How Does Presence of Essential Tremor Affect Progression of Parkinson Disease?

Among patients with Parkinson disease (PD), a positive history of essential tremor (ET) was found to be associated with slower progression of motor and nonmotor symptoms, with no impact observed on survival, according to study findings published in Scientific Reports.

In recent years, the relationship between ET and PD has become a controversial debate. “In the past, they were regarded as 2 different conditions that represent the most common tremor disorders in adults,” explained the study authors. “However, subsequent studies indicated that the presence of ET significantly increased the risk of conversion to PD.”

In light of these emerging studies, some scholars have sought to classify the coexistence of both conditions as Essential tremor-Parkinson disease (ET-PD) syndrome, which is characterized by patients with a long-lasting history of ET that then developed into PD. For those with ET-PD syndrome, research has indicated that patients are older at onset of PD diagnosis, exhibit less severe PD, and are on a lower dosage of levodopa compared with patients with PD without preexisting ET.

Researchers of the present study sought to further examine the effect of preexisting ET history on the progression of PD. Of a total of 785 patients recruited (male, n = 407; mean [SD] age, 60.5 [11.8] years), 61 (7.8%) reported a history of ET based on retrospective interviews or past medical records. The mean (SD) age of PD onset was 58.9 (11.8) years, with mean (SD) PD disease duration indicated as 1.6 (0.8) years.

Patients were recruited and followed up from March 2009 to July 2020. Cox proportional hazard ratios with inverse probability of treatment weighting (IPTW) were utilized to estimate HRs in both patient cohort groups.

Cox regression models after IPTW showed that positive history of ET provided a protective effect against progression of severity for motor and nonmotor symptoms in PD, when compared with patients with PD without history of ET:

• United PD Rating Scale III 14-point increase (HR, 0.301; 95% CI, 0.134-0.678; P = .004)
• time to akinesia and rigidity 8-point increase (HR, 0.417; 95% CI, 0.218-0.796; P = .008)
• time to conversion to Hoehn and Yahr stage 3 (HR, 0.356; 95% CI, 0.131-0.969; P = .043)
• time to develop dyskinesia (HR, 0.160; 95% CI, 0.037-0.698; P = .015)
• time to Montreal Cognitive Assessment 3-point decrease (HR, 0.389; 95% CI, 0.160-0.946; P = .037).

Conversely, there was no significant difference observed with time to tremor 4-point increase (HR, 1.638; 95% CI, 0.822-3.266; P = .161) and time to death (HR, 0.713; 95% CI, 0.219-2.319; P = .574) when comparing both patient cohorts.

“Our study indicated that ET history in PD patients was associated with a benign prognosis with slower motor and non-motor progression and had no impact on survival,” concluded the study authors.

Reference

Ou R, Wei Q, Hou Y, et al. Association between positive history of essential tremor and disease progression in patients with Parkinson disease. Sci Rep. Published online December 10, 2020. doi:10.1038/s41598-020-78794-1