Implantation Programming

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).

Table 1: Indications and applications for neurostimulation and intrathecal drug delivery

Neurostimulation Intrathecal drug delivery
Neuropathic pain ● Cancer pain
● Failed back surgery syndrome (FBSS)* ● Pancreatitis
● Chronic regional pain syndromes ● Spinal cord injury
● Radiculopathy ● Osteoarthritis
● Diabetic neuropathy ● Dystonia
● Postherpetic neuralgia ● Osteoporosis
Peripheral nerve injury ● Spinal stenosis
Ischaemic pain ● Osteomyelitis
● Peripheral vascular disease (PVD) ● Coccygodynia
● Refractory chronic angina pectoris  
Deafferentation pain  
● Stump pain  
● Phantom limb pain  
● Spinal cord injury  
● Spinal stenosis  
● Causalgia  
* Most FBSS pain comprises a nociceptive as well as a neuropathic component


Figure-1: The implanted neurostimulation system

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
● 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.

Figure-2: The Synergy Internal (IPG) neurostimulation system.

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.


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).

Table 2: Optimal lead positioning

Paraesthesia coverage Area of stimulation
Upper limb C3-C5
Precordium T1-T2
Lower back and lower limb T8-T9
Foot T12-L1

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.

Figure-3: The Synergy neurostimulator with leads

Figure-4: Percutaneous leads 

Figure-5: Surgical leads

Table 3: Percutaneous and surgical leads

  Percutaneous leads  Surgical leads
Insertion procedure ● Percutaneous ‘wire’ type devices can be inserted via a Tuohy needle ● Electrodes are either improved multi-contact versions of the percutaneous wire type or plate electrodes requiring surgical insertion by laminotomy or partial laminectomy
Benefits ● Often used in neurostimulation trials because less invasive than surgical lead implant ● Enables lead to be securely anchored in place, reducing lead migration both laterally and longitudinally
● Different electrode dimensions and spacing ● Generally accepted that surgical leads should be used when repeated migrations occur and when the voltage needed is high and needs to be reduced
● Surgical implantation with plate electrodes provide a greater variety of stimulation patterns as well as a broader area of paresthesia

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.


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.

Figure-6: Electric circuit


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.

Figure-7: Waveform


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.

Table 4: Therapeutic parameters of neurostimulation

  Symbol Definition Therapy  affect Programming
Amplitude Volts (V) The strength of the pulse Intensity of the paraesthesia Start at 0 V and increase until paresthesia felt
Pulse width Microseconds (μs) The length of time that a particular pulse is delivered Paraesthesia spread or patient’s feeling of paraesthesia  Start at 180 μs
Rate Pulses per second (pps) or Hertz (Hz) The number of times per second that a pulse is delivered Sensation of the stimulation comfort Start at 40 pps and try not to go over 80pps. If the rate is too low, the patient will feel a ‘bumpy’ effect. If the rate is too high, no additional therapeutic effect will occur and the battery will drain prematurely

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.

Figure-8: Synergy with 1 lead

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.

The Medtronic Synergy™ neurostimulation system
Backed by two decades of Medtronic’s experience in implantable technologies, the Synergy™ system was developed to address the limitations associated with single-lead neurostimulation systems and to meet the need for greater flexibility in managing different types of pain and
laterality of pain. The Synergy™ neurostimulation system is the first totally implantable dual channel system available for the management of chronic, intractable pain.
Furthermore, Synergy™ is Medtronic’s most powerful neurostimulation system, providing greater pain relief for a longer duration and greater convenience with less frequent battery replacement. Neurostimulation with Synergy™ increases the chance of successfully managing more types of pain amongst a wider range of patients:
● The dual-stimulation capability of Synergy™ confers greater flexibility, on either one or two leads, to generate paraesthesia and therefore to provide more effective pain relief. If one lead is not sufficient to cover pain adequately, a second lead can be added at a later date
● The use of dual channels, which means that up to 8 electrode options can be programmed to two independent patterns of stimulation, enables physicians to cover changing pain patterns over time
Components of the Synergy™ neurostimulation system
The Synergy™ system consists of an implantable pulse generator, an implantable lead (or leads), an extension (or extensions), a patient programmer and a physician programmer (N’Vision™).
The Synergy™ implantable pulse generator
The Synergy™ implantable pulse generator (IPG) generates electrical impulses to create paraesthesia. The IPG contains a special battery and electronics to create these impulses. It is typically implanted under the skin in the abdomen and is connected to one or two leads which are implanted near the spinal cord.
Medtronic extensions come in a large variety of sizes ranging from 25–66cm in length. The extension is placed under the skin and conducts electrical pulses from the implanted Synergy™ neurostimulator to the lead.
Leads and electrodes
The lead is a special polyurethane-encased wire, available in lengths of 28cm, 33cm, 45cm and 56cm, which is connected to a set of electrodes. The lead conducts electrical pulses from the extension to the electrodes, which deliver the impulses to large nerve fibers in the dorsal column or the periphery. Electrodes are fixed at the end of the lead typically in groups of four or for the octal lead in groups of eight. The Synergy™ neurostimulation system can be used with one lead or two leads, on one or two channels.

Figure 9:The Synergy™ neurostimulation system with leads Figure 10 : Synergy™ EZ patient programmer

Synergy™ EZ patient programmer
Synergy™ also comes with a patient programmer called Synergy™ EZ. This is a small, hand-held device that allows patients to make adjustments to the neurostimulation parameters within a range set by their clinician. Depending on the patient’s need for pain control, patients can also use this programmer to turn Synergy™ on and off and to check the battery status of the neurostimulator. A 9 V battery is required to operate Synergy™ EZ. Synergy™ has been shown to provide effective pain relief
Results of the Synergy™ system clinical trial, which took place in nine centers throughout Europe, Australia and Canada and involved 69 patients, found that patients who were new to treatment and those in whom other SCS systems had previously failed responded well to treatment with Synergy™. Furthermore, at implantation, mean paraesthesia coverage was 88.1%, indicating almost complete pain coverage by Synergy™

Table 5: Improvement in pain scores with the Synergy™ system (n=69)

  Patients new to SCS Patients switching SCS to Synergy
  N Mean N Mean
Baseline 39 7.53 13 7.38
Month 3 33 3.97 9 5.01
Change 32 3.34 9 2.77
Baseline vs Month 3 p-value <0.001 0.055
Pain was measured using a VAS in 69 patients with chronic pain treated with the Synergy system

Neurostimulation with Synergy™ also provided documented patient satisfaction. At three months, in answer to the question: ‘Based on your experience so far, would you have agreed to this stimulation therapy?’ a total of 93.6% of patients answered yes. In answer to the question: ‘Are you satisfied with the pain relief produced by the Synergy™ system?’ a total of 78.7% of patients answered yes. Synergy™ has also been shown to significantly improve a patient’s quality of life by improving mobility and independence, enabling patients to participate in daily activities and return to work, reducing depression and improving psychological well-being.

What’s Up
Inomed ISIS Intraoperative neurophysiological monitoring started to function in all our related surgeries.
Oct /07/2009
The author celebrating 30 years experience in neurosurgery.
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