“…However, the new classification describes ET as a tremor “syndrome.” This change has stimulated scientific debates as to whether ET should be regarded as a “disease” or a “syndrome” or whether the various variants of ET should be simply considered subtypes, such as ET-PD, ET-dystonia, ET-ataxia, and other, since one may with time evolve into another ( 10 , 183 – 185 ). A recent study using multimodal investigations, including objective gait assessment, neuropsychological assessment, and optical coherence tomography (OCT) for retinal thickness measurement, provided objective evidence for the existence of two ET subtypes ( 186 ). Using cluster analysis one subtype, characterized by midline tremor, cognitive decline and thin retinal inner layer, suggests that this subtype of ET is more likely to be associated with neurodegeneration.…”