High-resolution ultrasound visualization of the recurrent motor branch of the median nerve: normal and first pathological findings
High-resolution ultrasound visualization of the recurrent motor branch of the median nerve: normal and first pathological findings
Georg Riegler 0 1 2 3
Christopher Pivec 0 1 2 3
Hannes Platzgummer 0 1 2 3
Doris Lieba-Samal 0 1 2 3
Peter Brugger 0 1 2 3
Suren Jengojan 0 1 2 3
Martin Vierhapper 0 1 2 3
Gerd Bodner 0 1 2 3
0 Department of Anatomy, Center for Anatomy and Cell Biology, Medical University of Vienna , Währingerstrasse 13, 1090 Vienna , Austria
1 Department of Neurology, Medical University of Vienna , Währingergürtel 18-20, 1090 Vienna , Austria
2 Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna , Währingergürtel 18-20, 1090 Vienna , Austria
3 Department of Surgery, Division of Plastic and Reconstructive Surgery, Medical University of Vienna , Währingergürtel 18-20, 1090 Vienna , Austria
Purpose To evaluate in a prospective study the possibility of visualization and diagnostic assessment of the recurrent motor branch (RMB) of the median nerve with high-resolution ultrasound (HRUS). Materials and methods HRUS with high-frequency probes (18-22 MhZ) was used to locate the RMB in eight fresh cadaveric hands. To verify correct identification, ink-marking and consecutive dissection were performed. Measurement of the RMB maximum transverse-diameter, an evaluation of the origin from the median nerve and its course in relation to the transverse carpal ligament, was performed in both hands of ten healthy volunteers (n = 20). Cases referred for HRUS examinations for suspected RMB lesions were also assessed. Results The RMB was clearly visible in all anatomical specimens and all volunteers. Dissection confirmed HRUS findings in all anatomical specimens. Mean RMB diameter in volunteers was 0.7 mm ± 0.1 (range, 0.6-1). The RMB originated from the radial aspect in 11 (55%), central aspect in eight (40%) and ulnar aspect in one (5%) hand. Nineteen (95%) extraligamentous courses and one (5%) subliga mentous course were detected. Three patients with visible RMB abnormalities on HRUS were identified. Conclusion HRUS is able to reliably visualize the RMB, its variations and pathologies. Key Points Ultrasound allows visualization of the recurrent motor branch of the median nerve. Ultrasound may help clinicians to assess patients with recurrent motor branch pathologies. Patient management may become more appropriate and targeted therapy could be improved.
Median nerve; Carpal tunnel syndrome; Ultrasound; Iatrogenic disease; Anatomical variation
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The recurrent motor branch (RMB), sometimes also referred
to as the muscular thenar branch of the median nerve,
classically supplies innervation to the thenar musculature, including
the abductor pollicis brevis, the opponens pollicis and the
superficial head of the flexor pollicis brevis (Fig. 1) [1, 2].
These contribute to the most important movements of the
hand: opposition and abduction of the thumb.
Damage to the RMB may lead to severely impaired
function in patients, with loss of dexterity, pinch and grasp
function. The main clinical relevance of the RMB is its
susceptibility to iatrogenic injury, due to its variants, during
decompression surgery for carpal tunnel syndrome (CTS) [3]. This is
because anatomical studies have shown that there is high
Fig. 1 Illustration of the regular branching of the median nerve with an
extraligamentous recurrent motor branch coursing toward the thenar
musculature
variability with regard to the origin from the median nerve [4,
5] and its course in relation to the transverse carpal ligament
(TCL) [4, 6–9]. Moreover, accessory motor branches have
also been described [2, 4, 10–12].
Isolated RMB neuropathies are either rare or possibly
underdiagnosed due to the lack of imaging modalities that
can depict the nerve. Among these RMB neuropathies,
anecdotal reports describe compression of the nerve due to
schwannomas [13–15], ganglia [16, 17], anomalous
anatomical structures [18, 19], long distance cycling [20] or cutting
injuries [21, 22]. Furthermore, there is an ongoing debate
about whether selective involvement of the thenar motor
fibres is a variant of CTS or an idiopathic entity [23–26].
