Frequency, Amplitude, and Waveform Characteristics of Physiologic and Pathologic Tremors

Abstract

Tremor is defined as a rhythmic, not necessarily sinusoidal periodic oscillation of one or more body parts. Tremor occurs in normal man as well as in patients with pathologic conditions and is one of the most distressing symptoms of various extrapyramidal movement disorders. There are differences between various forms of tremor, which can be distinguished by clinical investigation alone. Physiologic tremor has a low amplitude and can not usually be seen with the naked eye. Pathologic tremors occur in a variety of diseases, of which essential tremor (ET) and Parkinson’s disease (PD) are the most frequent ones. In ET, the tremor is usually present during posture and action, whereas it is present during resting conditions in PD. Moreover, ET is a mono-symptomatic disease, but PD is characterized by additional symptoms (bradykinesia and rigidity). However, these differences are not seen in every case and it has been estimated that in the early stage of the disease, 20% of patients with ET are misdiagnosed as having PD and vice versa (L. Findley, personal communication). This is due to the fact that there is considerable overlap of tremor characteristics, especially in the early phase of the disease. The tremors of PD may present with atypical characteristics and if additional symptoms of PD are lacking or are only mild, a differential diagnosis may be difficult [4]. Hence, the development of additional criteria based on objective measurement would be helpful for differential diagnosis.