Various novel and non-invasive brain stimulation methods have come in last two decades
Transcranial Magnetic Stimulation
- Magnetic Seizure Therapy
- Deep Brain Stimulation
- Vagus Nerve Stimulation
Although they differ in means of application and degree of invasiveness, all of these procedures are essentially electrical in nature.
Transcranial Magnetic Stimulation
- Brief historical Overview
- The Machine
- How to Administer
- How Does it Act
- Adverse Effects
- Therapeutic Applications
Faraday in 1831 showed that a time-varying magnetic field can induce a current in a nearby conductor
- The French scientist d’Arsonval in 1896 reported on the first application of TMS in humans. He induced phosphenes (a flickering-light sensation not elicited by visual perception), vertigo, and syncope in subjects whose heads were placed in a large electromagnetic coil.
- In 1959, Kolin demonstrated for the first time that an alternating magnetic field can stimulate the sciatic nerve of a frog.
- In 1965, Bickford induced muscle twitching in humans by applying a pulsed magnetic field to ulnar, peroneal, and sciatic nerves.
Dr. Anthony Barker and colleagues at the University of Sheffield, England developed the first modern TMS device in 1985. They found that by placing the coil over the motor cortex, finger and foot movement could be induced.
The stimulating coil acts as the first coil, space as the medium for the flow of magnetic field and the electrically conducting living tissue acts as the second coil.
High intensity current is rapidly turned on and off in the coil through discharge of capacitors. This produces a magnetic field that passes through the scalp, and the oscillations in the magnetic field induce an electrical current that stimulates neurons. This magnetic field has a transient strength of about 1.0 to 1.5 Tesla and is able to activate neurons 1.5 to 2 cm from the surface of the coil .
If TMS pulses are delivered repetitively and rhythmically, it is called repetitive TMS (rTMS).
With the development of machines that allow transcranial magnetic stimuli to be delivered at very fast rates, this is called high frequency or fast rTMS (>1 Hz)
Two distinct parts:
- High current pulse generator producing discharge currents of 5000 amperes or more
- Stimulating coil producing magnetic pulses with field strengths of 1 tesla or more with a pulse duration of around 1 ms.
How to Administer
- The amount of electricity needed to cause changes in the cortex varies from person to person, and also from one brain region to the next
- One commonly used method for standardizing and adjusting the amount of electricity delivered and induced by TMS across different patients is to determine each person’s motor threshold (MT).
- The MT is commonly defined as the minimum amount of electricity needed to produce movement in the contralateral thumb when the coil is placed optimally over the primary motor cortex.
- MT can be determined by using electromyograph (EMG) recordings
Diagram for procedure of repetitive TMS
Treatment being carried out through repetitive TMS
How does it acts??
- High frequency rTMS (>1 Hz.)increases cortical excitability and enhances regional blood flow.
- Low frequency rTMS (<1 Hz.) has just the opposite effect
- Excitability apparently follows reduction of intracortical inhibition (GABA- receptor based) or enhancement of intracortical excitation (glutamate receptor based) or both
- The result
Several neurophysiological changes found after TMS:
- Increased firing rate of dopaminergic neurons in the ventral tegmental area and substantia nigra
- Elevated taurine, serine, and aspartate in the hypothalamic paraventricular nucleus
- Release of monoamines in the hippocampus
- Increased hippocampal serotonin and 5-hydroxyindoleacetic acid
- Selectively increased 5-HT1A binding sites in frontal cortex, cingulate cortex, and anterior olfactory nucleus
- Down-regulation of cortical beta adrenergic receptors
- Muscle tension type headache and discomfort at the site of stimulation (most common, around 20%)
- Seizures can rarely occur.
- Scalp burns from electrodes
- No conclusive evidence of cognitive deficits
- TMS could cause the movement of paramagnetic objects of head and eye
- TMS can also cause heating of metallic implants, and the inactivation of pacemaker, medication pumps, or cochlear devices
The of metal anywhere in the head, excluding the mouth.
Ω. Individuals with cardiac pacemakers and implanted medication pumps should not participate in rTMS studies without a clear potential benefit.
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