Neural stimulator back pain control

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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Information from Staodyne Corporation and the University of Michigan Medical Center

Transcutaneous electrical nerve stimulation (TENS) currently is one of the most commonly used forms of pain control.

Many reports confirm the use of TENS for various types of conditions such as low back pain (LBP), myofascial and arthritic pain, sympathetically mediated pain, bladder incontinence, neurogenic pain, visceral pain, and postsurgical pain. Because many of these studies were uncontrolled, there has been ongoing debate about how much more TENS is than placebo in reducing pain.

The currently proposed mechanisms by which TENS produces neuromodulation include the following:

• Presynaptic inhibition in the dorsal horn of the spinal cord
• Endogenous pain control (via endorphins, enkephalins, and dynorphins)
• Direct inhibition of abnormally excited nerves

It appears that electrical stimulation delivered by a TENS unit reduces pain through nociceptive inhibition at the presynaptic level in the dorsal horn, thus limiting its central transmission. The electrical stimuli activates afferent input inhibits propagation of nociception blocking transmission of pain signals to the dorsal horn.

The mechanism of the analgesia produced by TENS is explained by the gate control theory proposed by Melzack and Wall in 1965. The proposed mechanism for "closing the gate" to pain transmission is inhibition of the C-fiber nociception by electrical impulses in activated myelinated fibers.

A TENS unit consists of one or more electric signal generators, a battery, and a set of electrodes. The units are small and programmable, and the generators can deliver stimuli with variable current strengths, pulse rates, and pulse widths. The usual settings for the stimulus parameters used clinically are the following:

• Amplitude - Current at low intensity, comfortable level, just above threshold
• Pulse width (duration) - 10-1000 microseconds
• Pulse rate (frequency) - 80-100 impulses per second (Hz); 0.5-10 Hz when stimulus intensity is set high

When TENS is used for pain control, patients are instructed to try different frequencies and intensities to find those that provide the best pain control for that individual. Optimal settings of stimulus parameters are subjective and are determined by trial and error. Electrode positioning is quite important. Usually, the electrodes are placed initially on the skin over the painful area, but other locations (eg, over cutaneous nerves, trigger points, acupuncture sites) may give comparable or even better pain relief.

The 3 options for the standard settings used in different therapeutic methods of TENS application include the following:

1. Conventional TENS has a high stimulation frequency (40-150 Hz) and low intensity, just above threshold, with the current set between 10-30 mA. The pulse duration is short (up to 50 microseconds). The onset of analgesia with this setup is virtually immediate. Pain relief lasts while the stimulus is turned on, but it usually abates when the stimulation stops. Patients customarily apply the electrodes and leave them in place all day, turning the stimulus on for approximately 30-minute intervals throughout the day. In individuals who respond well, analgesia persists for a variable time after the stimulation stops.

2. In acupuncturelike settings, the TENS unit delivers low frequency stimulus trains at 1-10 Hz, at a high stimulus intensity, close to the tolerance limit of the patient. Although this method sometimes may be more effective than conventional TENS, it is uncomfortable, and not many patients can tolerate it. This method often is considered for patients who do not respond to conventional TENS.

3. Pulsed (burst) TENS uses low-intensity stimuli firing in high frequency bursts. The recurrent bursts discharge at 1-2 Hz, and the frequency of impulses within each burst is at 100 Hz. No particular advantage has been established for the pulsed method over the conventional TENS method.

Patient comfort is a very important determinant of compliance and, consequently, the overall success of treatment. The intensity of the impulse is a function of both pulse duration and amplitude. Greater pulse widths tend to be more painful. The acupuncturelike method is less tolerable because the impulse intensity is higher.

The amount of output current depends on the combined impedance of the electrodes, skin, and tissues. With repetitive electrical stimuli applied to the same location on the skin, the skin impedance is reduced, which could result in greater current flow as stimulation continues. A constant current stimulator, therefore, is preferred to minimize sudden uncontrolled fluctuations of current intensity related to changes in impedance. An electroconductive gel applied between the electrode and skin serves to minimize the skin impedance. Skin irritation can occur in as many as 33% of patients, at least in part, due to drying out of the electrode gel. Patients need to be instructed in the use and care of TENS equipment, with particular attention to the electrodes.

