Table 1
A Summary of Evidence Supporting the Association of ET and PD
| Phenomenologic motor non-motor ET–PD phenotype other | Overlapping clinical motor features, e.g. rest tremor, cogwheeling, postural instability26,28–30,58 Slowed reaction times31,32 Preserved olfaction in tremor dominant PD patients with family history of tremor41 Cognitive dysfunction (primarily frontal/executive problems)42–46 Increased frequency of anxiety, depression, and apathy45,47,112,113 Increased frequency of RBD and daytime somnolence22,45,47,48 Emergence of PD after long-standing ET27 PD asymmetry correlating with historical tremor asymmetry and severity27,36 Increased incidence of rest and action tremor in ET–PD group versus PD38 Increased cerebellar signs and more diffuse tremor in ET–PD patients than isolated PD patients39 Increased risk of melanoma in PD and ET49,50 Decreased risk of PD and ET among smokers51–53 |
| Epidemiologic | Increased prevalence of PD in ET patients relative to general population54–57 Increased prevalence of ET in family members of PD patients60–65 Increased prevalence of tremor dominant PD in ET families37 Increased family history of tremor in relatives of ET–PD patients than isolated PD patients38,40 |
| Imaging | Hyperechogenicity in substantia nigra of ET patients associated with progression to PD68 Reduced striatal dopamine uptake on DaTscan69–71 |
| Pathologic | Overlapping cerebellar pathology of changes in climbing fiber density and torpedoes74,75,77,78 Lewy bodies in brains of ET patients75,77 |
| Genetic (strong family history and putative genes) | LINGO193–96 LINGO296,97 HS1BP398,99 DNAJC13100 HTRA2102 NAACP-Rep1103 CACNA1G (Cav3.1) |