Article Text

Download PDFPDF

Bifid median nerve: a notable anomaly in carpal tunnel syndrome
Free
  1. Muralidhar Reddy Yerasu1,
  2. Mahmood Ali1,
  3. Ravichander Rao2 and
  4. Jagarlapudi M K Murthy1
  1. 1Neurology, CARE Hospital, Banjara Hills, Hyderabad, India
  2. 2Plastic surgery, CARE Hospital, Banjara Hills, Hyderabad, India
  1. Correspondence to Dr Muralidhar Reddy Yerasu; muralidharnims{at}gmail.com

Statistics from Altmetric.com

Description

A man in his 30s presented with pain and paresthesias of right lateral three fingers and right half of palm for 45 days. Symptoms were intermittent initially, especially at night. After writing semester exams, he reported increased severity and intermittent fasciculation in the right thenar muscles. There was no history of diabetes, hypothyroidism or other systemic diseases. Examination revealed a positive carpal tunnel compression test and mild wasting of abductor pollicis brevis (APB) consistent with carpal tunnel syndrome (CTS). The Boston Carpal Tunnel Questionnaire Symptom Severity Score (BCTQ-SSS) was 3.73, and Functional Severity Score (FSS) was 3. Electrodiagnostic testing showed severe right CTS and fibrillations in APB. Ultrasonography of the carpal tunnel and median nerve showed a bifid median nerve (BMN) at the wrist along with pulsating persistent median artery (PMA) in the centre on both sides. The combined median nerve cross-sectional area (CSA) at the wrist and PMA diameter were 11.5 mm2 and 4 mm on the right side, respectively (figure 1). The median nerve CSA and PMA diameter on the left were 9.8 mm2 and 3 mm, respectively (figure 2). He underwent carpal tunnel release on the right side (figure 3). BCTQ-SSS and FSS were 1.54 and 1.37, respectively, 1 month after surgery.

Figure 1

Ultrasound image of two nerve trunks of the right median nerve (yellow arrows) with the persistent median artery (red arrow).

Figure 2

Ultrasound image of two nerve trunks of the left median nerve (yellow arrows) with the persistent median artery (red arrow).

Figure 3

Perioperative photograph showing two nerve trunks of the right median nerve (yellow arrows) and the persistent median artery (red arrow).

Lanz et al first described BMN.1 Prevalence ranged between 2% and 26% in ultrasound studies.2 The medial and lateral branches are ulnaris and radialis trunks, respectively. BMN is frequently associated with PMA, persistent median vein (PMV) and aberrant muscles.3 PMA is an accessory artery that normally regresses after 8 weeks of intrauterine life. The reported prevalence of PMA in postnatal life is 10%–20%.3 PMV is reported in 4.9%.2 Anomalous muscles associated with BMN include accessory first lumbrical, accessory palmaris longus, prolonged flexor superficialis muscle and palmaris profundus.2 Both MRI and ultrasonography can diagnose median nerve anomalies. The latter has the advantage of being an easy to do, convenient, cheap and dynamic imaging modality. Ultrasound transducers with frequencies of 12 MHz and above can easily detect the median nerve at the wrist. The sum of the CSA of two trunks represents the size of the median nerve in BMN. Colour and pulsed doppler are used in identifying the PMA and PMV. BMN was reported to cause CTS more often than single trunk median nerve.2 4 However, this conclusion was proven wrong in some studies.5 It was shown by Mitchell et al that BMN with a larger radialis trunk is more prone to develop CTS.6 Nerve CSA ≥12 mm2 on ultrasound is considered to be suggestive of CTS in BMN.7 Previous studies showed that larger PMA diameter (>3 mm), internal thrombus, aneurysm and calcified plaques were independent risk factors for entrapment at the wrist.8

In conclusion, although BMN alone is inconsequential in CTS, specific characteristics such as larger BMN and especially radialis trunk, larger and aneurysmal PMA with thrombosis can cause CTS. Moreover, diagnosing this anomaly before carpal tunnel release helps the surgeon to decompress both the trunks and avoid accidental vessel injury.

Patient’s perspective

It was a scary experience to lose the ability to use my hand for daily activities while being in constant pain. I was diagnosed with some rare cause, and it added more to the apprehension. However, the timely intervention resulted in significant improvement and has restored almost normal function in my hand.

Learning points

  • Anomalous high bifurcation of the median nerve proximal to the carpal tunnel is called bifid median nerve (BMN). It is usually accompanied by persistent median artery, persistent median vein and aberrant muscles.

  • Specific characteristic features such as larger BMN (≥12 mm2) and especially radialis trunk, larger and aneurysmal persistent median artery with thrombosis can cause carpal tunnel syndrome.

  • Surgeons should be acutely aware of this anomaly before surgery for successful carpal tunnel release.

Ethics statements

Patient consent for publication

References

Footnotes

  • Contributors MRY has identified the case, confirmed the diagnosis, did the electrodiagnostic and ultrasound studies, written the manuscript; MA has collected the case details and drafted the initial manuscript; RR has done the surgery and managed the case; JMKJ has done the final revision of the manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Case reports provide a valuable learning resource for the scientific community and can indicate areas of interest for future research. They should not be used in isolation to guide treatment choices or public health policy.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.