A median nerve block is the production of local anaesthesia in the distribution of the median nerve at the wrist. It is useful as anaesthesia for procedures such as carpal tunnel decompression.
Prior to the procedure, the patient is advised to report any symptoms of paraesthesia - pins and needles - or shooting pain in the distribution of the median nerve. This can result from the needle irritating the nerve itself and is a symptom that should prompt slight withdrawal of the needle tip.
The palmaris longus tendon is identified by resisted flexion. Less than 5ml of a local anaesthetic solution such as 1% lignocaine or 0.25% marcaine is injected along the radial border of this landmark at a distance of about 1cm proximal to the proximal wrist crease.
If the palmaris longus is congentially-absent, the flexor carpi radialis tendon is identified and, at the same distance from the wrist and 1cm towards the from its ulnar side, the needle is inserted.
The needle must be passed through the palpable resistance of the antebrachial fascia/flexor retinaculum to reach the carpal tunnel. A reliable sign of suitable depth of penetration is the sudden reduction in resistance to flow on attempting to inject. The block may take 10-15 minutes to reach full effect, dependent on the local anaesthetic used.
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