Can tremor be used to measure the effect of chronic mercury exposure in human subjects?

Clinical and quantitative studies in which tremor was used to evaluate the effect of chronic exposure to organic and inorganic mercury in human subjects during the last 20 years were identified in MEDLINE and BIOLOGICAL ABSTRACTS. In the selected studies, methods varied widely by types of transducers used, anatomical locations of tremor testing, tasks executed by the subjects, types of tremor recorded and durations of trials. Measurements of tremor in these studies also varied by frequency, amplitude, waveform and reproducibility. Typically measures of variability were not included. Results obtained depended heavily on the methods and analyses used. We suggest that, although many contradictions and discrepancies have been noted in these studies, tremor can probably be used to measure the effect of chronic mercury exposure. This would involve more rigorous and uniform recording procedures and analyses. Specific suggestions proposed include: realizing the limitations of accelerometric data; considering the effect of weight on the tested limb; controlling recording conditions; selecting samples more carefully; recording other physiological variables besides tremor; using tasks and manoeuvres that unmask subclinical abnormalities in tremor; reporting frequency, amplitude, waveform and variability of tremor; understanding the limitations of power spectra analysis; separating tremor characteristics according to types of mercury exposure; and considering supraspinal influences that both vision and voluntary movements have on tremor.