when it was present in at least 5 of 10 trials. Tremor axes were determined by the directions of maximal tremor power assessed over 360°.
There were 29 cases (mean age, 68.8 Ϯ 10.6 years). Fifteen (51.7%) had a tremor severity rating of 2 or 3 (moderate or severe tremor), 12 (41.4%) had a rating of 1 (mild tremor), and 2 (6.9%) with head tremor had no clinically detectable tremor on either right-or left-hand spirals.
The neurologist identified an axis in one or both spirals in 23 (79.3%) of 29 cases; the nurse practitioner, in 22 (75.9%). Seven (58.3%) of 12 cases with mild tremor had an identifiable axis on both spirals. The neurologist identified an axis in the right hand in 20 cases; in 19 (95%) of these, the axis was detected in segments equivalent to 2 or 3 o'clock. Eighteen ET cases had an axis detectable in the left hand; in 15 (83.3%) of these, the tremor axis was detected by the neurologist in segments equivalent to 10 -12 o'clock (oriented at 90°relative to that drawn with the right).
SA detected a tremor axis in 25 (86.2%) right hands, in 21 (75.0%) left hands, and in one or both hand-drawn spirals in 28 (96.6%). The axis was detected in segments equivalent to 2 and 3 o'clock (Fig. 1) in 18 (72.0%) of 25 cases with detectable axis in the right hand and in segments equivalent to 10 -12 o'clock in 16 (76.2%) of 21 cases with an axis detectable in the left hand.
Action tremor in ET has several definable features. For example, the amplitude of the kinetic tremor is on average greater than the amplitude of the postural tremor 3,4 and the tremor often worsens over time. 5 As shown in this study, an axis with a characteristic orientation is also present in most but not all patients with ET. The diagnostic utility of the axis is not known; additional research will involve the enrollment of patients with Parkinson's disease and other tremor disorders. The basis for the axis, in terms of central vs. mechanical-reflex mechanisms, is unclear and deserves further study.
Acknowledgments: This study was supported by the National Institutes of Health (R01 NS42859 and R01 NS039422).