This article presents two considerations. First, since SCI-related symptoms may include a peripheral component, SCS may help improve intractable pain. Second, sub-perception SCS using 1-kHz stimuli may ameliorate neuropathic pain, even if the SCS electrode cannot be placed in the appropriate space.
According to the International Spinal Cord Injury Pain Basic Data Set classification, SCI-related neuropathic pain can imply an at-level or below-level neurological level of injury (NLI) [2, 3]. At-level neuropathic pain is defined as pain within a region spanning one dermatome rostral and three dermatomes caudal to the NLI, while below-level refers to neuropathic pain located more than three dermatomes below the NLI. Although it remains unclear whether at-level and below-level pain share identical mechanisms or have different etiologies, it is conceivable that at-level pain results from damage to roots and nerves at or around the injury level  while below-level pain has been hypothesized to be induced by interruption of ascending sensory tracts or development of phantom pain in deafferented regions. Patients with at-level pain experience pain in response to tactile stimuli about twice as frequently as patients with below-level pain . Thus, peripheral nerves may be more involved in at-level, rather than below-level, pain. Although we do not know to what extent at- or below-level SCI pain is associated with peripheral nerves, it is possible to determine whether the SCI pain includes peripheral components or not using peripheral nerve blocks. In our patient, the severe pain in the bilateral upper extremities—approximately three dermatomes below the C4 NLI—was highly suggestive of at-level neuropathic pain. The temporary symptom improvement conveyed through the use of an epidural block implied a peripheral component. Although SCI-related pain is considered as central neuropathic pain, our patient’s symptoms may have resulted from a combination of factors, including peripheral nerve involvement.
SCS is a low-risk and cost-effective treatment that produces significant neuropathic pain relief, especially in patients with post-lumbar surgery syndrome (PLSS) and complex regional pain syndrome [8, 9]. Published studies and evidence-based guidelines recommend SCS for peripheral neuropathic pain and ischemic pain rather than central pain. Moreover, patients with SCI might respond to SCS for pain at the injury level as opposed to diffuse pain below the level of injury .
SCS improves pain through modifying the stimulation parameters (i.e., electrode position, frequency, amplitude, and pulse width). Tonic SCS has been achieved with about 40- to 80-Hz frequency and a range of 200- to 500-μs pulse width for pain relief and comfortable paresthesia, applied in accordance with gate control theory . Tonic SCS delivers mild electrical pulses and stimulates Aβ fibers in the dorsal column, subsequently producing an analgesic effect via two routes. Antidromic stimulation of Aβ fibers leads to the modulation of GABAergic interneurons in the dorsal horn, whereas orthodromic stimulation of Aβ fibers activates the descending pain modulation system (DPMS) in the supraspinal areas. However, despite several mechanisms, in tonic SCS, it is considered that paresthesia overlap in the painful area is essential for ameliorating pain symptoms . Recently, several programming approaches, known as sub-perception or high-dose SCS, to increase charge delivery involving kHz frequency and burst have been proposed to provide further pain control. These programs may significantly alleviate neuropathic pain without paresthesia overlap [12, 13]. Several clinical studies have reported that 1-kHz high-frequency SCS for patients with chronic back and leg pain provided improvement in pain relief, QOL, and patient satisfaction [14,15,16]. A study using functional MRI in patients with PLSS indicated that high-dose SCS may modulate the brain and brainstem regions of the DPMS . Moreover, in a study using functional MRI, De Groote et al. reported that 10-kHz high-frequency SCS affected pain awareness through involving the dorsolateral prefrontal cortex and the right anterior insula . When a supraspinal effect is one of the important mechanisms of sub-perception SCS, it may be more effective for relieving SCI pain than tonic SCS because many patients with SCI have epidural adhesions due to a past history of spinal surgery. No preclinical or clinical study has examined the efficacy of sub-perception SCS on SCI pain. Therefore, its utility and indications require further investigation using randomized controlled trials.
In conclusion, 1-kHz high-frequency SCS relieved SCI-related chronic refractory pain of our patient. When SCI-related symptoms include a peripheral component, SCS may have a notable effect on intractable pain. Furthermore, even in cases where the SCS electrode leads cannot reach the target area, the use of sub-perception SCS is advisable and should be considered.