This article reviews the use of transcranial magnetic stimulation in the diagnosis and treatment of neurologic disorders as distinct from electrical stimulation of the brain. It involves noninvasive stimulation of the cerebral cortex by externally applied magnetic fields. Favorable results have been reported in the treatment of depression, epilepsy, Parkinson disease, multiple sclerosis, stroke rehabilitation, and chronic pain. Some of the reported results are still conflicting, and mechanisms of action have not been fully elucidated. Despite these limitations, the method remains promising for further investigations for application in neurologic disorders.
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• Transcranial magnetic stimulation involves noninvasive stimulation of the cerebral cortex using externally applied magnetic fields.
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• Transcranial magnetic stimulation has been used extensively for investigation, diagnosis, and management of neurologic disorders.
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• Changes in the nervous system following transcranial magnetic stimulation, such as biochemical and electrophysiological changes, have been well documented.
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• Usefulness of transcranial magnetic stimulation has been reported in several neurologic disorders, but has not been confirmed by clinical trials.
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• Further studies including controlled clinical trials are needed for establishing efficacy and safety of transcranial magnetic stimulation.
Historical note and terminology
The application of transcranial magnetic stimulation involves noninvasive stimulation of the cerebral cortex using externally applied magnetic fields. The treatment of illnesses with magnetized iron-containing stones was practiced in ancient Egyptian and Greek medicine. In 18th century Europe, Franz Mesmer claimed to heal the sick with magnetism, believing that magnetic forces held a special power over human behavior. Magnetic fields were then applied in the treatment of neurologic disorders. Various reports in 20th century European medical literature indicate the use of electromagnetism in the treatment of peripheral neuropathies and neuromuscular disorders. In the 1990s, considerable publicity was given to claims that magnets promoted the healing of various disorders of the body. Rarely were these claims supported by controlled studies. It was in this atmosphere, one of popular interest in magnetic therapy, that transcranial magnetic stimulation evolved as a scientific tool and gained acceptance in neuropsychiatry.
The first scientific attempts to use magnetic energy to alter brain activity were conducted by D'Arsonval in 1898 and Thompson in 1910 (02). They built magnetic stimulators powerful enough to stimulate retinal cells and evoke perception of light flashes in human subjects. However, they were not powerful enough to activate the cerebral cortex. Merton and Morton showed in 1980 that it was possible to stimulate the motor area of the cortex through the intact scalp (35). They used a brief, high-voltage, electric shock to produce a motor evoked potential. A suitable instrument with adequate power to activate cortical neurons was not designed until 1985 (02). It was shown that transcranial magnetic stimulation achieved the same effect as electrical stimulation of the cortex. Yet, contrary to the conditions of electrical stimulation, transcranial magnetic stimulation achieved its ends by using painless means. This device was adopted by the neurologists for measuring nerve conduction time. Single-pulse transcranial magnetic stimulation has moved into routine clinical neurophysiology laboratory. In 2009, the FDA approved the NeuroStar TMS System ® (Neuronetics), a transcranial stimulation device for treatment of major depressive disorder resistant to antidepressant medication.
This article reviews the applications of transcranial magnetic stimulation in the diagnosis and treatment of neurologic disorders as distinct from electrical stimulation of the brain, as well as from the uses of magnetic fields over the whole body for healing of various diseases including neurologic disorders. Some of these methods fall under the category of alternative or complementary healing arts and have not been scientifically validated. Low-frequency picotesla range magnetic fields are under investigation in patients with Alzheimer disease. The basis of this investigation concerns a patented bioresonator technology using electromagnetic fields that promotes the regeneration of damaged neuronal cells.
Magnetoencephalography, a recording of the magnetic fields of the brain, is recognized as a complement to EEG in neurophysiology. Although EEG records the electrical potentials on the scalp, magnetoencephalography records the magnetic fields produced by the same electrical activity. Although both EEG and magnetoencephalography result from the same electrical activity in the brain, the EEG can better pinpoint this activity in 1 direction and magnetoencephalography in another. The physical properties of these fields are such that electrical and magnetic fields are aligned at right angles.