No handshake agreement on postural tremor subtypes in Parkinson disease?

The tremor of Parkinson disease (PD) is classically defined as a resting tremor, but postural tremor is a frequently observed accompaniment.¹ The postural tremor may be mild and inconsequential, thus ignored by both patient and physician. Conversely, it can be more bothersome and disabling than rest tremor. In this situation, its specific pathophysiology assumes clinical importance, to guide treatment. It is broadly divided into re-emergent tremor² (resting tremor that reappears after a latency with fixed posture) or postural tremor (immediate onset with posture). The latter is further subdivided into enhanced physiologic tremor, essential tremor (ET), or dystonic tremor, based on key clinical characteristics.³ Surface electrophysiology is a useful diagnostic aid to classify these postural tremor subtypes. Its utility lies in recording tremor frequencies under different conditions: rest, posture, action, and limb weighting.⁴ For re-emergent tremor, resting and postural tremor frequencies are similar, whereas ET postural tremor is at least 1.5 Hz higher than PD rest tremor.³ With enhanced physiologic tremor (a peripherally generated tremor), weighting the limb changes the tremor frequency, which distinguishes it from centrally generated tremor conditions (such as ET and PD).⁴ Phasic co-contraction of EMG burst activity in forearm muscles suggests dystonic tremor.⁴ Moreover, electrophysiology can assess for distractibility during contralateral limb tapping, a hallmark of functional tremor.^3,4 Despite these differences, there may be clinical and electrophysiologic overlap, and the postural tremor of PD may elude specific diagnosis.