The prevalence of diabetes mellitus (DM) in industrialized countries is estimated to be about 7.3% and its incidence has been growing in recent years. The prevalence of diabetic neuropathy in the diabetic patient population is up to 50%. When limb surgery is necessary, it is reasonable to assume that diabetic patients will benefit from a peripheral regional anesthesia because of the severe comorbidities associated with DM. On the other hand, the use of regional anesthesia (RA) has generally not been recommended in patients with preexisting neuropathies mainly because of medical liability issues, as worsening neuropathy could be attributed to nerve damage caused by the regional anesthetic. The current state of the art of peripheral regional anesthesia for the identification of correct placement of an injection needle suitably close to the target nerve is to elicit a motor response by current injection through the needle. Constant reduction of the current as the nerve is approached ensures close proximity so that an effective nerve block is obtained when the local anesthetic is delivered through the needle, and absence of a motor response at 0.3 mA is generally accepted as a safety marker to avoid harmful intraneural injection. An important deficit in our understanding is whether diabetic neuropathy influences the stimulation parameters for peripheral nerve stimulation (PNS), possibly decreasing safety.

The currently proposed research is guided by the hypothesis that nerves in patients with DM are more resistant to stimulation and the current thresholds for PNS have to be set much higher to prevent injections from occurring within the epineurium. The investigators will examine the effect of DM on nerve excitability in a blinded, prospective, observational case control trial. Accordingly, the investigators have defined following aims:

Specific Aim 1: To characterize the required stimulation current in patients with and without diabetes mellitus.

Specific Aim 2: Follow-up to examine if the rate of adverse neurologic events is higher in diabetic patients.

Specific Aim 3: Guided by the results, formulate recommendations for the performance of regional anesthesia in patients with a history of DM.

These experiments will provide better understanding of the needle-current-nerve relationship during peripheral nerve stimulation. Findings from this study will have a major impact on patient safety, especially in the subgroup with preexisting neuropathy, undergoing regional anesthesia.

Inclusion Criteria:

• Patients of both sexes
• age more than 18 years
• American Society of Anesthesiology status 1 to 4
• signed informed consent

Exclusion Criteria:

• Pregnancy
• Allergies to local anesthetics
• Unwillingness or incapability to sign informed consent
• Any preexisting neurological deficit of the lower limb that cannot be attributed to diabetic neuropathy
• Ongoing dual anti-platelet therapy

Surgical patients scheduled for lower limb surgery and are eligible for a popliteal sciatic nerve block