IMPLANTATION TECHNIQUES AND POSTOPERATIVE MANAGEMENT FOR NEUROSTIMULATION

The overall implantation procedure for a neurostimulation system generally involves three stages: ● Lead placement and intraoperative test stimulation ● Screening test period ● Complete system implantation with either the totally implantable system or the radio frequency system (if the screening test has been successful)

Preoperative set up Selecting the leads Selecting the appropriate leads is essential for successful neurostimulation. The process of lead selection is based on patient information (chronology and symptomatology of pain), the physicians diagnosis of the patient’s pain type and pattern e.g. simple pain or complex pain, and physician preference. Two basic types of neurostimulation leads can be used with neurostimulators: ● Percutaneous leads – Leads that are inserted through a needle into the epidural space ● Surgical leads – leads that are implanted via a laminectomy Percutaneous leads have become increasingly popular because the implantation procedure is substantially less invasive. However, it is generally accepted that surgical leads should be used when repeated migrations occur or when the voltage required for paraesthesia is high. Selecting the implant site The table below summarizes optimal lead sites by vertebral body and pain location. It is important to note that when the back is stimulated so are the legs. For percutaneous leads the entry level is 1–2 vertebral bodies below the target vertebra. For surgical leads, the incision is usually made half a vertebrae below (or at the bottom of) the target vertebra.

Initial lead placement The protocol for initial lead placement is based on lead type. A summary of two common techniques used for percutaneous and surgical leads, respectively, is highlighted below. Percutaneous leads Prior to beginning initial lead placement, the physician uses fluoroscopy to mark the insertion or entry level for the lead(s). Lead placement is performed under local anesthesia within the sterile field. All the accessories necessary for placement are provided in a lead kit. If two leads are used, two lead kits are required. The physician will begin the initial lead placement procedure by: ● Identifying the vertebral body using fluoroscopy and administering an anesthetic along the incision line (Figure 1)

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● Using a long needle to provide a marker at the midbody of T10 to provide a constant point of reference (Figure 2)
● Making a small puncture wound and inserting the Tuohy needle under fluoroscopic guidance (Figure 3)
● Introducing the lead and threading it to the target vertebra (Figure-4)
● Attaching the screening cable to the proximal end of the lead when the lead is in place (Figure 4 and 5)
At this point the physician will pass the end of the screening cable out of the sterile field to the nurse. With the screener off, the nurse will connect the cable to the screener (temporary power source) and intraoperative test stimulation will begin (Figure 6).
 

Surgical leads Surgical lead placement is usually performed under general anesthesia, though sometimes under local anesthesia, within the sterile field. The physician will begin the lead placement procedure by: ● Identifying the vertebral body using fluoroscopy and administering an anesthetic along the incision site. ● Making the skin incision and then using electrocautery to expose the interlaminar space (Figure-7) ● Completing the laminectomy by exposing the dura matter (Figure-8) ● Introducing the lead blank and then passing the lead through the ligament window (Figure-8 and 9) ● Passing the lead cephalad (towards the head) until optimal position is achieved (Figure 10) ● At this point the physician will pass the end of the screening cable out of the sterile field to the nurse. With the screener off, the nurse will connect the cable to the screener (temporary power source) and intraoperative test stimulation will begin. There are two different types of temporary screener, one for the single lead and one for the dual lead (Figure 11)

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Intraoperative test stimulation
The purpose of intraoperative test stimulation is to create an appropriate field of paresthesia to cover the patient’s pain pattern. The location and strength of the paresthesia are optimized by manipulating:
● Lead position along the length of the
patient’s spinal cord
● Stimulation parameters (amplitude, pulse width and rate)
● Lead electrode selection (i.e. changing the location and number of anodes and cathodes on the lead).

