Teaching Case

A 55-year-old patient without any significant medical history was referred to the radiology department by the general practitioner due to persisting pain in her left wrist and numbness of fingers which lasted for approximately 2 years.

Ultrasound examination (US), performed with a high-frequency linear transducer, showed on the level of the left distal forearm proximal to the level of transverse carpal ligament (TCL) incidental finding of a bifid median nerve (BMN). The BMN manifested itself as two oval hypoechoic structures with resemblance to nervous tissue, accompanied by an anechoic tubular structure (Fig. 1) which showed a flow pattern on Color Doppler Ultrasound (CDUS) (Fig. 3), representing persistent median artery (PMA). The examination included morphology, echotexture, and calibre change of the median nerve (MN) and PMA. Ultrasound examination of the contralateral wrist showed normal MN (Fig. 4).

Magnetic resonance imaging (MRI) of the left wrist on PD weighted sequences showed degenerative changes that were presumably the cause of the left wrist pain. The MN was seen at the level of the distal forearm (Fig. 5). Proximally to the TCL, it was divided into BMN shaped like two oval high signal intensity structures on PD weighted sequences with PMA located between the branches (Fig. 6).

Neither US nor MRI showed any pathological abnormalities associated with BMN or PMA.

The median nerve is a nerve that originates from medial and lateral cords of the brachial plexus and contains mixed sensory and motor fibres from roots C5-TH1 [1]. At the level of the wrist, the median nerve is the only nerve that passes through the carpal tunnel and is located between the transverse carpal ligament and tendons of the flexor digitorum superificialis muscle [2, 3].

The median artery is an important artery in embryonic development whilst it represents the dominant blood supply to the hand and normally regresses after the embryonic stage [4]. After 8 weeks of gestation, it is replaced with ulnar and radial arteries [5]. However, it may persevere as PMA that accompanies the median nerve and appears as two types: an antebrachial type that provides blood supply to the median nerve and does not reach the hand; and a palmar type that contributes to the arterial supply of the hand [6].
Lanz classified anatomical variations of the median nerve into four groups: Group I - variations of the course of the thenar branch; Group II - accessory branches at distal carpal tunnel; Group III - high division of the median nerve; Group IV - accessory branches proximal to the carpal tunnel [7]. The reported pooled prevalence of a high division of the median nerve resulting in BMN (Lanz group III) was 2.6% [8]. The prevalence of PMA associated with Lanz group III variation is 63% [8]. Al-Quattan [4] classified six subgroups of BMN according to its associated anomalies, and the one that we are presenting belongs to subgroup II, which represents the BMN with persistent median vessels without any vessels pathology or any other abnormality.

The BMN is considered to be an independent risk factor for the development of carpal tunnel syndrome (CTS) because of its rather high cross-sectional area that occupies more space in the carpal tunnel [9]. In addition, the PMA may dilate or become thrombosed and cause CTS [9]. Ultrasound imaging provides a reliable tool in detecting BMN morphology [10]. As a fast, inexpensive and effective method, in conjunction with CDUS, it can be used to obtain relevant information [11]. It is of great importance that the surgeon is aware of the existence of anatomic variations preoperatively [12]. Therefore, before carpal tunnel release, ultrasound screening can be used to recognise those at increased risk of iatrogenic nerve damage and thus allowing better surgical planning [8].
 

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