A total number of 19 patients participated in the study. The mean age was 53 years (range: 25–80) and the patients were using DRG-S for a mean of 17.2 months (range: 4–102). The most common pain etiology was chronic regional pain syndrome (CRPS) (7 subjects), followed by postsurgical pain after implantation of joint prosthesis (4), postherpetic neuralgia (3), nerve injury after resection of neurinomas (2), traumatic nerve injury (2), and diabetic polyneuropathy (1). Fourteen patients had a PainDetect Score of 12 or higher (76.7%), indicating higher probability of neuropathic pain. Patients reported a mean VAS of 8.6 (SD 1.0) before the implantation of the DRG-S system and a mean baseline VAS of 3.9 (SD 1.9). All patients had already been programmed in the clinical routine and had reached a stable therapeutic response. All patients had a stimulation frequency of 20 Hz at study start.
Even at subthreshold level with corrected amplitude, some patients experienced at higher frequencies a change in the paresthesia field. Amplitude was reduced in these cases. No patient had painful paresthesia nor motor stimulation.
Results for mean VAS for 20 Hz, 40 Hz, 60 Hz, 80 Hz, and sham stimulation were 3.7 (SD 1.9), 4.9 (SD 2.2), 5.8 (SD 1.9), 5.8 (SD 1.9), and 8.6 (SD 1.3) respectively (Table 1). 20 Hz achieved significantly lower pain intensity than 40 Hz (p = 0.004) and any other tested stimulation parameters (p < 0.001). 40 Hz did not result in significantly better results than 60 Hz (p = 0.086), nor did 60 Hz have lower pain intensities than 80 Hz (p = 0.695) (Fig. 1). Although the overall trend and statistics favor lower stimulation frequencies, two patients preferred higher stimulation frequencies and reported better pain control. In both cases, amplitude remained at the necessary level for subthreshold stimulation.
The same trend was seen with the McGill Pain Questionnaire, which resulted in 30.8 (SD 15.8), 33.1 (SD 17.3), 35.9 (SD 16.9), 36.3 (SD 14.2), and 46.5 (SD 17.2) points. In this case, statistical significance was only achieved when comparing MPQ results of 20 Hz and 80 Hz (p = 0.047). When analyzing quality of life, EQ-5D indexes were 0.76 (SD 0.16), 0.69 (SD 0.26), 0.59 (SD 0.30), 0.58 (SD 0.30), and 0.24 (SD 0.37). The index for 20 Hz was not significantly higher than for 40 Hz (p = 0.071), but than for 60 Hz and 80 Hz (p = 0.001).
Beck Depression Inventory resulted for the same groups 9.9 (SD 7.8), 10.8 (SD 7.1), 11.9 (SD 8.9), 13.6 (SD 8.7), and 15.5 (SD 10.2) points. Under 20 Hz, BDI was not significantly lower than under 40 Hz (p = 0.19), but under 60 Hz (p = 0.033) and 80 Hz (p = 0.005). Table 2 shows comprehensive data with the mean difference and statistical significance.
Although only assessed in a very basic fashion (increase in medication yes/no), the lowest number of patients reported an increased need for analgesic medication during 20 Hz stimulation (9 subjects), and 13 patients referred increased analgesics intake during 40 Hz stimulation and 16 subjects under 60 Hz and 80 Hz, whereas all 19 patients reported an increase during sham stimulation.
When stratified by PainDetect, a higher overall VAS and a higher mean difference in the VAS between stimulation frequencies were observed in the patients with a score > 12 without reaching statistical significance. The overall observation regarding better pain control with lower frequencies was still observed.