To date, evaluation and localization of the RMB has been
restricted to electrophysiological assessment [26] and clinical
testing using landmarks [27, 28].
High-resolution ultrasound (HRUS), using linear,
highfrequency probes, offers excellent tissue differentiation for
the examination of superficial structures and may facilitate
imaging of the RMB. As this has been described for the
palmar cutaneous branch of the median nerve [29], which can be
assumed to have a comparable diameter, we hypothesized that
RMB evaluation would be possible with HRUS. This may
open the possibility of diagnosing pathologies related to the
nerve, or allow for pre-surgical evaluation or marking in case
of suspected variations, and, thus, reduce the risk of iatrogenic
injuries.
Therefore, this study aimed to: (i) confirm the correct
identification of the RMB by HRUS with ink-marking and
consecutive dissection in anatomical specimens; (ii) provide the
first measurements of RMB diameter, evaluating the origin,
course and possible accessory branches in healthy volunteers;
and (iii) present cases with RMB pathology found with
HRUS.
Ultrasound technique
This prospective study was approved by the ethics committee
of the Medical University of Vienna (EC-number 1529/2015)
and was conducted between 1 February 2015 and 1 December
2015.
HRUS examinations were performed using a GE LOGIQ e
(GE Healthcare, Wauwatosa, WI, USA) ultrasound (US)
platform with high-frequency probes (GE L8-18i-D, GE
L10-22RS). Two radiologists carried out all examinations. One had
more than 20 years’ experience (G.B.) and one had 4 years’
experience (G.R.) in peripheral nerve imaging. Both raters
were present during the collection of the subjects. G.B.
performed all the interventions on all the anatomical specimens.
G.R. collected all the images of healthy individuals, with G.R.
watching the procedure.
The examination followed a standardized assessment
protocol that started with the transverse view of the
median nerve or its digital cutaneous branches at the level
of the metacarpal bodies III-IV. The probe was moved
proximally until a tubular structure arising from the
median nerve, in most cases curving radially and proximally
and coursing toward the thenar musculature, was
presumed to be the RMB. Subsequently, the origin was
assessed by turning the probe until the longitudinal axis
of the nerve was visible. In most cases, the RMB formed
an approximately 45° angle with the median nerve.
Subsequently, the nerve was followed until its entrance
into the thenar musculature. To avoid confusion with the
palmar cutaneous branch or the palmar digital branch of
the median nerve, the RMB had to enter the thenar
musculature in contrast to the other branches. To avoid
confusion with vessels, colour Doppler was used. Probe
positioning, probe track and measurement of the RMB
diameter is presented in Fig. 2. The normal presentation of
the RMB at its origin is presented in Fig. 3.
A second possibility to locate the RMB (considered by
the authors to be more difficult) was to start with the
transverse view of the median nerve 3 cm proximal to
the pisiform bone. Following the median nerve, the probe
was moved distally until its subdivision into terminal
branches. At this level, the probe was moved proximally
and distally to identify the RMB. After identification, the
assessment of the nerve was performed in a manner
similar to that described above.
Fig. 2 (a) Probe positioning at the origin of the recurrent motor branch
(RMB). The red dotted line indicates the track of the probe for full RMB
examination. To obtain transverse views of the RMB probe, the
orientation needs to be perpendicular to the dotted line. (b) Example of
RMB transverse diameter measurement (arrow) in a healthy volunteer
(0.8 mm). APB abductor pollicis brevis muscle, FT flexor tendon, MN
median nerve
Ultrasound in anatomical specimens
Ultrasound in healthy volunteers
In four randomly selected fresh anatomical specimens in
the legal custody of the Department of Systematic
Anatomy, Medical University of Vienna, HRUS was
performed as described above in both wrists (n = 8). After
locating the RMB, a small amount of blue dye mixed
with glue (0.1 ml) was injected into the nerve/adjacent
to the nerve under HRUS guidance (22-gauge needle,
inplane technique). Subsequent anatomical dissection was
performed to confirm the exact location of the dye
injection. A plastic surgeon (M.V.) and anatomist (P.B.) who
performed the dissections determined the exact location
of the dye injection. Correct dye injection was noted if at
least some amount of the dye was injected into the nerve
sheath.
