Peripheral magnetic stimulation (PMS) or so-called transcutaneous magnetic stimulation is a noninvasive method of delivering a rapidly pulsed, high-intensity magnetic field to the periphery other than the brain. Interest in the research and clinical applications has increased in the over the last three decades as it is considered a novel, painless, and easy approach for many neurological and musculoskeletal conditions.
Humankind has been trying to use the magnetism to treat illness for more than thousands of years. Almost 190 years ago, Faraday discovered that a time-varying current creates a magnetic field that can induce another current in a nearby conductive medium. Around 60 years ago, Kolin et al. first demonstrated that an alternating magnetic field could stimulate a nerve in an animal model. In 1982, a group of researchers from the University of Sheffield was the first to report developing a practical magnetic peripheral stimulator and using it to stimulate human peripheral nerves. This magnetic stimulator's main difference from the previously developed pulsed electromagnetic field device (PEMF) was that it had a much higher peak magnetic field strength.
The time-varying current flow passing to the coil creates the magnetic field around the coil. When the pulse of magnetic field passes into the body, it will induce a voltage difference between any two points. This creates an electric field and induces electrons to flow between these two points. Unlike the electrical stimulation, the magnetic stimulation does not need a traverse of electric current through electrodes, skin, and tissue interface. The magnetic field acts as the vehicle to induce ions to flow and does not stimulate the nervous tissue itself. However, once the ion flow is created, the mechanism of both electrical and magnetic stimulation at the neural level is the same which are axon depolarization and the initiation of the action potential. Because of the higher stimulation threshold of the cell bodies, peripheral magnetic stimulation will stimulate axons rather than cell bodies.
The magnetic field provides many advantages. First, the magnetic field can pass any medium, even a vacuum space, without attenuation of energy and allows penetration to deep tissue such as spinal nerve roots or deep muscles. The magnetic field only decreases inversely proportional to the distance away from the generator coil. Owing to this characteristic, no mechanical contact is necessary which makes it applicable to patients with extreme hypersensitivity or allodynia to skin touch. Similarly, because the magnetic field can pass through clothing, the patient does not need to undress. Moreover, due to no charged particles being injected into the skin and superficial tissue and weak recruitment ability of cutaneous sensory afferent fiber, the magnetic stimulation rarely causes pain during clinical practice.
Possible Mechanisms of Action
Many researchers have been trying to identify the mechanisms of action underlying the effect of peripheral magnetic stimulation; however, no clear conclusion has been made. The one strong postulate is that peripheral magnetic stimulation can recruit peripheral afferents which potentially influences cerebral activation and neuroplasticity. Peripheral magnetic stimulation is thought to be another useful method to induce proprioceptive afferents resembling movement therapy that has already demonstrated to increase motor control in stroke patients. Peripheral magnetic stimulation triggers massive proprioceptive afferents when applied to muscles via two pathways. The first pathway is the direct activation of sensorimotor nerve fibers. The other is the indirect activation of mechanoreceptors in the muscle fiber. Evidence shows an increase in regional cerebral blood flow by PET scan of the premotor cortex, parietal areas, and motor cingulum in the lesioned hemisphere in stroke patients after applying peripheral magnetic stimulation on the paretic muscles. Peripheral magnetic stimulation is also shown to normalize the activation patterns of the frontoparietal networks of motor planning and leads to some functional improvement. Other supporting findings include that peripheral magnetic stimulation might have effects on the homeostasis of cortical excitability. Additional possible underlying mechanisms for the use of peripheral magnetic stimulation in many applications such as spasticity reduction, strength improvement, and pain control are still being investigated. Interestingly, spasticity reduction is consistently reported after peripheral magnetic stimulation application over both spinal nerve roots and muscles.
The equipment consists of a high current pulse generator able to produce a large amount of electric discharge current (several thousand amperes). The current flows through a stimulating coil, generating magnetic pulses with field strength up to several Teslas. Heat is an unavoidable by-product derived from magnetic pulse generation; therefore, the coil needs to be contained in an air- or oil-cooling system.
Many types of the coil have been manufactured. Two frequently used are the round coil and figure-8 coil. The choice of the coil depends on the focality and depth of penetration on the target. The round coil is less focal but produces a deeper magnetic field with a stimulated area equivalent to its diameter. The figure-8 coil produces a stronger magnetic field at the center with an accurate focus. When the coil is distant from the target, the magnetic field from the figure-8 coil declines faster than the round coil. The orientation of the coil also matters. Placing the coil in flat, tangential orientation with the longitudinal axis of the conductive structure is the most effective way to stimulate structures beneath.