Screening test period The basic screening protocol, which should be followed in every diagnostic test stimulation procedure, is carried out as follows: ● The screener is turned on and the amplitude is set to 0 V, the pulse width to 210 msec and the rate to 40 Hz ● To start, the centre bipole electrode selection should always be used. Center bipole consists of one positive electrode and one negative electrode on each lead (Figure 12) ● The amplitude is increased to the point where the patient begins to feel a tingling sensation ● The amplitude is increased until the patient’s pain threshold (intensity of a noxious stimuli necessary for a person to perceive pain) is reached ● The amplitude is reduced to the point where the patient feels comfortable, yet strong, paresthesia ● Microadjustments in lead position are made until the optimal lead position is located (trolling) ● The pulse width is gradually adjusted until it covers the breadth of the patient’s pain. Some patients with back and leg pain may require a pulse width higher than 330 μs ● At the end of the screening period, all system settings are carefully documented. Normally, amplitude and pulse width are usually the only parameters adjusted during intraoperative test stimulation. The rate should remain at 40 Hz.

Figure-12: Center Bipole electrode selection.

Patient evaluation
Patient evaluation, which occurs between diagnostic test stimulation procedure and complete system implant, may last from a day to several weeks. During patient evaluation, patients receive a screener with which to adjust (within a clinician-defined range) the stimulation parameters established during intraoperative test stimulation. Patients should fully test the neurostimulation system under their normal, day-to-day activities, such as walking, sitting and working. Patients are encouraged to try different amplitudes to determine optimal device settings. Some centers encourage patients to document the setting after each use along with the duration of the stimulation and time of day. The screening test period is also an opportunity for the patient to develop an understanding of the technology and realistic expectations of the therapy.
Patient evaluation allows the physician and the patient to:
● Evaluate the impact of stimulation on the patient’s pain and daily life
● Optimize settings for the best pain relief
● Identify the patient’s energy requirements
● Determine whether the patient is comfortable both with the operating system and with the feeling of paresthesia
If the patient does not respond positively to neurostimulation during the screening test period, the lead is removed and the patient is often referred for an intrathecal drug delivery trial. However, if the patient experiences pain relief of at least 50% during test stimulation, the physician and the patient may agree to proceed with the implantation of a complete system.

Complete system implant
If the diagnostic test stimulation procedure was successful, a complete system can be implanted. Neurostimulator selection is based on initial stimulation parameters, patient evaluation data and lead requirements.
Complete system implant is usually scheduled two weeks after the diagnostic test stimulation procedure to provide the patient with ample time to evaluate neurostimulation and to reduce the likelihood of infection.
Possible implant sites include the upper abdominal wall, the upper buttocks, or a subclavicular (below the collar bone) pocket. The choice of implant site is determined by patient and physician preference. The most common site is the upper abdominal wall (Figure 13–16).
 

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Programming stimulation parameters
Stimulation parameters are programmed according to patient evaluation data and the parameters determined during intraoperative test stimulation. There are two considerations with respect to device programming:
● Maximizing patient comfort
● Extending battery life (e.g. rates greater than 50 Hz substantially shorten battery life)
In order to extend battery life, some neurostimulators can be programmed to cycle. Cycling means that the device can be programmed to turn on and off for specified periods of time.
 

Postoperative management
Once the neurostimulation system is implanted, patients are closely monitored. Postoperative care involves:
● Management of complications
● Patient education
● Care of the operative site
 

Management of complications
Potential postoperative complications include:
● Surgical complications
● System complications
Surgical complications
Patients who have poor nutritional status, are small in build and/or thin, or who have generally poor health are at greater risk for post-surgical infections. Potential surgical complications include:
● Infection
● Hematoma
● Epidural hemorrhage
● CSF leakage
● Pain and discomfort
● Seroma at the neurostimulator or receiver site
System complications
Complications with a neurostimulator system may occur and include:
● Battery failure and/or battery leakage
● Lead and/or neurostimulator erosion or migration
● Connection problem
● Wire breakage
● Short circuit
In addition, the patient may experience:
● Loss of pain relief
● Loss of stimulation
● Undesirable change in stimulation described by some patients as uncomfortable ‘jolting’ or ‘shocking’
● Radicular chest wall stimulation
● Allergic or immune system response to the implanted materials
During the postoperative period, patients are advised to limit their activities to reduce the risk of lead movement and subsequent loss of stimulation effectiveness. Postimplant instructions are listed in the table below.