Electrical Nerve Stimulation for Chronic Median Nerve Pain

Fig. 30.1

Diagram of the gate control theory proposed by Melzack and Wall. The T cell is a neuron with ascending fibers in the spinothalamic, spinoreticular, and spinomesencephalic tracts. The SG cell is a neuron in the substantia gelatinosa of the dorsal horn. Impulses from large fibers excite SG neurons and T cells. In contrast, impulses from small fibers have an inhibitory effect on SG neurons and an excitatory effect on SG cells. SG cell activity has an inhibitory effect on both large and small fibers

Reynolds reported that electrical stimulation of the periaqueductal gray region of the mesencephalon produces analgesia in rats and Mayer suggested that the mechanism of stimulation-produced analgesia might be similar to one of administrations of morphine [6, 7]. Han et al. showed that electrical nerve stimulation increases the concentrations of β-endorphins and met-enkephalin in blood and cerebrospinal fluid [8]. These researchers also found that administration of a δ-opioid receptor antagonist into the rostroventromedial medulla inhibits the pain relief provided by electrical nerve stimulation [9]. Thus, electrical nerve stimulation may enhance the release of endogenous opioids in the spinal cord as well as activating descending pain inhibitory pathways.

Electrical stimulation may provide pain relief through other mechanisms as well. For example, Linderoth reported that electrical stimulation on the dorsal column induced serotonin and substance P release and pain relief in cats [10]. Sluka and colleagues reported that high-frequency electrical stimulation reduces the expression and secretion of excitatory neurotransmitters, such as glutamate, in the spinal cord in rats [11]. In other studies, Radhakrishnan et al. showed that several types of spinal 5-HT receptor modulated the effect of pain relief induced by TENS, and Sluka and coworkers found that spinal serotonin concentrations increased during and immediately after treatment with low-frequency TENS [12, 13]. Currently, these mechanisms are considered to be part of the more broadly defined gate control theory.

Indications for Peripheral Nerve Stimulation (PNS)

There are many indications for electrical nerve stimulation therapy, including the following:

1.

Nerve injury (traumatic or iatrogenic)
2.

Mononeuropathy (idiopathic, entrapment)
3.

Postsurgical neuropathic pain (post-carpal tunnel release, neurolysis, nerve graft, etc.)
4.

Complex regional pain syndrome
5.

Plexopathy

Clinicians should examine whether temporary partial or complete analgesia is achieved by xylocaine block of the affected nerves. When the block provides pain relief, electrical nerve stimulation can be expected to have an analgesic effect as well. TENS does not have any predictive value for PNS, even for patients that failed to experience pain relief with TENS [14].

Contraindications

Patients with a number of medical conditions should not undergo electrical nerve stimulation.

These include the following:

1.

Any pain of unknown etiology
2.

Active infections
3.

Any debilitating illness
4.

Previous electrical accident
5.

Cardiac pacemakers
6.

Cardiovascular disease
7.

Cerebrovascular disease
8.

Pregnancy
9.

Epilepsy
10.

Metallic implants beneath electrode positioning sites
11.

Communication difficulties (including younger children and those with psychiatric disorders)
12.

Patients considered otherwise inappropriate candidates by the physician

Furthermore, physicians must consider whether patients with an abnormal coagulation profile or those taking anticoagulants are suitable candidates for treatment.

Complications

Patients undergoing PNS occasionally need revision or removal surgery owing to adverse events, as follows [15–18]:

1.

Infection (3.6–17.9%)
2.

Migration of the electrodes (9–25%)
3.

Leads or device failure (3.6%)
4.

Hardware irritation and discomfort (4.5–50%)
5.

Postoperative perineural fibrosis (frequency unknown)

Technique

It is recommended that clinicians perform a two-stage surgical protocol for PNS. The first stage is intended to determine whether chronic pain can be alleviated by electrical stimulation therapy.

First Stage of Surgery

Under general anesthesia or regional block, the affected nerve is exposed and neurolysis is performed. Subsequently, the electrodes are placed proximal to the apparent or suggested site of the lesion (Fig. 30.2). Although a percutaneous procedure to locate electrodes can also be used, the complication of electrode migration occurs at a substantially high frequency with this procedure. Furthermore, the direct placement of the electrodes can reduce power requirements for stimulating the nerve. Electrodes should be sutured securely to the fascia adjacent to the nerve to prevent migration. The proximal tip of the leads connected to the electrodes can be passed subcutaneously through a small incision in the axillary area. The leads can be connected to an extracorporeal electrical stimulator. After this first stage of surgery, electrical stimulation is performed with different parameter settings (pulse width, amplitude, frequency) to determine the optimal conditions for pain relief. It is recommended that technicians follow the protocols provided by the manufacturers of the electrical stimulating equipment (e.g., Resume, Medtronic, Minneapolis, MN). This procedure can be carried out 1 week to a month prior to the second stage. When the effectiveness of electrical stimulation is confirmed in the patient, the surgery to embed the implantable stimulator, the second stage of the operation, can be performed. In the case of no significant improvement, and in cases where the patient does not consent to further treatment, the electrodes should be extracted.

Fig. 30.2

Photograph showing placement of a sheet-type electrode underneath the affected median nerve during surgery. This photograph is taken from the paper by Mirone et al. [19]

Second Stage of Surgery

Under general anesthesia, the externalized leads are replaced with new sterile leads to mitigate the risk of infection. The original electrode placement site should be left undisturbed. The new leads are passed subcutaneously from the original incision site in the axial area to a prepared subcutaneous pocket with a small incision in the anterior chest wall. An implanted stimulator can be embedded into the pocket and connected to the leads. The stimulator can be turned on or off and its settings can be adjusted by remote control. A well-trained physician or medical technician can adjust the stimulator settings and instruct the patient to continue the electrical stimulation therapy and to turn the unit off when not needed to preserve battery life and to help maintain the effectiveness of treatment.

Clinical Efficacy of Electrical Nerve Stimulation

Physicians have two options for electrical nerve stimulation therapy. The first is the transcutaneous method (TENS), and the second is direct stimulation (PNS).

Cochrane’s review (2008) concluded that there is insufficient clinical evidence to recommend therapy with TENS. The more than 100 reports on TENS reviewed in Cochrane’s paper varied substantially in the parameters used for stimulation therapy. This may in part underlie the lack of evidence for the effectiveness of electric nerve stimulation using TENS [20]. However, it may be premature to conclude that TENS has no clinical efficacy, particularly because of the lack of uniform conditions, for instance, stimulation setting or patient selection.

As mentioned previously, an important criterion for the use of PNS is whether pain relief is obtained by a local block. PNS can be an effective treatment in appropriate patients. There are very few reports that focus exclusively on the use of PNS for chronic median nerve pain . Only Deer et al. reported prospective study to evaluate the effect of a new type implantable device named StimRouter (Bioness Inc., Valencia, CA) for the patients with chronic pain due to carpal tunnel syndrome. The study lasted for a very short period because of trial study and included eight patients wherein surgery was performed to insert the electrode and who underwent daily stimulation for 5 days. The patients presented decreasing pain intensity and opioid consumption during the therapy. After explant of the device and terminated the PNS treatment, the pain intensity increased back, but mean study satisfaction score among all of the patients was 96% [21].

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