Electricity has been used to treat pain since ancient times, as early as 1559 when Dioscorides reported the use of the marine torpedo to relieve persistent headaches.
Shealey was the first to stimulate the spinal cord by placing electrodes in the subarachnoid space on the surface of the dorsal column of the spinal cord through a tapered plate incision in 1967. Subsequently, by 1975, Dooley proposed a new method of placing electrodes into the dorsal epidural space of the spinal cord by percutaneous puncture.
Early stimulation systems consisted mainly of radiofrequency-driven receivers. By the mid-1970s, Cordis produced the first generation of pulse generators powered by lithium batteries. The electrodes soon evolved from the initial monopoles to bipoles. Of particular note was an important invention by Jose Waltz and Neuromed in the early 1980s, namely a transdermal quadrupole electrode, which featured an external sensor that could be used to adjust the stimulation parameters of the implanted stimulator at will without damage.
In the late 1970s, SCS became popular throughout Europe and the United States. Doctors used the SCS method to treat thousands of patients, including almost all types of pain. Inappropriate patient screening, technical failures of the implanted devices, and the lack of experience of the physicians, of course, led to unacceptable treatment results. Therefore, the SCS approach was at a low point since the 1980s. It is only in the last decade that the method has been reintroduced for the treatment of chronic intractable pain of non-malignant origin.
Neurophysiological mechanisms
The exact mechanism of dorsal spinal cord electrical stimulation for the treatment of chronic pain is still not well understood. The activation of large spinal cord afferent fibers undoubtedly plays an important role. It is well documented that the effect of SCS is related to the activation of segmental spinal cord activity in the dorsal horn of the spinal cord by retrograde conduction of action potentials evoked by electrical stimulation in the dorsal column of the spinal cord caudally, as well as the activation of nerve cells in the brainstem by upward conduction of action potentials in the dorsal column of the spinal cord, which in turn activates the activity of the downstream inhibitory system of the brainstem. In terms of neurochemistry, animal studies suggest that stimulation induces the release of 5-hydroxytryptamine, SP, and GABA in the dorsal horn of the spinal cord. However, animal experimental evidence has not been confirmed in humans.
Clinically, the most effective stimulation is achieved when the electrodes are placed within 3 mm of the midline of the dorsal spinal cord, but in general, the best stimulation of midline structures (such as the lower back, buttocks, and perineum) is achieved when the electrodes are appropriately located in the midline. The distance of the electrodes from the midline can produce different stimulation effects, depending on the level of the spinal cord and the part of the body to be stimulated. For example, stimulation of the C2 ganglion and thoraco-abdominal wall regions can be easily obtained with electrodes placed off the midline; upper extremity stimulation can be fairly easily obtained with electrodes placed on or off the midline. If bilateral stimulation is required, electrodes should be placed as close to the midline as possible. In the lower thoracic spinal cord, laterally placed electrodes can induce stimulation responses in the anterior lower extremity, while stimulation responses in the posterior lower extremity will come from midline placed electrodes. As a general principle, the stimulus response induced by electrical stimulation usually extends caudally from the level of placement and occasionally cephalad; an upper extremity stimulus response has been obtained with stimulation of the mid-thoracic segment, an effect that is not constant and cannot be expected.
SCS does not have exactly the same effect on all types of pain; for example, for cancer pain treatment SCS is not very effective due to excessive injurious afferent stimulation in advanced cancer pain. SCS is very effective for stable biting-like and burning pain, but not for episodic pain.
Equipment
Electrodes
There are two types of stimulation electrodes: transdermal placement type and flat type. The current transdermally placed electrodes are 4 contacts, 8 contacts, and 16 contacts. Generally, one or two 8-contact electrodes are used for limb pain, while two or three 8-contact electrodes or 16-contact electrodes are used to treat pain in the mid-axis region. The flat type electrodes require surgical placement and very small cone bone resection and are currently mostly 8-contact electrodes. Both types of electrodes are safe and effective under contemporary technology. The advantage of the percutaneous step is that no surgical incision is required; a screening test can be performed on the patient before permanent implantation is performed. At the same time, the percutaneously placed electrodes can be extended forward in the epidural space at will and tested to stimulate several spinal cord segments. The percutaneous insertion of multiple parallel electrodes allows the physician to construct a stimulating electric field with the appropriate contour for the specific pain site.
Pulse generators and receivers
SCS refers to the delivery of square wave pulses to the epidural cavity through implanted electrodes to stimulate the spinal cord and other nerve structures. The fully implanted pulse generator is powered by a lithium battery and is controlled by a transcutaneous telemetry, which, once turned on, no longer requires patient input and can be switched on and off and adjusted to a range of stimulation parameters by a small magnet that the patient always carries with him. The battery life depends on the mode of use and the choice of parameters (voltage, frequency, wave width, etc.) and is typically 2 to 7 years. The entire program adjustment needs to be done by the program controller in the hands of the doctor.
