Contralateral spread of unilateral tremor in Parkinson’s Disease

__Background__ Parkinsonian tremor usually starts unilaterally. The mid-term prognosis of this lateralized tremor is unknown, as is the development of tremor in the contralateral arm. __Objective__ To investigate the emergence of contralateral tremor in the Parkinson-Progression-Marker-Initiative database, with data available for 7 years. __Methods__ Tremor Amenable for surgery (TAS) was defined as any rest, postural or kinetic tremor with amplitude >1 cm (MDS-UPDRS score ≥2) as this criterion is commonly accepted for inclusion in surgical studies. Tremor was analyzed by side mainly in the off-medication state. __Results__ At baseline, 348 (87.7%) of the 397 patients with Parkinson’s disease had tremor at least on one side of the body. 183 (46%) had only mild tremors but 165 (41.6%) had TAS. 159 patients (40.1%) had lateralized TAS and 6 (1.6%) had bilateral TAS. Among patients with unilateral TAS, 40 patients (25.8%) developed contralateral TAS at 3 years, 49 patients (30.8%) at 5 years, and 61 patients (39%) at 7 years. The side more affected by tremor was also more affected by other cardinal symptoms. In 159 patients with initially unilateral TAS, tremor severity did not increase on the tremor-dominant side over the 7-year period. However, there was an increase in tremor on the contralateral side. This was associated with a clear increase in bradykinesia and rigidity on both sides. __Conclusion__ The study findings may prove beneficial in counselling patients with TAS, and may also provide an explanation as to why the worsening of tremor is not correlated with overall disease progression.