Prodromal Parkinson's Disease in Essential Tremor: the TITAN study
Background: The relationship between essential tremor (ET) and Parkinson's disease (PD) remains controversial. Beyond viewing ET as a discrete risk factor for PD, recent frameworks propose that an ET phenotype may represent a clinical presentation of prodromal PD (pPD), consistent with the current reconceptualization of ET as a syndrome. Whether co-occurring subtle motor signs alter pPD probability in ET remains unknown. Methods: Using the MDS research criteria, we calculated pPD probability in a large cohort of ET patients with and without subtle motor signs (rest tremor, hypomimia, isolated rigidity, reduced arm swing, altered repetitive movements, global slowing). Multivariable regression was used to identify independent predictors of pPD probability. Results: Among 599 ET patients (median disease duration: 12 years), only 6 (1.0%) met criteria for probable pPD. Although ET patients with subtle motor signs exhibited higher continuous pPD probability scores than those without, the frequency of possible or probable pPD did not differ significantly between groups. In multivariable regression, neither ET nor individual subtle motor signs, but hypomimia, were independent predictors of pPD probability, which was primarily driven by older age and male sex. Conclusions: Long-standing ET, whether isolated or accompanied by subtle motor signs, is not associated with pPD, with the possible exception of co-occurring hypomimia.