Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy of the upper extremity and is typically characterized by pain, paresthesia, nocturnal symptoms and functional impairment in the median nerve (MN) distribution. Although the diagnosis is primarily based on clinical findings and may be supported by electrophysiological studies, these assessments may not fully explain symptom mechanisms, particularly in patients with anatomical variations or dynamic nerve behavior.

High-resolution ultrasonography (US) has become an increasingly valuable modality in the evaluation of CTS, as it allows direct visualization of the MN, surrounding soft tissues and potential structural causes of compression. In addition to static parameters such as cross-sectional area (CSA), fascicular edema and vascularity, dynamic US can demonstrate motion-related abnormalities that may contribute to intermittent or position-dependent symptoms. Furthermore, techniques such as sono-palpation and sono-Tinel can integrate imaging findings with symptom reproduction, thereby strengthening the clinical relevance of sonographic observations.

Anatomical variations of the MN and its branches are clinically important, particularly during interventional procedures. The palmar cutaneous branch (PCB) usually arises proximal to the wrist crease; however, variant origins may alter both the clinical presentation and the safest interventional approach. Recognition of such variations is essential to avoid iatrogenic injury and to tailor US-guided treatment.

Herein, we present a patient with CTS in whom high-resolution dynamic US revealed MN edema, increased vascularity, snapping of the MN over the flexor tendons and a rare anatomical variation involving the PCB. This case highlights the role of dynamic US as both a diagnostic extension of clinical examination and a guide for precise, minimally invasive management.

A 57-year-old female patient presented with a two-year history of pain and paresthesia, predominantly at night, involving the base of the left thumb and the volar aspects of the left first three fingers. She also reported a sensation of impingement in the left wrist, occasionally occurring during wrist and finger flexion. A blind injection had been administered at another clinic, which was ineffective. The remainder of her medical history was unremarkable. On physical examination, Tinel’s and Phalen’s signs were positive on the left side, while neurological assessment revealed hypesthesia in the thenar eminence (confirmed by pinprick and light touch testing) but no motor weakness in the affected hand. Motor and sensory nerve conduction studies of the median and ulnar nerves performed one year earlier had indicated moderate CTS; however, the detailed report was not available for review. US (Logiq Fortis R4 HDU, GE Ultrasound Korea, Ltd) examination revealed swollen/edematous MN with the CSA of 16 mm² at the level of the pisiform bone (reference value for CTS: >10 mm²),^{Reference Roomizadeh, Eftekharsadat and Abedini1} along with mildly increased vascularity (Figure 1A, 1B, 1C). Vascularity was most prominent along the periphery of the MN on transverse color and power Doppler imaging, a pattern consistent with distension of the extrinsic epineurial vascular network rather than intrafascicular flow (Figure 1B, 1C).^{Reference Ricci, Ricci and Cocco2} Sono-palpation also reproduced similar symptoms in the same region. Additionally, the PCB of the MN was observed to arise within the carpal tunnel, representing an anatomical variation (Video 1). Dynamic US evaluation demonstrated snapping of the MN in both lateral-to-medial and medial-to-lateral directions over the flexor digitorum superficialis tendons, just proximal to the wrist crease (Video 2), which was accompanied by the onset of paresthesia and pain. These findings were considered a plausible explanation for the paresthesia associated with the patient’s sense of impingement. The absence of a positive pronation test and Hagert’s triad did not suggest a proximal MN entrapment. Accordingly, a diagnosis of CTS involving the PCB was established. Under US guidance, perineural hydrodissection was performed using a 27-gauge needle and an in-plane, radial-to-ulnar approach to separate the MN and its PCB from the adjacent flexor retinaculum. Hydrodissection targeted the interface between the MN/PCB and the overlying flexor retinaculum, as dynamic imaging identified this as the principal site of nerve excursion; the deeper interface between the nerve and the flexor tendons, where inflammatory adhesions may also occur, was not specifically addressed and should be considered in future evaluations. A total of 1 mL of methylprednisolone and 4 mL of isotonic saline were injected (Video 3). A splint that holds the wrist in a neutral position was also prescribed. At the three-week follow-up visit, the patient reported marked clinical improvement, with a post-injection CSA of 14 mm.^{Reference Ricci, Ricci and Cocco2} The patient remains under uneventful follow-up.

