Identification of psychogenic, dystonic, and other organic tremors by a coherence entrainment test

The differentiation of psychogenic from organic tremors, particularly those of a dystonic nature, can be difficult on clinical grounds. Entrainment of tremulous movements of different body parts into a single rhythm has been used clinically as a means of distinguishing these tremor forms, based on the inability of a patient with hysterical tremor to generate voluntary tapping oscillations independent of their ongoing tremor oscillation. The coherence entrainment test is a quantified electrophysiological entrainment test performed on accelerometer or surface EMG tremor signals. This test was carried out on 25 patients referred with suspected psychogenic tremor or dystonic tremor and on 10 normal subjects attempting to tap two independent voluntary oscillations. Using established and new clinical diagnostic criteria, patients were assigned the following final clinical diagnoses: 6 cases of clinically definite dystonic tremor, 5 cases of probable dystonic tremor, 2 cases of classic essential tremor, 5 cases of clinically definite psychogenic tremor, 3 cases of probable psychogenic tremor and 4 uncertain cases. On comparing these clinical diagnoses with those reached by a coherence entrainment test subsequently carried out on each patient, there was 100% concordance in both clinically definite and clinically probable patients. In uncertain cases, when later clinical information came to light, this also corroborated with the coherence entrainment diagnosis. No normal subjects were able to "mimic" organic tremor. The coherence entrainment test appears to be a sensitive and specific means of distinguishing psychogenic tremor from dystonic and other organic tremors.