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CHAPTER 96  Spinal Cord Stimulation

own animal model using monophasic pulses. 60 Because the DCs are neither activated nor blocked by the HF SCS, the mecha­ nisms hypothesized may be segmental. In a rat study with bipha­ sic stimulation, Shechter et al. 64 compared 20%, 40%, and 80% of motor threshold (MT) as stimulation amplitudes. They used 50 Hz, 1 kHz, and 10 kHz applied for 30 minutes on 3 consecu­ tive days. The only amplitude producing relevant data was 40% of the MT. For this amplitude, some effects of SCS emerged over the 3 days, and actually, the frequency 1 kHz proved slightly better—or at least equally effective as the 10 kHz SCS. It must be remembered that clinical experiences demon­ strate that HF SCS ( . 800 Hz) might result in uncomfortable sensory experiences as soon as the amplitude is beyond the sen­ sory (paresthetic) threshold. 13 Very recent preclinical studies (unpublished data 65,66 ) using HF SCS (2 to10 kHz) where DC activation was also studied have yielded interesting results. Application of 10-kHz SCS in the rat through conductive agar from needle electrodes directly over the L5 segment demonstrated no evidence of a DC fiber conduction block nor activation. Stimulation for several hours did not induce asynchronous firing in myelinated primary sen­ sory neurons. In an inflammatory pain model producing more long-lasting pain, SCS using 20% MT for up to 135 minutes, which was verified to be subthreshold for activation of A b pro­ jection neurons, occurred after 45 to 90 minutes when com­ pared to control. The present view of HF SCS mechanisms may be summarized as follows: HF SCS must be applied with low amplitude—below paresthesia threshold; otherwise, it can be very uncomfortable. There seems to be neither activation nor block of the dorsal col­ umns with HF SCS. Although PW is short and amplitude low, HF delivers more energy, and thus, rechargeable or wireless de­ vices have to be used. The dorsal columns do not seem to be involved in the ef­ fect, 63,67 thus clearly distinguishing HF SCS from conventional SCS. As yet unpublished data indicate that HF SCS might

induce a slowly building up, inhibitory effect directly onto su­ perficial neurons in the DH. 65,66 BURST SPINAL CORD STIMULATION The use of irregular stimulation patterns including “burst stim­ ulation” originates from De Ridder’s work with cortical stim­ ulation, but “modulated stimulation” has been used earlier in the clinic. In the 1970s, burst transcutaneous electrical stimu­ lation (TENS) was launched as a variant to steady frequency TENS with very low “electro-acupuncture-like” frequency (1 to 5 Hz) as compared to normal TENS at about 100 Hz. 68 At this time, hypotheses about different mechanisms for burst and HF TENS were discussed, and the burst TENS was ap­ plied more for nociceptive pain types, and some data pointed to a possible mediation by release of endogenous opioids. As already mentioned, recent animal studies further indicate that the antinociceptive effect of low-frequency SCS may depend on opioid mechanisms. 29 One manufacturer marketed for a period during the 1980s an external stimulator that could give vari­ able stimulation patterns (“modulated SCS”), but it never be­ came popular, and the apparatus disappeared from the market. Burst SCS was presented as a stimulation mode, which would also be effective for the midline or axial low back pain com­ ponent of failed back surgery syndrome (FBSS). 69,70 De Ridder argues that bursts or irregular firing are similar to normal nerve activity and exert more prominent effects on supraspinal re­ lays (e.g., the thalamus, as “a wake-up call to the brain” to activate neurons). 71 De Ridder et al. 70 have, on the basis of “source localized electroencephalogram (EEG)” investigations of patients with burst SCS, advanced the idea that burst SCS could activate cortical areas involved in the modulation of pain perception (Fig. 96.3). In a very recent study, De Ridder and Vanneste 72 presented data from five patients undergoing tonic, burst, and sham stimulation. In a source-localized EEG subtraction and conjunction analysis, they showed that burst and tonic stimulation share activation of some cortical areas

Neuromodulation Nov 2015

FIGURE 96.3  Present hypotheses for mechanisms behind effects of burst stimulation of the spinal cord. Burst spinal cord stimulation (SCS) is hypothe- sized to especially modulate the activation of the medial (affective/attentional) pathway (right) . Conventional SCS more acts on the lateral spinothalamic tract which conveys information of nociception (strength; site). (Adapted from De Ridder D, Vanneste S. Burst and tonic spinal cord stimulation: different and common brain mechanisms. Neuromodulation 2016;19(1):47–59.)