Nature and Working of TMS

A nerve cell’s plasma membrane maintains electrical potential difference within and outside of the cell. An external electrical stimulus in the vicinity of the membrane area will change the characteristics of the membrane of the cell and the cell is excited. These electric stimuli can be generated externally in a non-interventional manner to stimulate nerve cells. Magnetic field with varying time durations can stimulate electric fields that will run across the brain. Transcranial magnetic stimulation (TMS) is one such method of providing stimulus to the cells.

TMS (a single-pulse variety, not repetitive) runs on the principle of placing a coil (an electromagnet) on the scalp of a patient that gets magnetized on application of electric current. The resultant short burst of magnetic pulse will traverse through the skull and across the brain, stimulating nerve cells. The frequency of the stimulation can be adjusted for desired results. Brain activity is reported to decrease with low frequency TMS and will increase with higher frequency TMS. Such changes in activity behavior will continue long after the stimulation procedure is stopped.

The equipment of magnetic stimulation consists of an electric circuit (a resonant circuit), which can generate measured alternating electric discharge using an electromagnet, a coil in the shape of conjoined loops (which resembles the numerical 8). The coil is held above the scalp, some three inches from the scalp. Generally, it is positioned on the left side of the forehead. The magnetic pulse from the energized electromagnet that travels through the brain stimulates nerve cells that are associated with mood behavior and depression.

The magnetic fields used in TMS are produced by passing current either through a hand-held coil or machine-driven one, whose shape determines the properties and size of the field. The coil is run by a mechanism which triggers the high electric charge that is needed in a very accurate and regulated manner. The required frequency of the electric charge for rapid rate TMS will be around 50 cycles per second. Lower frequency levels will also stimulate brain areas and these low levels can be maintained by adjusting the settings of the apparatus.

Transcranial magnetic stimulation has the capability to have an effect on brain activities such as mood and feelings, memory and remembrance, motor function and movement, speech and vocal functions, visual perception and eyesight, and reaction time and stimulus response. These effects will last for some time after TMS is stopped, but eventually the effects will disappear.

Transcranial magnetic stimulation does not require any anesthesia as it does not use electric charge nor does it induce any seizures in the patient. The pulse will be shorter, but the stimulation may last for about half an hour. There is no need for any hospitalization and the patient can be discharged the same day.

Safety Aspects of TMS

Normally, the transcranial magnetic stimulation is not considered harmful. The studies on animals have shown no effects on brain matter or its structure, even after repeated stimulations. Certain unusual and rare effects have been reported in treatment of people. Patients complained of having seizures with the stimulation. Certain do-and-don’ts instructions are drawn on the usage of TMS, like frequency, duration, amount of electric charge, patient’s history, equipment standards, etc. In comparison to electroconvulsive therapy, TMS has fewer side effects.

The non-interventional nature of TMS is still reported to produce certain mild after effects in patients. The major risk considered is occurrence of seizures. International Society for Transcranial Stimulation has laid down the procedures to deal with such possibilities and to provide emergency help. The frequent adverse effects are:

• Mild to severe headaches
• Dizziness or faintness
• Temporary hearing loss from the coil noise
• Irritation and pain on the scalp at the stimulation point
• Twitching and stinging of facial muscles
• Temporary memory disturbances
• Nausea
• Mild convulsions of facial muscles

Efficacy of TMS

In order to treat any disease, it is required to understand the causative factors and their functions. Recent advances made in TMS experimental applications in treatments of such illnesses as depression, schizophrenia, obsessive-compulsive disorders, Parkinson’s disease, Huntington’s disease, etc., raises the hope of increased utilization of TMS in future. The efficacy of TMS has still to be proved convincingly and conclusively. TMS is also being tried to understand and treat auditory hallucinations and other neurological disorders in patients. It may be noted that Food and Drug Administration in America has not approved transcranial magnetic stimulation procedure as yet.