Re-emergent tremor of Parkinson's disease

Objective-To characterise postural tremors in patients with Parkinson's dis ease. Rest tremor is a well recognised cardinal symptom of Parkinson's dise ase, but postural tremors associated with the disease may cause more disabi lity than the more typical rest tremor. Postural tremor of Parkinson's dise ase has been attributed to enhanced physiological tremor, clonus, or coexis tent essential tremor. It is postulated that one type of postural tremor in Parkinson's disease represents a rest tremor that re-emerges after a varia ble delay while maintaining posture, hence "re-emergent tremor". Methods-Accelerometry, peak frequency, peak frequency amplitude, root mean square (RMS) amplitude, and latency were determined in 18 patients (mean ag e: 63.2 (SD 9.8) years) with Parkinson's disease who had clinically evident postural tremor, 20 (mean age: 66.9 (SD 5.8) years) with typical essential tremor, and seven (mean age: 68.7 (SD 15.3) years) with the combination of pre-existing essential tremor and subsequent Parkinson's disease (essentia l tremor/Parkinson's disease). Latency, the time interval starting with the assumption of an outstretched posture and ending with the onset of postura l tremor, was measured by marking the start time by a pulse produced from i nterrupting a beam to a photocell when the arm reached a horizontal positio n. Results-The latency for the re-emergent tremor (9.37 (SD 10.66) s), present in 12 of 18 patients with Parkinson's disease, was significantly (p<0.0005 ) longer than the latency for postural tremor of essential tremor (1.29 s i n one patient, absent in 19 others); five of seven essential tremor/Parkins on's disease patients had an observed latency (6.57 (SD 8.23 s) which was a lso significantly (p<0.005) longer than that for essential tremor. There wa s no difference in the mean tremor frequency (similar to 5.5 Hz) between th e re-emergent tremor and the more typical Parkinson's disease rest tremor. The amplitudes were generally higher for the postural tremor associated wit h Parkinson's disease compared with those of essential tremor. Conclusion-These studies suggest that the re-emergent tremor of Parkinson's disease can be differentiated from the postural tremor of essential tremor and that it may share pathophysiological mechanisms with the: more typical rest tremor.