The ability to relieve pain is very variable and unpredictable, depending on the source or location of pain and whether it is acute or chronic. Pain mechanisms are complex and have peripheral and central nervous system aspects. Therapies should be tailored to the specifics of the pain process in the individual patient. Psychological issues have a very strong influence on whether and how pain is experienced and whether it will become chronic. Most effective pain management strategies require multiple concurrent approaches, especially for chronic pain. It is rare that a single modality solves the problem.

Static or electromagnetic fields have been used for centuries to control pain and other biologic problems, but scientific evidence of their effect had not been gathered until recently. This review explores the value of magnetic therapy in rehabilitation medicine in terms of static magnetic fields and time varying magnetic fields (electromagnetic). A historical review is given and the discussion covers the areas of scientific criteria, modalities of magnetic therapy, mechanisms of the biologic effects of magnetic fields, and perspectives on the future of magnetic therapy.

In the past few years a new and fundamentally different approach has been increasingly investigated. This includes the use of magnetic fields (MF), produced by both static (permanent) and time-varied (most commonly, pulsed) magnetic fields (PEMFs). Fields of various strengths and frequencies have been evaluated. There is as yet no “gold standard”. The fields selected will vary based on experience, confidence, convenience and cost. Since there does not appear to be any major advantage to any one MF application, largely because of the unpredictability of ascertaining the true underlying source of the pain, regardless of the putative pathology, any approach may be used empirically and treatment adjusted based on the response. After thousands of patient-years of use globally, there very little risk has been found to be associated with MF therapies. The primary precautions relate to implanted electrical devices and pregnancy and seizures with certain kinds of frequency patterns in seizure prone individuals.

Magnetic fields affect pain perception in many different ways. These actions are both direct and indirect. Direct effects of magnetic fields are: neuron firing, calcium ion movement, membrane potentials, endorphin levels, nitric oxide, dopamine levels, acupuncture actions and nerve regeneration. Indirect benefits of magnetic fields on physiologic function are on: circulation, muscle, edema, tissue oxygen, inflammation, healing, prostaglandins, cellular metabolism and cell energy levels.

Most studies on pain use subjective measures to quantitate baseline and outcome values. Subjective perception of pain using a visual analogue scale (VAS) and pain drawings is 95% sensitive and 88% specific for current pain in the neck and shoulders and thoracic spine.

Measured pain intensity (PI) changes with pain relief and satisfaction with pain management. A 5%, 30%, and 57% reduction in PI correlated with "no," "some/partial," and "significant/complete" relief. If initial PI scores were moderate/severe pain (NDS > 5), PI had to be reduced by 35% and 84%, to achieve "some/partial" and "significant/complete" relief, respectively. Patients in less pain (NDS < or = 5) needed 25% and 29% reductions in PI. However, relief of pain appears to only partially contribute to overall satisfaction with pain management.

Several authors have reviewed the experience with PEMFs in Eastern Europe and the West. PEMFs have been used extensively in many conditions and medical disciplines. They have been most effective in treating rheumatic disorders. PEMFs produced significant reduction of pain, improvement of spinal functions and reduction of paravertebral spasms. Although PEMFs have been proven to be a very powerful tool, they should always be considered in combination with other therapeutic procedures.

Since the turn of this century, a number of electrotherapeutic, magnetotherapeutic and electromagnetic medical devices have emerged for treating a broad spectrum of trauma, tumors and infections with static and PEMFs. Their acceptance in clinical practice has been very slow in the medical community. Practitioner resistance seems largely based on confusion of the different modalities, the wide variety of frequencies employed (from ELF to microwave) and the general lack of understanding of the biomechanics involved. The current scientific literature indicates that short, periodic exposure to pulsed electromagnetic fields (PEMF) has emerged as the most effective form of electromagnetic therapy.

The ability of PEMFs to affect pain is dependant on the ability of PEMFs to positively affect human physiologic or anatomic systems. Research is showing that the human nervous system is strongly affected by therapeutic PEMFs. Behavioral and physiologic responses of animals to static and extremely low frequency (ELF) magnetic fields are affected by the presence of light. Light strengthens the effects of PEMFs.

One of the most reproducible results of weak, extremely low-frequency (ELF) magnetic field (MF) exposure is an effect upon neurologic pain signal processing. PEMFs have been designed for use as a therapeutic agent for the treatment of chronic pain in humans. Recent evidence suggests that PEMFs would also be an effective complement for treating patients suffering from acute pain. Static magnetic field devices with strong gradients have also been shown to have therapeutic potential. Specifically placed static magnets reduce neural action potentials and alleviate spinal mediated pain. The placebo response may explain as much as 40% of an analgesia response. The central nervous system mechanisms responsible for the placebo response are an appropriate target for magnetic therapies. Magnetic field manipulation of cognitive and behavioral processes is seen in animal behavior studies and in humans. This may also be one of the mechanisms of the use of MFs in managing pain.

