Intraoperative physiology techniques to inform targeting

Background

Spinal cord stimulation (SCS) has been employed for the treatment of intractable pain most commonly for failed back surgery syndrome and complex regional pain syndromes. It is generally accepted that for successful SCS treatment there is the superposition of SCS-induced paresthesias overlapping the regions of perceived pain. This has become true for both paresthesia-based and paresthesia-free stimulation as physiologic placement is paramount. This is especially true when utilizing traditional tonic stimulation which has become an option on each SCS system.

The most straightforward way to confirm pain-paresthesia during lead implantation is via verbal feedback from the conscious patient. In this way, the implanter can achieve optimal lead placement by adjusting the lead location within the epidural space based on the patient's report of perceived paresthesia. However, there is stress/discomfort with awake procedures, there is risk of oversedation in a prone nonintubated patient, and sometimes it is not possible to perform interventions in an awake patient. Although there is a common acceptance within the field for awake placement, there is no published data specifically investigating awake placement. Its general acceptance was largely in part because an awake patient gives you a marker of safety, and confirmation of lead positioning in the placement of spinal cord stimulators. The difficulty also arises in that noncooperative patients for awake procedures are often sedated which removes the ability to have these two factors.

For these reasons, there is the option of placing the patient under general anesthesia; this may be especially true for surgical lead placement because a laminotomy is required, but also for percutaneous placement as well. In this scenario, it is imperative to have a method of cord protection for which neuromonitoring has been widely accepted in spinal surgery. Asleep lead placements, however, do not allow for verbal feedback from the patient during the procedure and could therefore contribute to suboptimal lead placement. It has been shown that anatomic midline does not correlate with physiologic midline at least 40% of the time. It has also been shown that placing a paddle lead without mapping will lead to revisions up to 20% of the time. This number is likely higher with percutaneous leads given the smaller range of coverage. Given these findings, protocols have been established to use neuromonitoring in asleep patients to also confirm lead placement and perform physiological mapping. Initial research and publications were performed over 10 years ago and have been improved upon. At present, there are numerous retrospective and prospective studies demonstrating its efficacy, and in several studies superiority to awake placements. In addition, protocols have been established and published. Perhaps the most landmark study was a prospective multicenter study directly comparing awake to asleep placement with neuromonitoring (“NAPS”) that demonstrated the use of neuromonitoring improved time efficiency by 25%, had superior paraesthesia coverage, and had a fifth of the adverse events.

The Neurostimulation Appropriateness Consensus Committee (NACC) guidelines were revised in 2017 commenting on confirmation of correct lead placement being advocated with either awake intraoperative confirmation of paresthesia coverage or use of neuromonitoring in asleep placement, such as EMG responses or SSEP collision testing. The observation of compound motor action potentials (CMAPs) or somatosensory-evoked potentials (SSEPs) within the painful dermatome(s) in response to intraoperative SCS can be used as a proxy for verbal confirmation of paresthesia coverage. CMAPs use myotomal coverage as a marker for dermatomal coverage. The NACC guidelines, combined with published evidence, have led to an increase in its use that not only includes surgeons but also interventional pain physicians.