Transcranial magnetic stimulation (TMS) to the motor cortex can be used to elicit a cortical silent period (CSP) which is an interruption of electromyographic (EMG) activity in the corresponding contracting muscle [4
]. The length of this silent period is a reflection of cortical excitability and is thus a useful measure in a variety of experimental settings, including healthy subjects and those with neurologic compromise, and may provide a clinical marker of certain movement disorders, such as focal hand dystonia (FHD).
The CSP is thought to represent GABA-B receptor-mediated inhibition of cortical excitability, in addition to spinal inhibitory mechanisms such as Renshaw inhibition [13
]. Altered CSP duration has been observed in neurologic conditions associated with corticospinal lesions and in some movement disorders [6
]. In subjects with hemiparetic stroke, decreases in ipsilesional CSP length paralleled clinical improvement [6
]. Shortened CSP has been observed in patients with focal hand dystonia (FHD) indicating a lack of cortical inhibition as a feature of this disorder [2
]. Other clinical conditions have also been correlated to CSP length; longer CSP duration is typically associated with motor neglect while shorter CSP lengths often occur with spasticity [7
]. These findings demonstrate a role for CSP measurement in understanding pathophysiology of certain neurologic disorders as well as monitoring cortical excitability changes over time [6
] and after an intervention [24
There are multiple approaches for calculating CSP which creates within-study reliability and between-study reproducibility concerns, reducing the generalizability of this measurement. The approach frequently used is a visually guided or “manual” calculation. However, often the definitions of CSP onset and offset are not explicitly stated. The CSP onset is described as either the stimulation artifact or the onset of the motor evoked potential (MEP) and CSP offset is defined as the return of spontaneous EMG activity or pre-stimulus muscle activity, but the exact parameters are not consistently reported [10
Computerized methods have been developed to automate the time consuming CSP calculation [8
]. One program, known as the Cusum method, has demonstrated excellent reliability with manual measurement by experienced raters and poor to moderate reliability with other automated methods [11
Despite these attempts to automate CSP calculation, the majority of researchers employ a manual calculation technique. Manual calculation, though time consuming, allows researchers to examine for errors, variability or patterns within a subject or group. Given the additional time required to perform manual calculation, it may be required that more than one researcher be involved to expedite data processing. To our knowledge, no reliability has been reported between different manual raters or compared raters of different experience level. Additionally, no study has determined the reliability of CSP calculation in a neurologically impaired population. The purpose of this paper is to present a manual method with clear definitions for calculating CSP length that can be employed successfully and reliably by raters after minimal training, encouraging a uniform definition of measurement. Standardization in definition will allow for improved ability to compare findings across different studies. The reliability and relative variation of CSP measurement between raters is reported in subjects with FHD and in healthy subjects during a single testing session. A secondary purpose of this study was to examine the relative variation between subject groups. We hypothesized that healthy subjects would display less variability than subjects with FHD.