Temporal discrimination in patients with dystonia and tremor and patients with essential tremor

To investigate whether psychophysical techniques assessing temporal discrimination could help in differentiating patients who have tremor associated with dystonia or essential tremor. We tested somatosensory temporal discrimination thresholds (TDT) and temporal discrimination movement thresholds (TDMT) in 39 patients who had tremor associated with dystonia or essential tremor presenting with upper-limb tremor of comparable severity and compared their findings with those from a group of 25 sex- and age-matched healthy control subjects. TDT was higher in patients who had tremor associated with dystonia than in those with essential tremor and healthy controls (110.6 ± 31.3 vs 63.1 ± 15.2 vs 62.4 ± 9.2; p < 0.001). Conversely, TDMT was higher in patients with essential tremor than in those with tremor associated with dystonia and healthy controls (113.7 ± 14.7 vs 103.4 ± 11.3 vs 100.4 ± 4.2; p < 0.001). Combining the 2 tests in a pattern for essential tremor (abnormal TDMT/normal TDT) and tremor associated with dystonia (normal TDMT/abnormal TDT) yielded a positive predictive value (PPV) of 86.7% and a negative predictive value (NPV) of 70.8% for diagnosing essential tremor and a PPV of 100.0% and NPV of 74.1% for diagnosing tremor associated with dystonia. TDT and TDMT testing should prove a useful tool for differentiating tremor associated with dystonia and essential tremor. Our findings imply that the pathophysiologic mechanisms underlying tremor associated with dystonia differ from those for essential tremor.