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Sonography and MR Imaging of Bifid Median Nerve with Anatomic and Histologic Correlation

¹ Department of Radiology, University of Michigan Medical Center, 1500 E. Medical Center Dr., TC-2910G, Ann Arbor, MI 48109-0326.
² Toledo Radiologic Associates, Toledo Hospital, Toledo, OH 43528.
³ Department of Pathology, University of Michigan Medical Center, Ann Arbor, MI 48109-0326.
⁴ Department of Radiology, Henry Ford Hospital, 2799 W. Grand Blvd., Detroit, MI 48202-2689.
⁵ Department of Plastic and Hand Surgery, Henry Ford Hospital, Detroit, MI 48202-2689.

Received March 28, 2000; accepted after revision May 5, 2000.

 
Presented at the annual meeting of the American Roentgen Ray Society, Washington, DC, May 2000.

Address correspondence to J. A. Jacobson.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. Imaging of a bifid median nerve has not been previously described in the radiology literature. We present three cases of bifid median nerve. The first is a patient with carpal tunnel syndrome seen on sonography and confirmed at surgery. The other two were found among 10 cadaveric specimens and were imaged with sonography and MR imaging. Confirmation of bifid median nerve in these two specimens was obtained using anatomic and histologic correlation.

CONCLUSION. Sonography and MR imaging can allow effective diagnosis and delineation of a bifid median nerve in the wrist. This diagnosis is important to make before carpal tunnel release or other wrist surgeries are performed to avoid nerve injury. Furthermore, the sonographic size criteria for diagnosing carpal tunnel syndrome in nonbifid median nerves may not be accurate in evaluating bifid median nerves.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
MR imaging is well established as an effective imaging method to evaluate disorders of the wrist including carpal tunnel syndrome [1]. Criteria for diagnosing carpal tunnel syndrome on MR imaging include swelling of the median nerve within and proximal to the carpal tunnel, and palmar bowing of the flexor retinaculum [2]. Sonography has also been proven effective in the diagnosis of carpal tunnel syndrome, relying on enlargement of the median nerve with an area greater than 9 mm² [3] or 10 mm² [4] at the level of the pisiform bone. Given the importance of median nerve size by either imaging method in the diagnosis of carpal tunnel syndrome, an understanding of median nerve variants that potentially affect nerve caliber is critical.

High division of the median nerve proximal to the carpal tunnel or bifid median nerve has been described in the surgery literature as a median nerve anomaly with an incidence of 2.8% [5,6,7,8] (Fig. 1A,1B). To our knowledge, imaging of a bifid median nerve has not been described in the radiology literature. It is also not known whether the size criteria for median nerve enlargement on sonography and MR imaging can be applied to bifid median nerves.

View larger version (51K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —Illustration of median nerves. (Adapted from [5]) Normal median nerve (solid arrow). Note flexor retinaculum (open arrow).

View larger version (59K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —Illustration of median nerves. (Adapted from [5]) Bifid median nerve (solid arrows) and persistent median artery (arrowhead). Note flexor retinaculum (open arrow).

We present sonography and MR imaging findings in three cases of bifid median nerves. One case is that of a patient with carpal tunnel syndrome with surgical correlation. Two additional cases were found among 10 cadaveric specimens and were confirmed with anatomic and histologic correlation.

Subjects and Methods

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Clinical Patient
A 16-year-old girl presented with a 6-month history of pain and numbness in the right hand while driving. Her physical examination and electromyographic findings were consistent with the diagnosis of carpal tunnel syndrome. She under-went sonography of the wrist to evaluate for carpal tunnel syndrome. Sonography was performed by a fellowship-trained musculoskeletal radiologist using an HDI 3000 equipped with a linear 10-MHz probe (Advanced Technology Laboratories, Bothell, WA). Transverse images were obtained at the level of the carpal tunnel. Anteroposterior and transverse dimensions of the median nerve trunks were measured and the cross-sectional area was calculated. Surgery of the wrist was performed by an orthopedic surgeon with extensive experience in wrist surgery. The surgeon had knowledge of the electromyographic and sonographic results before surgery.

Cadaveric Specimens
Institutional review board approval was obtained. Ten random nonembalmed cadaveric upper extremities were examined on sonography by a fellowship-trained musculoskeletal radiologist with musculoskeletal sonography experience. An HDI 5000 equipped with a 12-MHz linear probe (Advanced Technology Laboratories) was used. The frozen cadaveric specimens were thawed before examination. Transverse images were obtained at the level of the radiocarpal joint, pisiform bone, and hamate bone.

MR imaging of the cadaveric wrists was completed using a 1.5-T magnet (Signa; General Electric Medical Imaging, Milwaukee, WI) and a dedicated wrist coil. The MR imaging parameters included T1-weighted (TR/TE, 450/15) axial images with a 10-cm field of view, 256 x 192 matrix, 2 repetitions, 3-mm slice thickness, and 3-mm spacing. MR images were reviewed by a fellowship-trained musculoskeletal radiologist and a musculoskeletal radiology fellow. By consensus, median nerve size and presence of bifurcation were recorded at the level of the radiocarpal joint, pisiform bone, and hamate bone. The cross-sectional area was calculated by multiplying the anteroposterior and transverse dimensions of the nerve trunks.

