Functional tremor is the most common type of FMD. Tremor associated with typical neurological disorders is defined as a rhythmic and oscillatory movement of a body part with a relatively constant frequency.
Functional tremor is typically a complex resting, postural, and action tremor. The only known organic tremor with such physical findings is a cerebellar outflow or midbrain tremor (ie, rubral tremor) . Any body part may be involved with functional tremor, but hand tremor, leg tremor, and whole body tremor are common sites . Remarkably, the fingers are often spared with much of the upper limb tremor occurring at the wrist.
A characteristic that suggests functional tremor rather than organic tremors is abrupt onset with immediate maximal severity, often precipitated by trivial emotional or physical trauma . By contrast, essential tremor typically starts bilaterally, though one side usually is affected more than the other, and gradually worsens over years to decades
With functional tremor, a classic but not universal finding is complete remission or entrainment (ie, a shift of tremor frequency to the speed of contralateral rhythmic movement) with distraction maneuvers such as repetitive tapping tasks with an uninvolved opposite hand or foot. In some patients with functional tremor, restraining the affected limb may precipitate tremor in other previously unaffected body regions. Another sign of a functional tremor is that it can be “chased.” That is, when an affected hand is restrained, the tremor may move to the arm, and when the arm is restrained, the tremor may move to the shoulder. Additionally, the affected limb may “fight” with the examiner when it is restrained.
Electrophysiologic studies may be helpful in documenting the frequency of the functional tremor and distractibility.