1) Introduction

2) Respiration & Airway Clearance (Cough)

3) Gastric Clearance & Colon Transfer

4) Bladder Control

Introduction: Nerves transmit signals through electrical and chemical changes in nerve cells called neurons. When properly applied to neurons, nerve roots, and peripheral nerves, pulsed magnetic fields can initiate these electro-chemical changes. This leads to a domino effect, causing similar changes in adjacent cells. This artificially created neural signal, called a neural “pulse,” can stimulate muscle contractions if activated on a motor nerve.

The use of magnetic fields to promote functional benefits, or as a rehabilitative or diagnostic tool, is called Functional Magnetic Stimulation (FMS). In humans with SCI, FMS has been used to stimulate breathing, enhance digestion, control urination, speed bodily wastes through the colon, and enhance circulation. (Other electromagnetic SCI applications have been discussed previously) At this time, FMS’SCI applications are limited to laboratory and clinical testing. No commercial FMS devices are available for SCI home use.

Similar to Functional Electrical Stimulation (FES), FMS requires that target muscles or organs are innervated (peripheral nerves remain intact) to affect bodily functions and muscle contractions. Like FES, FMS is applied below the injury level to facilitate functions that have been lost or impaired.

FMS systems consist of three components: a high-current pulse generator, magnetic coil(s), and power source.

FMS pulse generators discharge output currents of 5,000 amps or more with pulse durations usually close to 250 microseconds. This is an extremely high electrical output that requires a powerful energy source. Internal circuitry in the pulse generator includes a storage capacitor that charges up to several thousand volts, control circuitry, and a thyristor - a solid state device able to switch large amounts of electrical current in a few microseconds.

To induce ions (electrically charged molecules) to flow in affected nerves and, in turn, signal propagation, FMS magnetic coils generate field strengths of two Tesla or more, a powerful electromagnetic force. The magnetic coil (MC) typically consists of one or more well-insulated copper coils, temperature sensors and safety switches - all housed in a molded plastic case. The leads that connect the pulse generator to MC must be of sufficient gauge to minimize energy loss under high-current conditions.

Either batteries or fixed electrical sources are used to generate FMS.

Unlike FES, which applies electrical current directly to the skin or nerves through external or implanted electrodes, FMS affects nerve cells through a pulsed magnetic field, which radiates from one or more MCs placed outside the body over target nerves, or nerve roots. This difference between FMS and FES presents benefits and drawbacks.

Benefits include:

1. FMS penetrates the body's tissues, including skin, fat, and bone, reaching peripheral nerves, the spinal cord, spinal nerve roots, the brain, and internal organs without surgery, high-density electrical current, or pain.

2. FMS induces an electric field under the area of application, providing a means for simultaneous stimulation of multiple nerves. [Note: If the functional stimulation of a muscle group is desired this property is beneficial, whereas it can be a drawback if the activation of a specific nerve or muscle is desired.]

3. FMS voids risks associated with surgically implanted FES electrodes, including, infection, bleeding, wire breakage, and implant failure.

4. FMS can be applied over clothing. It does not require direct skin contact, or electrolytic gels to enhance electrode conductivity.

Drawbacks include:

1. FMS can activate inappropriate nerves that lie within its field, because FMS radiates a magnetic field rather than a focused electrical current.

2. Present FMS systems can be unwieldy and cosmetically noticeably for individual use. Technical improvements in signal generators, MC and battery design are hoped to reduce these concerns.

According to Dr. Vernon Lin (Irvine, CA), a leading pioneer in FMS for SCI applications, two technical factors have slowed FMS development for widespread home and clinical use - MC field dispersion and battery size. However, changes in MC design are improving the ability to target specific nerves with FMS. Likewise, improvements in battery technology are making portable FMS a possibility for individual use. Lin cautions that the small market represented by potential FMS users coupled with its present costs ($40,000 for hospital or laboratory systems) are slowing its commercial development. Nevertheless, he believes that FMS will eventually serve multiple SCI applications, including daily functional use, the retraining and reconditioning of disused limbs, and to restore the conductivity of dormant nerves.

