Electrical peripheral nerve stimulation (PNS) is often used to place needles correctly during peripheral nerve block. The current recommendation is to use stimulation currents between 0.3 and 0.5 mA, but a published study in the British Journal of Anesthesia suggests that these thresholds might not be sufficient to protect against intraneural injuries in patients with diabetic neuropathy.
In the study, even some healthy patients were found to require currents exceeding 0.5 mA during needle placement, suggesting this threshold might be inappropriate for a larger population of patients than previously thought and that other techniques might be warranted to better protect against nerve injuries during needle placement.
“Electrical impedances are not uniform across different body regions and the mix of tissues surrounding a peripheral nerve is not homogeneous, which challenges the use of PNS alone to detect optimal needle position,”the study investigators wrote. “While the generally recommended stimulation current is 0.3–0.5 mA, this threshold or even higher currents cannot exclude intraneural needle placement.”
The study population included 55 patients with diabetes and 52 without diabetes who were undergoing lower limb surgery and were eligible for popliteal sciatic nerve block. Prior to enrollment, all patients were screened for neuropathy using a variety of clinical assessment tools, including a tuning fork test, pin prick test, temperature discrimination test, reflex testing, and electroneurography examinations. Compared with the nondiabetic patients, those with diabetes tended to weigh more, have a higher body mass index, and have more signs of neurological dysfunction on neuropathy assessments, including decreased perception of light touch, pressure, vibration, temperature, and painful stimuli, as well as deficits in tendon reflex and neural conduction.
During popliteal sciatic nerve block, ultrasonography was used to determine the minimal stimulation current required to produce motor activity of the tibial nerve (TN) and common peroneal nerve (CPN). This was achieved by gradually increasing the current until a response of the respective muscles was elicited, and the result was confirmed by gradually reducing the current until the distal motor response disappeared. After these measurements were taken, 1 mL of glucose 5% was injected to facilitate post-hoc verification of needle placement by blinded investigators reviewing the ultrasound images and videos of the procedures.
Although stimulation currents were not found to differ between diabetic and nondiabetic patients, individuals who exhibited diminished pressure perception required increased thresholds to produce motor activity of the CPN (median 1.30 vs 0.57 mA for those with normal perception; P = .042). Threshold requirements for the CPN were even greater among those with decreased pain sensation (1.60 vs 0.50 mA for those with normal sensation; P = .038). “[We observed] a direct correlation between the duration of diabetes mellitus and the stimulation threshold for the CPN, the nerve that is typically more affected by diabetic neuropathy than the TN,” the study investigators wrote.
Despite the use of stimulation currents >0.5 mA, 4 patients with diabetes and 1 without diabetes ended up with intraneural needle placements. Additionally, 14 patients (12 with diabetes and 2 without) required currents of 2 mA or higher to elicit a motor response, with needle-nerve contact observed in 1 such patient in the diabetes group. Slowed ulnar nerve conduction velocity was predictive of elevated mean stimulation current (P = .002).
“[Our findings show] that nerve stimulation is not as reliable as previously thought and even more so in patients with diabetic neuropathy,” the study investigators note. “It is therefore advisable to use additional tools such as ultrasound to decrease the risk of potentially harmful intraneural injections.”
Heschl S, Hallmann B, Zilke T, et al. Diabetic neuropathy increases stimulation threshold during popliteal sciatic nerve block. Br J Anaesth. 2016;116(4):538-545.