History and Pathophysiology

Electricity has been used to modulate pain since before the era of modern medicine, through various basic means. The Egyptians were already using eels, catfish, or torpedo fish, which generated electrical discharges for the treatment of various medical conditions. Romans were prescribed contact with a living torpedo fish for analgesia, with one species capable of voltages as high as 220 volts. With the publication of this book, the field of PNS in the modern era is officially 50 years old, following the original work of Wall and Sweet in 1967. They tested a small group of patients after first testing infraorbital nerve stimulation on themselves. This first use of PNS was based on suppositions from the gate control theory, wherein large myelinated fiber stimulation might block transmission of smaller unmyelinated pain fiber transmission to the central nervous system. Despite advances in the understanding of pain pathophysiology since that time, there is no current unifying theory of how neuromodulation affects chronic pain. In addition to gate control, suppressed firing of neuromas, selective modulation of pain-neurotransmitters, and spinal/supraspinal descending modulation are all possible. Prolonged analgesia (minutes to hours) may occur after 30-minute applications of balanced-charge kilohertz frequency alternating current (KHFAC) that may lead to a true nerve conduction block wherein action potentials under the blocking electrode are desensitized (action potential progression is arrested). More research is required to better understand the mechanisms of “standard frequency PNS” as compared with KHFAC.

Evidence

In a prospective multicenter, randomized, double-blinded, partial crossover study performed by Deer and colleagues a new technology that uses a flexible percutaneously inserted electrode using ultrasound (US) guidance was examined. The efficacy and safety of this new PNS device (StimRouter, Bioness) was demonstrated and is now US Food and Drug Administration (FDA) approved to treat patients with intractable neuropathic chronic pain of peripheral origin. One exciting use of this technology is for axillary nerve stimulation in chronic poststroke shoulder pain (see Fig. 74.1 , Bioness StimRouter). In another trial of PNS, an experimental feasibility study for a novel KHFAC device was tested on lower extremity postamputation pain. The trial tested various domains of pain, function, medication use, and patient satisfaction and suggested evidence of both safety and efficacy for both residual limb pain and phantom limb pain. This trial demonstrated the exciting finding of an enduring analgesic effect, persisting despite cessation of the stimulation, for minutes to hours, suggesting that noncontinual use may be possible in selected pain syndromes.

The Bioness StimRouter peripheral neurotranstimulator Electrical Pulse Transmitter (EPT) showing Anchor, stimulating electrodes and receiver.

Technical Considerations

For each of the limb nerves described in the anatomy sections later, there are universal considerations. Sunderland noted significant variability in fascicle number, location, and size within a given nerve trunk. The complex fascicular arrangement of upper extremity nerves is an important consideration when attempting to stimulate a sensory fascicle. Briefly, a peripheral nerve will have one to several internal fascicles that routinely change locations within the nerve topography. Thus, if the desired fascicle is on the medial peripheral aspect of the nerve, it would be ideal to locate the target electrodes as close to that area as is feasible. Often, the location of these fascicles is an advantage of the percutaneous approach. An open neurosurgical approach allows only intraoperative motor testing with a nerve stimulator, unless the operator performs a wake-up test. In the upper extremity, peripheral nerves were mapped as to the variability in the internal structure of the nerves. The key nerves of interest are usually superficial enough to be seen well under US. US also allows visualization of surrounding key soft tissue structures, and in each case, care should be taken to not pierce muscle compartments or vascular structures along the needle/lead path to the nerve. For implantation cases the lead can be anchored to the superficial muscle fascia with a strain relief loop. As nerves will normally move within the neurovascular compartment as much as several millimeters, redundancy of available contacts or advanced programming capabilities are important ( Fig. 74.2 ). Note that these technical considerations are merely examples and not intended to be all inclusive.

Dissection of final placement of percutaneous electrode at median nerve in a fresh cadaver model.

Radial Nerve

Anatomy: The radial nerve in the midhumeral area is very close to the lateral surface of the humerus, in a shallow groove called the radial groove (at a point 10–14 cm proximal to the lateral epicondyle). The deep branch of brachial artery runs lateral to the radial nerve.

