Epidural Spinal Cord Stimulation
Spinal cord stimulation was first hosted in 1967 as a technique for treating chronic back pain. In this treatment, the nerves in the spinal column, also called the spine or backbone, are imparted mild electrical impulses or shocks. The impulses are supplied through leads that are implanted into the epidural space. The location of these leads is adjoining the lower facet of the spinal cord between T9 and L1. The implantation is carried out under fluoroscopic control in a relatively minor surgical procedure. The leads are supplied current from the pulse generator positioned between the skin and the fascial layers (connective tissue fibers, largely collagen). The electric impulses interfere with and modify the nerve activity to minimize the sensation of pain propagation to the brain. An additional available feature is programmability of electrode activation. Either constant-voltage or constant-current pulse trains can be chosen. Facility to minimize stimulus energy requirements is provided. The technique is vulnerable to migration of the lead. Shunting of the stimulus currents by the cerebrospinal fluid (CSF) and various other complications diminish its clinical efficacy.
KeywordsSCS Dorsal column stimulation Back pain Paresthesia Dura mater Epidural anesthesia Gate control theory
17.1 Introduction and Historical Glimpses
Chronic pain is the principal cause of bodily and poignant anguish. It leads to domestic and societal commotion, frailty, and nonattendance of work . The first clinical application of dorsal or posterior column stimulation (DCS) was documented by Shealy et al. ; “dorsal” means situated at the backside. They inserted the stimulator into patients suffering from cancer pain in 1967 . This technique was portrayed as an original analgesic (pain relieving, palliative) method. It could alleviate pain in a multiplicity of chronic pain syndromes. Nevertheless, it was proved in later years that a greater number of structures in the nervous system might be activated by application of an electrical potential to the dorsal epidural space. By delivering electrical impulses of small amplitudes, unswervingly into the spinal cord in the region popularly known as epidural space, the straight spreading of pain signals traveling along the spinal cord towards the brain is interfered with and disturbed. Therefore, the term “dorsal column stimulation (DCS)” transformed to “spinal cord stimulation (SCS) .”
The SCS is tactically designed to exchange the unpleasant sensory feeling of pain with a more gratifying tingling feeling. This tingling feeling is referred to as paresthesia. “Paresthesia” is a condition causing a patient to feel a sensation of numbness, burning, or tingling. It may appear like itching or prickling with no obvious durable effect on the body. This condition is vastly different from paralysis, which involves the loss of both movement and sensation. Paresthesia is only accompanied by loss of sensation. The perception of paresthesia in the region where pain is felt “masks” pain signals.
Pursuant to the first-time implantation of a stimulation system for spinal cord therapy wayback in 1967, techniques of neuromodulation have been widely practiced for treating patients with chronic pain. Evidence has mounted in support of the benefits of SCS in managing constant, obdurate pain of the torso and limbs. Many examples can be quoted. These include unilateral/bilateral pain insolent to orthodox and surgical treatments for chronic low back pain syndrome, radiculopathy (disease of spinal nerves/nerve roots), postsurgical pain, degenerative disk disease or herniated disk (slipped/ruptured disk), peripheral causalgia (burning pain due to peripheral nerve injury), epidural fibrosis (scar tissue formation near the nerve root), arachnoiditis (arachnoid inflammation), and complex regional pain syndrome. SCS has also been investigated to treat paralysis, spasticity (stiff/rigid muscles), gastrointestinal issues, and urinary system dysfunction. The stimulation site on the spine determines which nerve networks are targeted.
Continuing pain reduction and improvement in health-linked quality of life has been achieved by spinal cord stimulation in individuals suffering from chronic reflex sympathetic dystrophy (RSD) syndrome of unknown pathophysiology, a burning pain with swelling and tenderness [4, 5], and intractable cancer pain [6, 7]. In 1989, FDA approved SCS as a means to relieve pain from nerve damage in the trunk, arms, or legs. SCS now covers nearly 70 % of all neuromodulation treatments.
