Practice Parameters for the Use of Spinal Cord Stimulation in the Treatment of Chronic Neuropathic Pain
Glossary & Acronyms
ACTION POTENTIAL – a rapid rise and fall in voltage and chemical gradients across a neuronal membrane, which can propagate over distance (most rapidly along a myelinated axon) and can be triggered or inhibited by electrical stimulation.ANODE - a stimulating contact programmed as a positive pole, which attracts negative ions.
ANODE – a stimulating contact programmed as a positive pole, which attracts negative ions.
CATHODE – a stimulating contact programmed as a negative pole, which attracts positive ions. Compared with an anode, a cathode initiates action potential generation (depolarization) at a lower amplitude threshold; thus, cathodal effects predominate.
CHANNEL – multiple independent channels allow simultaneous delivery of pulses of different amplitudes to different contacts. (A generator that allows rapid sequential delivery of pulses to different contacts is not, strictly speaking, a multichannel device; instead it is a “gated single channel” generator.)
CLOSED LOOP STIMULATION – a type of stimulation that adjusts automatically (typically in amplitude) in response to one or more inputs, such as the amplitude of evoked compound action potentials (usually recorded from the spinal cord via electrode contacts that are not in use for stimulation). Thus, the dose of stimulation can automatically be kept at optimum therapeutic levels despite 1) the movement of the spinal cord in relation to the electrodes or 2) other physiologic changes in the patient’s body and the stimulation system, such as increased electrode impedance with scarring. Other inputs, such as accelerometers, can also be used to “close the loop.”
COMPLEX REGIONAL PAIN SYNDROME (CRPS) – formerly known as reflex sympathetic dystrophy, is characterized by extremity pain, sensitivity, and sometimes changes in color or appearance, with these symptoms out of proportion to any underlying injury or other apparent cause.
COMPOUND ACTION POTENTIAL – compared with individual elements, multiple neurons or axons firing simultaneously will, in aggregate, produce a larger signal that can be recorded at a greater distance from a larger electrode (e.g., a typical stimulator contact).
CONSTANT CURRENT STIMULATION (see also CONSTANT VOLTAGE) – stimulation in which the rate at which charge is delivered stays constant. If the impedance or resistance changes, the device will adjust the voltage to keep current at the requisite level.
CONSTANT VOLTAGE STIMULATION (see also CONSTANT CURRENT) – stimulation in which the voltage stays constant. If the impedance or resistance changes, the current will change accordingly.
CONTACT – the electrically conductive portion of the electrode assembly, resembling and sometimes referred to as a “button.” An individual contact can be programmed as an anode, a cathode, or neither (i.e., off).
CONTACT COMBINATION – assignments (anode or cathode or off) of individual contacts. Modern implanted electrodes have multiple contacts.
CONTACT ARRAY – an assembly of multiple stimulating contacts. A single percutaneous electrode, thus, can create a longitudinal array. Parallel longitudinal arrays can be created with parallel percutaneous electrodes or a single plate/paddle electrode with contacts prefabricated in two or more columns. Plate or paddle multicolumn arrays have fixed offsets between columns.
CONTACT COMBINATION – anode or cathode or on/off assignment of contacts in an array.
CONVENTIONAL SPINAL CORD STIMULATION – see TONIC.
DORSAL COLUMN STIMULATION (DCS) – the original terminology for SCS, reflecting electrode placement directly over the dorsal columns as well as the low threshold for recruitment of the large myelinated fibers therein. DCS went out of favor as it was recognized that SCS activates multiple structures, not merely the dorsal columns.
DURA – the tough membrane that surrounds and protects the brain and the spinal cord and (along with the underlying arachnoid membrane) contains cerebrospinal fluid.
ELECTRODE – an assembly comprising contacts, wire, insulating spacers, catheter(s), and backing material. “Electrode” is generally applied to the part of the assembly that contains the contacts, while “lead” is best used to describe the cable or wire leading between the electrode and the implanted generator. Using “electrode” to refer solely to a contact is imprecise because it ignores the other components of the assembly. Using “lead” to refer to an electrode is commonplace but imprecise; the electrode does not lead anywhere.
ELECTRODE ARRAY – an arrangement of multiple contacts in an assembly, which can (for example) be oriented longitudinally or transversely, in one, two, or more columns. Multiple arrays can be placed to create larger arrays, e.g.,percutaneous single columns in parallel can create a multicolumn array.
EVOKED POTENTIAL (EP) – an electrical signal resulting from stimulation, typically a compound action potential. EP’s may be recorded from the brain, the spinal cord, and/or peripheral nerves, via surface (skin) electrodes or implanted electrodes (including unused contacts in an array), and they may in turn be used to guide implantation and/or adjust stimulation in real time.
FAILED BACK SURGERY SYNDROME – persistent or recurrent pain after one or more surgical procedures on the spine.
FREQUENCY – as used in neurostimulation, which is delivered as a series of pulses, “frequency” refers to pulse repetition rate. (Strictly speaking, each pulse has a broad spectrum of frequencies.)
GUARDED CATHODE – an arrangement of contacts with two programmed as anodes bracketing a cathode. The boundary created by the anodes for the depolarizing effect of the cathode helps define the area of paresthesia. In general (statistically), paresthesia-based SCS patients prefer a guarded cathode array.
