4
CORTICOSPINAL SILENT PERIOD AND IPSILATERAL SILENT PERIOD
When recorded in a tonically contracted muscle, MEP produced by a TMS pulse delivered to the contralateral motor cortex is always immediately followed by a period of electrical silence. This silence has a mixed cortical and spinal origin (corticospinal silent period, CSP). Spinal mechanisms are involved in the early part of the CSP, i.e., about the first 50ms (Fuhr et al., 1991), while the latter part of the CSP takes its origin in intracortical inhibitory controls of the motor cortex with gamma-aminobutyric (GABA) mediation, involving mostly type B receptors (Siebner et al., 1998; Stetkarova and Kofler, 2013), particularly at high intensity of stimulation (Kimiskidis et al., 2006).
Diagnostico \ Tratamento
Casos
a. Reabilitação Neurologica e Psiquiatrica
b. Artigos
c. Pros\ Cons
Descrição de casos dos pacientes tratados no hospital.
PRINCIPLES OF NEURONAVIGATION
A common method to identify a brain area relates to the effects produced by its stimulation. Examples of cortical functional localization include the production of movements following precentral gyrus stimulation, speech arrest following left frontal stimulation (Pascual-Leone
et al., 1991), or phosphenes following occipital cortex stimulation (Amassian et al., 1998).
MOTOR THRESHOLD
In most applications and studies, the intensity of TMS is individually adjusted to the rest motor threshold (RMT), defined as the minimal
intensity of a TMS pulse delivered to the motor cortex to elicit a reliable MEP of minimal amplitude (>50)
MOTOR EVOKED POTENTIALS
The main use of TMS in clinical neurophysiology is to assess the conduction of the descending corticospinal (or corticonuclear) pyramidal tract by recording MEPs.
The MEPs are recorded over target muscles with surface electrodes and bipolar belly-tendon montage. The placement of the electrodes, as well as band-pass and amplification settings, are identical to those used for recording compound muscle action potentials (CMAPs)
THETA BURST STIMULATION
Introdução
TBS consists of short bursts of three low-intensity pulses with inner high frequency (50Hz, within the gamma range) that are delivered every 200ms, i.e., at 5Hz (within the theta range)
- TMS can serve various purposes for diagnosis or treatment.
- TMS is mainly dedicated to the recording of motor evoked potentials (MEPs).
TMS & fMRI
- MEP recording allows investigation of corticospinal conduction time and cortical motor control in clinical practice.
- MEP recording is a reliable method to perform functional mapping of muscle representation within the motor cortex.
Conclusion
PRINCIPLES OF TRANSCRANIAL
MAGNETIC STIMULATION
TMS delivered as repetitive trains of stimulation (rTMS) may activate, inhibit, or otherwise interfere with the activity of neuronal cortical networks, depending on stimulus frequency and intensity, and brain induced electric field configuration
rTMS
- TMS is based on the scientific principle of electromagnetic induction discovered by Faraday in 1831.
- The magnetic field pulse delivered by a stimulating coil applied on the scalp is able to pass through skull bone without being attenuated and to generate an electric field when entering the brain.
- The intensity of the induced current is sufficient to produce action potentials and to activate brain networks safely and painlessly. The first TMS machines for clinical use were built in the mid-1980s (Barker et al., 1985).
Therefore by modifying brain functions, with after-effects lasting beyond the time of stimulation, rTMS opens exciting perspectives for therapeutic applications, especially in the domain of depression and chronic pain syndromes.
- Repetitive TMS (rTMS) was introduced in the early 1990s and required a specific set of stimulators able to overcome the recharging time of magnetic stimulators to maintain the same output level, even with extremely brief ISIs. Repetitive TMS has a modulatory effect on cortical excitability, which outlasts the stimulation period and can be used in a variety of indications, delivered to either motor or non motor brain regions.