BACKGROUND AND AIMS
Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy of the upper extremities, causing pain, paraesthesia, numbness, and weakness in the territory corresponding to the median nerve.1-3 Although nerve conduction studies have been proposed for use in its diagnosis, the electrodiagnostic severity of CTS may not be associated with its clinical severity. The strength-duration time constant (SDTC) is a property of nodal membrane and, while it depends on a number of factors, its measurement may shed light on axonal properties when taken in conjunction with measurements of axonal excitability.2 For example, SDTC increases with demyelination as the exposed membrane is enlarged by inclusion of the paranodal and internodal membrane, it decreases with hyperpolarisation, and it increases with depolarisation.3 The Boston carpal tunnel questionnaire (BCTQ) is an easy, brief, self-administered tool for assessing symptom severity and functional status in CTS; recently, a Greek version has been validated.1 The aim of this study was to correlate BCTQ with electrodiagnostic measurements including nerve axonal excitability.
MATERIALS AND METHODS
BCTQ was administered to 29 consecutive patients (25 females, four males; age: 57.48±15.48 years) referred to the authors’ laboratory with symptoms consistent with CTS. All of the patients and 19 age-matched control subjects underwent motor conduction study and excitability measurements using Qtrac software.
RESULTS
For the 29 patients with CTS, the average SDTC was 0.54±0.13 μsecs and median nerve compound muscle action potential (CMAP) was 11.62±3.27 mV. Only SDTC was found to be strongly correlated with the BCTQ score, whereas the latency and amplitude of CMAP were not. The amplitude of CMAP correlated only with the functional status scale of the BCTQ.
CONCLUSION
A previous study compared the strength-duration behaviour of motor and sensory fibres in human subjects, determined the best method for deriving a SDTC, and evaluated the reproducibility of time-constant measurements to determine whether they could be suitable for clinical use.4 The precise value of the time constant depends on a complex interaction of biophysical variables and on experimental technique. However, such measurements may prove a useful additional tool for probing the pathophysiology of peripheral nerve disorders in human subjects, particularly when combined with other measures of axonal properties such as excitability, latency, or CMAP. In this study, these measurements of SDTC may shed light on axonal properties in patients with CTS and could constitute a useful, relatively simple technique in clinical practice.
