Advancement
Current use
Future impact
Rechargeable IPG
Yes
Large
Enhanced battery life
No
Important for smaller IPG
Different targets
Yes
Important for DRG-SCS
Increased frequency
Yes
Makes miniaturization difficult
Bluetooth and microwave technology
Yes
Uncertain
Improved energy conduction in leads & materials
No
Uncertain
Remote energy delivery
No
In concept, very important
Enhanced spinal cord lead arrays and Artificial Intelligence programming
No
May be the next critical step
The humble beginnings of ancient civilizations utilizing the electrophysiology of fish in order to adjust afferent sensory input to the cortex of patients has now transformed into a field of science that is boundless. Applications are breaking away from theory and becoming the forefront of options for patients with complex pain diagnoses and other disease processes. This chapter focuses on the future of neuromodulation with respect to hardware and programming, neuroanatomical targets, and novel neuromodulation strategies.
22.2 Current and Short-Term Advances: Miniaturization
The size of the implantable programmable generator (IPG) has been a quandary for the device companies. Larger, bulky IPGs have a greater potential for longevity and enhanced battery capacity, but a large object in the subcutaneous tissues can produce pain and cosmetic problems.
22.3 Modern Advances and Future Enhancements
22.3.1 Stimulation Frequency
For more than four decades, the current has been delivered to implants either as a constant current or a constant voltage mode with frequencies that are considered “low” or normal, unusually ranging from 30 to 100 Hz. In recent years, this situation has been an area of investigation. In the United States, investigators in an early pilot study found that stimulation at high frequencies of 10 kHz could improve pain without eliciting a paresthesia. This finding led to an interest in investigating a new mode of spinal cord stimulation (SCS), termed HF10, which has now advanced to approval in the European Union (EU) and Australia. Interesting evidence from multiple centers in those two regions have shown improvement in patients with axial back pain and failed back surgery. There has also been efficacy in patients who had earlier failed conventional frequency stimulation.
The lead placement for HF10 has been shown to be similar to that for conventional SCS. Because no paresthesia has been elicited, the trialing has the advantage of being efficient, but not being able to get immediate feedback from the patient is a disadvantage. Interestingly, the use of 5000 Hz in Switzerland was found not to create pain relief, as compared to sham stimulation. This finding suggests that a particular neurotransmission change may occur at 10 kHz that is unique to that frequency.
In the future, investigations may include ultra-high frequency (Ultra HF) or more research into the basic science of the effect of frequency on neural plasticity and neurotransmitters.
22.3.2 Waveform Delivery
Waveform Shape: In addition to innovations with frequency, new devices will have changes in the way waveforms are delivered. For decades, the traditional waveforms have been rectangular. These conventional waveforms have been used in all devices in the field, including those targeting deep brain, the vagal nerve, peripheral nerves, and the spinal cord.
Some new, non-rectangular waveforms have been investigated in recent years, including triangular waveforms and other novel attempts. Each of these has suggested an improvement in energy efficiency.
Waveform Programming: In recent years, work by De Ridder and others has led to an interest in the programming of the waveform. This work is seated in the neurophysiology of the brain and the “language of the brain” in regards to conventional tonic waveform delivery verses a focus on the “burst” delivery. This landmark work has been based on preliminary work by prominent neurophysiologists that suggested three types of neuronal communication in the brain: regular spiking, fast spiking, and bursting. The theory of burst stimulation involves changing current delivery to accommodate to these different brain nerve systems. In this mode, there is a 40 Hz burst mode with five spikes at 500 Hz per burst. The pulse width is fixed at 1 ms, with 1-ms interspike interval delivery. The delivery is in a constant current mode. This burst-type stimulation is free of paresthesia in over 85 % of patients and has shown statistically significant improvement in axial back pain when compared with conventional tonic stimulation. The ability to go back and forth from tonic SCS to burst SCS may lead to lower failure rates and improved long-term cost efficiency.
22.3.3 Closed-Loop Feedback Systems
The use of closed-loop neurostimulation has been studied in the treatment of seizure disorders, and now is being applied to SCS and the treatment of pain. These results have been promising and have shown improved efficacy and outcomes. In the treatment of pain, researchers in Australia have used closed-loop systems to provide internal stimulation feedback that may enable the system to change its programming and response automatically, thus improving outcomes, reducing system variability, and improving patient pain relief and satisfaction.
22.3.4 Material Engineering
An engineering focus going forward will be on the materials used for leads, contacts, and wiring. New design concepts have included micro-texturing and new metal use. Goals of this engineering include MRI compatibility, improved efficiency, and the ability to deliver more complex programming. Part of this materials engineering is also important for the ability to make more complex arrays while simultaneously limiting battery size and improving energy efficiency.
22.3.5 Current and Future Neuroanatomical Targets
In the current field of neurostimulation, common targets are the dorsal columns of the spinal cord, the peripheral nerves, and the brain, with targets in the cortex, the thalamus, and other deep brain structures. Table 22.2 examines current targets and newly approved and clinically evolving targets.
Table 22.2
Traditional and selected evolving targets for neurostimulation
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