Cortical Silent Period
The technique of magnetic stimulation is superior to conventional electrical stimulation in the stimulation of deep and less accessible nerves. Responses are quick to obtain without needing stimulation site preparation. Measurements available from the waveforms will include conduction latency, response amplitude (either base-line to first peak, or peak to peak value), and the threshold of stimulation. Other parameters also noted, which are perhaps used less frequently, include morphology (number of response phases), response area, power spectrum, silent period, fatigue and central recovery time. The silent period is a physiological phenomenon whereby a second, much delayed EMG response is seen after stimulation when the muscle is facilitated.
An example application of cortical silent period (CSP) is to evaluate cerebellar dysfunction in patients with MS. Studies have found a relationship between cerebellar dysfunction and SP prolongation, with CMCT and SP analyses providing complementary tests in evaluating motor pathways of patients with MS. It is suggested that, SP can be applied in patients with pure cerebellar dysfunction and it can be a valuable test in subclinical cases with MS (Tataroglu et al 2003).
CSP can also be used to assess the level of intracortical inhibition in patients with untreated idiopathic generalized epilepsy (IGE). A study by MacDonell et al (2001) showed that CSP duration was increased at all stimulus intensities, indicating that intracortical inhibition is increased in patients with IGE.
The effect of drugs on corticomotor excitability can be measured using twin pulse TMS. The silent period may be shortened or lengthened by the use of a drug. Many clinics worldwide find this a very useful method for an early assessment of how an individual reacts to a drug. If a drug does not appear to be having the desired effect on a patient, this can be detected early and the drug changed for a more effective alternative.
References
- Macdonell et al., Neurology, 2001.
