High-resolution Ultrasound of the Thenar Motor Branch of the Median Nerve

Transcription

1 High-resolution Ultrasound of the Thenar Motor Branch of the Median Nerve Poster No.: C-1727 Congress: ECR 2016 Type: Scientific Exhibit Authors: F. Zaottini, J. Smith, S. Airaldi, C. Martinoli ; Genova/IT, Rochester/US Keywords: Trauma, Diagnostic procedure, Ultrasound, Neuroradiology peripheral nerve, Musculoskeletal system, Extremities DOI: /ecr2016/C-1727 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 19

2 Aims and objectives The thenar motor branch (TMB) of the median nerve (MN) - which is also referred to as the recurrent motor branch of the median nerve - is a relevant structure within the carpal tunnel region as it supplies the primary motor innervation to the thenar muscles (i.e. opponens pollicis, abductor pollicis brevis and superficial part of flexor pollicis brevis) in most subjects (Figure1-2). As the median nerve courses through the carpal tunnel, it splits into two or three common interdigital nerves. In its typical configuration, the TMB arises from the most radial interdigital nerve at the carpal tunnel outlet and travels vertically in a palmar direction around the distal boundary of the flexor retinaculum to enter the thenar eminence. However, in some individuals the TMB arises from the median nerve within the carpal tunnel and directly penetrates the TCL to enter the thenar musculature (transligamentous course). High-resolution ultrasound of the TMB may be challenging due to its small size, vertical orientation relative to the skin surface, curvilinear course around or through the TCL, and multiple potential anatomic variations. To the best of our knowledge, no attention has still been given in literature to the imaging evaluation of the TMB of the MN. Accordingly, the aim of this study was to describe the normal appearance of this small nerve branch by means of high-resolution ultrasound (US) and to assess the value of this technique in detecting and characterizing TMB neuropathy. Page 2 of 19

3 Images for this section: Fig. 1: Anatomical views of the ventral aspect of the wrist and hand. After removal of the flexor retinaculum, the median nerve (void arrow) is seens dividing into a large radial trunk (thin arrow) and an intermediate (2) and ulnar (3) common digital nerves. The radial trunk gives off the radial collateral nerve of the index, the radial and ulnar collateral nerves of the thumb and the TMB (blue arrow) that assumes a recurrent course to supply the thenar eminence muscles. Note the respective position of the adductor pollicis (AddP), abductor pollicis brevis (ABPB) and opponens pollicis (OppP) relative to the nerve branches. Page 3 of 19

4 Fig. 2: Anatomical views of the ventral aspect of the wrist and hand. Closer look to the origin of the TMB (blue arrow) of the MN. This image shows the intimate relationships between flexor tendons and common digital nerves. Page 4 of 19

5 Methods and materials Ten unembalmed cadaveric wrist-hand specimens were evaluated by placing ml of diluted colored latex adjacent and into in the presumed TMB using direct 16-7MHz ultrasound visualization. At a minimum of 24 hours post-injection, specimens were dissected to identify the course of the TMB and determine whether the sonographicallyguided latex injection had accurately localized the TMB. Presence of latex within and/ or around the TMB versus no latex within the TMB, thickness of the vertical nerve segment measured to the nearest 0.1mm, and variations in the TMB origin, course, or number were recorded. Between March 2011 and July 2015, n=15 patients who had unexplained or disproportionate thenar atrophy or persistent symptoms or signs of thenar motor deficit were found having an abnormal TMB at 17-5MHz US. Seven had concurrent carpal tunnel syndrome, four underwent carpal tunnel release, three had a history of penetrating trauma and one had a mass along TMB course. Correlative 1.5T MRI was performed in six patients. Imaging findings from the patients group were compared with those obtained in n=24 wrists from n=12 healthy volunteers who were free of MN neuropathy and symptoms suggesting TMB neuropathy. Informed consent was obtained from all patients and members of the control group before ultrasound was performed. High-resolution ultrasound was performed with "small parts" broadband linear array transducers (frequency band, 17-5MHz and 12-5MHz). Patients were seated in front of the examiner with the wrist resting on the examination table in an extended position, palm up. The scanning technique basically relied on transverse planes from proximal to distal along the short axis of the MN within the carpal tunnel. To better recognize this small nerve, the distal end of the transducer could be pivoted to orient the scanning plane in the long-axis of the vertical segment of the TMB coursing around the distal TCL. This manoeuvre helped to distinguish the nerve as a continuous threadlike hypoechoic structure from the adjacent soft-tissue echoes as well as to identify the point at which the TMB arises from the MN and crosses the retinaculum (Figure 2). Correlative MR imaging was obtained in n=6 patients using a 1,5 Tesla equipment using a flexible surface-coil and the following protocol: T1-weighted SE (TE=15.0ms TR= ms; matrix: 384x384, slice thickness: 3,5mm, field-of-view: 11.0 x 11.0cm, NEX: 4) and fat-suppressed T2-weighted turbo SE. All acquisitions were obtained on axial planes. Patients were examined in a supine position while keeping the upper arm alongside the body with extended elbow and supinated wrist. Page 5 of 19

