently performing a study on ET for which patients taking GABA-ergic medication are excluded. 26 ET patients from this pool, who have never taken primidone, were recruited together with 6 additional ET patients, chronically taking primidone, to assess the effect of primidone on GABA MRS. Demographic information and medical history questionnaires were collected for each subject. The Montreal Cognitive Assessment (MoCA) was performed as a brief assessment of cognitive function⁷.The Washington Heights-Inwood Genetic Study of ET (WHIGET) tremor rating scale was used to rate postural and kinetic tremor⁸. MRS exams were conducted on a 3T Siemens Tim Trio scanner, equipped with a 32-channel head coil. Volumes of interest (VOIs) were centered on left and right cerebellar dentate nucleus (25mm×25mm×25mm). For GABA detection the MEGA-PRESS J-editing sequence was used (TR/TE=1500/68 ms, 392 averages)⁹. High-resolution MPRAGE images were acquired (TR/TE/TI=2300/2.91/900ms, voxel size: 1.0×1.0×1.2mm3, GRAPPA=2) for tissue segmentation. The spectra were quantified using LCModel 6.3-0L¹⁰, with basis sets generated from density matrix simulations using chemical shifts and J-coupling values from Kaiser et al¹¹. The voxel placement and an example of an LCModel fitted spectrum are shown in Fig. 1. GABA levels corrected for CSF were obtained as described by Chowdhury et al¹². All statistical analyses were performed in SPSS (version 21.0). Student’s t test, Fisher’s exact tests, and Mann Whitney tests (for variables not normally distributed) were used to compare demographic and clinical features of ET cases on primidone vs. ET cases off primidone. The correlations between dentate GABA levels and demographic and clinical features were assessed using Student’s t test as well as correlation coefficients. Linear regression models were established to analyze the association between primidone status and GABA concentrations, adjusted for potential confounds.The two groups did not differ in their demographic and clinical features (Table 1). While right dentate GABA was significantly higher than left dentate GABA in all ET patients (p<0.001), both right and left dentate GABA concentrations were similar in the two groups (right: 2.21±0.46 [on primidone] vs. 1.93±0.39 [off primidone], p=0.15; left: 1.61±0.35 [on primidone] vs. 1.67±0.34 [off primidone], p=0.72) (Fig. 2). Dentate GABA concentration was not correlated with age, gender, years of education, or total tremor score. Higher right dentate GABA concentration was weakly associated with a lower MoCA score (r = -0.42, p = 0.03). In the six medicated cases, there was no association between the daily primidone dose and either right dentate GABA levels (Pearson’s r = 0.11, p = 0.89) or left dentate GABA levels (Pearson’s r = -0.24, p = 0.76). In a linear regression model that adjusted for potential confounds (number of drinks of ethanol per month, tremor duration, MoCA score), taking primidone was not associated with either the right dentate GABA concentration (beta = 0.27, p = 0.17) or the left dentate GABA concentration (beta = -0.24, p = 0.22).To our knowledge, this is the first study to examine the effects of current, daily administration of primidone on MRS-assessed dentate GABA concentration. Our data addresses a question about a major potential confounder in studies of ET pathophysiology. The fact that no difference was found in dentate GABA concentrations between patients taking primidone or not, as well as the lack of any association between daily primidone dose and dentate GABA concentration suggests that it is not necessary to exclude ET patients on primidone from MRS studies of dentate GABA concentration.