Measurement of Tremor
For more than a century instrumental
methods are used to describe and quantify tremor, in order to answer pathophysiological or
pharmacological questions or to monitor patient therapy.
Quantities that are usually recorded include:
EMG activity of the affected muscles (in parkinsonian and
essential tremor usually the wrist extensors and flexors),
the acceleration of the hand using piezoresistive accelerometers,
the displacement of the hand using potentiometers or
modern video or laser equipment, or
the force tremor using piezoresistive force transducers.
Characterization of Tremor
From these recordings as well as from clincal observation several characteristics of
tremor may be derived:
A)
The Amplitude/Intensity
, of course, is the most important characteristic. It clearly (by definition)
distinguishes normal tremor from all pathological forms.
It correlates with disability and embarrassment. The correlation of different measures
of (EMG-based, clinical rating, disability measures) is currently under investigation in our lab.
B)
The Tremor frequency
: From all of the abovementioned traces (the EMG has to be
demodulated / rectified the Tremor frequency can be obtained by using suitable
spectral estimation and FFT techniques. Although the tremor frequency may vary between
patients as well as in one patient from time to time, there are typical frequency ranges for the
different forms of tremor: Essential Tremor: 4-12 Hz, Parkinson's Disease: 3-8 Hz, and cerebellar
tremor: 1.5-4 Hz.
C)
Context dependence
: Tremor may be different in different situations. A tremor that is present during
constant activation of the affected muscles (e.g. holding the hands stretched out) is called a
postural
tremor. A pure
resting
tremor is present only while the muscles are not
voluntarily activated. Essential tremor is mostly postural, while Parkinsonian tremor usually
is stronger under resting conditions. More unusual forms of tremor are movement tremor,
occurring during movement, and
intention
tremor, that is becoming most prominent while the
hand is approaching a target the patient is aiming at.
D)
Agonist-antagonist interaction
. The way extensors and flexors cooperate in order to generate the
involuntary oscillations is another important characteristic of tremor. In Parkinsonian tremor, extensors and
flexors are commonly believed to be activated alternatingly, while in essential tremor simultaneous
contraction is more common.