Medical complications arising from use of TENS are rare; however, skin irritation is a frequent problem and often is due partly to the drying out of the electrodes. Sometimes individuals react to the tape used to secure the electrodes. Skin irritation is minimized by using self-adhesive disposable electrodes and repositioning them slightly for repeated applications. The use of TENS is contraindicated in patients with demand-type pacemakers because their stimulus outputs may drive or inhibit the pacemaker.

A variety of newer transcutaneous or percutaneous electrical stimulation modalities recently has been developed.

• Interferential current therapy (IFC) is based on summation of 2 alternating current signals of slightly different frequency. The resultant current consists of cyclical modulation of amplitude, based on the difference in frequency between the 2 signals. When the signals are in phase, they summate to an amplitude sufficient to stimulate, but no stimulation occurs when they are out of phase. The beat frequency of IFC is equal to the difference in the frequencies of the 2 signals. For example, the beat frequency and, hence, the stimulation rate of a dual channel IFC unit with signals set at 4200 and 4100 Hz is 100 Hz.

• IFC therapy can deliver higher currents than TENS. IFC can use 2, 4, or 6 applicators, arranged in either the same plane for use on regions such as the back or in different planes in complex regions (eg, the shoulder).

• Percutaneous electrical nerve stimulation (PENS) combines advantages of both electroacupuncture and TENS. Rather than using surface electrodes, PENS uses acupuncturelike needle probes as electrodes, placed at dermatomal levels corresponding to local pathology. The main advantage of PENS over TENS is that it bypasses the local skin resistance and delivers electrical stimuli at the precisely desired level in close proximity to the nerve endings located in soft tissue, muscle, or periosteum.

Literature on the use of TENS in a variety of medical conditions reports a wide range of outcomes, from very positive to negative effectiveness. Currently, there is an overall consensus favoring the use of TENS, with authorities differing on its value in different clinical situations. Generally, TENS provides initial relief of pain in 70-80% of patients, but the success rate decreases after a few months or longer to around 20-30%. To exclude a false-negative response, a trial of TENS for at least 1 hour should be given to confirm potential benefit from subsequent continuous use.

According to Johnson, the time from the start of stimulation to the onset of analgesia varies from almost immediate to hours (on average 20-30 minutes in over 75% of patients and 1 hour in 95% of patients). The duration of analgesia also varies considerably, continuing only for the duration of stimulation in some patients and providing considerable prolonged poststimulation relief in others. The same TENS protocol may have different degrees of antinociception in acute experimental pain compared with chronic clinical pain in patients with chronic LBP.

Patients differ in their stimulus preferences and in their rates of compliance. In Johnson's study of compliance in patients who benefited from TENS, 75% used the device on a daily basis. Patients showed individual preferences for particular pulse frequencies and patterns and consistently adjusted their stimulators to these settings on subsequent treatment sessions.

Indications for the use of TENS

• Neurogenic pain (eg, deafferentation pain, phantom pain), sympathetically mediated pain, postherpetic neuralgia, trigeminal neuralgia, atypical facial pain, brachial plexus avulsion, pain after spinal cord injury (SCI)

• Musculoskeletal pain: Examples of specific diagnoses include joint pain from rheumatoid arthritis and osteoarthritis, acute postoperative pain (eg, postthoracotomy), and acute posttraumatic pain. After surgery, TENS is most effective for mild-to-moderate levels of pain, and it is ineffective for severe pain. The use of TENS in chronic LBP and myofascial pain is controversial, as placebo-controlled studies fail to show statistically significant beneficial results. Uncertainty also exists about the value of TENS in tension headache.

• Visceral pain and dysmenorrhea are other conditions in which TENS has been applied successfully.

Other disorders: TENS has been used successfully in patients with angina pectoris and urge incontinence, as well as in patients requiring dental anesthesia. Reports discuss use of TENS to assist patients in regaining motor function following stroke, to control nausea in patients on chemotherapy, as an opioid-sparing modality in postoperative recovery, and in postfracture pain.