Subsequently, 3-mm sectioning of the frozen cadaveric wrists was completed using a band saw. Sections of the volar soft tissues were embedded in paraffin, prepared for histologic analysis using H and E stain, and reviewed by a pathologist.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Clinical Patient
In the 16-year-old girl (subject 1), transverse sonographic images at the level of the carpal tunnel showed a bifid median nerve with the radial trunk measuring 3.9 x 2.0 mm (area, 7.8 mm²) and the ulnar trunk measuring 3.5 x 2.0 mm (area, 7.0 mm²) (Fig. 2A,2B,2C). Each nerve trunk was abnormally hypoechoic compared with the alternating hypo- and hyperechoic fascicular pattern of a normal peripheral nerve [9]. At surgery, a bifid median nerve was identified originating proximal to the carpal tunnel (Fig. 2A,2B,2C). Both nerve trunks of the bifid median nerve were compressed within the carpal tunnel.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —16-year-old girl with carpal tunnel syndrome and bifid median nerve. Axial sonogram of abnormal wrist and asymptomatic contralateral wrist shows normal median nerve (curved arrow and "1") on left side (lt) and bifid median nerve (ulnar trunk = open arrows and "1," radial trunk = solid arrows and "2") on right side of image (rt). L = lunate.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —16-year-old girl with carpal tunnel syndrome and bifid median nerve. Sagittal sonogram of abnormal wrist shows ulnar trunk (open arrows, between x's, "1") on left side of image and radial trunk (solid arrows, between +'s, "2") on right side of image of bifid median nerve. L = lunate. Note abnormal hypoechogenicity of median nerve and flexor retinaculum (arrowhead).

View larger version (133K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —16-year-old girl with carpal tunnel syndrome and bifid median nerve. Intraoperative photograph of volar wrist shows ulnar trunk (open arrows) and radial trunk (solid arrows) of bifid median nerve.

Cadaveric Specimens
Sonography of the 10 random cadaveric specimens revealed a bifid median nerve of the wrist in one specimen (subject 2; 70-year-old man) and a partially duplicated median nerve in another specimen (subject 3; 70-year-old woman).

The radial trunk of the bifid median nerve in subject 2 measured 3 x 2 mm (area, 6 mm²), and the ulnar trunk measured 4 x 2 mm (area, 8 mm²) at the level of the radiocarpal joint and the pisiform bone on MR imaging (Fig. 3A,3B,3C,3D). At the hamate bone, the radial trunk measured 1 x 3 mm and the ulnar trunk measured 1 x 3 mm. Each nerve trunk showed the normal alternating hyperechoic and hypoechoic fascicular pattern of a peripheral nerve on sonography. Anatomic sectioning and subsequent histologic examination confirmed the MR imaging and sonographic findings (Fig. 3A,3B,3C,3D). A 1-mm-diameter tubular structure coursed longitudinally and bifurcated between the two nerve trunks. Histologic correlation indicated that the tubular structure interposed between the two trunks of the median nerve represented a vascular structure consistent with a persistent median artery.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —Cadaveric wrist specimen from 70-year-old man with bifid median nerve. Axial sonogram of volar wrist shows ulnar trunk (open short arrows) and radial trunk (solid short arrows) of bifid median nerve. Note persistent median artery (long arrow). R = radius, t = flexor digitorum tendons.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —Cadaveric wrist specimen from 70-year-old man with bifid median nerve. Sagittal sonogram of volar wrist (proximal is left, distal is right) shows radial trunk of bifid median nerve (solid short arrows). Note normal alternating hypoechoic and hyperechoic appearance of median nerve. (R = radius).

View larger version (108K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —Cadaveric wrist specimen from 70-year-old man with bifid median nerve. Axial T1-weighted MR image shows ulnar trunk (open short arrow) and radial trunk (solid short arrow) of bifid median nerve. Note persistent median artery (long arrow). t = flexor digitorum tendons.

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —Cadaveric wrist specimen from 70-year-old man with bifid median nerve. Anatomic section of volar wrist shows ulnar trunk (open short arrows) and radial trunk (solid short arrows) of bifid median nerve. Note persistent median artery (long arrow). t = flexor digitorum tendons.

In subject 3, the median nerve had an elliptic shape and measured 3 x 5 mm on MR imaging at the level of the radiocarpal joint. At the level of the pisiform bone, the median nerve was bilobed; the ulnar lobe measured 2 x 2 mm (area, 4 mm²) and the radial lobe measured 2 x 3 mm (area, 6 mm²) (Fig. 4A,4B,4C,4D). At the level of the hamate bone, the median nerve measured 1 x 6 mm. The median nerve showed the normal alternating hyper- and hypoechoic fascicular pattern of a peripheral nerve on sonography. Anatomic sectioning and histologic examination confirmed the sonographic and MR imaging findings (Fig. 4A,4B,4C,4D).

