34.1 Introduction

The mechanistic basis of neuromodulation derives from the gate control pain theory, by which a non-noxious stimulus suppresses pain signaling. ¹ The understanding that pain pathways can be modified in this manner has altered the focus of chronic pain treatment. The publication of two seminal studies has led to reversible neuromodulation replacing ablation as a standard of care. Shealy et al., published their work on dorsal column or spinal cord stimulation in 1967. ² That same year Wall and Sweet introduced the concept of peripheral nerve stimulation (PNS). ³ Additional therapeutic modes that have since been studied are: deep brain stimulation (DBS), motor cortex stimulation (MCS), and intrathecal (IT) drug delivery. The technology behind neuromodulation continues to improve and the indications for its use have expanded and diversified.

34.2 Spinal Cord Stimulation

SCS is most effective in the treatment of neuropathic extremity pain. It is generally ineffective for nociceptive pain, except for ischemic injury pain. The benefit in ischemic pain may be from improved blood flow and not altered pain pathways. ⁴ SCS improves quality-of-life and pain levels in patients with failed back surgery syndrome, or complex regional pain syndrome. ^{5 , 6} Most studies of SCS are retrospective and show time related diminishing effectiveness. ⁷ Yet, at least 50% of patients remain satisfied with the quality of their pain relief. ⁸

SCS use in the treatment of chronic pain is FDA-approved in the United States. The stimulus source is either a percutaneously placed epidural lead or surgically-placed paddle (▶ Fig. 34.1). Standard stimulation is at a tonic frequency of 40–80 Hz. Paresthesia-free stimulation is a recently introduced concept in the field. It has the potential to improve the treatment of axial neuropathic pain and salvage patients who do not respond to conventional SCS. ^{9 , 10} Dorsal root ganglion stimulation also has the potential to provide targeted modulation in the treatment of focal neuropathic or nociceptive pain. ¹¹

34.3 Peripheral Nerve Stimulation

The most common PNS modality is percutaneous occipital nerve stimulation (ONS) for occipital neuralgia. ¹² Although most studies contain small patient populations without a control or comparison group, significant and sustained pain alleviation is consistently described. ¹³ Additional indications for the use of PNS include ONS for chronic migraine, sphenopalatine ganglion stimulation for cluster headaches, trigeminal neuralgia, post-stroke central pain and post-herpetic neuralgia. ^{14 , 15 , 16} Peripheral nerve field stimulation, or the targeting of a nondermatomal areas of pain with a subcutaneously inserted electrode has been described in the treatment of low back, chest and abdominal wall and joint pain. ¹⁶

PNS implantation is done by percutaneous insertion techniques that are relatively simple procedures with a low risk of injuring neurovascular structures and few absolute contraindications. There is great interest in the development of new such stimulation devices and to define appropriate and broad clinical indications. However, most PNS devices are not FDA-approved and are used off-label.