Peripheral Magnetic Stimulation

Function

Physiology

The time-varying current flow passing to the coil creates the magnetic field around the coil. When the pulse of the magnetic field passes into the body, it will induce a voltage difference between any 2 points. This creates an electric field and induces electrons to flow between these 2 points. Unlike electrical stimulation, magnetic stimulation does not need a traverse of electric current through electrodes, skin, and tissue interfaces. The magnetic field acts as the vehicle to induce ions to flow and does not stimulate the nervous tissue. However, once the ion flow is created, the electrical and magnetic stimulation mechanism at the neural level is the same: axon depolarization and initiating the action potential.[4] Because of the higher stimulation threshold of the cell bodies, PMS will stimulate axons rather than cell bodies.[1] The magnetic field provides many advantages. First, without energy attenuation, the magnetic field can pass any medium, even a vacuum space. This allows penetration to deep tissue such as spinal nerve roots or deep muscles. The magnetic field decreases inversely proportional to the distance away from the generator coil. Owing to this characteristic, no mechanical contact is necessary, making it applicable to patients with extreme hypersensitivity or allodynia to skin touch. Similarly, the patient must not undress because the magnetic field can pass through clothing. Moreover, due to no charged particles being injected into the skin and superficial tissue and the weak recruitment ability of cutaneous sensory afferent fiber, magnetic stimulation rarely causes pain during clinical practice.[4]

Possible Mechanisms of Action

Many researchers have been trying to identify the mechanisms of action underlying the effect of PMS; however, no clear conclusion has been made. The 1 strong postulate is that PMS can recruit peripheral afferents, potentially influencing cerebral activation and neuroplasticity. PMS is thought to be another useful method to induce proprioceptive afferents resembling movement therapy, which has already been demonstrated to increase motor control in stroke patients. PMS triggers massive proprioceptive afferents when applied to muscles via 2 pathways.[1] 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 PMS on the paretic muscles. PMS also normalizes the activation patterns of the frontoparietal networks of motor planning and leads to some functional improvement. Other supporting findings include that PMS might affect the homeostasis of cortical excitability. Additional underlying mechanisms for PMS in many applications, such as spasticity reduction, strength improvement, and pain control, are still being investigated. Interestingly, spasticity reduction is consistently reported after PMS application over spinal nerve roots and muscles.[5]

Device

The equipment consists of a high-current pulse generator able to produce a large electric discharge current (several thousand amperes).[1] The current flows through a stimulating coil, generating magnetic pulses with field strength up to several Teslas. Heat is an unavoidable by-product of magnetic pulse generation; therefore, the coil must be contained in an air or oil cooling system. Many types of coils 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 coil declines faster than the round coil. The orientation of the coil also matters. Placing the coil in a flat, tangential orientation with the longitudinal axis of the conductive structure is the most effective way to stimulate structures beneath.[6][7]

Parameter  

Different parameters have been speculated to create different preferential activation. Until now, there is no consensus regarding a standardized protocol for PMS. The following are common parameters for PMS:

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 inhibit overactive spinal circuits of muscle spasticity briefly, 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" periods in the intermittent protocol has not been determined. The increase in the "off" period during treatment may reduce the risk of excessive heating originating from the coil.[5]

Total Number of Stimuli

For transcranial magnetic stimulation (TMS), the total number of magnetic pulses obtained is 1 important factor in determining effectiveness; however, its role in PMS has not been determined.[5]

Frequency

As with the total number of stimuli, 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.[8] Its influence on the effect of PMS remains inconclusive.[5]

Intensity

PMS intensity is indicated using Tesla units or a percentage of the maximal stimulator output. However, 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, the depth of target tissues, and the 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.[5]

Issues of Concern

Unlike TMS, PMS's safety data remain insufficient. Since both PMS and TMS have similar physics properties, the safety data of TMS is referenced.

