Surgical Procedures for Pain Control

Surgical procedures for pain control may be considered as falling into one of three categories: anatomic, ablative, or augmentative ( ). Anatomic procedures address an abnormality presumed to be the cause of a complaint of pain, e.g., a herniated disc compressing a nerve root. Ablative procedures, e.g., rhizotomy or cordotomy, attempt to block transmission of pain by removing the neural substrate. Augmentative or neuromodulation procedures attempt to interfere with pain transmission by modulating the function of the intact nervous system. For 50 years spinal cord stimulation (SCS) has been the most common augmentative/neuromodulatory procedure using a device implanted surgically for chronic use ( ).

Historical Uses of Electrical Stimulation

Electrical stimulation has been used for medical purposes, particularly the relief of pain, for more than 5000 years. In 3100 BC the King Narmer papyrus highlighted using the electricity-emitting Nile catfish for pain control, and in the first century AD Scribonius Largus described the use of another bioelectric generator, the torpedo fish ( ). As artificial methods of generating electricity were developed in the 17th and 18th centuries, medical applications were prominent and a number of well-known historical figures were involved: Galvani, Volta, Franklin, Faraday, Duchene de Boulogne, Kratzenstein, Antoine Louis, Sherrington, Adrian, and others. In the late 1800s and early 1900s electrical stimulation devices were marketed for the treatment of pain, among other ailments, but this met with limited success in the absence of an accepted scientific rationale.

The Gate-Control Theory of Pain

The modern era of electrical stimulation for pain relief dates to the publication of the “gate theory” in by Melzack and Wall. The balance of activity between large and small fibers in the peripheral nervous system, according to this theory, determines whether pain is signaled centrally. An excess of small-fiber activity will open, and an excess of large-fiber activity will close, a metaphorical “gate” in the dorsal horn of the spinal cord. As large fibers in a mixed peripheral nerve have a lower threshold than small fibers for depolarization by an externally applied electrical field, stimulation at the appropriate amplitude can selectively activate the large fibers, closing the “gate.”

Peripheral Nerve Stimulation

Dr. William Sweet, then chairman of the Department of Neurosurgery at Massachusetts General Hospital, collaborated with Wall in a series of experiments with direct stimulation of peripheral nerves, beginning in October 1965. The resulting report, “Temporary Abolition of Pain in Man,” was published in Science in 1967 ( ). This work involved percutaneous placement of needle electrodes to study the effects of stimulation on acute as well as chronic pain. Chronically implanted peripheral nerve stimulation devices remain particularly useful for a subpopulation of patients with otherwise intractable pain in the distribution of a single peripheral nerve.

Dorsal Column Stimulation

One of the physicians who had worked with Sweet in Boston, C. Norman Shealy, became the first to implement the concept of electrically stimulating the spinal cord in humans. The dorsal columns of the spinal cord contain large-diameter primary afferents—the same neurons that may be recruited selectively in a mixed peripheral nerve because of their lower threshold for depolarization. In the spinal cord these fibers are conveniently segregated from motor fibers in an accessible location. Electrical stimulation of the dorsal columns of the spinal cord, it was reasoned, can have equivalent effects to peripheral nerve electrical stimulation and should address pain problems in the distribution of multiple peripheral nerves or segments on both sides of the body.

The first “dorsal column stimulation” (DCS) implant, later termed SCS, was performed 50 years ago, on March 24, 1967, and reported by Shealy later that year ( ). After his 1-year fellowship in Boston with Professor Sweet ended in 1963, Shealy had joined the neurosurgery faculty at (now Case) Western Reserve in Cleveland, where he collaborated with biomedical engineers Thomas Mortimer and James Reswick. Mortimer, a graduate student at the time, wrote his PhD thesis on the development of animal models, followed by successful clinical application of devices he designed and built ( ). The first use of DCS reported by Shealy was a cancer patient whose pain was relieved through implantation of an intradural but extramedullary electrode via high-thoracic laminectomy.

Early Devices

By the 1960s solid-state electronics had made portable and even implantable devices practical. Even before publication of the gate theory by Melzack and Wall, in 1962–63, Shelden had delivered 14 kHz simulation to the trigeminal nerve using a subcutaneously implanted passive receiver, powered by radiofrequency (RF) coupling between its antenna coil and a parallel coil, worn on the overlying skin and connected to a transistorized oscillator, worn externally by the patient.

In the mid-1960s Medtronic Inc. (Minneapolis, MN, USA) began to develop this technology for stimulation of the carotid sinus in the neck. Torresani in Marseilles had introduced this as a method to control hypertension, angina pectoris, and certain cardiac arrhythmias ( ). The first generation of SCS systems used similar RF technology. Fig. 45.1 (from Mortimer’s PhD thesis) shows a system implanted by Shealy.

The first radiofrequency SCS system implanted in 1967. Plainly visible are antenna coils in the external transmitter and implanted receiver which couple through the patient’s skin to power the implant.

Reproduced with permission from Mortimer, J.T., 1968. Pain Suppression in Man by Dorsal Column Electroanalgesia (thesis). Case Western Reserve University Engineering Design Center, Cleveland, Ohio, Report EDC 4-68-21.

