Ultrasonography of a bifid median nerve causing carpal tunnel syndrome: MSUS or MRI, which is better?

Excerpt

The patient, a 43-year-old woman, with bilateral carpal tunnel syndrome (CTS). Electrophysiological studies disclosed bilateral severe CTS. High-resolution musculoskeletal ultrasound (MSUS) examination was done, by the first author, using linear probe 18 MHz (TOSHIBA ApIio 400). In (Fig. 1) the median nerve cross-sectional area was 22 mm² (A) on the right hand (star) (3), normal reported up to 15 mm². Interestingly, her left wrist transverse volar scan showed a bifid median nerve (asterisk) in grey-scale mode (1) with persistent median artery (straight arrow) in power Doppler mode (2). This morphological anomaly consisted of two unequal median nerve trunks: a large radial trunk (15 mm²) and a small ulnar trunk (4 mm²). The sum of both cross-sectional areas was 19 mm² (B + C) (4). A small branch can be seen from the radial trunk (curved arrow) with same nerve echogenicity and contained within the hyperechoic epineurium; this was confirmed on dynamic proximal scanning as it was seen originating from main nerve trunk in the forearm. The total cross-sectional area of this neurovascular bundle was 45 mm². Confirmatory MRI, fat suppressed PD-weighted axial (5) proved the same diagnosis; however, its resolution was inferior to MSUS. A thorough survey of the English literature disclosed that the incidence of bifid median nerve is 3%; however, with the aid of high-resolution MSUS, this incidence might increase up to 19% [1]. The implication of such preoperative diagnosis either by MSUS [2] or MRI [3] can be utilized to preferentially choose the conventional surgery over endoscopic release which might be contraindicated if such anomaly was known [4]. Adequate median nerve exposure minimizes the risk of inadvertent injury to the persistent median artery or smaller nerve branches. Noteworthy, MSUS is a convenient imaging modalities to detect such anomalies and aid their managements [5, 6]. …