Is a peripheral nerve stimulator (PNS) helpful in nerve localization in obese patients?

From the Guidelines

A peripheral nerve stimulator (PNS) can be helpful in nerve localization in obese patients, but its effectiveness is limited by technical difficulties, including maintaining optimal contact between the stimulating electrode and the nerve, and dissociating PNS from brachial plexus stimulation. When performing nerve blocks or other procedures requiring precise nerve identification in obese patients, a PNS can significantly improve accuracy and success rates, but it may require significant pressure on the soft tissues of the neck, which can be painful for the subject and awkward for the operator, particularly in obese or old subjects, or those with hypertrophy of neck muscles 1. The increased adipose tissue in obese patients often makes anatomical landmarks difficult to palpate and visualize, creating challenges for traditional nerve localization techniques.

Key Considerations

• A PNS works by delivering small electrical currents to elicit muscle contractions or paresthesias, confirming proximity to the target nerve, but the stimulus intensities required to achieve supramaximal stimulation can be uncomfortable 1.
• The technical expertise required for effective PNS may be a source of variability in research studies and limit its use in the clinical field, particularly in demanding settings such as the ICU or exercise 1.
• Combining PNS with ultrasound guidance may provide better outcomes, as this dual approach compensates for the limitations of each individual technique and increases the safety profile by improving visualization of surrounding structures and reducing the risk of intravascular injection or nerve injury.

Limitations of PNS

• Maintaining optimal contact between the stimulating electrode and the nerve can be difficult, especially in obese patients 1.
• It may be impossible to dissociate PNS from brachial plexus stimulation, particularly at high current intensities, which can be a source of discomfort for the subject and can theoretically modify the characteristics of the rib cage, with which the contraction of the diaphragm interacts 1.

From the Research

Peripheral Nerve Stimulator in Obese Patients

• The effectiveness of peripheral nerve stimulators (PNS) in nerve localization for obese patients has been studied in various research papers 2, 3, 4, 5.
• A study published in 2014 compared the effectiveness of ultrasound guidance versus nerve stimulation for lateral popliteal-sciatic nerve blocks in obese patients, and found that ultrasound guidance reduced procedural time and procedure-related pain, and increased patient satisfaction compared to nerve stimulation 2.
• Another study published in 2021 discussed the role of peripheral nerve stimulation in the era of ultrasound-guided regional anesthesia, and found that nerve stimulation can serve as a monitor against needle-nerve contact and may be useful in avoiding nerves that are in the needle trajectory during specific ultrasound-guided techniques 3.
• A case study published in 2009 described the use of ultrasound-guided placement of a permanent percutaneous femoral nerve stimulator lead for the treatment of intractable femoral neuropathy in a morbidly obese patient, and found that the patient had good coverage over the anterior thigh and below the knee after the implant 4.
• A retrospective review of 528 consecutive patients who received preoperative ultrasound-guided interscalene nerve blocks found that an elevated body mass index (BMI) was associated with an increased time required for block placement, intraoperative fentanyl administration, peak post-anesthesia care unit (PACU) pain scores, and PACU opioid administration 5.

Limitations of Peripheral Nerve Stimulators

• The studies suggest that while peripheral nerve stimulators can be helpful in nerve localization, they may not be as effective as ultrasound guidance in obese patients 2, 3.
• The use of peripheral nerve stimulators may require more time and may be associated with more complications, such as needle-nerve contact, compared to ultrasound guidance 3.
• The effectiveness of peripheral nerve stimulators may depend on the specific nerve block being performed and the individual patient's anatomy and physiology 4, 5.

Comparison with Ultrasound Guidance

• The studies suggest that ultrasound guidance is generally more effective than peripheral nerve stimulation for nerve localization in obese patients 2, 3, 5.
• Ultrasound guidance can provide real-time visualization of the nerve and surrounding anatomy, allowing for more precise placement of the needle and reduced risk of complications 2, 4.
• However, peripheral nerve stimulators can still be a useful adjunct to ultrasound guidance, particularly in cases where the nerve is difficult to visualize or the patient has a complex anatomy 3, 4.