HIGH DENSITY STIMULATION – relatively high frequency (typically as high as a given generator will allow, but not as high as 10 kHz) increases the average current delivered and thus the “dose” rate. This of course depletes the power source (battery) more rapidly than would a lower frequency. The amplitude for high density stimulation is typically adjusted below (e.g., to 75% of) that required to elicit paresthesia (see PARESTHESIA).
HIGH FREQUENCY – in current parlance, pulse repetition rates at and above 1 kHz.
IMPLANTED PULSE GENERATOR (IPG or internally powered generator) – a battery powered implanted device that sends pulses to a programmable combination of contacts on an implanted electrode.
LAMINECTOMY – removal of all (or some) of the bony roof of the spinal canal, providing access for placement of one or more electrodes, particularly insulated paddle or plate electrodes. The posterior midline spinous process is typically removed along with at least a portion of the lamina on either side to allow electrode placement in the midline.
LAMINOTOMY – removal of part of one lamina to allow the introduction of an electrode from one side.
LEAD – the insulated wire that connects (leads) from the implanted generator to the electrode contacts. (This term, pronounced “leed” is often, but inappropriately, used to refer to an entire electrode assembly.)
MONOPOLAR STIMULATION – the use of one active electrode contact near the stimulation target along with a remote indifferent electrode, such as the metallic case of an implanted generator. The active electrode is typically a cathode and the indifferent electrode an anode, minimizing power requirements. "Monopolar” is a misnomer, however, because stimulation, of course, requires two poles.
MULTICHANNEL STIMULATOR – allows simultaneous delivery of pulses of different pulse amplitudes to different contacts. (A multicontact stimulator that allows rapid sequential delivery of pulses to different contacts is not, strictly speaking, a multichannel stimulator.)
PARESTHESIA – a tingling or buzzing sensation caused by stimulation. Until recently clinicians and researchers believed that stimulation in general, and SCS in particular, had to induce paresthesia that covered as much of the painful area of the body as possible in order to provide pain relief. This overlap, however, did not guarantee pain relief. Thus, pain/paresthesia overlap was thought to be necessary but not sufficient, and researchers paid a great deal of attention to ways to optimize pain-paresthesia coverage. Even if eventual stimulator settings used currently will be subliminal (with no perceived sensation), evoking paresthesia can still be useful during implantation to optimize electrode placement and during programming to optimize contact selection.
PERCUTANEOUS or CATHETER ELECTRODE – an electrode that can be inserted through a needle, in a manner that is less invasive than laminectomy/laminotomy.
POSITION (or POSTURAL) ADAPTIVE – the distance between the spinal cord and epidural electrodes changes with body position; generally, the appropriate SCS stimulation amplitude is lowest when the patient is lying supine. Adjustments to stimulation can be made manually by the patient or automatically (e.g., with equipment that senses body position or evoked potentials.) See CLOSED LOOP.
POWER SOURCE – stimulation may be delivered from an internally powered generator (IPG) containing a battery or from a passive implant powered by radiofrequency or microwave signals from an external device. A rechargeable implant combines these approaches.
PULSE – a single stimulation wave (or phase).
PULSE AMPLITUDE – the strength or intensity (height) of a pulse wave, measured in volts or milliamperes.
PULSE WIDTH – the duration of a pulse, measured in microseconds.
PULSE RATE – the frequency (number per unit time) of pulses, expressed in pulses per second or as hertz (Hz).
STIMULATION (PULSE) PARAMETERS – the pulse amplitude, width, and repetition rate.
SURGICAL PLATE/PADDLE ELECTRODE – an array of contacts on an insulated curved or planar substrate, facing the targeted spinal cord or nerve(s).This reduces power consumption as well as the extraneous stimulation that can cause side effects. Because of their size and shape, paddle or plate electrodes must be inserted via an open surgical exposure.
TARGET – a physical or anatomic location where stimulation energy is focused or a stimulating contact is positioned for maximum or optimum effect (defined, for example, as “most effective,” “best ratio of therapeutic to side effects,” “most energy efficient,” etc.). This is sometimes referred to as the "sweet spot.”
TONIC (or MONOTONIC) – stimulation using pulse parameters that never vary from one pulse to the next – to be distinguished, for example, from burst stimulation, in which the pulse repetition rate (or interpulse interval) changes periodically. Conventional or “traditional” stimulation at frequencies in the 10’s and 100’s of Hz is “tonic,” but so is 1 kilohertz (kHz) or 10 kHz stimulation; thus, “tonic” should not be used to distinguish them.
TRADITIONAL SPINAL CORD STIMULATION – tonic stimulation delivered in the 10’s and 100’s of Hz at an amplitude eliciting paresthesia but below motor or discomfort threshold.
WAVEFORM – generally and broadly speaking, a representation (usually graphical) of how voltage or current varies with time. Electrical stimulation devices emit a series of pulses, the parameters of which (as well as the interpulse interval) may vary from one pulse to the next.
Acronyms
ADL = activities of daily living
CRPS = complex regional pain syndrome
DCS = dorsal column stimulatoin
DRG = dorsal root ganglion
FBSS = failed back surgery syndrome
Hz = hertz
IPG = implanted pulse generator
kHZ = kilohertz
MRI = magnetic resonance imaging
NPRS or NRS = numeric (pain) rating scale
PHN = postherpetic neuralgia
PLS = phantom limb syndrome
PNS = peripheral nerve stimulation
QofL or Q of L = quality of life
RCT = randomized controlled trial
SCS = spinal cord stimulation
VAS = visual analogue scale