Some of the mechanisms of PEMF effects
Magnetotherapy is accompanied by an increase in the threshold of pain sensitivity and activation of the anticoagulation system. PEMF treatment stimulates production of opioid peptides; activates mast cells and increases electric capacity of muscular fibers. Long bone fractures that did not unite over 4 months to 4 years are repaired in 87% of cases with 14-16 hr of daily PEMF treatment. Several of these devices are FDA approved. PEMF of 1.5- or 5-mT field strength, proved helpful edema and pain before or after a surgical operation.

PEMF for 15-360 minutes increases amino acid uptake about 45%. PEMF for 2 hour induces changes in transmembrane energy transport enzymes, allowing energy coupling and increased biologic chemical transport work.

The density of pigeons’ brain mu opiate receptors decreases by about 30% and therefore their pain perception. A 2 hr exposure of healthy humans was found to reduce pain perception and decreased pain-related brain signals. Biochemical changes were found in the blood of treated patients that supported the pain reduction benefit.

Normal standing balance is subject to control by the vestibular area of the brain. PEMF couple with muscular processing or upper body nervous tissue functions. 200-uT PEMFs cause a significant improvement in normal standing balance in adult (18-34 year old) humans. Further evidence of the sensitivity of the nervous system on MFs.

Various MFs with different characteristics reduce pain inhibition in various species of animals including land snails, mice, pigeons, as well as humans. 0.5 Hz rotating MF, 60 Hz ELF magnetic fields and even MRI reduces analgesia induced by both exogenous opiates (i.e. morphine) and endogenous opioids (i.e. stress-induced). Reduction in stress-induced analgesia can be obtained not only by exposing animals to a variety of different magnetic fields, but also after a short-term stay in a near-zero magnetic field. This suggests that even for magnetic field, as for other environmental factors (i.e. temperature or gravity), alterations in the normal conditions in which the species has evolved can induce alterations in physiology as well as in behavior.

MFs applied to the head or to an extremity, for from 1 to 60 minutes, with intervals from several minutes to several hours, randomly sequenced with sham exposures allowed study of brain reactions by various objective measures. From these multiyear studies, the brain shows a non-specific initial response. The changes were "modulatory", meaning that the brain was found to sense EMF exposures vs. sham exposures. The sensory reactions were a weak pain, tickling, pressure, etc. sensations, mediated by the body’s peripheral sensory systems. Reactions could be prevented by local anesthesia of the exposed area. EEGs showed increased low-frequency rhythms, more pronounced when brain damage was present. This explains the common perception of relaxation and sleepiness with MFs. Cell analysis showed that all types of brain cells react to EMFs but astrocytes were most sensitive. They are involved in memory processes and slow wave brain activity.

The benefits of PEMF use may last considerably longer than the time of use. In rats, a single exposure produces pain reduction both immediately after treatment and at 24 hrs after treatment. The analgesic effect is still observed at 7th and 14th day of repeated treatment and even up to 14 days after the last treatment.

PEMFs promote healing of soft tissue injuries by reducing edema and increasing resorption of hematomas. Low frequency PEMFs reduce edema primarily during treatment sessions. PEMFs at very high frequencies (PRFs) for 20-30 minutes cause edema decreases lasting several hours. PRFs induce vasoconstriction at the injury site. They displace negatively charged plasma proteins found in traumatized tissue. This increases lymphatic flow, an additional factor in reducing edema.

In rats exposed for 20 min daily on 3 successive days to PEMFs of 50 mG, the pain threshold increased progressively over the 3 days. The pain threshold following the third magnetic field exposure was significantly greater than those associated with morphine and other treatments. Brain injured and normal rats both showed a 63% increase in mean pain. PEMFs may be very helpful in patients with closed head injuries. The mechanism probably involves the longer acting endorphins rather than enkephalins.

Chronic pain is often a result of aberrantly functioning small neural networks involved in self-perpetuated neurogenic inflammation. High intensity pulsed magnetic stimulation (HIPMS) noninvasively depolarizes neurons and can facilitate recovery following injury. Patients suffering from posttraumatic or postoperative low-back pain, reflex sympathetic dystrophy, peripheral neuropathy, thoracic outlet syndrome and endometriosis had pain relief. Up to ten,10-min exposures to 1.17 T at a rate of 45 pulses/minute were applied to the areas of maximal pain for 6 treatments. One patient became pain free after 4 HIPMS treatments. All patients reported some pain relief. Maximum pain relief occurred 3 hr after treatment. Two patients had complete pain relief and 3 had partial pain relief that lasted for 4 months. The others had pain relief that lasted for 8-72 hours.