17.2 Epidural Space and Epidural Anesthesia
Epidural anesthesia is the utmost universally practiced method for pain relief during labor. It is a local anesthesia. It occludes pain only in a specific region of the body. An epidural provides only analgesia or relief from pain. It does not aim to provide anesthesia, which causes total lack of feeling. The epidural obstructs the nerve impulses starting from the spinal segments on the lower side. The result is a decrease in responsiveness in the lower half of the body.
The procedure for providing epidural anesthesia is as follows: The middle back is washed in waistline area with an antiseptic solution to counter any chances of infection. A local anesthetic is infused by injecting into a small region on the back to create the feeling of numbness. A needle pierces the numbed region encircling the spinal cord in the lower back. A catheter is guided through this needle into the epidural space. The needle is then carefully withdrawn. But the catheter is left in its position. The catheter is securely fixed by adhesive tape to the back to check it from sliding off. Medicine is supplied by giving injections periodically. It may also be delivered in continuous infusion.
17.3 SCS Equipment
17.3.1 The Hardware and the Electrodes
By programming the system, the following parameters are set at definite values: pulse amplitude, width of the pulse, and frequency of repetition of pulses. The rostrocaudal position (between head and tail) of the multiple-contact lead is alterable. The position is changed to provide electrical stimulation at many levels of the spine.
The placement of the cathode is done at a location between the dorsal median sulcus (longitudinal groove marking the midline of the medulla oblangata) and the dorsal root entry zone (DREZ) area. Upon stimulation, the current starts flowing from the cathode (negatively charged electrode) towards the anode (positively charged electrode). Fundamentally, every neural structure situated proximally to the cathode can receive activation at sufficiently high electrical stimulation. But current follows the path of least resistance. It therefore flows through anatomical structures having low electrical resistivity. Cerebrospinal fluid carries approximately 90 % of injected current because of its lowest electrical resistivity. Then the longitudinal white matter follows. Due to its anisotropic or directionally dependent characteristics, transverse white matter is more resistive than gray matter. Epidural fat and dura mater likewise possess high resistivity. Vertebral bone has the highest electrical resistivity. Therefore, it practically behaves as an insulator. Hence, the besieging tissues, e.g., the heart and structures of the pelvis, are protected from receiving stimulation.
The first-generation electrodes were unipolar. One lead was positioned at the stimulation site and the other at an area of zero potential (the earth or a large conducting body). Unipolar electrodes provided paresthesia in a restricted zone. Therefore, a new lead design was later developed. The number of electrodes in this design varied from four to eight. Two kinds of leads are presently used: the percutaneous lead and the paddle lead. The percutaneous electrode is inserted via Tuohy needles (hollow hypodermic needles). It is a perfect lead for both provisional and final implants. For placing the paddle lead, surgery is necessary by orthopedic neurosurgical procedures known as laminotomy or partial laminectomy. In laminotomy, a small part of the lamina is removed while in laminectomy, it is completely removed. The paddle lead offers greater stability and lesser tendency for migration. It is especially befitting for patients who have a history of lead relocation. It is also appropriate for the patients in whom assignment of the trial lead was previously demanding.
Programmable multiple-electrode arrays have shown superiority over the devices with a solitary channel. The reason is that they permit anode–cathode guarding and changes of polarity. In this manner, they enable optimal current steering.
Axonal activation and paresthesia (tingling/pricking sensation) distribution are determined by the comparative positions of the electrodes (cathodes and anodes). They are also impacted by their distances from the spinal cord. Using a two-channel pulse generator and applying pulses differing in simultaneity, a profounder dissemination of the pulses into the spinal cord is attained without creating a high-intensity electric field. The idea of electrical field routing through selectively engaging axonal nerve fiber tracts in the dorsal columns was publicized by a transverse tripole array (+, −, +) system. Electrical steering of the paresthesia across the axial back region is simplified. At the same time, the stimulation of the nerve roots was minimized.
17.3.2 Implantable Power Sources
There are three different types of implantable power sources to be discussed in the forthcoming subsections.