Safety Considerations

Heating

The temperature increase is affected by the coil type, cooling system, target tissue positions relative to the coil, and stimulation parameters. Different tissues have different thresholds for 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 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 PMS heats certain implants. It is advisable first to measure the heating with planned parameters outside the body if it is still uncertain. Furthermore, applying PMS over tumors is contraindicated.[9]

Force and Magnetization

The magnetic field emitted from the coil exerts an attractive force on ferromagnetic objects, meaning that the magnetic force can move the object. 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. No safety data on PMS is applied to these kinds of ferromagnetic objects. Some experts suggest that principles of MRI safety for patients with implants can be adapted as a guide for PMS.[9]

Induced Voltage

The magnetic field pulse can potentially damage the circuits of electronic implants, such as deep brain stimulation and cochlear implants. Based on previous TMS studies, applying TMS to patients with implanted stimulators with some distance between the coil and the internal pulse generator appears safe. However, no comprehensive information about safe distance exists, even for TMS. Some TMS guidelines 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.[9] 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 a 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.[3]

Adverse Effects

PMS is considered a painless approach. Some pain and discomfort were reported in studies that used triple stimulation techniques deploying suprathreshold PMS. These adverse effects are likely associated with the intensity of PMS.[5]

Considerations on Patient Selection

Pediatric Patients

TMS safety guidelines concluded that single-pulse and paired-pulse TMS are safe for children aged 2 years and older, although there are few studies of PMS conducted in children. One study deployed PMS to 5 cerebral palsy patients with a mean age of 8 to evaluate spasticity reduction. The study did not state any adverse effect after PMS stimulation of tibial and common peroneal nerves.[9]

Pregnancy

It is suggested that direct magnetic stimulation on the lumbar spine should be avoided. Pregnant women should stay at least 70 cm away from the coil.[9]

Clinical Significance

Many studies have demonstrated that PMS is advantageous in many medical conditions. However, more evidence is needed to prove the effectiveness of PMS in certain clinical settings. The following are possible indications for the use of PMS:

Myofascial Pain Syndrome

Smania et al reported significantly better long-term (follow-up after 3 months) outcomes in both subjective and objective clinical evaluations of PMS in treating upper trapezius myofascial pain compared to TENS and sham ultrasound therapy.[10]

Traumatic Brachial Plexopathy

A study conducted by Khedr et al showed significant improvement in electrophysiologic parameters, handgrip strength, and pain scores by applying both suprathreshold and subthreshold PMS over upper trapezius muscle compared to sham treatments.[11]

Post-traumatic Peripheral Neuropathic Pain

A case series reporting 4 patients with neuromas and 1 with inguinal nerve entrapment was treated with low-frequency (0.5 Hz) PMS. Allodynia was resolved after treatment. After treatment, a 60% to 100% reduction in pain scores was observed.[12]

Acute Low Back Pain

A pilot study was carried out by Lim et al, in which immediate pain relief was reported after PMS application on patients' most tender points. The study also showed better results for functional questionnaires after 10 PMS sessions than sham treatment.[13]

Chronic Low Back Pain

Several studies were conducted to investigate the effects of PMS in patients with chronic low back pain. The investigators successfully showed relations between the use of PMS and reactivation of short-interval intracortical inhibition of the primary motor cortex, which is usually absent in patients with chronic pain. They also emphasize that combining PMS and motor training positively affects pain, function, and lumbopelvic spine motor control.[14][15]

Spasticity Reduction

Many studies with varying protocols aimed to identify the antispastic effect of PMS. Some studies applied PMS over spinal nerve roots, while others applied it over spastic muscles. All studies reported consistent results that spasticity decreased after each PMS session. Nevertheless, understanding how PMS could reduce spasticity needs further investigation.[1][16]

Increase Muscle Strength

A recently published study investigated the effect of PMS on vastus lateralis muscles in patients after hip replacement surgery. After 15 sessions of PMS, muscle strength improved but without a significant difference compared with the sham treatment. However, other functional outcomes seemed to be better in the PMS group. The author explained that it might be related to the proprioceptive effect of PMS on the brain.[17]

Dysphagia

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 PMS to swallowing exercises for a week. Although, no single patient could change their mode of nutritional intake.[18]

Enhancing Healthcare Team Outcomes

PMS, or transcutaneous magnetic stimulation, is a non-invasive method of delivering a rapidly pulsed, high-intensity magnetic field to the periphery other than the brain. Interest in research and clinical applications has increased over the last 3 decades as it is considered a novel, painless, and easy approach for many neurological and musculoskeletal conditions.[1] Any healthcare professional can use the technique, but solid evidence for its efficacy is still lacking. Patients with mild to moderate pain due to the musculoskeletal system may try PMS. Still, patients should be warned that the benefits are often not immediate, and the therapy may require multiple sessions.