Published online by Cambridge University Press: 23 December 2009
Tremor is defined as a “… rhythmical, involuntary oscillatory movement of a body part …” (Deuschl et al., 1998). These involuntary movements can easily affect the voluntary movements of reaching and grasping up to the total loss of control in patients with severe tremor disorders. The following chapter will review the clinical characteristics and pathophysiological concepts of the most frequent and pathophysiologically important tremor disorders and link the findings to the control of grasping and other hand functions.
Any movement or isometric contraction is accompanied by the mostly invisible normal physiological tremor. The limits between normal and pathological tremors can be difficult to define. A pragmatic clinical approach is to define abnormal tremor whenever it is visible to the naked eye. The frequency of physiological tremor is usually greater than 7–8 Hz. It has recently been proposed that any tremor at lower frequencies is likely to be pathological (Elbe et al., 2005), but in cases of gradual transitions this clinical criterion can be problematic.
Theoretically tremor oscillations can emerge from two basic mechanisms. Any movable limb can be regarded as a pendulum with the capability to swing rhythmically (oscillate). These oscillations will automatically assume the resonant frequency of this limb which is dependent on its mechanical properties; the greater its weight the lower its resonant frequency, the greater the joint stiffness the higher its frequency (Elbe & Randall, 1978; Lakie et al.,1986). Any mechanical perturbation can activate such an oscillation.