6 Results In the cadaveric study, the presumed vertical segment of the TMB was identified and targeted for latex injection. Subsequent dissection confirmed that n=10 sonographicallyguided injections placed latex into and around the TMB of the MN. In this series, all 10 TMBs appeared to travel through a fibrous tunnel formed by the distal edge of the flexor retinaculum and the thenar fascia. No cadaveric specimen demonstrated a true transligamentous course of the TMB. These data are reported elsewhere [1]. In the group of healthy subjects, the TMB was identified in 8/24 wrists at 17-5MHz ultrasound as a very thin linear fascicle detaching from the radial side of the MN and running vertically alongside the ulnar boundary of the abductor pollicis brevis (Figure 3). The TMB then curved to assume a more horizontal course entering the thenar musculature (Figure 4). In the patients group with carpal tunnel syndrome, high-resolution ultrasound demonstrated an abnormally swollen and hypoechoic TMB in association with increased cross-sectional area of the MN, indicating a coexisting compression of the MN trunk and the TMB at the level of the passage through the flexor retinaculum (Figure5-6). The distal edge of the flexor retinaculum appeared thickened in 3/7 (42%) patients. Four other patients had undergone carpal tunnel release and presented with motor deficit in the territory of TMB distribution. None of them, however, revealed either discontinuity of the TMB along the surgical access or a terminal neuroma. The TMBs in these patients were swollen suggesting nerve compression, that may have been possibly established prior to and not following surgical decompression. The ultrasound appearance of these nerves was nearly similar to that observed in the non-operated cases with concurrent MN entrapment. An anomalous transligamentous course and ulnar-sided origin of the TMB was observed in two cases (Figure 7-8). In the 3/15 patients who received a penetrating wound over the ventral wrist, the diagnosis of TMB involvement was based on detection of either a terminal neuroma along the course of the nerve (n=1) or encasement of the nerve bundle by hypoechoic scarring tissue (n=2) (Figure 9-10). Different from the cases described above, the site of nerve lesion was related to the location of the injury and occurred distal to the vertical segment of the TMB. In one patient of our series, a neurogenic mass was detected arising from the TMB. It had a solid hypoechoic appearance and expanded throughout the vertical segment of the TMB (Figure 11 to 16 ). After surgical removal, histopathology revealed a myofibroblastic tumor of the TMB. Page 6 of 19

7 Images for this section: Fig. 3: Ultrasound anatomy of the TMB of the MN.Level of origin and vertical segment. Transverse 17-5MHz US image of the median nerve (MN) just beyond the distal boundary of the flexor retinaculum. One of the most radial fascicles of the median nerve is seen deflecting from the nerve trunk to assume a vertical serpiginous course (void arrow) to approach the thenar muscles. Note the flexor tendons (ft), the abductor pollicis brevis (AbPB) and a superficial thenar branch of the radial artery (thin arrow). Page 7 of 19

8 Fig. 4: Ultrasound anatomy of the TMB of the MN.Distal intrathenar segment. Transverse 17-5MHz US image demonstrates the TMB (large arrow) and a superficial division of the radial artery (narrow arrow) as they course over the tubercle (asterisk) of trapezium and within the fascicles of the abductor pollicis brevis. ft, flexor digitorum tendons. Page 8 of 19

