An atypical presentation of carpal tunnel syndrome

A 45-year-old woman presented with persisting pain and numbness in the right wrist and hand for months. She had previously undergone an urgent percutaneous transluminal coronary angioplasty with stenting of the right circumflex artery for a non-STEMI infarction 8 months before, for which follow-up was uncomplicated. 

Radiography did not show any abnormalities. 

Focused ultrasound showed a tubular hypo-echogenic structure, which could be visualised at the ulnar side of the median nerve. This structure had a slight compressive effect on the median nerve. There was peripheral vascularisation around this structure on duplex ultrasound investigation, but no major blood flow was noted. The radial artery and ulnar artery had a patent aspect. There was no visualisation of musculotendinous pathology. 

Further investigation with magnetic resonance imaging (MRI) was performed.

3-Tesla MR showed a linear structure lying ulnar to the median nerve with discrete contrast enhancement and surrounding oedema. The median nerve had a bifid configuration.

Background
The carpal tunnel is formed by the flexor retinaculum and the carpal bones. The flexor retinaculum is a continuation of the antebrachial fascia and is attached to the pisiform, hamate, scaphoid and trapezium. It contains nine tendons: four flexor digitorum superficialis tendons, four flexor digitorum profundus tendons, and the flexor pollicis longus tendon. The median nerve itself lies between the flexor digitorum profundus and the flexor digitorum superficialis tendons and gradually progresses to a more superficial volar position in the axial plane in around two out of three cases. In one-third of subjects the nerve exhibits either a radial or ulnar curve. [1] It has a mixed motoric-sensory function. Motoric branches innervate the thenar eminence: the opponens pollicis and abductor pollicis brevis muscles are innervated by the thenar branch of the median nerve, whereas the flexor pollicis brevis is normally innervated by the ulnar nerve, though sometimes with a mixed innervation by the ulnar and the median nerve. Sensory branches innervate the radial side of the 4th digit, as well as digits one till three.
Imaging perspective
Normally there are no vascular structures within the carpal tunnel. Two known variants, however, do have vascular structures coursing the carpal tunnel.
- Normally the superficial palmar arch and the deep palmar arch supply blood to the digits. Respectively, they branch from the ulnar and the radial artery. The superficial palmar arch lies volar to the flexor retinaculum, although a known variant of the normal anatomy with a superficial position under the antebrachial fascia is seldom reported.
- An embryological remnant of the axial artery can persist as a side-branch of the ulnar artery: the so-called persistent median artery. This is often associated with a high division of the median nerve, which then divides into two separate branches, both passing through the carpal tunnel: the so-called bifid median nerve. If a bifid median nerve is present, the artery lies in between the two branches. If not, then the artery lies ulnar to the median nerve. [2]
Take-home message
The above-mentioned vascular structures should always be reported when incidentally found on ultrasound examination. Not only are they surgically relevant, but they also have pathological potential as demonstrated by the case above.
Complete relief of the symptoms has been achieved after open carpal tunnel release.
Written informed patient consent for publication has been obtained.