Comment: A tale of the lost rib — Revisiting anatomy and volume optimisation

Dear Editor,

We read the article ‘Comparison between ultrasound-guided subpectoral intercostal plane approach and pectoserratus plane approach for intercostobrachial nerve block in surgeries involving posterior and medial aspects of the arm – A randomised comparative trial’.[1] We commend the authors for addressing an important and often overlooked aspect of regional anaesthesia, explicitly targeting the intercostobrachial nerve (ICBN), which is notoriously difficult to anaesthetise reliably using standard brachial plexus blocks. However, we would like to raise a few observations that merit consideration.

Firstly, the depiction of the pectoralis minor muscle (PmM) in relation to the second rib appears anatomically imprecise. Classical anatomical references describe the PmM originating from the third to fifth ribs [Figure 1a], rather than the second, which raises questions about the claim that the second rib lies deep to the bulk of this muscle.[2] Suppose the probe is positioned at the lateral third of the clavicle and directed caudally. In that case, it is more plausible that the rib visualised beneath the PmM is the third or lower [Figure 1b-d]. Although the superior border of the PmM may appear to approach the second rib [Figure 1c], it typically does not cover it sufficiently to obscure it on sonography [Video: https://tinylink.info/10qG1]. Real-time ultrasound often diverges from textbook anatomy due to variations in patient habitus, probe tilt, and scanning plane. It could create a misleading impression of PmM overlying the second rib in lateral or oblique views with caudal angulation. This sonographic misrepresentation likely reflects an infraclavicular view, rather than a true subpectoral intercostal plane.

Figure 1.

Anatomy and sonoanatomy of right-sided pectoral region. a: right chest wall showing the relationship between ribs and the pectoralis minor muscle (adapted from the Elsevier Complete Anatomy, version 11.2.1 (21136) by 3D4 Medical Ltd.); b, c, d: the sonoanatomy at different transducer positions (inset pictures) across the infraclavicular fossa - from cephalomedial to caudolateral scan on a volunteer; R1, R2, R3 = first, second, and third ribs, respectively; PmM = pectoralis minor muscle; PMM = pectoralis major muscle; SAM = serratus anterior muscle; ICM = intercostal muscles

Secondly, the needle trajectory described—caudocranially directed towards the lower border of the ‘second rib’ medial to the axillary vessels—resembles an infraclavicular approach. The injectate lies in the same fascial plane as the brachial plexus cords and is prone to spread medially. This territory overlaps with what has been previously described by Chen et al. as the costoclavicular-serratus anterior muscle space (CC-SAS), where local anaesthetic (LA) spreads between the serratus anterior and subclavius or PmM, providing rapid and complete upper limb analgesia.[3] The authors’ technique, therefore, may reflect a CC-SAS or infraclavicular variant rather than a distinct block.

Thirdly, the ICBN, typically a lateral cutaneous branch of the second intercostal nerve, exiting the thorax near the midaxillary line, branches early and travels across the intercostal and serratus anterior muscles towards the axilla. An injection performed medially or distally within the subpectoral plane may not reliably capture the main trunk, especially if administered at or below the lateral border of PmM. This is because the ICBN may have already branched or formed plexiform communications with the medial brachial cutaneous nerve, posterior brachial cutaneous nerve, anterior branch of the lateral cutaneous branch of intercostal nerves (T3, T4), or axillary cutaneous branches of the medial and posterior cords.[4,5,6,7] Although the ICBN is known to be incidentally anaesthetised in costoclavicular,[3] infraclavicular,[8] supraaxillary,[6] and pectoserratus plane blocks[9] due to fascial continuity along its course, such involvement is often inconsistent and unpredictable. For reliable ICBN blockade, particularly in axillary, anteromedial, or posterior arm procedures, a field block using targeted subcutaneous infiltration at the desired cutaneous territory (e.g. axilla or posteromedial upper arm) or under ultrasound guidance may offer a safer, more precise, and reproducible alternative to indirect blockade by an interfacial plane block. It avoids unnecessary high-volume LA deposition and inadvertent spread to brachial plexus elements and directly addresses the nerve’s terminal sensory distribution.

Lastly, in the study, the volume and concentration of LA appear arguably excessive for targeting a single sensory nerve, ICBN. A more judicious approach could have involved further dilution of the LA concentration. A simple infiltration using a diluted LA solution of 5–10 mL at the surgical site or axilla may suffice in most cases. Ultrasound-guided ICBN block will reduce the LA volume and improve the efficacy.[10,11]

In conclusion, we sincerely appreciate the authors’ efforts to explore this challenging anatomical region and encourage further research with careful anatomical validation and volume optimisation to refine the efficacy and safety of such blocks.

Disclosure of use of artificial intelligence (AI)-assistive or generative tools

The AI tools or language models (LLM) have not been utilised in the manuscript, except that software has been used for grammar corrections and references.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.