SSPINAL CORD STIMULATION

Neurostimulation and intrathecal drug delivery (IDD) – are indicated to reduce and control chronic, intractable pain. Which type of therapy to use can generally be determined by the indication, type of pain, the pain pattern and the screening period. Studies have showed that both therapies provide effective pain relief and significant improvements in quality of life for people suffering from severe, chronic pain conditions. In addition, both therapies are reversible and are preceded by screening tests which help predict the patients most likely to benefit from treatment.
Both therapies act on specific structures of the nervous system to reduce and control pain. However, the mechanism of action and type of pain most responsive to each therapy differ. Neurostimulation is most effective for neuropathic pain, while IDD is perceived to be most effective for nociceptive pain (Table 1).

Patients with chronic deafferentation or neuropathic pain who are refractory to previous medical and surgical therapies may benefit from neurostimulation of the motor cortex. Motor cortex stimulation involves implanting an electrode over the motor cortex region of the brain
corresponding to the area of pain. It is a reversible procedure and carries less potential risk to the brain than other surgical procedures.

Overview of neurostimulation
Neurostimulation systems use an implanted lead to deliver low-voltage electrical stimulation to selected nerves or anatomic structures. Neurostimulation is divided into subcategories based upon the type of nerve that is being stimulated. Spinal cord stimulation (SCS) involves stimulation of the dorsal column of the spinal cord by placing electrodes in the space above the spinal cord. Neurostimulation can also be used on the peripheral nerves by stimulating a specific nerve branch in the affected limb. This site-specific electrical stimulation inhibits or blocks the sensation of pain in a targeted region of the body.
In the present document we have focused on the most frequently used target for neurostimulation for the treatment of pain. This is often referred to as ‘spinal cord stimulation’ (SCS) or ‘stim’.

The mechanism of action for neurostimulation
The mechanism of action for neurostimulation is based upon the use of electricity (Melzack R, Wall PD. Science 1965; 150:971–979). The use of electricity for pain relief is based on the gate control theory, which suggests that a metaphorical ‘gate’ exists in the spinal cord that allows or prohibits the transmission of pain signals to the brain.

Neurostimulation systems
Neurostimulation systems consist of three basic components that generate and deliver electricity in the form of short bursts or pulses to large nerve fibers in the dorsal column or the periphery:
● Power source
● Extension
● Surgical or percutaneous leads

create paraesthesia. There are two types of power sources:
● Internal – Implantable pulse generator
(IPG)
● External – Radio frequency
(RF) system

Implantable pulse generator (IPG) power source
In an IPG system the entire system, including the battery, are implanted within the patient’s body. IPG systems are capable of meeting the needs of most patients with chronic pain, except those with very high expected energy consumption. For these patients an RF system is recommended.

Radio frequency (RF) power source
An RF system consists of two components:
● A transmitter and antenna that are worn externally
● A receiver that is surgically implanted
The external transmitter sends RF signals through the skin to the implanted receiver, which is surgically placed under the skin. The receiver processes the RF signals from the transmitter and generates electrical pulses for neurostimulation.

Generally patients prefer IPG systems because the totally implantable IPG systems are seen as more comfortable, more convenient, and more cosmetically appealing than external neurostimulators. In addition, unlike RF systems, IPG systems do not cause skin irritations. For these reasons, patients will often be more compliant, and overall therapy will be more effective. Patients using IPG systems may also have greater ease in daily activities, such as working, exercising and sleeping.

Extension
The extension cable connects the power source to the lead. Using an extension rather than a lead connected to the IPG gives additional benefits in terms of being easier to handle in case of revisions and offering more comfort for patients.

Leads and electrodes
The lead is a polyurethane-encased wire connected to a set of electrodes. The lead conducts electrical pulses from the extension to the electrodes, which deliver the pulses to large nerve fibers in the dorsal column or the periphery. Electrodes are fixed at the end of the lead, usually in groups of four. Optimal lead positioning always requires the cooperation of the patient and is the key to success with this therapy (Table 2).
 

For spinal cord stimulation (SCS), leads are placed in the epidural space (between the vertebrae and the dura matter) so that the electrodes are close enough to the dorsal horn to stimulate specific large nerve fibers. Leads can be implanted into the spinal column in one of two ways (Table 3):
● Percutaneously through a needle
● Surgically
The functioning neurostimulation system provides a flow of electrical pulses from the power source through the extension and lead to the electrodes. The electrical pulses are then conducted into the dorsal column
of the spinal cord or the periphery to produce paraesthesia.

The basic science of neurostimulation
Electricity, which is the flow of electrons from a negatively charged area or pole, to a positively charged pole is the key principle of neurostimulation. The movement of electrons between these two poles is called an electric current.

