Establishing the definition and inter-rater reliability of cortical silent period calculation in subjects with focal hand dystonia and healthy controls

The purpose of this paper is to describe a clearly defined manual method for calculating cortical silent period (CSP) length that can be employed successfully and reliably by raters after minimal training in subjects with focal hand dystonia (FHD) and healthy subjects. A secondary purpose was to explore intra-subject variability of the CSP in subjects with FHD vs. healthy subjects. Two raters previously naïve to CSP identification and one experienced rater independently analyzed 170 CSP measurements collected in 6 subjects with focal hand dystonia (FHD) and 9 healthy subjects. Intraclass correlation coefficient (ICC) was calculated to quantify inter-rater reliability within the two groups of subjects. The relative variability of CSP in each group was calculated by the coefficient of variation (CV). Relative variation between raters within repeated measures of individual subjects was also quantified by CV. Reliability measures were as follows-mean of three raters: all subjects: ICC = 0.976; within healthy subjects: ICC = 0.965; in subjects with FHD: ICC = 0.956. The median within-subject variability for the healthy group was CV = 7.33% and in subjects with FHD:CV = 11.78%. The median variability of calculating individual subject CSP duration between raters was CV = 10.23% in subjects with dystonia and CV = 10.46% in healthy subjects. Manual calculation of CSP results in excellent reliability between raters of varied levels of experience. Healthy subjects display less variability in CSP. Despite greater variability, the CSP in impaired subjects can be reliably calculated across raters.