Ten healthy volunteers were recruited via notices at the
Department of Biomedical Imaging and Image-guided
Therapy and word-of-mouth acquisition. Written informed
consent was obtained from all volunteers. Inclusion criteria
were age over 18 years, and exclusion criteria were known
polyneuropathy, known myopathy, chronic disease known to
cause peripheral neuropathy, current or previous CTS and
previous hand surgery.
T h e R M B w a s a s s e s s e d o n b o t h s i d e s ( n = 2 0 ) .
Measurements of the maximum transverse diameter were
obtained immediately after the separation from the median nerve
using the platform software of LOGIQ e. The origin of the
RMB with respect to the median nerve was assessed
according to Mackinnon and Dellon (4). The site of origin of the
Fig. 3 (a, b) Example of
sonographic findings of the
recurrent motor branch (RMB) at
its origin from the central aspect
of the median nerve in a
radioulnar transverse view. (c, d)
Example of sonographic findings
on a long-axis view of the RMB at
its origin from the central aspect
of the median nerve coursing
proximally toward the thenar. The
median nerve and flexor tendons
are obliquely projected. Note the
approximately 45° angle of the
RMB with the median nerve. FT
flexor tendon, MN median nerve,
DIST. distal, PROX. proximal
RMB from the median nerve was classified as either ulnar,
central-volar or radial, since the intermediate type (one-third
the distance between the radial and central aspect) described
by Mackinnon and Dellon [5] is not clearly distinguishable
with US.
The course of the RMB in relation to the transverse carpal
ligament (TCL) was evaluated using the classification of Lanz
[8]: extraligamentous – origin and course distal to the TCL;
subligamentous – origin within the carpal tunnel, winding
around the distal edge of the TCL; transligamentous – piercing
the TCL. Furthermore, the presence of accessory branches
was evaluated.
Ultrasound in patients
Between 1 February 2015 and 1 December 2015, we
monitored patients who were referred to the Department of
Biomedical Imaging and Image-guided Therapy, and in whom
RMB pathologies were detected with HRUS. The referring
diagnosis of all screened patients was clinically or
electrophysiologically diagnosed carpal tunnel syndrome. All
patients were referred to our department for preoperative
evaluation prior to carpal tunnel surgery. HRUS examinations were
performed using the same assessment protocol as described
above.
Descriptive statistics were performed using IBM SPSS
Statistics for Windows Version 22.0.0.2 (IBM, Armonk, NY,
USA). Metric data (nerve diameter) are presented as mean ±
standard deviation and range (minimum–maximum).
Ultrasound in anatomical specimens
The RMB was clearly visible in all anatomical specimens.
Dissection confirmed the correct identification of the RMB
(100%) on both sites in all subjects (n = 8). An example of a
dissection finding is shown in Fig. 3.
Ultrasound in healthy volunteers
Table 1 shows a summary of all demographic findings and
measurements. Five females and five males (mean age,
31.5 years; age range, 27–54 years) were included in the
study. The RMB could be visualized in both wrists of all
volunteers (n = 20). Assessment of the nerve was possible
from its origin until its ramification into terminal branches.
Some of these branches could even be visualized in the thenar
musculature. Sonographically, the RMB appeared as a
hypoechoic, round dot in the transverse view, with a small
surrounding hyperechoic border. While the hypoechoic dot
was clearly depictable, the surrounding hyperechoic tissue,
which we presumed to be the epineurium, was not well
distinguishable from the adjacent tissue. Individual fascicles of
the RMB could be seen in only a few cases.
The mean transverse diameter was 0.7 mm ± 0.1 (range,
0.6–1). The maximum detectable intraindividual side
difference was 0.03 mm.
The RMB originated from the radial aspect in 11 hands
(55%), the central aspect in eight hands (40%), and the ulnar
aspect in one hand (5%).
An extraligamentous course was seen in 19 hands (95%),
and a subligamentous course in one hand (5%). No
transligamentous course was observed. One accessory branch
arising from the radial aspect of the median nerve, with a
maximum transverse diameter of 0.6 mm, was detected
(Fig. 4 and Movie).
