Functional tremor (FT) is a common form of functional movement disorder.  Diagnosis is based on clinical history and examination.  Despite characteristic features, the diagnosis of FT can be challenging.  While there are numerous studies evaluating FT, there is a paucity of quantitative data on the characteristic frequency of FT using 3-D motion analysis and kinematics.

A retrospective study was performed using data from Emory University's motion analysis laboratory database.  Ten patients with functional tremor (FT), ten with Parkinson’s tremor (PT), and ten with Essential tremor (ET) were used for comparison.  3-D motion data was obtained in a dedicated laboratory with patients performing a set of standardized examination tasks.  Statistical analysis using ANOVA and F-test compared groups for tremor frequency.

The average frequency of FT at rest, with posture, and with action were 5.2 ± 1.5 Hz (n=7), 5.2 ± 1.6 Hz (n=8), and 6.9 ± 1.0 Hz (n=3) respectively.  There was a significant difference in frequency between the three groups with action (p=0.00002), but not at rest (p=0.44) or with posture (p=0.74).  The variance of tremor frequencies at rest was significantly more in the FT group when compared to the PT group (p=0.0003) or ET group (p=0.02).  The variance of tremor frequencies with posture was also significantly more in the FT group when compared to PT group (p=0.003) or ET group (p=0.003).

There is considerable overlap in the clinical presentation of different tremor syndromes, making diagnosis difficult.  The variance in frequency within the FT group compared to either PT or ET group may help distinguish such conditions in a quantitative manner.  For this reason, quantitative kinematics may be helpful in differentiating FT from PT and ET.