Different parameters have been speculated to create different preferential activation. Until now, there is no consensus regarding a standardized protocol of peripheral magnetic stimulation. The following are common parameters for peripheral magnetic stimulation:
Duty cycle: on and off periods
Two different protocols regarding the duty cycle include (1) the continuous protocol (only “on” during the whole treatment session), which is hypothesized to briefly inhibit overactive spinal circuits of muscle spasticity and (2) the intermittent protocol, which imitates physiological muscle contraction and relaxation and generates proprioceptive afferent inducing neuroplasticity. However, the optimal length of the "on" and "off" period in the intermittent protocol has not been determined. The increase in the "off" period during treatment may reduce the risk of excessive heating originated from the coil.
Total number of stimuli
For transcranial magnetic stimulation (TMS), the total number of magnetic pulses obtained is one important factor to determine effectiveness; however, its role in peripheral magnetic stimulation has not been determined.
Like the total number of stimuli, the frequency is another major factor of TMS. Low-frequency stimulation (less than 1 Hz) has inhibitory effects, while high-frequency stimulation (more than 5 Hz) initiates excitatory effects in the brain. Its influence on the effect of peripheral magnetic stimulation remains inconclusive.
Peripheral magnetic stimulation intensity is indicated by using tesla units or in a percentage of the maximal stimulator output. But the real magnetic field strength that reaches the target structure cannot be determined. Factors affecting the strength are the type of coil used for stimulation, depth of target tissues, and geometry of the area beneath the coil. Therefore, the intensity is roughly measured by observing whether there is a muscle contraction, and it would be reported as subthreshold and suprathreshold stimulation. Almost all studies used for suprathreshold stimulation are based on the rationale that muscle contraction would produce proprioceptive afferents to induce neuroplasticity.
Unlike TMS, the safety data regarding peripheral magnetic stimulation remain insufficient. Since both peripheral magnetic stimulation and TMS have similar physics properties, safety data of TMS is referenced.
Temperature increase is affected by the coil type, its cooling system, target tissue positions relative to the coil, and stimulation parameters. Different tissues have different thresholds to thermal damage, depending on exposure time and temperature. Most tissues can tolerate minutes of heat up to 43 degrees Celsius. Concerning excessive heat, the manufactured coils always have heat sensors that will automatically stop the coils when the temperature reaches around 40 degrees Celsius. Implants can heat as well and might cause thermal damage to surrounding tissues. No specific data has been provided on how peripheral magnetic stimulation heats certain kinds of implants. It is advisable to measure the heating with planned parameter outside the body first if it is still uncertain. Furthermore, applying peripheral magnetic stimulation over tumors is contraindicated.
Force and magnetization
The magnetic field emitted from the coil exerts an attractive force on ferromagnetic objects, meaning that the object can be moved by the magnetic force. A study indicated that stainless steel aneurysm clips in the brain were barely moved by TMS less than 0.0003 mm. Thus, this shift has no clinical significance. There are no safety data of peripheral magnetic stimulation applying over these kinds of ferromagnetic objects. Some experts suggest that principles of MRI safety for patients with implants can be adapted as a guide for peripheral magnetic stimulation.
The magnetic field pulse can potentially damage the circuits of electronic implants such as deep brain stimulation and cochlear implant. Based on previous TMS studies, it appears to be safe to apply TMS to patients with implanted stimulators with some distance between the coil and the internal pulse generator. However, there is no comprehensive information about safe distance, even for TMS. Some TMS guidelines even suggest that life-sustaining implants anywhere in the body like prosthetic cardiac valves are absolute contraindications. Any electronic devices carried by both operators and patients should be removed to prevent possible damage. The heart is also a conductive structure. There was a concern that the magnetic field would interfere with the cardiac electrical conduction; however, stimulating the cardiac muscles requires extremely high energy. Two mechanisms are proposed: (1) First, the magnetic stimulators produce current with a shorter duration which cannot stimulate cardiac muscles. (2) The second reason relates to the distance of the heart away from the coil. The current produced by the magnetic field decreases with an increased distance from the stimulating coil. Also, the location of the heart makes it hard to stimulate.
