New stimulation methods
In recent years, higher frequency stimulation patterns have increasingly been used for spinal cord stimulation, including burst (500 Hz intermittent) and 10 K (10 000 Hz continuous). Both result in greater energy transfer to the spinal cord without the patient experiencing paraesthesias, thereby enabling double-blind, placebo-controlled studies to be performed.
These types of stimulation exceed the neurons' maximum firing frequency of around 250 Hz (14), and knowledge of the associated physiological effects is limited, especially in light of the absence of paraesthesias. Burst stimulation may also result in supraspinal effects, including changes in the emotional components of pain and a reduction in patients' attention to pain (15).
In randomised controlled trials including mostly patients with leg and back pain following lumbar spinal surgery, both burst and 10 K stimulation showed superior efficacy to conventional spinal cord stimulation (16, 17). One of the trials was a crossover study in which patients were randomised to receive three months of burst followed by three months of conventional stimulation, or vice versa (16). With burst stimulation, 60 % of patients reported a reduction in pain intensity of at least 30 % compared to baseline, a reduction that is considered clinically significant. With conventional tonic stimulation, 51 % of patients reported a similar effect. In total, 69 % of patients described a clinically significant reduction in pain with one or both of the stimulation methods. After the study period, most patients opted to continue with burst stimulation, either because it provided better pain relief or because it did not induce paraesthesias. The Oswestry Disability Index revealed a statistically significant improvement in functioning with both burst and conventional stimulation, with no difference between the two.
Although the scientific justification for using spinal cord stimulation to treat neuropathic pain is based largely on studies of patients with either leg/back pain after lumbar spinal surgery or with painful diabetic polyneuropathy, it is standard practice to offer the treatment to patients with peripheral neuropathic pain of other origins as well (8).
Stimulation of the dorsal root ganglion (DRG stimulation) can also be an option in some cases, e.g. for patients with neuropathic pain in the groin (18), where it can be difficult to achieve an effect with spinal cord stimulation.
Because the new stimulation methods are often simply compared directly to conventional spinal cord stimulation in head-to-head studies, the scientific justification for the new protocols remains limited. Patients may also have high expectations of technologically advanced treatments, leading to placebo effects (19). A systematic review from 2020 examined eight randomised studies in which paraesthesia-free spinal cord stimulation was compared to placebo/sham treatment (20). A meta-analysis found that active treatment led to a statistically significant reduction in pain intensity of 1.15 points on a 10-point scale.