Tremor is rhythmic movement resulting from involuntary alternating or synchronous contractions of muscle antagonists. Tremor may be observed in several conditions, and it is important to consider the characteristics of the tremor to narrow in on its cause.
PD tremor and essential tremor (ET) may be confused. ET is much more prevalent and results in an action tremor, not a rest tremor, and often affects hands, head or voice; the latter two are not seen in PD. PD's resting tremors are observed with the body part completely at rest, and subsides with action or the assumption of a posture. Onset is typically asymetrically in one hand, although it may be observed elsewhere. Early in the disease, tremor may be intermittent and may be precipitated by axiety, stress, or clenching of the contralateral hand.