Ultrasound-guided posterior antebrachial cutaneous nerve block utilising the 'fat-filled flat tunnel': Description of technique and cutaneous sensory block area

The aim of the study was to investigate an ultrasound-guided posterior antebrachial cutaneous nerve block based on visualising the nerve within the fat-filled flat tunnel and describe the area of cutaneous sensory loss. A total of 12 healthy volunteers were included in the study. The posterior antebrachial cutaneous nerve was identified within the fat-filled flat tunnel in the upper arm using high-frequency ultrasound. The nerve was blocked using an in-plane needle guidance technique with 1 ml 2% lidocaine. Sensory loss to pinprick was evaluated 15 minutes after performing the block and the cutaneous sensory block area mapped. Ultrasound visualisation of the posterior antebrachial cutaneous nerve in the fat-filled flat tunnel was possible in all volunteers. The median distance of the posterior antebrachial cutaneous nerve to the lateral epicondyle of the elbow was 67.5 (range 54-105) mm. Loss of sharpness sensation to pinprick extended from the posterior aspect of the distal upper arm and posterior forearm to the wrist. The median cutaneous sensory block area was 103 (range 61-341) cm(2). Two volunteers had a sensory block over the anterior forearm of 29 and 10 cm(2) respectively. This amounted to 11% and 4.5% of the total cutaneous sensory block area. One volunteer had a sensory block over the dorsum of the hand of 39 cm(2) (15% of the total cutaneous sensory block area). The results of this study indicate that the fat-filled flat tunnel can be a useful sono-anatomical landmark in identifying the posterior antebrachial cutaneous nerve and may serve as a target for injection. Although sensory block is predominantly confined to the posterior distal arm and forearm, inter-individual variability in the area and distribution was observed. Sensory block in the anterior forearm and dorsum of the hand can occur.