Ulnar sparing/Lower trunk sparing: A more accurate descriptor than ulnar nerve sparing in supraclavicular blocks

Dear Editor, The term “ulnar nerve sparing” is frequently used to describe incomplete anaesthesia following a supraclavicular brachial plexus block, particularly when local anaesthetic spread to the “corner pocket”, the inferomedial aspect of the brachial plexus adjacent to the subclavian artery, is inadequate.[1-3] However, this is anatomically imprecise as the neural element present at this location is not just the ulnar nerve but the lower trunk, and an inadequate block can result in sparing of the entire ulnar aspect, that is, the C8 and T1 innervated region of the upper limb. A literature search did not reveal any prior use of the term ‘ulnar sparing/lower trunk sparing’ in regional anaesthesia, making this a novel, anatomically precise descriptor for incomplete supraclavicular blocks. In clinical practice, patients who demonstrate preserved cold sensation in the little finger (ulnar nerve territory) following supraclavicular brachial plexus block also retain sensation in the tip of the middle finger (median nerve territory).[4] Anatomically, the lower trunk of the brachial plexus, formed by the ventral rami of C8 and T1, gives rise to the ulnar nerve and also contributes fascicles to the median and radial nerves (both with root values from C5 to T1), as well as to the medial cutaneous nerves of the arm and forearm [Figure 1a]. While the medial pectoral nerve also shares this root origin, it is less relevant in the context of upper limb anaesthesia.Figure 1: (a) Schematic representation of the brachial plexus, showing the origins of the radial, median, ulnar, and medial cutaneous nerves and local anaesthetic deposition during supraclavicular block (b) Dermatomal distribution of the cervical and thoracic roots along the preaxial and postaxial bordersIn a previous study, while consistent sparing of the median nerve was noted in all patients who demonstrated ulnar nerve sparing, radial nerve sparing was not reported as the dermatome assessed for radial nerve was the anatomical snuffbox.[4] The anatomical snuffbox lies in the preaxial segment and receives fascicular contributions from C6 rather than C8/T1 [Figure 1b]. Assessing the C8 and T1 components of the radial nerve is inherently difficult as the medial forearm extensors cannot be tested in isolation, and there is significant dermatomal overlap with the ulnar nerve. Consequently, exclusive evaluation of postaxial radial innervation remains difficult. The term “ulnar nerve sparing” underrepresents the full extent of neural sparing that can occur with incomplete blockade of the lower trunk. In the context that all nerves receiving fascicles from C8 and T1 get spared, rescue blocks targeting only the ulnar nerve may prove inadequate.[4] Instead, a more comprehensive approach addressing the median and radial nerves is required. For example, if a costoclavicular or infraclavicular block is chosen as a rescue technique, it should include all three cords of the brachial plexus. The medial cord targets the ulnar nerve, the posterior cord for the radial nerve, and the lateral cord for the median nerve. Likewise, if a rescue block is performed at the axilla, blockade of the ulnar, median, and radial nerves is necessary. Moreover, at the axillary level, the ulnar nerve lies close to the medial cutaneous nerves of the arm and forearm, which would also be blocked. This benefit may not be achieved with more distal approaches. We advocate replacing the term “ulnar nerve sparing” with the more anatomically precise “ulnar sparing/lower trunk sparing”. This refinement in nomenclature would enhance anatomical understanding and improve clinical decision-making. 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. Authors contributions SR: SR was involved in concepts, literature search, manuscript preparation, editing, review and approval. JN: Manuscript preparation, editing, review and approval. RD: Manuscript preparation, editing, review and approval. Supplementary material None Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.