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Bifid median nerve with persistent median artery in carpal tunnel syndrome: assessment with US
Alvarez-Buylla Hospital, Mieres, Asturias. Spain
Mar Perez-Peña firstname.lastname@example.org
Figure 1: Transvese US at the level of carpal tunnel shows a bifid median nerve with two enlarged nerve trunks (radial trunk:white thick arrow, ulnar trunk:white thin arrow) joined by a septum (yellow arrow),underneath the carpal transverse ligament. P: pisiform , S:scaphoid
Figure 2: Doppler ultrasound shows a pulsatile vessel with red arterial flow (white arrow) within the cleft between the two branches of median nerve, consistent with persistent median artery. P:pisiform, S:scaphoid
Figure 3: Cross sectional areas of lateral (left:0,6mm2) branch (white thick arrow) and medial (right:0,5mm2) branch (white thin arrow) of enlarged bifid median nerve, with a total cross sectional area of 11mm2, consistent with carpal tunnel syndrome. P:pisiform, S:scaphoid
Figure 4: Comparison of enlarged right carpal tunnel bifid median nerve (right:A:between callipers) with contralateral (left:B:between callipers ) not enlarged bifid median nerve.
Video 1: Pulsatile persistent median artery in between the two branches of bifid median nerve
Video 2: Bifurcation of median nerve at the level of carpal tunnel
53-year-old woman with paresthesia, pain and weakness of the right hand worsened by sleep, which is mitigated by shaking the hand. She also suffers from sensory deficit in the median innervated region of the hand. Electrophysiologic studies depicted malfunction of the median nerve.
Gray scale ultrasound (US) examination performed on a transverse plane with a 8-14 MHz linear probe shows bifid median nerve with two enlarged nerve trunks joined by a septum underneath the transverse carpal ligament (fig.1). Doppler ultrasound shows vascular pulsatile structure lying anteriorly, in the cleft between the two trunks, consistent with persistent median artery (fig.2). Both, ulnar and radial branches of the bifid nerve are thickened, with an area of 6mm2 for lateral and 5mm2 for medial trunks with a total area of 11mm2 (fig.3), findings consistent with carpal tunnel syndrome.
On the contralateral carpal tunnel there is also a bifid nerve with persistent artery with no signs of enlargement. (fig.4).
Videos 1 and 2 also show the findings described.
A bifid median nerve is an uncommon anatomical variation in the forearm that it can be accompanied by a persistent median artery with an incidence described in the literature of less than 3%.
Although the median nerve usually divides into two or three branches after exiting the distal edge of transverse carpal ligament, sometimes it may divide in two bundles in the distal forearm and appear as a bifid median nerve in the carpal tunnel.
A bifid median nerve may be accompanied by an accessory artery, the persistent median artery of the forearm, which lies in between the two nerve bundles. The artery and bifid nerve can be enclosed by a common epineurium.
Although questioned by authors, bifid median nerve may more commonly develop carpal tunnel syndrome because of its relatively higher global cross sectional area. For bifid median nerve, the size criterion for the diagnosis of carpal tunnel syndrome is slightly higher than for a non-bifid median nerve. Data suggest an optimal cut-off value of 11mm2 for median nerve cross sectional area in bifid nerves.
The diagnosis of carpal tunnel syndrome is traditionally based on clinical history, physical examination, and electrophysiologic study results, but ultrasonography (US) has revealed to be an accurate and useful diagnostic tool in these patients.
US allows depiction of anatomical variants such as bifid median nerve, as well as other causes of compression of median nerve in the carpal tunnel, such as ganglia, tenosynovitis or tumors in the flexor compartment of the wrist. It also provides measurement of median nerve cross sectional area, that contributes to the diagnosis of this condition.
As shown in this case, bifid median nerves associated with a persistent median artery are important to diagnose for their clinical and surgical implications. The knowledge of the existence of bifid median nerve is important in planning surgical decompression of median nerve to avoid nerve injury or potential relapse if decompression of both branches has not been done. It is also important to diagnose a persistent median artery to prevent its section when performing the surgery.
Surgical decompression by transection of the transverse carpal ligament is the indicated treatment of carpal tunnel syndrome. Currently, endoscopic carpal tunnel release is the treatment of choice, but the risk of median nerve injury increases when anatomic variation is present, because of difficulty in seeing the branches at operation. In the case of a bifid median nerve the two branches may be separately constricted and require separate decompression. For these reasons in cases of bifid median nerve an open carpal tunnel release is preferred by surgeons.
OUTCOME & PROGNOSIS:
The patient had an uneventful postoperative course with almost complete resolution of preoperative symptoms in the postoperative visit. The resolution of symptoms was complete and even the sleep disturbance was solved. She was able to perform regular duties with her hand.
1. Ultrasound should be performed preoperatively in carpal tunnel syndrome to reveal morphological variants of the median nerve or other conditions that may compress the nerve in the carpal tunnel.
2. Bifid median nerve with a persistent median artery in the carpal tunnel may be associated with a higher incidence of carpal tunnel syndrome and is very important to diagnose for its clinical and surgical significance.
I declare that consent from the patient was obtained to publish this case.
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