Sonoanatomy: With the probe placed in the posterolateral midhumeral area, one will be able to identify the triceps muscle and immediately deep to the triceps muscle the hypoechoic shadow of the humerus. The radial nerve has a honeycomb appearance sandwiched between the triceps and humerus. Color power Doppler can be used to identify the deep brachial artery lateral to the radial nerve.

Scanning technique: Using linear 38-mm, high-frequency 10- to 12-MHz probe, optimize machine image by selecting the appropriate depth, gain, and focus range. Usually beginning at the lateral aspect of the elbow and, with the probe in short axis view (a transverse orientation to the arm), continue cephalic alignment (move the probe in the longitudinal axis to the scanned structure) moving the probe proximally until the desired view is identified ( Fig. 74.3 ).

A cartoon showing placement of a peripheral nerve stimulation electrode and needle with percutaneous ultrasound-guided technique. A, Anterior; L, lateral; M, medial; P, posterior.

From Huntoon MA, Huntoon EA, Obray JB, et al: Feasibility of ultrasound-guided percutaneous placement of peripheral nerve stimulation electrodes in a cadaver model. Part I, lower extremity, Reg Anesth Pain Med. 33:551-557, 2008.

Needle insertion/electrode placement: The needle can be advanced in plane from posterolateral to anteromedial to lie between nerve and humerus. Piercing the lateral head of the triceps muscle may be unavoidable.

Potential patients could include those with posterior interosseous neuropathies or resistant lateral epicondylitis (tennis elbow) patients.

Ulnar Nerve

Anatomy: The ulnar nerve is superficial to the medial head of the triceps muscle. In anatomic feasibility studies, the nerve was easily identified at a point 9–13 cm proximal to the medial epicondyle in the medial/posterior arm.

Sonoanatomy: With the probe placed in posteromedial midhumeral area you will be able to identify the medial head of triceps muscle immediately under the skin; the ulnar nerve is located, using color power Doppler to identify the vascular structures accompanying the ulnar nerve.

Scanning technique: Using linear 38-mm, high-frequency 10- to 12-MHz probe, optimize machine image by selecting the appropriate depth, gain, and focus range. US scanning can commence at the elbow and, with the probe in a short axis view (transverse orientation to the arm), continues alignment (move the probe in the longitudinal axis to the scanned structure) of the probe proximally until the nerve fascicular arrangements can be well identified (honeycomb appearance).

Needle insertion/electrode placement: The needle may be advanced from posterior to anterior on the medial aspect of the arm to lie between nerve and humerus, staying superficial to the medial head of the triceps. Electrode orientation can be transverse or longitudinal depending on device specifics.

Ulnar nerve placements are perhaps the most facile of all the upper extremity nerves because the nerve lies superficial to the medial head of the triceps muscle. Caution is important to avoid injury to the medial cutaneous nerve of the arm, as well as the recurrent ulnar collateral artery.

Median Nerve

Anatomy: The median nerve enters the antecubital fossa medial to the biceps muscle and its tendon, medial to the brachial artery. The nerve passes between the two heads of the pronator teres muscle and then passes under the sublimis bridge of the two heads of the flexor digitorum superficialis. The common neural fascicular communications between the median and ulnar nerves in the forearm are an important consideration in terms of expected stimulation patterns.

Scanning technique: Using a linear 38-mm, high-frequency 10- to 12-MHz probe, optimize machine image by selecting the appropriate depth, gain, and focus range. US scanning can commence at the antecubital crease with the probe in a short axis view (transverse orientation to the arm); the brachial artery serves as a good landmark. Medial to the artery there is a hyperechoic honeycomb appearance (median nerve); continue to scan either distally or proximally until the desired location is identified.

Needle insertion/electrode placement: Median nerve stimulation may be accomplished either superior to the elbow or inferior. In some cases, during anatomic testing, the US probe was placed in the longitudinal plane with the nerve to allow more electrode contact (see Fig. 74.1 ).