Introduction
One of the greatest advantages of neuromodulation therapies, and in contrast to ablative procedures of the past is that the anatomical and physiological substrate of the therapy may continue to be manipulated long after the implant surgery is accomplished. The adjustment of stimulation parameters postoperatively is called programming and allows the clinician to modify the effect of the implanted device. It is the goal of programming to optimize the delivery of the electrical therapy by changing specific parameters of the stimulation signal. Even though each area of neurostimulation therapy affects different clinically relevant modalities, and the specific neural elements that are targeted with stimulation may be different, the underlying approach behind programming is similar. When approaching the neuromodulation patient, the clinician needs to know the goals of the programming session (e.g. why surgery was performed and the expected reduction in symptoms), the potential side effects (both acceptable and unacceptable), the specific neural elements involved, and the time course for the benefits of the therapy to take effect. This chapter will describe a common methodology for programming and then use examples encountered in common neuromodulation programming situations.
Overview
Programming is the term given to the spatial and temporal adjustment of the stimulation signal parameters used to treat specific neurologic disorders. In all approved electrical delivery systems, there is a standard set of parameters that can be adjusted which are:
- 1.
amplitude
- 2.
pulsewidth
- 3.
frequency
- 4.
polarity ( Fig. 16.1 ).
Figure 16.1
Stimulus waveform parameters.
The clinical relevance of each of these parameters is discussed in Chapters 6 , 7 , and 8 of this book. Most systems (Boston Scientific, Valencia, CA; Medtronic Inc., Minneapolis, MN; St Jude Medical, Plano, TX) allow for the activation of at least one of a multiple of electrical transfer surfaces (interfaces) that make contact with tissue. These interfaces are known as the electrodes. The polarity of the electrodes includes either the cathode or the anode and both are needed for a complete circuit. The cathode is defined as the electrode which is negative in the initial phase of the stimulus waveform (see Chapters 6 , 7 , and 8 for a more detailed description of these parameters). The polarity of the electrodes is the most commonly modified parameter, other than amplitude. Pulsewidth and frequency tend to be adjusted later either to reduce adverse effects or to focus the effect of neural stimulation on a specific neural element (e.g. cell body, axon, and axon size) .
When first approaching a programming session, irrespective of the therapy modality, the clinician and the patient need to discuss the desired benefit and also the acceptable adverse effects. Even though this has been discussed prior to the surgical intervention, it is important to keep these two opposing stimulation effects in the forefront during the application of the therapy. Also, the mindset during initial programming is different than during follow-up programming or complication and change assessment; during initial programming the process is evaluation while during follow up the process is fine-tuning, while during troubleshooting it is evaluation and hardware troubleshooting, described in Chapter 15 , is searching. in all, however it is only four parameters that are accessible by the programmer.
Figure 16.2 shows a flow diagram representing the overall pathways for successful programming. Therapies but may differ on the internal details of each decision or action point, but the main points in the figure are similar for all therapies.
Process
Hardware evaluation
All neuromodulation systems rely a continuity of stimulation current from the stimulation generator (more commonly called the pulse generator) to the tissue, so it is critical that implanted hardware be functioning. The implanted hardware, as described in Chapters 6 , 9 , 10 , and 11 , includes the pulse generator (PG), the electrode, or tissue–stimulation interface, and the connections between the two. Many new devices also include external devices that can recharge the implanted battery and also allow the patient to adjust various parameters on their own. At all visits, including the initial visit, testing of this hardware should be the first agenda item during a patient programming visit. The time required for this is minimal yet, as described in the troubleshooting chapter ( Chapter 15 ), it is both helpful in determining basic issues such as the lack of proper charging, or creating a record of the patient specific normative electrical parameters of impedance and current which can be helpful at future visits. Hardware evaluation consists of three components:
- 1.
system continuity
- 2.
battery state and longevity
- 3.
utilization history.