The RF drive system (RF system) consists of a signal receiver placed under the skin and a sensor powered by a nickel alkaline battery carried outside the body. When used, the sensor antenna is placed on the skin on the surface of the signal receiver and then connected to the sensor, so that the stimulation signal can be transmitted percutaneously. The most used system abroad is the rechargeable one, which can charge the implanted pulse generator through the skin, increasing the output stimulation parameters while greatly extending the life of the pulse generator and reducing the average cost of treatment for the patient.
Insertion Technique
Electrode placement is usually done under local anesthesia with intravenous sedation, but occasionally general anesthesia is required. The patient needs to be awake and fully cooperative with the physician because the identification of the exact distribution of the stimulus response to the test stimulus and the motor stimulus is a critical step in ensuring the success of the treatment. In general, electrode placement under general anesthesia is only required for patients whose insertion site is in the C1-C4 medullary segment or who have a history of surgical incision in the area to be inserted.
The patient is usually placed in a prone position during electrode placement. The electrode placement point should be at least 2 spinal cord segments below the target level so that the epidural segment of the electrode has a certain length, which is conducive to its stability and reduces electrode displacement. Clinically, for lower back pain, the electrode placement point can be selected at T12, L1, L2, while the upper extremity target point needs to be placed at T4 and 5 segments. There are several methods that can be used to identify the epidural space. Tactile feedback from the needle tip as it passes through the ligamentum flavum is a very important piece of information, but sometimes it is not completely reliable. The most common method is by using a syringe with low frictional resistance (not a disposable plastic syringe)
to identify the loss of resistance. Introducing the Seldinger wire through the puncture needle and observing its travel pattern and orientation under x-ray also helps to identify its exact location.
Once it is determined that the electrode has entered the spinal canal, the next goal is to clarify whether the electrode is located in the epidural space rather than the subarachnoid space. When the electrode is in the subarachnoid space, the guidewire travels with little or no resistance and can oscillate widely to either side, whereas when the guidewire is in the epidural space, the only way to move it forward is through special manipulation. Another phenomenon is that when the electrodes are located in the subarachnoid space, a motor or sensory response can be induced using very low stimulation intensity.
Procedure and electrical stimulation parameters
Precise implantation of the SCS system is one of the key factors for the success of the treatment. Another key factor is the adjustment of the electrical stimulation parameters and careful follow-up. Three variables need to be determined after the system is implanted; the recognition threshold, which represents the voltage level at which the patient begins to feel the stimulus response; the tolerance threshold, which represents the voltage level at which the patient feels the stimulus response is too strong to produce unpleasant sensations or induce motor contractions; and the usable range, which is the voltage difference between the tolerance threshold and the recognition threshold. The wide usable range will give a wider choice of stimulation parameters.
Once the stimulator has been placed, the workload of the test stimulation should begin. The purpose of the test stimulation is to find the optimal electrical stimulation parameters and to enable the stimulation response area to adequately cover the painful area, otherwise it will be difficult to obtain the therapeutic effect of pain relief.
Once the ideal electrode point and optimal stimulation voltage have been selected, the next step should be to choose other electrical stimulation parameters. Another more important parameter should be the stimulation frequency, and although most patients choose a frequency of 20-1001-Iz, we already know that there is sometimes considerable individual variation between individuals. The wave width can affect the voltage to some extent. Sometimes it takes 4-5 months to find the best stimulation parameters for a particular patient
Complications
The most fatal complication of SCS is secondary to spinal cord compression injury due to nerve root or spinal cord injury during placement or intracanalicular hematoma.
Electrode displacement usually occurs within a few days of placement. The incidence of dislocation is significantly higher with the currently used percutaneous electrodes than with plate electrodes.
The reported incidence of infection in implanted devices ranges from 0.5%-15%. Infections usually involve the pulse generator and RF receiver of the implant, the wires connecting the electrodes, and occasionally the epidural space. Infection can occur within days to years after placement and manifests as persistent redness and swelling of the skin on the surface area of the placement device and pressure pain. The ultimate management of this persistent infection is complete removal of the device and administration of antimicrobial agents intravenously for 6 weeks.
Intractable cerebrospinal fluid leaks can occur after percutaneous or incisional plate electrode placement and present clinically with headache and accumulation of cerebrospinal fluid at the pulse generator placement site. Simple treatment is to have the patient use a fully tensioned lap band for 2-3 weeks to compress the pulse generator and the path of the lead. If simple treatment fails, a small amount of autologous blood may be injected into the epidural space of the spinal canal to promote adhesions or early surgical exploration and repair of the leak.
Conclusion
In the 25 years since its introduction, the SCS approach has been slowly but steadily evolving, with refinements in the principles of indication screening and improvements in technique, making it a very safe and reliable method for the treatment of chronic pain. Of course, as with other treatments, the results of SCS are far from satisfactory, and its long-term effects are difficult to assess with certainty. Nevertheless, SCS has established itself as an important treatment for chronic pain that cannot be underestimated.