Is Constant Current or Constant Voltage Spinal Cord Stimulation Superior for the Suppression of Nociceptive Visceral and Somatic Stimuli? A Rat Model

Spinal cord stimulation programming: a crash course

The aim of this comprehensive review is to provide an instructional guide for providers regarding the parameters and programming of spinal cord stimulation (SCS) devices. Knowing these fundamentals will aid in providing superior pain relief to patients. SCS has four programmable parameters: contact (electrode) selection, amplitude, pulse width, and frequency. Each parameter needs to be accounted for when assessing which program works for which patient. Traditional open-loop systems allow for different "programs," or combinations of these four parameters, to be pre-set by the provider and medical device representative. These allow for flexibility in the type of stimulation delivered to the patient depending on activity. Patients are also given control over programs and changing the amplitudes of these programs. However, some open-loop systems place the burden of toggling between programs to manage pain control on patients, though this tends to be less in subparesthesia programs. Newer closed-loop systems make it possible for stimulation settings to automatically adjust in response to accelerometry and evoked compound action potential feedback, and therefore have the potential to streamline the patient experience. This article provides practitioners with the basic knowledge of SCS parameters and programming systems. Understanding their use is essential to providing optimal pain relief to patients.

Spinal Cord Stimulation With "Conventional Clinical" and Higher Frequencies on Activity and Responses of Spinal Neurons to Noxious Stimuli: An Animal Study

OBJECTIVES

Spinal cord stimulation (SCS) at both conventional and higher frequencies may effectively reduce pain, but optimal parameters need to be established. This study investigated how SCS at different frequencies and pulse widths acutely modulates nociceptive activity of wide dynamic range (WDR) and high threshold (HT) dorsal horn neurons in rats at a stimulus amplitude that influences both local circuits and dorsal column fibers.

MATERIALS AND METHODS

L₂ -L₃ and L₆ -S₂ spinal segments were exposed for SCS and spinal neuronal recordings, respectively. Responses to pinch of a hindpaw were recorded before and after SCS (40 or 200 µsec pulse width at 50, 500, 1 kHz and 10 kHz, amplitude: 90% of motor threshold) for 5 or 20 min. Pinch responses were tested within 30 s after SCS ceased (first pinch) and at ∼4 min intervals until response recovery.

RESULTS

1) SCS for 5 min suppressed averaged first pinch responses, except for 40 µsec/50 Hz. 2) Only SCS with 40 µs/1 kHz suppressed more spinal neurons than 200 µsec/50 Hz. 3) All SCS parameters at 5 min increased pinch responses for a small population of cells, with the incidence being greater for WDR than for HT neurons. 4) SCS at 1 kHz (40 or 200 µsec) for 20 min reduced the response to the second pinch as compared with baseline responses. In addition, no neurons exhibited increased pinch responses.

CONCLUSIONS

Compared with a typical low frequency SCS (200 µs/50 Hz) or high-frequency SCS at 10 kHz, at an amplitude designed to influence both local spinal circuits and dorsal column fiber tracts, 1 kHz SCS suppressed nociceptive responses of more spinal neurons and/or demonstrated longer persisting suppressive effects. SCS at 1 kHz surpassed both low-frequency (50 Hz) and high-frequency (10 kHz) SCS application in this normal animal model.

High-Frequency (1 kHz) Spinal Cord Stimulation-Is Pulse Shape Crucial for the Efficacy? A Pilot Study

OBJECTIVES

Conflicting data regarding the efficacy of high-frequency spinal cord stimulation (HF SCS) has prompted the issue of the possible importance of the shape of the stimulating pulses. The aim of this pilot study was to compare HF SCS applied with monophasic and biphasic pulses of two different durations with conventional SCS in a rat model of neuropathic pain.

MATERIALS AND METHODS

Rats were operated with lesions of sciatic nerve branches according to the spared nerve injury procedure (SNI). Animals, which developed pathological tactile hypersensitivity after surgery, were implanted with four-polar miniature SCS leads. SCS was applied during 60 min with either conventional current parameters (monophasic pulse width [PW]: 200 μsec; 50 Hz and amplitude 80% of the motor threshold [MT]), or with high-frequency SCS (1 kHz) with monophasic or biphasic pulses, the latter with pulse widths of either 24 (12 + 12) or 48 (24 + 24) μsec. The outcomes were examined regarding change of tactile hypersensitivity during the one-hour SCS period and with two tests of thermal sensitivity.