Creation of an electrical circuit
An electric current requires a complete electrical circuit for the electrical pulses to follow as they travel from the negative to the positive pole. Three components are necessary to create a complete electrical circuit:
● Power source
● Conductor
● Resistance

Because the full electrical circuit needs to be maintained, if there is no paraesthesia this could imply that the circuit is open.

Power source
The power source provides the source of electrons. The power source for a neurostimulation system is the IPG.

Conductor
A conductor provides the path of least resistance for the electrical pulses to move from one pole to another. In a neurostimulation system, the conductor comprises of the extension and the lead.

Resistance – ohms
Resistance, also referred to as impedence, is the opposition of a material to an electric current. Resistance is an electrical circuit that acts to restrict the flow of electricity through the conductor and is measured in ohms. The amount of electric current that travels through a material depends on the type of material and its physical dimensions. In a neurostimulation system, body tissue and the material from which the leads and extensions are made all provide some degree of resistance. Tissue resistance should normally be in the range of 50–2000 ohms, with a maximum reading of 4000 ohms. If the patient is no longer experiencing paraesthesia then an impedence measurement test will need to be performed.

Parameters of an electrical pulse
A neurostimulation system delivers electricity into the neural targets in the dorsal column or periphery via electrical pulses (stimulus) rather than a continuous electric voltage. Pulses differ from a steady electric current in several ways, including:
● The parameters of the pulse
● The way in which the parameters are measured
Electrical pulses are represented by a waveform (Figure 7). A waveform provides three parameters unique to electrical pulses:
● Amplitude – the strength of the pulse which is measured in volts (V)
● Pulse width – the length of time that a particular pulse is delivered which is measured in microseconds (μs)
● Rate – the number of times per second that a pulse is delivered which is
measured in pulses per second (pps) or Hertz (Hz)

Therapeutic parameters
The parameters of a waveform are also the therapeutic parameters of neurostimulation and produce the effect of paraesthesia (Table 4):
● Amplitude – Affects the intensity of the paraesthesia
● Pulse width – Affects the paraesthesia spread or the patient’s feeling of paraesthesia
● Rate – Affects the sensation of stimulation
Amplitude, measured in volts (V), affects the intensity of the paraesthesia. By increasing the amplitude, the patient will feel the paraesthesia more strongly. Paresthesia that is too strong may cause a patient extreme discomfort or even pain. Pulse width, measured in microseconds (μs), affects the lateral breadth or ‘broadness’ of the paraesthesia. By increasing the pulse width, the patient will feel the paraesthesia spread to a larger area of the painful region.
The third parameter, rate, which is measured in pulses per second (pps) or Hertz (Hz), controls the ‘smoothness’ of the paraesthesia.

The primary goal of neurostimulation device programming is to individualize stimulation parameters to adequately mask the patient’s pain pattern, by manipulating the three parameters to provide the desired pain relief.

Creation of an electric current
A neurostimulation system is programmed for certain electrodes on the lead to have a positive charge and other electrodes to have a negative charge, producing unipolar or bipolar stimulation. To operate properly, neurostimulation systems must have at least one positive electrode(called an anode) and one negative electrode (called a cathode).

Single-lead stimulation and dual-lead stimulation
Depending on the capabilities of the power source, two stimulation options are available:
● Single-lead stimulation
● Dual-lead stimulation

Single-lead stimulation
Single-lead stimulation is typically used for patients with simple pain because the paresthesia generated by a single lead can effectively mask the patients pain. In general, single lead stimulation requires less energy than dual lead-stimulation.
Some physicians place a single lead on the midline of the spinal cord in order to generate paresthesia on both sides of the body. However, single-lead systems are not effective in treating complex pain patterns because positioning and maintaining a single lead on the midline of the spinal cord is often difficult. Furthermore, single-lead systems do not always provide adequate paresthesia coverage in complex pain patterns.

Dual-lead stimulation
In recent years, dual-lead and dual-channel stimulation systems such as Synergy have become available. The dual stimulation capability of such devices gives the physician greater flexibility in managing hard to treat pain conditions including prominent low back pain or multiple pain foci. In particular, dual lead stimulation has proved to be beneficial for patients with chronic low back pain and leg pain associated with failed back surgery syndrome (FBSS). The great advantage of such a system is that if one lead is not sufficient to cover pain adequately a second one can be added latter.

Advantages of dual-lead stimulation
Dual-lead stimulation systems provide the physician with more programming options. This enables them to provide more effective pain coverage and therefore greater pain relief. The use of dual-lead stimulation systems also enables a reduction in analgesic use. Depending on the capabilities of the power source, dual-lead systems can provide two stimulation modes:
● SingleStim mode
● DualStim mode
Stimulation modes refer to the delivery configuration of the stimulation programs. In SingleStim mode, the two leads are programmed to the same amplitude, pulse width and rate, thereby producing stimulation from one channel. In the DualStim mode, the amplitude and pulse width can be programmed in two different ways on both leads providing stimulation from two channels.