9 Fig. 5: Carpal tunnel syndrome.longitudinal oblique 17-5MHz US image shows the origin of the TMB (arrowheads) from the radial side of the median nerve (MN). The TBM forms a loop passing between the median nerve and the ulnar boundary of the abductor pollicis brevis muscle (AbPB). fpl, flexor pollicis longus tendon Fig. 6: Carpal tunnel syndrome.transverse 17MHz US image obtained just beyond the distal edge of the flexor retinaculum reveals a swollen median nerve (MN) and TMB (arrowhead) in the context of a compressive neuropathy. fpl, flexor pollicis longus tendon. ft, flexor digitorum tendons. Page 9 of 19

10 Fig. 7: Anatomical variations of TMB. Different cases in patients with Carpal tunnel syndrome.transligamentous course of the TMB. Transverse 17-5MHz US image of the distal carpal tunnel reveals the abnormal course of a swollen TMB (void arrow), that crosses the flexor retinaculum (arrowheads) to move forward the thenar eminence muscles. The TMB pierces the retinaculum just superficial to the median nerve (thin arrow). Page 10 of 19

11 Fig. 8: Carpal tunnel syndrome.ulnar-sided origin of the TMB. Beyond the distal boundary of the retinaculum, transverse 17-5MHz US image demonstrates the trifurcation of the median nerve splitting into a radial trunk (1) and intermediate (2) and ulnar (3) common digital nerves. A swollen TMB (arrow) is seen arising from the ulnar side of the radial trunk. ft, flexor digitorum tendons. Fig. 9: TMB nerve injury.following a penetrating injury by a knife, transverse 17-5MHz US image shows a stump neuroma (arrow) of the TMB affecting the horizontal course of this small nerve bundle as it passes superfcial to the abductor pollicis brevis muscle (AbPB). Note the soft-tissue hypoechoic scar (arrowheads) related to the knife wound. ft, flexor digitorum tendons. Page 11 of 19

12 Fig. 10: TMB nerve injury.transverse fat-suppressed T2w tse MR image shows increased signal intensity (arrow) in the opponens pollics, abductor pollicis brevis and superficial belly of the flexor pollicis brevis related to early denervation changes in the TMB territory. fpl, flexor pollicis longus tendon. Page 12 of 19

13 Fig. 11: Myofibroblastic tumor of the TMB. Trasnverse 17-5MHz US image reveals a "C"-shaped elongated solid hypochoic mass (arrows) arising from the radial side of the median nerve and extending superficial to the flexor pollicis longus tendon (fpl) just beyond the distal edge of the flxor retinaculum. The thenar musculature (asterisk) appears diffusely hyperechoic as a results of denervation atrophy. Fig. 12: Myofibroblastic tumor of the TMB.A closer look to the thenar eminence demonstrates increased echogenicity and loss in bulk of the opponens pollicis (asterisk). Met, first metacarpal Page 13 of 19

14 Fig. 13: Myofibroblastic tumor of the TMB.Transverse T1w tse shows denervation changes (asterisk) affecting the bulk of muscles in the thenar eminence. The tumor (arrows) appears isointense to muscles on T1w sequences. Page 14 of 19

15 Fig. 14: Myofibroblastic tumor of the TMB.Transverse fat-suppressed T2w tse shows denervation changes (asterisk) affecting the bulk of muscles in the thenar eminence. The tumor (arrows) appears hyperintense on T2w MR images. Page 15 of 19

16 Fig. 15: Myofibroblastic tumor of the TMB.Surgical view demonstrates the tumor (arrow) connected with the median nerve (arrowhead) Page 16 of 19

17 Fig. 16: Myofibroblastic tumor of the TMB.Surgical specimen. Page 17 of 19

18 Conclusion High-resolution ultrasound can identify the TMB of the MN and characterize its abnormalities, providing unique information about this small nerve branch of the hand. Page 18 of 19

19 References Al-Quattan. Variations in the course of the thenar motor branch of the median nerve and their relationship to the hypertrophic muscle overlying the transverse carpal ligament. J Hand Surg Am 2010; 35: Smith J, Barnes DE, Barnes K, Strakowski J, Martinoli C. Sonographic visualization of thenar motor branch of the median nerve: a cadaveric validation study. J Hand Surg (manuscript submitted) Page 19 of 19