Peripheral magnetic stimulation is considered a painless approach. Some pain and discomfort were reported in studies that used triple stimulation techniques deploying suprathreshold peripheral magnetic stimulation. It is likely that these adverse effects are associated with the intensity of peripheral magnetic stimulation.
Considerations on Patient Selection
TMS safety guidelines concluded that single-pulse and paired-pulse TMS is safe for children aged 2 years and older, although there are few studies of peripheral magnetic stimulation conducted in children. One study deployed peripheral magnetic stimulation to five cerebral palsy patients with a mean age of 8 years old to evaluate spasticity reduction. The study did not state any adverse effect after stimulation of tibial and common peroneal nerves by peripheral magnetic stimulation.
It is suggested that direct magnetic stimulation on the lumbar spine should be avoided. Women who are pregnant should stay at least 70 cm away from the coil.
Many studies demonstrated peripheral magnetic stimulation to be advantageous in many medical conditions. However, more evidence is needed for proving the effectiveness of peripheral magnetic stimulation in certain clinical settings. The following are possible indications for use of peripheral magnetic stimulation:
Myofascial pain syndrome
Smania et al. reported significantly better long-term (follow up after 3 months) outcome in both subjective and objective clinical evaluations of peripheral magnetic stimulation in treating upper trapezius myofascial pain compared to TENS and sham ultrasound therapy.
Traumatic brachial plexopathy
A study conducted by Khedr et al. showed significant improvement of electrophysiologic parameters, hand grip strength, and pain scores by applying both suprathreshold and subthreshold peripheral magnetic stimulation over upper trapezius muscle in comparison with sham treatments.
Post-traumatic peripheral neuropathic pain
A case series reporting four patients with neuromas and one patient with inguinal nerve entrapment was treated with a course of low frequency (0.5 Hz) peripheral magnetic stimulation. Allodynia was resolved after treatment. A reduction of 60% to 100% of pain scores was observed after treatment.
Acute low back pain
A pilot study was carried out by Lim et al. Immediate pain relief after peripheral magnetic stimulation application on patients' most tender points was reported. The study also showed better results of functional questionnaires after ten sessions of peripheral magnetic stimulation compared to sham.
Chronic low back pain
Several studies were conducted to investigate the effects of peripheral magnetic stimulation in patients with chronic low back pain. The investigators successfully showed relations between use of peripheral magnetic stimulation and reactivation of short-interval intracortical inhibition of primary motor cortex which is usually absent in patients with chronic pain. They also emphasize that a combination of peripheral magnetic stimulation and motor training has a positive outcome in pain, function, and lumbopelvic spine motor control.
Many studies with varying protocols aimed to identify the antispastic effect of peripheral magnetic stimulation. Some studies applied peripheral magnetic stimulation over spinal nerve roots, while the others applied over spastic muscles. All studies reported consistent results that spasticity decreased after each peripheral magnetic stimulation session. Nevertheless, understanding how peripheral magnetic stimulation could reduce spasticity needs further investigation.
Increase muscle strength
A recently published study investigated the effect of peripheral magnetic stimulation over vastus lateralis muscles in patients after the hip replacement surgery. After 15 sessions of peripheral magnetic stimulation, muscle strength improved but without a significant difference compared with the sham treatment. However, other functional outcomes seemed to be better in the peripheral magnetic stimulation group. The author explained that it might be related to the proprioceptive effect of peripheral magnetic stimulation on the brain.
Eight patients with stroke and dysphagia (7 out of 8 patients had subcortical strokes) were reported to have some reduction in penetration-aspiration episodes after applying additional peripheral magnetic stimulation to swallowing exercises for a week. Although, no single patient could change their mode of nutritional intake.
Peripheral magnetic stimulation (PMS) or so-called transcutaneous magnetic stimulation is a noninvasive method of delivering a rapidly pulsed, high-intensity magnetic field to the periphery other than the brain. Interest in the research and clinical applications has increased in the over the last three decades as it is considered a novel, painless, and easy approach for many neurological and musculoskeletal conditions. The technique can be used by any healthcare professional but solid evidence for its efficacy are still lacking. For patients with mild to moderate pain due to the musculoskeletal system, one may try PMS but patients should be warned that the benefits are often not immediate and the therapy may require multiple sessions.
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