The real-time visualization of a needle and nerve during an ultrasound-guided nerve block can be challenging. These difficulties may partly explain the systemic complications of local anesthetics under ultrasound. Injection of small amounts of a solution around the anesthetized nerve (hydro-dissection) has been proposed to enhance contrast outlining its borders and also to improve the visualization of the needle tip. The glucose solution 5% solution is interesting because it allows, unlike saline, to maintain the motor response with neurostimulation. The hydro-dissection can be particularly useful when one suspect hypoechoic vessels near the nerve to be anesthetized. Thereby, the nerve well demarcated and separated from the vessels, injection of local anesthetic is performed in the circumferential diffusion space (like a small pocket) without redirecting needle.

The influence of this hydro-dissection on the nerve block efficiency is unknown. The nerve block quality can be improved because the entire anesthetic is injected in contact with the nerve, but it can also be reduced due to the dilution of the local anesthetic by the glucose solution.

In this randomized study, the investigators test the hypothesis that hydro-dissection does not alter the nerve block onset time.

Inclusion Criteria:

• Adult patients with an ASA physical status I-II scheduled for elective ambulatory surgery of the hand or wrist involving the median nerve

Exclusion Criteria:

• Patients with type 1 or 2 diabetes mellitus,
• History of clinical or laboratory evidence of abnormal bleeding,
• Infection at the injection site,
• Allergy to local anesthetic,
• Preexisting central or peripheral muscular or neurological disease (for example: carpal tunnel syndrome)