Contraindications for the use of TENS

• TENS should not be used in patients with a pacemaker (especially of the demand type).
• TENS should not be used during pregnancy because it may induce premature labor.
• TENS should not be applied over the carotid sinuses due to the risk of acute hypotension through a vasovagal reflex.
• TENS should not be placed over the anterior neck because of possible laryngospasm due to laryngeal muscle contraction.
• The electrodes should not be placed in an area of sensory impairment (eg, in cases of nerve lesions, neuropathies), where the possibility of burns exists.
• A TENS unit should be used cautiously in patients with a spinal cord stimulator or intrathecal pump.

A number of studies have compared TENS to other similar therapeutic modalities, including PENS, IFC, and acupuncture.

• In one study of elderly patients with chronic LBP, both acupuncture and TENS had demonstrable benefits, with the acupuncture group demonstrating improvement in spinal flexion.

• In patients with chronic LBP and sciatica, PENS was more effective than TENS in providing short-term pain relief and improved function, including an improved quality of sleep and sense of well-being.

• Overall, 91% and 73% of patients, respectively, chose PENS as the preferred modality for pain relief in LBP and sciatica.

• PENS has been used successfully for pain relief in patients with acute herpes zoster and cancer with bony metastases.

• Both IFC and TENS had a statistically significant effect on median nerve excitation threshold in young women.

Faculty of the Multidisciplinary Pain Center at the University of Michigan Medical Center under the direction of Dr. Vildan Mullin have started admitting patients to the University Hospital for implantation of dorsal column stimulators and intrathecal morphine pumps. Dr. Jeffrey Rosenthal, Assistant Professor of Anesthesiology and a faculty member in the pain center, describes the new program.

“We have always worked closely with the Department of Neurosurgery, screening patients and subsequently referring them for implantation. What is new and exciting is that pain management fellows, under our supervision, will begin treating these patients independently, including operating room management,” Rosenthal said.

Intrathecal morphine pumps and spinal stimulators are two methods utilized in controlling chronic pain. They are near end-of-the-line therapy for very difficult pain problems. Delivery of morphine to the spinal space is a very potent and powerful method. The ratio of oral to spinal morphine is 300:1. A typical patient considered for a morphine pump, currently, has been diagnosed with cancer and is suffering excessive side effects, or incomplete pain relief from their opioid medication. Typical treatments have been tried and failed.

“In our pain clinic, the most common indicator, for dorsal column stimulation is treatment of chronic low back and leg pain. Most patients have undergone multiple spinal surgeries for complaints of back and leg pain, yet have failed to find relief. They have been through physical therapy, are taking drugs used for the long-term management of chronic back pain, have undergone injection therapy and, despite these measures, continue to complain of pain and seek treatment,” Rosenthal said.

Patients are brought into the hospital for an anticipated overnight admission. Dorsal column stimulation involves the placement of a neural stimulator electrode in the epidural space somewhere between the cervical and caudal region. The electrode is gently advanced under fluoroscopic guidance to a position overlying the spinal segments to be stimulated. The electrode is attached to a pulse generator, located externally during the trial phase, and then implanted subcutaneously following a successful trial.

While these two invasive and exciting therapies are not new, they are a new path in which the Chronic Pain Clinic is venturing. For the first time, Anesthesiologists at the U-M are admitting their own patients to the hospital and managing every aspect of their care, including surgical implantation in the Main University Hospital.

“We have sought and been granted the opportunity to admit our patients for whatever the need may be. This opportunity will be most commonly utilized for patients known to the Chronic Pain Clinic, however, if another situation arises that we feel warrants inpatient care, we have the privilege to admit and primarily manage in the Main University Hospital.

“Currently, the plan is to have Pain Clinic faculty and fellows responsible for coordinating admission, care, and discharge of inpatients admitted onto our service. This is not a service that will perform inpatient consults, which is a role of the acute pain service,” Rosenthal said.

“It’s a direction in which we want to take the Multidisciplinary Pain Clinic because we feel there is enough data to support its efficacy,” Rosenthal said. “At this point in time it has been demonstrated that these surgically-based methods are safely performed in the hands of an anesthesiologist. The more complicated cases, requiring open surgical technique, will continue to be referred to Neurosurgery.

“A multidisciplinary approach for the management of chronic pain remains the gold standard in terms of outcome. We will continue to consult other services, when indicated, to provide the best overall care for our patients.”

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