Spinal Cord Stimulation for Peripheral Neuropathic pain

Recently, neuromodulatory treatments with a weak electric current, spinal cord stimulation, have been stated to be effective in properly selected subgroups of patients suffering from chronic neuropathic pain and other particular pain conditions. Neuropathic pain is a chronic condition, very impairing to quality of life, with usually modest relief by pharmacological treatments. Several studies have demonstrated that significant pain relief and improved functional ability are achievable after SCS. Recent developments of the stimulation methods, however, have yet to be appraised. Pain relief via SCS, first reported in 1967 in a case report, requires a battery-powered pulse generator associated with an epidural electrode. This is an effective treatment for selected patients with peripheral neuropathic pain, but it is also in use for complex regional pain syndrome, refractory angina pectoris, and ischemic pain from peripheral vascular disease.

Understanding Neuropathic Pain

Neuropathic pain is estimated to affect 7–10% of the population and is defined as pain caused by a lesion or disease affecting the somatosensory system. Anatomically corresponding to the site of injury or disease, there is pain distribution. Peripheral or central neuropathic pain, dependent on the affected part of the nervous system, may occur concerning pathogenesis. Sensory changes correspond to the affected neural structure and are usually found in the innervation territory. Autonomic and motor signs occasionally co-occur; imaging or neurophysiological examinations will then likely show concordant pathological findings.

Most patients with neuropathic pain do not receive disease-specific therapy. Though some antidepressants and gabapentinoids are known to produce analgesia, the majority either do not achieve an adequate response or bear intolerable side effects. Spinal cord stimulation is a very effective modality for rigorously selected patients with highly disabling peripheral neuropathic pain of significant impairment in functional and quality-of-life status.

Pain Conditions for SCS

Several pain conditions for which spinal cord stimulation is used are supported by scientific evidence and clinical experience. These include:

• Peripheral neuropathic pain
• Radiculopathy
• Polyneuropathy
• Peripheral nerve injury
• Stump pain after amputation
• Phantom limb pain
• Complex regional pain syndrome (Type 1 and Type 2)
• Refractory angina pectoris
• Specific peripheral vascular diseases

Patient Selection and Contraindications

To be considered suitable for spinal cord stimulation, a patient's pain must cause severe distress and persist for at least 3–6 months. The most common complaint in our department is painful radiculopathy. While TENS is not a dependable predictor of SCS, neuropathic pain often appears as chronic pain with a biopsychosocial component. Neurostimulation is offered only as an end-stage treatment at the Department of Pain Management and Research, Oslo University Hospital, after assessment by an interdisciplinary team. This may include medical, physical therapy, nursing, and psychology specialities, but is not limited to these.

Absolute contraindications to SCS involve the presence of coagulopathies, pregnancy, and infection at the site of implantation. Relative contraindications to implantation of an SCS include mental impairment, psychiatric disorders, and substance abuse that are significant enough to cause interference with treatment.

Trialing, Assessment, and Permanent Implantation

A permanent spinal cord stimulation system is typically preceded by implantation of the electrode percutaneously with an epidural needle and trialed for a week with an external pulse generator. Patients keep records of their pain intensity and ability to do personally selected activities on a patient-specific functional scale. About 70% of patients with peripheral neuropathic pain who meet the criteria for testing report sufficient improvement to be offered a permanent system.

In cases where degenerative changes exclude percutaneous placement, the electrode implantation can be performed by a neurosurgeon using microsurgical techniques. Severe complications are unlikely to happen; however, in such a case, their management would require close collaboration with the neurosurgical department.

Patients can choose between different stimulation programs using a handheld device. In general, the pulse generator has to be replaced after some years, but a rechargeable variant has to be discussed in very high consumption. Programs of intermittent stimulation can prolong the life of the pulse generator. Most modern systems are MRI compatible, although only some type of scanners or protocols enable this.

Conventional Spinal Cord Stimulation

Traditional tonic spinal cord stimulation involves continuous electrical stimulation at 40–100 Hz, which provokes projected paraesthesias corresponding to the regions where the dorsal columns are stimulated. It is not known how SCS achieves pain relief, but experimental studies suggest neurochemical changes in the dorsal horn of the spinal cord that are responsible for inhibiting the hyperactivity of the nociceptive system.

A 2016 systematic review of studies of traditional SCS for the treatment of neuropathic pain yielded four randomized controlled trials. Three of these concluded that six months of SCS were more effective at producing significant pain relief compared with conservative treatment for diabetic polyneuropathy and leg pain following lumbar spinal surgery. The fourth study concluded better pain relief from SCS than from reoperation for patients with leg pain after lumbar spinal surgery.

New Stimulation Methods

New stimulation methods represent developments either in the type of nerve or area stimulated or sometimes in the pattern of stimulation applied. Higher frequency stimulation patterns for SCS, for instance, include the more recent developments of burst and 10 K. These patterns impart more energy into the spinal cord without paraesthesia, enabling double-blind, placebo-controlled studies.

These high-frequency patterns of stimulation are above the maximum firing frequency of neurons—approximately 250 Hz—and their associated physiological effects are poorly understood, particularly in the absence of paraesthesias. On the other hand, the supraspinal effects of burst stimulation are postulated to reduce patients' attention to pain by modulating the emotional components of pain.

Randomised controlled trials in patients with predominantly leg and back pain following lumbar spinal surgery document that burst and 10 K stimulation are more effective than standard SCS. In one cross-over study, 60% of the patients had a clinically relevant pain reduction using the burst stimulation, compared with 51% using conventional tonic stimulation. Most patients preferred to keep receiving burst stimulation owing to better pain alleviation or absence of paraesthesias.

Complications

Percutaneous electrode implantation with subsequent spinal cord stimulation is a non-destructive treatment that has a low potential for major complications. The most frequent problems relate to the migration and breakage of the electrode. Less than 3% of patients in the past five years report superficial infections. Fewer than 1% of all cases involve serious complications like epidural infection or epidural hematoma.

Summary

Spinal cord stimulation is able to produce clinically significant analgesia with associated functional improvement for carefully selected patients with peripheral neuropathic pain. Ongoing advances in stimulation methods and pulse generators have brought about the possibility of greater personalization of treatment.

However, the scientific knowledge base for SCS remains limited. Mechanisms of action and factors predicting therapeutic efficacy are poorly understood; most randomized controlled trials were sponsored by industry and, at best, compared conventional SCS with medical management. Efficacy of newer stimulation methods has been investigated primarily in head-to-head studies versus conventional SCS. Placebo effects can be examined by sham-controlled studies using paraesthesia-free stimulation methods.

Better equipped with an understanding of the intricacies of spinal cord stimulation and its evolving methodologies, professionals will be well on their way to individualize therapies according to the needs of their patients and achieve better clinical outcomes in patients suffering from chronic neuropathic pain.