Persistent median artery detected during ultrasound-guided median nerve block: Clinical and anaesthetic implications

Dear Editor,

Vascular anomalies of the forearm are uncommon but clinically important incidental findings during ultrasound-guided peripheral nerve blocks (PNBs) and arterial cannulation.[1] Point-of-care ultrasound (POCUS) is an essential tool for identifying such variants, ensuring success, and avoiding iatrogenic complications.[1,2] We report a case of a persistent median artery (PMA) identified during ultrasound-guided median nerve (MN) block and discuss its clinical implications.

A 30 year old male presented with left third and fourth metacarpal fractures for fixation under regional anaesthesia (RA). After performing a successful superficial radial nerve block using a high-frequency linear transducer probe (HFL38x, 13–6 MHz, 6 cm, The SonoSite EDGE II; SonoSite, Inc., Bothell, WA, USA), the anaesthesiologist proceeded with a MN block in the distal forearm. POCUS revealed a pulsatile vascular structure immediately adjacent to MN [Figure 1a]. Doppler confirmed arterial flow, and the vessel was traced to originate from the ulnar artery at the proximal forearm, consistent with a PMA [Figure 1b]. The artery showed a continuous longitudinal course alongside the MN without branching toward the typical ulnar artery distribution. This persistent parallel trajectory is characteristic of a PMA rather than an aberrant ulnar artery branch, which usually diverges toward the ulnar or interosseous compartments. The diameter and cross-sectional area of the PMA were 1.47 mm and 1.7 mm², respectively. The diameter and cross sectional area of the ulnar artery and radial artery were 2.15 mm and 3.63 mm², and 1.87 mm and 2.75 mm², respectively. The POCUS examination of the contralateral forearm showed normal vascular anatomy. Because of the close association between PMA and MN, the block was performed at the axilla, and an ulnar nerve block at the usual mid-forearm site was achieved to provide surgical anaesthesia. The surgical procedure was uneventful.

Figure 1.

(a) Transverse ultrasound image of the distal forearm showing neurovascular structures. The median nerve appears as a hypoechoic oval structure with an internal fascicular pattern, located centrally. Adjacent to it lies the persistent median artery (PMA), a small, circular, anechoic vessel coursing alongside the nerve. Laterally, the radial artery appears as a larger, round, anechoic vessel. Lateral to the radial artery, the superficial radial nerve is visualised. (b) Doppler ultrasound showing arterial flow in the PMA. Top: B-mode with colour Doppler of the PMA showing arterial flow; bottom: spectral Doppler waveform with peak systolic velocity indicated.

PMA represents a remnant of the embryonic axial artery and has variable prevalence reported across populations.[3] PMA variants are commonly described as either antebrachial (incomplete) or palmar (complete/dominant). The artery most frequently arises from the anterior interosseous artery and, less commonly, from the ulnar artery.[3] PMA is frequently associated with MN anomalies, including a bifid MN, and when it traverses the carpal tunnel, it may predispose to thrombosis and compressive neuropathy, which may manifest as carpal tunnel syndrome (CTS).[3,4] The literature reports that PMA complicates forearm vascular and surgical procedures, as well as the management of nerve-related conditions in the forearm, wrist, and hand.[3,4]

Identification of PMA is essential for forearm RA, as it can alter the usual sonoanatomy of the MN block. Its close proximity to the nerve increases the risk of inadvertent intravascular injection, vascular puncture, and local anaesthetic systemic toxicity.[2,3] If any vascular structure is detected adjacent to the MN, the operator should reconsider the block site and consider alternative approaches, such as proximal nerve blockade or a change of needle trajectory to avoid neurovascular injury.[2,3] The vascular anomaly should be recorded and communicated for future reference. The Allen’s test and its modified form are simple bedside screens of radial–ulnar collateral flow, but assume the palmar arch is supplied only by the radial and ulnar arteries, so they have important limitations. PMA can alter palmar perfusion and produce false reassuring or false positive results. Therefore, when anatomic variants are suspected, or the modified Allen’s test is abnormal or equivocal, palmar arch anatomy needs to be confirmed with colour Doppler POCUS or plethysmography.[5,6] The ulnar artery variant of PMA is clinically important because its larger diameter increases the risk of CTS and MN compression in case of thrombosis, and it can be helpful for preserving hand perfusion when both the radial and ulnar arteries are injured or thrombosed.[3,4,6]

To conclude, ultrasound in RA should not be confined to real-time needle guidance alone; rather, it should begin with a deliberate and systematic anatomical survey using grayscale and colour Doppler scout scan before any needle insertion. The POCUS evaluation should be part of the routine evaluation before PNBs, vascular catheterisation, and surgical procedures to detect vascular variants.[1,2,3] Landmark-guided wrist blocks should be avoided, as they carry risks of inadvertent intravascular injections.

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

During the preparation of this manuscript, the authors used Microsoft Copilot (Microsoft Corporation, Redmond, WA, USA) for language editing, grammar correction, and improving the clarity and readability of the text. The authors critically reviewed and verified all AI-assisted content and take full responsibility for the accuracy, originality, integrity, and final content of the manuscript.

Author contributions

CRM and RVR: Conceptualisation, Writing – original draft, review and editing. RB and SS: Writing – original draft, software and investigation, and writing – review and editing. CRM: Overall supervision. All authors have read and agreed to the content of the final manuscript.

Declaration of patient consent

The authors certify that they have obtained all appropriate consent forms from the patient. In the form, the patient consented to the images and other clinical information to be reported in the journal. He understands that his name and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.