188.8.131.52 The Conventional Non-rechargeable Unit
The main component of the conventional unit is an implanted pulse generator. The pulse generator houses the integrated circuitry and batteries. It delivers low-voltage stimulation for pain relief. The pulse generator is accompanied by an external handheld, telemetry device. This is the external controller which activates the pulse generator and programs its parameters. It works transcutaneously. It is used to switch stimulation on or off and to attune the intensity of stimulation between the bounds set by the doctor. The patient too can control the stimulation parameters. The life of the battery is limited. The life expectancy of the battery is determined by the length of time over which it is used. Other decisive factors influencing battery life are the levels of the exploited parameters, viz., width of the pulse, the applied voltage, frequency, etc. This kind of unit generally lasts for 3–5 years before needing surgical replacement.
184.108.40.206 Rechargeable Unit
This unit is supplied to patients who require a high amplitude of stimulation for relieving pain. It lasts for a long period of 10–25 years. Recharging is done through an induction coil worn by the patient over the implanted device. During recharging, stimulation can be continued. The rechargeable unit is acquiring popularity among recipients due to its small dimensions and easy upkeep.
220.127.116.11 Radio-Frequency Unit
This power source is a radio-frequency device. Patients requiring high power adjustments are prescribed this kind of unit. These are the patients with pain in the back or legs. A radio-frequency unit consists of an implantable receiver and an external transmitter with antenna knotted on a belt or kept in a pocket by the patient. Energy emitted from the transmitter is picked up by the receiver. It travels along the lead and transmits impulses to stimulate the nerves along the spine near the implanted electrodes.
17.4 Mechanisms of Action
Spinal cord stimulation is an irrefutable outgrowth of the well-known gate control theory (GCT) of pain propounded by Melzack and Wall in . This theory describes gracefully and succinctly the attenuation of spinal pain transmission by activation of afferent fibers. The technical context of the SCS trials is provided by GCT in terms of a draft outline for probing the exchanges between native and remote excitatory and inhibitory organizations in the dorsal horn. The dorsal horn is a longitudinal subdivision of gray matter from posterior spinal cord to dorsal roots.
The expected mechanisms of pain relief by SCS are predominantly described in terms of gating. SCS closes the gate to incoming signals of pain from the peripheral nerves and spinal cord. It was hypothesized in the GCT theory that the gate was opened by excessive small fiber activity. Also, an excessive large fiber activity closed the gate. It was proposed in the theory that when the large non-nociceptive (not pain-related) myelinated (with myelin sheath) fibers of the peripheral nerves were stimulated, the activities of the small nociceptive (pain-related) projections in the dorsal horn of the spinal cord were repressed. In comparison to small fibers, the activation threshold of large fibers for depolarization by an electric field was lower. Further, large fibers could be stimulated selectively.
The correct mechanisms of pain reprieve caused by stimulation of the spinal cord still continue to elude scientists and remain shrouded by mystery. Existence of other possible dominant mechanisms has been conjectured. Animal studies have shown that SCS props up the activation of chemical substances such as gamma-aminobutyric acid (GABA)-B and the adenosine A-1 receptors. These may modulate the pain. Explicitly, a faulty local GABAergic function partially uplifts the basal levels of exciting neurotransmitters in neuropathic pain states. It was shown that SCS prompted the liberation of neurotransmitters concerned with the variation and control of pain signals in the spinal cord, such as GABA, substance-P, and serotonin.
In the model of pain for animals with injury to sciatic nerve (a large nerve that runs through the buttock down the back of each leg), SCS inhibited the over-excitability of the extensive active span cells in the dorsal horn. This signifies that the major antinociceptive (anti-pain) effects of SCS occur via A-fibers, which are the nerve fibers in nerve trunks and peripheral nerves with the highest conduction velocity. Using SCS, it is possible to eliminate peripheral ischemic pain caused by inadequate local blood supply due to the blockage of blood vessels leading to that region. It may do so by rebalancing the oxygen supply/demand ratio, as evidenced by anti-ischemic and antianginal characteristics. At low stimulation intensities, SCS enfeebles over-excitation of the sympathetic nervous system. Antianginal effect might similarly spring up from the subdual of the central nervous system. Other reasons could be the steadying of the inherent cardiac nervous system. Liberation of adenosine is another possibility. With increase of stimulation level, the nitric oxide reliant on liberation of the calcitonin gene-related peptide could perhaps induce vasodilatation, i.e., the dilatation of blood vessels. This may produce anti-ischemic effects.
