Emergence of Parkinson's disease in essential tremor: a study of the clinical correlates in 53 patients.

Patients with essential tremor (ET) may develop Parkinson's disease (PD); however, few studies have examined the clinical features of this combination syndrome. Fifty-three patients with ET-PD were studied and compared to 53 PD and 150 ET patients. Although the latency from onset of ET to PD was brief (<5 years) in 38.5%, in a sizable proportion (30.8%), it was very long (>20 years). The gender distribution of ET-PD (67.9% male) was identical to that of PD (67.9% male), yet differed from that of ET (50.0% male) (P = 0.02). The initial cardinal sign of PD was rest tremor in 100% of patients. In ET-PD, the side of greatest initial ET severity usually matched that of greatest PD severity (P < 0.05). In ET-PD, male gender predominated and the sidedness of the ET and PD usually matched. The co-occurrence of the two diagnoses in the same patient may be mechanistically related.