RESULTS

Conventional monophasic SCS, as well as HF SCS applied with monophasic PW = 24 μsec or with biphasic PW = 48 (24 + 24) μsec, had similar suppressive effects on tactile hypersensitivity. Solely, HF SCS applied with biphasic pulses with a total PW of 24 (12 + 12) μsec demonstrated no effect. Thermal hypersensitivity was unaffected by HF SCS with all pulse varieties.

CONCLUSIONS

There is no significant difference in efficacy between HF SCS applied with low amplitude ("subparesthetic") monophasic and biphasic pulses. However, short PWs providing only 12 μsec of cathodal stimulation was ineffective, presumably because of insufficient electric charge transfer from the lead contacts to the nervous tissue.

Burst and Tonic Spinal Cord Stimulation Differentially Activate GABAergic Mechanisms to Attenuate Pain in a Rat Model of Cervical Radiculopathy

OBJECTIVE

Spinal cord stimulation (SCS) is widely used to treat neuropathic pain. Burst SCS, an alternative mode of stimulation, reduces neuropathic pain without paresthesia. However, the effects and mechanisms of burst SCS have not been compared to conventional tonic SCS in controlled investigations. This study compares the attenuation of spinal neuronal activity and tactile allodynia, and the role of γ-aminobutyric acid (GABA) signaling during burst or tonic SCS in a rat model of cervical radiculopathy.

METHODS

The effects of burst and tonic SCS were compared by recording neuronal firing before and after each mode of stimulation at day 7 following a painful cervical nerve root compression. Neuronal firing was also recorded before and after burst and tonic SCS in the presence of the GABAB receptor antagonist, CGP35348.

RESULTS

Burst and tonic SCS both reduce neuronal firing. The effect of tonic SCS, but not burst SCS, is blocked by CGP35348. In a separate study, spinal cord stimulators were implanted to deliver burst or tonic SCS beginning on day 4 after painful nerve root compression; allodynia and serum GABA concentration were measured through day 14. Burst and tonic SCS both reduce allodynia. Tonic SCS attenuates injury-induced decreases in serum GABA, but GABA remains decreased from baseline during burst SCS.

CONCLUSION AND SIGNIFICANCE

Together, these studies suggest that burst SCS does not act via spinal GABAergic mechanisms, despite its attenuation of spinal hyperexcitability and allodynia similar to that of tonic SCS; understanding other potential spinal inhibitory mechanisms may lead to enhanced analgesia during burst stimulation.

Comparison of burst and tonic spinal cord stimulation on spinal neural processing in an animal model

OBJECTIVES

Spinal cord stimulation (SCS) using bursts of pulses suppressed neuropathic pain as well or better than tonic stimulation and limited the incidences of parasthesias. The present translational study explored possible differences in mechanisms of burst and tonic SCS on nociceptive spinal networks and/or the gracile nucleus supraspinal relay.

MATERIALS AND METHODS

Visceromotor reflexes (VMRs, a nociceptive response) or extracellular activity of either L6-S2 spinal neurons or gracile nucleus neurons were recorded during noxious somatic stimulation (pinching) and visceral stimulation (colorectal distension [CRD]) in anesthetized rats. A stimulating (unipolar, ball) electrode at L2-L3 delivered 40 Hz burst or tonic SCS at different intensities relative to motor threshold (MT).

RESULTS

Average MTs for burst SCS were significantly lower than for tonic SCS. Burst SCS reduced the VMR more than tonic SCS. After high-intensity SCS (90% MT), spinal neuronal responses to CRD and pinch were reduced similarly for burst and tonic SCS. At low-intensity SCS (60% MT), only burst SCS significantly decreased the nociceptive somatic response. Tonic but not burst SCS significantly increased spontaneous activity of neurons in the gracile nucleus.

CONCLUSION

Based on the clinically relevant burst versus tonic parameters used in this study, burst SCS is more efficacious than tonic SCS in attenuating visceral nociception. Burst and tonic SCS also suppress lumbosacral neuronal responses to noxious somatic and visceral stimuli; however, burst SCS has a greater inhibitory effect on the neuronal response to noxious somatic stimuli than to noxious visceral stimuli. Reduced or abolished paresthesia in patients may be due in part to burst SCS not increasing spontaneous activity of neurons in the gracile nucleus.