Spinal Cord Stimulation

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Spinal cord stimulation (SCS) is a common therapeutic technique for treating medically refractory neuropathic back and other limb pain syndromes. The common technique used historically for placing SCS leads required direct interaction with the patient, combining a sedative anesthetic technique and awakening the patient during the surgery. This approach required the patient to be alert enough to respond to sensations generated by the stimulation and directions from the surgical team. However, these responses have been demonstrated to be unreliable or misleading for several reasons: 1) the potential wide variety of pain responses of patients during changing levels of sedation, 2) the variable responses to anesthetics, 3) the variable array of pain distributions, 4) the often inadequate ability of the patient to differentiate their pain syndrome from the effects of surgery or the feeling of the stimulation at the time of testing, 5) positional changes due to spinal cord (cord) movement, and 6) cord location relative to the lead (the actual device containing all of the electrodes or contacts) in only the prone position.1, 2, 3, 4, 5 Newer multicontact designs of paddle leads have improved the ability to capture pain relief even when the lead has not been ideally positioned or moves slightly during surgery. However, these new designs still require the lead to be placed in an appropriate mediolateral position relative to the specific morphology of the dorsal column (DC) fibers and entering dorsal nerve roots. The center of the cord may be more than 2 mm from the canal center in 40% of patients,5, 6 especially when using fluoroscopy due to parallax and visual-alignment errors. The cord itself may also be rotated slightly, making one side of the DC closer to the electrode even if it is located perfectly along the midline.

It is critical that leads be placed at the appropriate cranial-caudal spine level in order to maximize the desired pain coverage. However, the cranial-caudal position is easier to locate given the use of trial lead information, the known segmental dermatomal distributions, and the length of the leads that can cover two or three vertebral levels. As a rule of thumb, lower back coverage is best obtained at T8, buttock and leg coverage at T9 or T10, and foot coverage below T10. Nonetheless, mediolateral electrode optimization remains very important for maintaining coverage once the lead is covered in an unknown amount of fibrosis, which changes the electrical characteristics of the surrounding tissue to an extent that often requires reprogramming.

Additionally, both sedative and awake procedures in prone patients have notable risks of complications. The risk of losing airway patency is high in prone patients who are overse-dated. More sedation is typically needed for tunneling and the creation of a pocket for the implantable pulse generator if the patient has only been sedated for the initial incision and dissection to the epidural space. Determining the optimal trade-off between patient comfort and oversedation is not always straightforward, and in rare situations the surgical wound will need to be packed on an emergency basis and the patient immediately returned to a supine position to allow intubation. Some data from analyses of the closed claims data of anesthesia cases suggest that the risk of severe respiratory depression with brain damage or even death could be as high as 6%.7

The use of neurophysiological mapping techniques allows these procedures to be performed under general anesthesia, which eliminates all of the above concerns. Two primary mapping techniques are currently used when placing SCS leads. The first technique is called compound muscle action potential (CMAP) activation, and is based on the antidromic activation of alpha motor neurons (MNs) (light green arrows in Fig. 1) through stimulation of the large Ia fibers of the DC.8, 9, 10, 11, 12, 13, 14, 15 This stimulation is applied via the electrode itself, and antidromically depolarizes the MN to induce a CMAP in the muscle innervated by that motor unit (Fig. 2). It is important to note that the stimulation needed to generate the CMAP response typically has a slightly higher intensity than that normally used postsurgery for pain therapy. This increased stimulation intensity is necessary to overcome the effects of anesthesia at the MN synapse and because these Ia fibers only constitute over 4,000 afferent synapses to the MN,16 while modeling and neurophysiological collision studies have demonstrated that the stimulation is in the DCs.8