Background
Recurrent and involuntary motor and vocal tics constitute the typical characteristics of patients with the syndrome of Gilles de la Tourette (GTS). Objective quantification of the frequency and intensity of tics in Tourette patients can contribute much to improvement of diagnostic criteria and evaluation of therapy efficacy. The difficulty of developing reliable methods for the assessment of tic severity relates to the variable presentation of tic symptoms, comprising aspects of localisation, frequency, intensity, complexity, and noticeability. Furthermore, there are time- and situation-specific variations within and across subjects in tic severity. At present, several clinician-based rating scales exist to assess the multiple clinical features that contribute to overall severity of Tourette's disorder, whereas videotape-based tic countings have been used to obtain a more quantitative index of tic severity and the temporal dynamics of tics. Much can be gained from objective tools that can describe the temporal and situational characteristics of tics in controlled and ambulatory situations.

Aim
In this study, we explored accelerometry as a novel tool to objectively quantify the frequency and intensity of motor tics and (non-)specific movements during period of rest and performance of various tasks in GTS patients.

Methods
Nine Tourette patients (2 females; mean age: 37.6 years, SD: 12.9) and 14 controls (7 females; mean age: 31.8 years, SD: 14.3) participated in a 1-hour session during which a video camera recorded the movements of the head during two pre-task periods of rest (5-min periods) and during performance of two tasks, e.g. a conversation of 5 min during which the subject talked with the researcher about his/her hobbies and the watching of a 15-min video of an entertaining program. Objective registration of head movements and head tics occurred by means of three uni-axial 5-g piezo-resistive accelerometers (IC-sensors 3031) that were attached to an elastic band, placed above the eyes of the subject. With these sensors both the position of the head in the lateral, sagittal and transaxial plane could be documented, as well as the actual acceleration of the head during movements in the three planes. The accelerometer signals were stored at a sample frequency of 64 Hz on a digital portable recorder. Two typical movement patterns were analysed: a) fast oscillatory head movements (about 3-4 per sec) which occurred in both patients and controls, and b) isolated fast head movements, primarily corresponding to isolated motor tics of the patients (head jerking). A head motility index was computed for each plane (lateral, sagittal, transaxial) in order to obtain a general motility measure: each of the 3 signals was high-pass filtered (0.5 Hz) and rectified, the resulting signals were integrated over 1-min intervals.

Results
Head motility levels in the lateral, sagittal and transaxial plane were significantly higher in the patients than in controls during all procedures (p<0.05). Both patients and controls showed a significant increase in head motility levels during conversation (p<0.05); both groups also showed a similar significant increase in specific oscillatory head movements during conversation (p<0.05). The number of isolated fast head movements was significantly higher in patients than in controls (p<0.05); these movements showed no increase during conversation but increased significantly while watching the videotape (p<0.05).

Conclusion
Our findings illustrate the potential of accelerometry to provide objective quantitative parameters of spontaneous movements in Tourette patients. In both standardized and ambulatory research, accelerometry may be used as an objective tool to quantify the severity and temporal dynamics of tics or (non-)specific movements in neuropsychiatric disorders.

© 2000 Noldus Information Technology b.v.