FMS APPLICATIONS FOR SCI

Respiration & Airway Clearance (Cough): The use of FMS for breathing assistance may offer future benefits superior to those presently gained through FES. A magnetic coil placed over the cervical C7 vertebra stimulates diaphragm-controlling phrenic nerves without surgery. Phrenic nerve stimulation using FMS and FES found that magnetic stimulation led to significantly higher diaphragm pressures. Researchers believe this difference stems from FMS activating diaphragmic and extradiaphramic musculature, which stiffens the upper thoracic cage, thereby allowing the diaphragm to function more efficiently. FES-phrenic nerve pacing (see FES section) has a more selective affect on the diaphragm.

FMS use for expiration (cough) is maximized by placing the MC over the T7 vertebra, stimulating the lower thoracic spinal nerve roots. This placement activates the lower intercostals and abdominal muscles. In humans with SCI, FMS-assisted exhalation resulted in improved expiration pressures, volume, and flow rates of 121%, 167%, and 110%, respectively. In addition, four weeks of FMS expiratory-muscle training led to significant improvements over baseline scores in voluntary maximum expiration pressures (116%), volume (173%), and flow rates (123%). 

At its present state of the art, however, FMS is too bulky for portable respiratory use. In addition, the large, round MCs most often used for respiratory applications can inadvertently activate nerves situated near target nerves. However, with continued technical advances, FMS may offer future respiratory benefits for individuals with SCI.

Gastric Clearance & Colon Transfer: Abnormalities in motor functions due to SCI can adversely affect the gastrointestinal tract, slowing the movement of solids though the stomach and colon (large intestine). In able-bodied subjects, the use of FMS with an MC placed over the thoracic T9 vertebra shortens the gastric emptying half-time (time required for the stomach to empty half its contents) by 8%. In individuals with SCI, FMS shortens gastric emptying half-time by 33%, clearing half the contents of one (SCI) subject's stomach 38 minutes faster than without FMS.   

FMS also increases rectal pressures and shortens colon transfer time - the time needed for solid wastes to pass through the colon. Placing an MC over the lower abdomen tenses abdominal musculature and elevates rectal pressure. Positioning the MC over the lumbosacral region further enhances colonic transfer by increasing rectal pressures in individuals with SCI by 83%. Using FMS, solid waste transit times through the sigmoid colon and the rectum decreased from an average of 50.4 hours to 34.8 hours. (Sigmoid colon is the large intestine’s final stage, connecting the descending colon with the rectum.)

Bladder Control: An initial clinical trial found that FMS could stimulate pulsed urination in 17 of 22 human subjects with SCI. This study also found that individuals with reflex or spastic bladders (spontaneously empty as a reflex when filled) responded well to using FMS for bladder control. Reflex bladders most often occur in individuals with lesions above the T12 vertebral level. FMS was ineffective for individuals whose bladders did not empty of their own accord (flaccid bladders), which can result from injuries below the T12 level. 

Safety: Whole body exposure to static (non-pulsed) magnetic fields of two Tesla is the current U.S. safety guideline for commonly used MRIs. FMS of the spinal cord applies a peak magnetic field of 1.1-1.4 Tesla (using a nine-centimeter MC and assuming a spinal cord depth of five centimeters below the MC). Because no evidence exists that suggest pulsed magnetic fields are less safe than static magnetic fields, and because FMS’ pulsed “on” time (250 microseconds) is so short, researchers expect that FMS will prove to be safe for SCI applications with continued use.

Because it exerts a powerful magnetic field, patients with metal implants in the abdomen or spine should not use FMS. Likewise, the presence of a cardiac pacemaker, or a similar device, is a contraindication to FMS in general.