Implications of Pass-over Brachial Plexus.

Sir,

Successful brachial plexus block requires detailed knowledge of gross anatomy, knowledge of the relevance of muscle twitch when a nerve stimulator is used, and knowledge of sonoanatomy when the block is planned under ultrasound guidance. The groove between the anterior and middle scalene muscle is the landmark where the drug is injected by landmark technique, and the stimulating needle is placed when a nerve stimulator is used to perform an interscalene block. Similarly, when ultrasound is used for performing the block, the two scalene muscles are identified, and the C5–C7 roots are traced in the interscalene groove.[12] However, sometimes the roots do not follow the rule of being placed in the groove. When they deviate from the regular position, the brachial plexus is also known by a different name. It is called a pass-through brachial plexus when the roots of brachial plexus pass through the anterior scalene muscle and a pass-over brachial plexus when the ventral rami of brachial plexus pass over the anterior scalene muscle.[3] Usually, C5 or C5–C6 roots travel this pathway. The possibility of all C5–C7 roots passing over anterior scalene muscle is rare. This anomalous location of the root is the reason for a failed or a patchy interscalene block when landmark technique is used and when the block is performed with a nerve stimulator. The block can fail with the use of ultrasound if the performer fails to recognize a pass-over or a pass-through plexus.

During a random neck scan of a patient who was not scheduled to undergo a surgery of the upper limb, we identified a C5–C6 nerve root passing over the anterior scalene muscle instead of the usual location that is in the interscalene groove [Figure 1]. In such situation, the C5–C6 roots have to be blocked separately in the substance of anterior scalene muscle. However, the problem with this injection in the belly of anterior scalene is that the injected local anesthetic might block the phrenic nerve as well.

Figure 1

The image shows C5–C6 root over the anterior scalene muscle rather than the groove between anterior and middle scalene muscle. The C7 root is seen in the usual location that is the interscalene groove. The interscalene groove is shown with the black line between anterior and middle scalene muscle

The phrenic nerve arises from C3 to C5 and is usually in proximity to the C5 root at the level of cricoid cartilage. From here, the nerve descends in a caudal direction over the anterior scalene muscle.[4] Therefore, once a pass-over brachial plexus is identified, a meticulous scan should be done to identify the phrenic nerve and a lesser volume of local anesthetic should be injected to avoid complications due to phrenic nerve block.

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