System continuity
Continuity assessment involves passing a known quantity of current, or generating a known voltage difference across the active and reference leads, from the pulse generator and then recording the variation in potential or current across the system. The method of testing is somewhat different for each manufacturer and thus the normative values are different. Also, since the implanted devices are in different tissues for different therapies and thus may overlie areas of differing biological material (e.g. CSF, gray matter, white matter, dura, blood), the exact values of the impedance can be highly variable. Thus, it is not the exact values that are important but their consistency, after an initial period of adjustment. Immediately after implant, and for about 3 months, tissue impedance changes due to the damage from the implant and the healing process. Devices that are passed through tissue, such as deep brain stimulation (DBS) leads, cause more damage than devices that are placed directly on the surface of the tissue, such as for peripheral nerve stimulation, or on neural coverings, such as spinal cord stimulation devices. Even with these devices there are impedance changes due to scarring and normal biologic foreign body reactions, yet they are less damaging under normal circumstances . By recording these values at each visit, the data can be used to evaluate potential continuity breaks, short circuits, or even device movements, which although unlikely, are possible . The details of localizing the points of these failures are described in Chapter 15 .
Battery state and longevity
Since there are so many different programming configurations, the exact life of the stimulation power supply cannot be determined. In all present devices, the power supply is a battery. For primary cell batteries (non-rechargeable), the described life span is about 5 to 7 years yet, in practice, it is usually between 2 and 5 years for DBS devices, and between 2 and 7 years for spinal cord stimulator systems. Factors that affect battery life are described in Chapter 11 yet, in general, the higher the amplitude of stimulation, the more time the stimulator is on (larger pulsewidth and higher frequency), and the greater area to which the stimulator is delivering energy, the shorter battery life will be. Also, in order to get more energy out of the batteries, specialized circuitry, in some devices, is designed to activate at certain values and will increase energy consumption, thus depleting the battery sooner. For rechargeable batteries, the life span is on the order of 9 years yet, as the battery gets older, the time between charges may get shorter, thus the amount of time the device can deliver therapy with the same charge is reduced. Finally, as battery use continues, the amount of energy output is decreased. This decrease can be either smooth and slow with the shape of a long hill, or quick with the shape of a cliff. Each manufactuter allows for checking of the battery status. Some devices, such as the Medtronic Soletra™, give battery output in volts, while others, such as the St Jude Libra XP™, give codes relating to the device status. Early systems, such as the Soletra, have slow or ramp-shaped battery energy depletion curve so, even though the manufacturer recommended battery replacement, at a device indication of ‘low’, our center found that when the battery voltage was at 3.65 volts or less a battery replacement was performed in order to assure constant therapy. As new batteries are introduced, the value at which the battery should be replaced needs to be adjusted. The new Activa™ (Medtronic) primary cell implantable pulse generator (IPG) has a more stable output and thus does not need to be replaced as soon. One important note is that with devices having a sharp energy reduction curve versus time, the time between implants needs to be watched more closely since there will be a quick reduction in therapy if the battery is not replaced. Finally, in order to increase shelf-life, some batteries include a special oxidative layer to impede small transfers of electrons from one polarity to the other. This layer can interfere with impedance and device testing when the battery is at certain values (e.g. the St Jude Libra™).
Utilization
As technology advances, the information stored in the device and accessed from the clinician programmers increases. Original devices (such as the Medtronic Itrel II and III) described length of time since last data reset, percent of device ‘on’ time, and number of device activations, or on/off cycles. Even these limited data were helpful in determining if the device was inadvertently shut ‘off’ or was inappropriately cycling on and off. For example, if a patient came into the clinic complaining of reduced stimulator efficacy and during interrogation of the device it was noticed that device was in the ‘off’ state, one could, utilizing the total time since the last programming session (assuming the device was reset) and the percentage of time the device was ‘on’ in that period, calculate the approximate date the device went ‘off’ (assuming one activation).
Days since device OFF = ( % device on ) ( total time since last reset ) 24
With this information, discussion with the patient and or family members can, in most cases, determine the exact situation that caused the device to turn off and then avoid it in the future. Newer devices are able to keep more detailed information of the exact times systems are in either the ‘on’ or ‘off’ state, charging status, program utilization, multiple program percent usage, and patient adjustment times. One major problem that patients with rechargeable systems run into is improper or incomplete charging. A good example of a regular charging cycle is shown in Figure 16.3 where you can see that the patient charges their system every other day to 100% charge thus never allowing for a low stimulation output potential. Using this graph, the clinician can determine if the device is being charged and if not get an understanding of why. If, for example, the patient states they charge every day it is most likely that the patient is not placing the charger over the IPG properly and thus the clinician can re-educate the patient on proper placement of the recharger to be able to get more efficient system utilization.