Ultrasound in patients
Of 189 patients with carpal tunnel syndrome, three patients
(1.6%) with RMB pathologies were identified and are
presented below. Figure 5 shows the HRUS findings of all patients.
Case 1 A 51-year-old female presented with a weakening of
the thenar musculature for the 6 months prior to presentation,
at the left wrist, sometimes combined with slight pain and
paraesthesias in the first finger on her left side. Clinical
examination revealed a loss of power (0/5) of thumb abduction,
thenar wasting and a mild hypoesthesia of the first digit.
Motor conduction studies showed severe axonal damage of
motor fibres on the left (compound muscle action potential:
left 1,500 μV, right 18,700 μV) and prolongation of distal
motor latency, while sensory testing revealed a preserved
sensory nerve action potential with a only slightly reduced
amplitude of 12 μV and a decreased antidromic conduction
velocity of 44 m/s. Sonographic assessment revealed a radially
originating, extraligamentously coursing and severely
thickened RMB (1.5 mm vs. 0.9 mm on the right). Further, the
whole median nerve also exhibited an increased
crosssectional area of 0.23 cm2 within/distal to the carpal tunnel
(upper limit normal ≤0.12 cm2). The clinically visible atrophy
was also documented with US (Fig. 5). In accordance with the
present findings, this was assumed to be CTS with severe
involvement of the RMB.
Case 2 A 49-year-old female presented with paresis of the
thenar muscles on her right side for the 4 months prior to
presentation. No paraesthesias or pain were reported.
Clinical examination revealed paresis of thumb opposition
(2/5) and mild paresis of thumb abduction (4/5). Atrophy of
the lateral thenar and the opponens pollicis muscle was
Table 1 Demographic
characteristics, measurement of
transverse diameter, origin,
course, and branches of the RMB
in healthy
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
RMB recurrent motor branch, No. number, M male, F female, y years, R right, L Left, mm miliimeter, TD
transverse diameter/mean cross sectional diameter, (a) accessory branch, Cen central, Rad radial, Uln ulnar,
Extra extraligamentous, Sub subligamentous
visible. Sonographic assessment revealed a radially
originating, extraligamentously coursing, moderately thickened RMB
(1.3 mm) and a normal cross-sectional area of 0.10 cm2
(standard value ≤0.12 cm2) of the median nerve. Further, atrophy
of the thenar musculature was assessed (Fig. 5). The patient
was operated upon approximately 2 months after HRUS
examination. There was a positive correlation between the
sonographic findings and surgery. Intraoperatively, the RMB was
thickened and seemed to be entrapped in ‘fibrous tissue’
directly after its origin from the median nerve, as classified by
the surgeons. The fibrous tissue was removed and neurolysis
was performed. A short-term follow-up 5 weeks after surgery
revealed improved power (3/5) of thumb opposition and
thumb abduction (5/5). In accordance with the present
Fig. 4 Example of finding in a dissection after high-resolution
ultrasound (HRUS)-guided, intraneural ink-marking of the recurrent
motor branch
findings, this was assumed to be a variant of CTS with
involvement of the RMB.
Case 3 A 45-year-old female presented with a severe paresis
of the thenar musculature on her right side for the 6 months
prior to presentation. No paraesthesias or pain were reported.
Clinical examination revealed plegia of thumb abduction
(0/5). A clear thenar atrophy was visible. Sonographic
assessment revealed a radially originating, extraligamentously
coursing and moderately thickened RMB (1.3 mm).
Moreover, the radial-sided motor fascicles within the median
nerve were clearly swollen. Further, atrophy of the thenar
muscles was assessed (Figs. 5 and 6). In accordance with the
present findings, this was assumed to be a variant of CTS with
involvement of the RMB.
This study confirms the reliable visualization of the RMB with
HRUS using US-guided ink-marking and consecutive
dissection in a series of anatomical specimens. Initial measurements
of the RMB in healthy volunteers showed a mean transverse
diameter of 0.7 mm ± 0.1 mm (range, 0.6–1). Subsequently, a
broad variation in point of origin and course was observed.