17.5 SCS Indications
To be an eligible candidate for SCS, the patient should have suffered from chronic pain for a period >1 year. Candidacy is decided through the individual’s qualifying physical and intellectual tests. The status of a patient’s health plays a vital role in determining whether the person is fit enough to draw healing advantage. It is abundantly clear that neuromodulation does not remove the underlying cause of pain. It is only a technique to provide relief from pain. Hence, the treatment is a part of a unified healthcare team approach. This approach seeks to provide long-term moral support and care to the patient. Only a collective effort by pain specialists, neurosurgeons, and rehabilitation physicians can offer effective SCS treatment.
A two-stage process is adopted for implanting the stimulator device. The first stage is a selection examination preceding the actual implantation to assess the suitability of the patient for permanent implantation. It is only when the patient is satisfied and convinced fully with the results of the trial that the SCS device is permanently implanted in the second stage.
In the first stage, the percutaneous method of electrode placement is applied. This method is less invasive. The electrode is inserted through a hollow needle. The doctor carefully looks at the image on the monitor and guides a hollow needle inside the epidural space in the region above the spinal canal. This needle serves as a passage through which thin wires are threaded. In each wire, there are a small number of electrical contact points near the end. By attaching the leads to the power supply, a mild current is supplied. The patient is asked to understand and assess the exact feeling. After the doctor has listened to the patient’s feedback, the positions of the electrodes are maneuvered for patient’s satisfaction. This is done until the area of pain is covered by a tingling sensation. For realization of analgesia, paresthesia must overlap the site of pain. Then only the pain effects are overshadowed by the paresthesia. The trial period lasts from 3 to 15 days. As the patient carries out routine activities, the degree of relief is observed. After the patient is contented and responds affirmatively, it is inferred that the exact position of lead has been located. If the pain is reduced by more than a factor of half from the baseline value, the trial is deemed to be successful. Then the second stage of implantation is completed, wherein the SCS lead is strongly fixed.
17.6 Discussion and Conclusions
SCS stands out as an anodyne and relaxing treatment for various ingrained neuropathic pains, i.e., for management of lingering pain conditions accompanied by tissue injury . It is useful to deal with assorted medical conditions of constant pain. One such condition is the failed back surgery syndrome (FBSS) , namely, continued back pain after surgery. Another situation is multifarious provincial pain syndrome called complex regional pain syndrome(CRPS) , a scarce form of prolonged pain typically affecting a limb such as an arm, a leg, a hand, or a foot, generally following an injury or trauma. Pain conditions associated with ischemic and coronary artery diseases [1, 11, 12], as well as cancer , are relieved using SCS. Time and again, the starting medical care acquirement expenditures of SCS gadgetry are compensated by a decrease in healthcare backup reinforcements and charges after implantation .
Differentiate between dorsal column stimulation and spinal cord stimulation. Why was the term, “dorsal column stimulation” changed to, “spinal cord stimulation”?
What is paresthesia? How does it differ from paralysis?
Explain the following terms: (a) dura mater, and (b) epidural space.
Name a popular method of pain relief during labor. How is this anesthesia carried out?
Describe the hardware of SCS equipment. What is the path of current flowing during spinal cord stimulation?
What are the two types of electrode leads available for SCS? Which type of lead is less prone to migration?
What are the three types of implantable power sources for SCS? Indicate the typical life span of each class. Discuss their relative benefits and limitations.
What is gate control theory of pain? How does it explain the pain relief obtained by SCS? What other mechanisms are likely to participate, as suggested by animal studies?
What type of patients can benefit from implantation of SCS equipment? What are the two stages in which this implantation is carried out? Why does a trial implantation precede the permanent one?
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