Spinal cord stimulation uses pulsed electrical energy near the spinal cord to manage pain. Initially, this technique applied pulsed energy in the intrathecal space. Presently, neuromodulation involves the implantation of leads in the epidural space. A similar principle utilizes the central nervous system and the peripheral nervous system stimulation in deep/cortical brain stimulation and peripheral nerve stimulation, respectively. Neurostimulation modalities arose as a response to treating the gate control theory of pain by Melzack and Wall. In summary, they posed that pain impulses provoked in the periphery, which are carried by C fibers and A-delta fibers, could be interrupted by stimulating larger A-beta fibers. This interruption is facilitated by the common nerve synapse location in the substantia gelatinosa of the dorsal horn. In other words, stimulation of the touch and vibration nerves “closes the gate” on ascending pain impulses that carry noxious pain stimuli cephalad. Multiple pain systems are responsible for the sensation of pain; these systems are composed of integrative neuronal sets (conduct excitatory or inhibitory signals on the nociceptors). The interrelation that exists among these three systems at all times is responsible for the perceived sensation of pain and the responses associated with it. First, nociceptors receive signals of noxious temperature, chemical, or mechanical stimuli (peripheral neurons). They send this information to second-order neurons located in the spinal cord, mainly in the dorsal horn (central pathways), which are then transmitted via projection neurons to the brainstem (integrative neurons).
Nociceptive fibers (peripheral pain receptors)
- Unmyelinated C fibers and lightly myelinated A-beta fibers (small nociceptive fibers, which conduct pain)
- Myelinated A-beta fibers (large non-nociceptive fibers, which conduct touch, pressure, and vibration)
- Central pathways (relay neuronal signals to higher brain structures)
- The primary integrative site in the brain is the thalamus, but other structures also participate in response to pain. Once the brain receives the pain signals, several reactions are generated almost immediately to modify and respond to these signals. These reactions include, but are not limited to, somatic and autonomic reflexes, negative or positive feedback to increase or reduce the pain, endocrine and emotional responses, cortical awareness, or the pain, as well as the memory of the event.
- The gate control theory of pain, mentioned above, is directly associated with these pain systems. It establishes that C fibers, A-delta fibers (nociceptive), and A-beta fibers (non-nociceptive) can all carry information from the injury site to two different cell types in the dorsal horn of the spinal cord, transmission cells, and inhibitory neurons. Both the nociceptive and non-nociceptive fibers can activate the transmission cells, opening the gate of signals sent to the brain. However, only the non-nociceptive fibers can activate the inhibitory cells, therefore closing the gate.
- Even though the gate control theory was the initial guiding mechanism of action, modern research has demonstrated that the underlying mechanisms are not clearly understood. There is evidence to suggest that dorsal column stimulation applies a different mechanism of analgesia when utilized for neuropathic pain versus ischemic pain. In neuropathic pain, evidence suggests that by altering local neurochemistry, stimulation suppresses hyperexcitability of the wide dynamic range neurons by increasing GABA and serotonin release, which suppresses levels of the excitatory cytokines glutamate and aspartate. On the other hand, the current belief is that ischemic pain alleviation occurs by alteration of sympathetic tone, achieved by restoring a favorable oxygen supply and demand balance.
Anatomy and Physiology
For the spinal cord stimulator leads to be introduced into the spinal cord, the epidural space needs to be accessed using an epidural needle. Therefore, there is relevant anatomy that merits consideration during this procedure.
- Vertebrae: Each vertebra is composed of a vertebral body (anterior) and a vertebral arch (posterior). The arch further divides into two lateral pedicles connected to two posterior laminae, a single spinous process, and two transverse spinous processes that extend laterally at the point where the pedicles connected to the laminae. The connection between two adjacent vertebrae at the level of the pedicles forms the foramina. There are 7 cervical, 12 thoracic, and 5 lumbar vertebrae, followed by 5 false or fixed vertebrae forming the bony sacrum and coccyx.
- Ligaments: After going through the skin and subcutaneous tissue, the first ligament encountered by the epidural needle is the supraspinous ligament, which connects one spinous process to another in adjacent vertebrae. The interspinous ligament then follows. The ligamentum flavum is much thicker and connects the lamina of adjacent vertebrae. Two other ligaments found on the anterior aspect of the spinal cord, are the anterior and posterior spinal ligaments, which connect adjacent vertebral bodies.
- Spinal cord: The spinal cord is surrounded by the dura mater (outermost layer), arachnoid mater, and pia mater (innermost layer, directly overlying the spinal cord). It extends from the medulla to the level of L1 in adults. At this level, one finds the conus medullaris. Below the conus, the spinal nerve roots become elongated and parallel, forming the cauda equina, which allows nerves to move freely within the CSF and makes this location preferable for the insertion of an epidural needle.
- Arterial supply: The arteries supplying the spinal cord derive from the vertebral arteries in the cervical spine, as well as the intercostal and lumbar arteries in the thoracic and lumbar spine. These arteries anastomose with other spinal cord vessels, forming the pial plexus. There are anterior and posterior branches, which supply the ventral and dorsal roots of the spinal cord. In the dorsal (sensory spinal cord), the posterior spinal arteries anastomose, protecting this area of the spinal cord from ischemia. On the other hand, there is a single anterior spinal artery that supplies the ventral (motor) spinal cord. One of the largest arteries supplying the anterior spinal cord is known as the artery of Adamkiewicz, which most commonly enters the vertebral canal through the L1 foramen. It supplies the lower two-thirds of the spinal cord. Damage to this artery from an improperly done epidural may lead to bilateral lower extremity paralysis.
- Venous supply: There is a vertebral venous plexus that drains into the vertebral canal. These veins empty into the azygos vein that ultimately empties into the superior vena cava (SVC). This plexus is of particular importance in patients with masses or increased intraabdominal pressure compressing the SVC. When this occurs, there is a backup of blood into the epidural space, which increased the risk of cannulating the veins with the epidural needle.
Enhancing Healthcare Team Outcomes
Spinal cord stimulation has proven its efficacy in refractory and difficult-to-treat pain syndromes. However, to enhance outcomes, it must also be cost-effective. In 2005, North and colleagues studied the cost-utility between SCS therapy and reoperation. The mean per-patient cost was $105928 for reoperation versus $48457 for SCS. Ultimately SCS was more effective and less expensive than reoperation in post-laminectomy syndrome patients. Another randomized controlled trial by North et al. determined that if SCS fails, reoperation is unlikely to succeed and should be discouraged.
Stimulation has shown a cost-benefit when compared to non-stimulation in the treatment of chronic back pain. Kumar et al. in 2002 followed 104 patients with failed back surgery syndrome, where 60 patients received implants. The control group (54 patients, non-stim) and the experimental group (60 patients, stim) were followed for five years. The average annual cost for the control group was $38000 versus $29000 for the stim group. The higher healthcare costs in the non-stimulator group were attributed to more medications, more follow-up visits, emergency center visits or hospitalizations, imaging (X-rays and MRIs), and rehab centers/physical therapy.
The interprofessional team is necessary for the best outcomes. Pain physicians and nurses, interventional radiologists, neurosurgeons, surgical nurses, radiology technicians, and pharmacists all participate in care. Nursing will provide followup and coordinate activities among other professionals and specialists with the surgeon. Pharmacists will oversee the patient's medication regimen, assist in preventing opioid misuse or dependence, and consult with the team regarding any potential drug interactions. The interprofessional approach will lead to improved patient outcomes. [Level 5]