Early implanted SCS systems were based on cardiac and carotid electrical stimulation designs, but whereas these used loops of twisted platinum tinsel wire in a Dacron filament matrix, which tended to create “hot” spots, SCS electrode development focused on plate electrode designs. Mortimer machined the electrodes for Shealy’s first patient from solid platinum. The earliest SCS electrodes were placed in the subdural or subarachnoid space, but complications such as cerebrospinal fluid (CSF) leak, fibrosis, increasing electrode impedance, and even spinal cord compression ensued. Charles Burton, a neurosurgeon at Johns Hopkins, developed the “endodural” method of placing the electrode between dissected layers of the dura. Early SCS investigators included Donlin Long, a neurosurgeon at the University of Minnesota, George Picaza of Memphis, Blaine Nashold of Duke, Reuben Hoppenstein of New York, and Don Dooley of Miami.

General Clinical Application

Between 1970 and 1973 DCS was used by a growing population of neurosurgeons in a growing number of patients suffering from a variety of pain syndromes. Not all these syndromes proved to be reasonable or practical uses of the method, and not all patients were reasonable candidates for the therapy. At that time multidisciplinary pain treatment programs were still in their infancy, and the importance of behavioral and psychological issues in patient selection was not widely appreciated. Implanting devices for pain control on the basis of a subjective complaint in the patients who complain most dramatically or doctor-shopped with the greatest determination is not a formula for success. For a few ensuing years, as selection criteria were refined and devices were improved, enthusiastic initial reports were tempered by mixed results.

Screening Methods

In an attempt to identify patients who might be the best candidates for SCS without subjecting them to laminectomy for electrode implantation, noninvasive transcutaneous stimulation methods, already in use ( ), were adapted as trialing tools. Shealy led the way in this application, beginning with the use of the Electreat, a handheld battery-operated device resembling a large flashlight which delivered a stinging, uncomfortable waveform. Norman Hagfors, an engineer, and Donlin Long, a neurosurgeon, developed a solid-state device which delivered a pulse train similar to that emitted by SCS devices: “transcutaneous electrical nerve stimulation” (TENS), a term coined by Burton, was put into use and proved to be adequate treatment all by itself in some patients. The first commercial TENS units were made by Medtronic; in the years since then, there have been approximately 100 different manufacturers of TENS devices. The role of TENS in its original purpose, identifying SCS candidates, has proven to be limited, but as some patients dislike the concept of using an electrical device or feeling any electrical sensation, a trial of TENS is a useful experience in a negative sense. Failure of TENS to relieve pain, on the other hand, has limited predictive value; apparently the mechanisms of action of SCS go beyond those of TENS.

To screen patients more meaningfully, percutaneous methods were developed for temporary SCS electrode placement, which has the distinct advantage over other prognostic tests (e.g., diagnostic/prognostic nerve block before anatomic or ablative surgery) that it can exactly emulate the long-term treatment. The potential morbidity of an SCS screening trial is of the same magnitude as that of the epidural injections used routinely for the treatment of pain of spinal origin.

In Hosobuchi et al. described a lateral percutaneous approach to the subarachnoid space at C1–C2, as was used for cordotomy, for trial stimulation, but of course this did not target the dorsal columns. Medtronic developed a “percutaneously inserted spinal cord electrode system,” a catheter with lead wire and single cylindrical contact that could be placed through a 16-gauge needle into the epidural space by a technique ( ) that remains in use today (see Fig. 45.2 ). Percutaneous electrodes were promptly adopted for chronic use, making permanent SCS implantation a minimally invasive procedure; Long reported the first case series ( ). Percutaneous methods made SCS accessible to other specialists; in the late 1970s Bruno Urban, an anesthesiologist at Duke, described not only midline epidural placement but also a novel transforaminal electrode placement method ( ). In the United States, by the turn of the 21st century anesthesiologists had become the predominant specialty group performing SCS procedures.

Percutaneous spinal epidural electrode placement via a specialized needle may be guided by fluoroscopy and mapping responses over multiple levels.

Figure courtesy of Stimwave, Inc.

As it became apparent that overlap of each patient’s topography of pain by SCS paresthesia was necessary for pain relief using traditional SCS, and that the major determinant of the location of stimulation paresthesia was the position of the stimulating contacts in the spinal canal, percutaneous techniques were used not only for therapeutic trials but also for mapping to optimize electrode position in the awake patient. As it often proved difficult to place percutaneous electrodes, prevent their subsequent migration within the epidural space, or create multicolumn arrays, paddles (surgically implanted electrodes) remained popular. Fig. 45.3 shows representative examples of both designs.

Representative multicontact electrodes of percutaneous (via a needle) and paddle (via laminectomy) design.

Photo by the author.

Device Manufacturers

In 1972 Charles D. Ray, a neurosurgeon and engineer, joined Medtronic and converted its effort from a research study into a research, development, and manufacturing program. He and Burton had worked together years before at Johns Hopkins; Ray helped to create a new department to support the Medtronic initiative at the Sister Kenny Institute in Minneapolis, and Burton came to run it. Thus neurostimulation was established as a modality with a clinical base near the major manufacturer in the field. Avery Laboratories in Long Island also became a manufacturer of neurostimulators, as did ClinTech of St. Louis, Missouri.