Ultrasound evidence of distal enlargement at entrapment sites following proximal peripheral nerve injury

BACKGROUND: Proximal peripheral nerve injuries may be accompanied by structural changes along the course of the nerve, including enlargement at distal sites. Ultrasound offers a noninvasive method to assess nerve caliber at anatomic tunnels where swelling may have clinical relevance. However, the prevalence and distribution of distal enlargement after proximal traumatic injury remain poorly defined. OBJECTIVE: To evaluate the frequency of distal nerve enlargement at common entrapment locations in patients with proximal upper extremity peripheral nerve injuries. METHODS: We retrospectively reviewed 76 patients (140 nerves) with proximal nerve injuries who underwent ultrasound evaluation. Cross-sectional area (CSA) was measured at standard entrapment sites in the upper extremities. Enlargement was defined as CSA exceeding published reference thresholds. Distal enlargement was analyzed by entrapment site, injury pattern, and mechanism of injury. RESULTS: Distal enlargement was identified in 66 of 140 nerve segments (47.1%) and in 44 of 76 patients (57.9%). Enlargement occurred across all injury patterns, with the highest prevalence in plexopathies (68.4%). It was also observed across all mechanisms of injury, including penetrating, blunt, and iatrogenic trauma, with the highest prevalence in iatrogenic injuries (80.0%). CONCLUSIONS: Distal nerve enlargement is a common ultrasound finding after proximal peripheral nerve injury. Although the mechanism cannot be determined from this retrospective, single-time-point dataset, the high prevalence underscores the importance of assessing for distal compression during evaluation and reconstructive planning. Future studies using standardized and serial ultrasound protocols are needed to clarify the temporal evolution of distal enlargement and its clinical significance.