Figure 1.

Ultrasound examination (using a 6–24 MHz hockey stick probe) demonstrating the edematous median nerve (encircled with yellow crosses) with a cross-sectional area of 0.16 cm² at the level of the pisiform bone (A). Longitudinal evaluation with power Doppler and superb microvascular imaging shows swollen fascicles (asterisks) and mild vascular activity (B, C).

Figure 1. Long description

Panel A: A grayscale ultrasound image showing a cross-sectional view of the wrist. The median nerve is encircled with yellow crosses, indicating an area of interest. The cross-sectional area of the median nerve is measured and labeled as 0.16 cm2. Panel B: A longitudinal ultrasound image with power Doppler imaging. The image shows swollen fascicles marked with asterisks and mild vascular activity indicated by color signals, with a color scale bar on the left side. Panel C: Another longitudinal ultrasound image using superb microvascular imaging. It also shows swollen fascicles marked with asterisks and mild vascular activity indicated by color signals, with a color scale bar on the left side.

Although the diagnosis of CTS is primarily based on clinical findings, both electrophysiological studies and morphological assessments provide valuable diagnostic insights.^{Reference Lin, Chang, Wu and Özçakar3} Particularly in the evaluation of neural structures, ultrasonographic assessment – whether static or dynamic – plays an indispensable role. In this case, US enabled the identification of fascicular edema and the rarely reported phenomenon of a “snapping” MN. Moreover, the integration of specialized examination techniques, such as sono-palpation and sono-Tinel, with advanced imaging modalities including power Doppler and superb microvascular imaging, further enhanced the diagnostic utility of US. US further provided identification of an anatomical variation, as well as guidance for interventional procedure. Regarding anatomical variations of PCB, it usually arises 4–5 cm proximal to the wrist crease, from the radial aspect of the MN. On the other hand, potential variations are well-recognized.^{Reference Lanz4} In our patient, US demonstrated that the PCB originated distally within the carpal tunnel. Although the PCB is typically described as a structure that should be preserved during MN injections, in this case, it was considered one of the affected structures due to the presence of hypesthesia in the thenar eminence. Therefore, simultaneous US-guided hydrodissection of both the PCB and the MN was performed, resulting in complete symptom resolution. Herein, hydrodissection was selected to improve the nerve’s autonomic regulation by decompressing the vasa nervorum and nervi nervorum.^{Reference Colorado, McNeill and Norbury5} Additionally, the injectate included a corticosteroid to reduce the fascicular edema. In conclusion, this case reminds us that high-resolution US is not merely a diagnostic tool for depicting the MN’s anatomy and motion but a dynamic extension of clinical reasoning – where sonography meets precision in the pursuit of minimally invasive care.^{Reference Yalçınkaya, Çolak, Abacıoğlu and Özçakar6–Reference Özçakar, Abacıoğlu and Leblebicioğlu8}

Several additional points merit comment. The thickening of the transverse carpal ligament at the tunnel outlet was likewise not systematically assessed in conjunction with the MN CSA at the inlet, an approach that could further support the diagnosis of CTS severity.^{Reference Wu, Chang and Tsai9} As with any single case report, these findings cannot be generalized, and the long-term durability of symptom relief beyond the three-week follow-up period remains to be determined; larger cohort studies with extended follow-up are warranted to confirm the reproducibility and durability of this diagnostic and therapeutic approach.

This case underscores that high-resolution, dynamic US extends beyond static anatomical assessment in CTS by enabling identification of rare anatomical variations and motion-related nerve pathology. The integration of dynamic evaluation and sono-palpation facilitates precise diagnosis and allows tailored, ultrasound-guided interventions. Consequently, US serves as both a diagnostic and therapeutic adjunct, supporting accurate, minimally invasive and patient-specific management of CTS.