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —Cadaveric wrist specimen from 70-year-old woman with partially bifid median nerve. Axial sonogram of volar wrist shows bilobed appearance of partially bifid median nerve (arrows). t = flexor digitorum tendons, L = lunate.

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —Cadaveric wrist specimen from 70-year-old woman with partially bifid median nerve. Sagittal sonogram of volar wrist (proximal is left, distal is right) shows partially bifid median nerve (arrows). R = radius, L = lunate.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C. —Cadaveric wrist specimen from 70-year-old woman with partially bifid median nerve. Axial T1-weighted MR image shows bilobed appearance of partially bifid median nerve (arrows). t = flexor digitorum tendons, L = lunate.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4D. —Cadaveric wrist specimen from 70-year-old woman with partially bifid median nerve. Anatomic section shows bilobed appearance of partially bifid median nerve (arrows). t = flexor digitorum tendons, L = lunate.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Anomalies of median nerve anatomy in the wrist described in the surgery literature consist of four different groups of variations. These include accessory branches proximal to the carpal tunnel, accessory branches in the distal carpal tunnel, variations in the course of the thenar branch, and high divisions of the median nerve (or bifid median nerve) [5]. The incidence of bifid median nerves was found to be 2.8% in a dissection study of 246 hands [5]. It is unclear why we identified two forms of bifid median nerves in 10 random cadaveric specimens. The clinical patient and one of the cadaveric specimens in this study had a division of the median nerve proximal to the carpal tunnel, whereas the median nerve became bilobed at the level of the pisiform bone in the other cadaveric specimen. Knowledge of median nerve anomalies is important before performing surgical intervention to avoid potential nerve injury.

The diagnosis of carpal tunnel syndrome on sonography and MR imaging relies on identification of median nerve enlargement [1,2,3,4, 10]. Monagle et al. [2] showed with MR imaging that the median nerve in carpal tunnel syndrome was enlarged both proximal to and within the carpal tunnel. Mean median nerve areas at the level of the pisiform bone were reported as 17.83 mm² with carpal tunnel syndrome and 10.20 mm² in an asymptomatic gender-matched group [2]. In contrast, Duncan et al. [3] reported that a median nerve area greater than 9 mm² at the level of pisiform bone was both sensitive and specific for the diagnosis of carpal tunnel syndrome on sonography. It is unclear why the mean median nerve areas in patients with carpal tunnel syndrome are different for these studies. Although this may be caused by differing patient populations, the sonographic measurements have been proven reproducible; the results of Duncan et al. were comparable with those of earlier works by Buchberger et al. [10]. This latter study concluded that the normal median nerve cross-sectional area (multiplying anteroposterior and transverse dimensions) should not be greater than 10.7 mm² at the level of the pisiform bone on sonography and MR imaging [10]. Regardless of imaging method, the importance of median nerve enlargement in carpal tunnel syndrome is emphasized. No study, to our knowledge, has addressed the issue of size criteria for carpal tunnel syndrome in the setting of a bifid median nerve.

In our clinical patient with carpal tunnel syndrome, the two trunks of the bifid median nerve were abnormally hypoechoic with areas of 7.8 mm² and 7.0 mm² on sonography at the level of the pisiform bone (Fig. 2A,2B,2C). Therefore, each trunk would be considered to be a normal size by established sonographic criteria. However, this patient had clinical, electromyographic, and surgical findings of carpal tunnel syndrome. This discrepancy was anticipated. It is presumed that the area of each nerve trunk of a bifid median nerve would be less than the area of the undivided nerve. Given the smaller area of each trunk, the size threshold for the diagnosis of carpal tunnel syndrome may not be met even in the presence of nerve swelling.

The cadaveric specimen with a complete bifid median nerve did not meet the sonographic size criteria for carpal tunnel syndrome, with the cross-sectional areas of each trunk measuring 6 mm² and 8 mm² (Fig. 3A,3B,3C,3D). The cadaveric specimen with a partially duplicated or bilobed median nerve had an estimated median nerve area of 10 mm² (Fig. 4A,4B,4C,4D). Unfortunately, we could not confirm if the individuals had symptoms of carpal tunnel syndrome before death. The tubular structure interposed between the bifid nerve trunks in subject 2 was consistent with a persistent median artery on sonography, MR imaging, and histology (Fig. 3A,3B,3C,3D). Lanz [5] described a similar finding in five of seven bifid median nerves.

In summary, sonography and MR imaging can effectively reveal the presence of a bifid median nerve. Given that our clinical patient with carpal tunnel syndrome did not reach the sonographic cross-sectional area threshold for this diagnosis, the established size criteria may not be applicable in patients with a bifid median nerve.

References

Top Abstract Introduction Subjects and Methods Results Discussion References

 

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