Essential Tremor: More Than a Motor Disorder? Robert D. Shura, Psy.D., Robin A. Hurley, M.D., Katherine H. Taber, Ph.D. FIGURE 1 AND COVER. RIGHT. It is gen- erally agreed upon that the action tremor characterizing essential tremor (ET) indi- cates dysfunction within the central motor control (cortico-bulbo-cerebello-thalamo- cortical) network. 1–3 Cover. Approximate anatomic locations for the inferior olive nucleus (blue) in the medulla and ventral intermediate nucleus (red) in the thalamus are color-coded onto representative MRI. FIGURE 2. Task-activated functional imaging has been used to identify areas that are more active during tremor in patients with ET. A functional MRI (fMRI) study that utilized electromyography (EMG) to identify tremor-related acti- vations in ET patients reported activations (approximate locations indicated in red) within all parts of the motor control network, as well as areas outside of this network. 4 Coherence and timing between tremor (EMG) and brain activity (magnetoencephalography, MEG, electroencepha- lography, and EEG) have also been used to identify brain areas contributing to tremor. Corticomuscular coherence localized to the motor cortex (approximate location in- dicated in green) in a MEG study. 5 This study also used coherence at the tremor frequency between the motor cortex and the rest of the brain (cerebro-cerebral coupling) to identify other areas of the tremor-related network (green). An EEG study that separated patients with ET by age at onset reported that corticomuscular coherence lo- calized (approximate location indicated in blue) to the motor cortex, the prefrontal cortex, and the thalamus in both early- and late-onset groups but also to the cere- bellum and brainstem only in the early-onset ET group (blue). 6 FIGURE 3. There is growing evidence supporting the pres- ence and importance of nonmotor features of ET. 7–9 Some of the findings reported in two recent studies that compared nonmotor symptoms of patients with ET to patients with Parkinson’s disease (PD) are color coded here by study and patient group. 10,11 The study that assessed nonmotor symp- toms in patients prior to formal diagnosis reported no differ- ence between ET and PD in overall symptom burden. 10 The study that compared ET and PD patients at a later point in time reported higher total symptom burden for the PD group. 11 It is noteworthy that in both studies, the PD group had higher frequency for some types of symptoms (e.g., changes in taste or smell, dizziness, acting out during dreams, double vision), suggesting phenotypic differences. neuro.psychiatryonline.org J Neuropsychiatry Clin Neurosci 30:2, Spring 2018 WINDOWS TO THE BRAIN
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