Fig. 5 (a, b) Example of
sonographic findings of the
recurrent motor branch (RMB)
originating from the ulnar aspect
of the median nerve. (c, d)
Example of sonographic findings
in the same volunteer. The RMB
crosses the anterior aspect of the
median and courses beneath the
transverse carpal ligament (TCL;
subligamentous) toward the
thenar musculature. FPB flexor
pollicis brevis muscle, FT flexor
tendon, MN median nerve, SUBL
subligamentous
Ultrasound measurements of the transverse diameter of the
RMB in healthy volunteers revealed clearly lower values than
previously described in anatomical studies. Wang and
colleagues [30] reported a mean diameter of 1.7 ± 0.3 mm in
Fig. 6 (a, b) Findings in patient 1, showing an enlarged recurrent motor
branch (RMB) (encircled) atop the enlarged median nerve just distal to
the carpal tunnel and a clear thenar atrophy on the patient’s left side. (c, d)
Findings in patient 2, showing a thickened RMB (encircled). The median
nerve is obliquely projected. (d) Intraoperative findings of the RMB after
removal of fibrous tissue surrounding the branch. (e, f) Findings in patient
3, with swollen radial-sided motor fascicles within the median nerve and a
clear thenar atrophy on the patient’s right side
seven fresh forearm amputation specimens, and Üstün et al.
[31] reported a mean diameter of 1.4 mm ± 0.12 mm in ten
fresh cadaver arms. This discrepancy in nerve diameter may
be due to the different visualization methods employed. In our
study, we could measure only the hypoechoic pattern of the
nerve without the surrounding neural tissue. Measurements of
the transverse diameter of the RMB with US have not yet been
reported. As presented, the transverse diameter in patients
clearly exceeded the upper limit observed in healthy
volunteers. Therefore, HRUS may help to evaluate RMB
pathologies, and our data may serve as a reference for further, more
detailed US characterizations of the RMB.
In addition to the depiction of the RMB, HRUS provides
further information about the course, origin and branching of
this nerve. The course of the RMB in relation to the TCL has
gained extensive attention in various studies, due to the fact
that the ‘anomalous’ trans- and subligamentous variants, in
particular, are at risk during both open and endoscopic surgery
[3, 7, 8, 32]. Our results are comparable with previous studies
[4, 6–9] (extraligamentous, 46–97%; subligamentous, 2–
34%; transligamentous, 1–23%), although we did not detect
a transligamentous course, which is mainly attributable to the
small study sample.
The origin from the radial aspect of the median nerve was
described as a possible site of entrapment because of separate
obliquely arranged fibres from the TCL encircling the RMB
[7]. As the radial origin is most common and the results of
previous studies are in accordance with our data (55% vs. 60–
80%) [4, 5], we think this condition can be easily detected by
HRUS. The less frequent origin from the ulnar aspect of the
median nerve (5% in our study vs. 1.1% in a study of 821
hands that had undergone carpal tunnel (CT) release surgery)
[9] represents a major risk for iatrogenic injury with the ulnar
side approach, as well as with the median approach, since the
nerve crosses the anterior aspect of the median nerve during
CT release [33].
Iatrogenic RMB injury during CT release seems to be a rare
complication, with approximately 0.5% in the reported
literature [3, 32]. Nevertheless, it represents a severe complication
also called the ‘million dollar injury’ due to the compensation
awarded in lawsuits because of the loss of thenar function [34,
35]. To date, no consensus exists about whether the branch
should be examined intra- or preoperatively to avoid damage.
Of 153 surgeons responding to a questionnaire, the majority
(>70%) did not explore the nerve routinely and did not
recommend doing so [36]. Other authors suggest exploring the
RMB intraoperatively, at least for some special conditions
[37]. Indeed, whenever the surgeon encounters muscle fibres
lying superficial to or interposed within the TCL, there is a
greater than 90% likelihood that the motor branch would be
anomalous [37]. The preoperative RMB localization is limited
to the use of surface landmarks, such as the Kaplan’s cardinal
and middle finger radial-side-line, or a physical examination
manoeuvre, such as the middle finger flexion test [27, 28].
Nevertheless, these tests are inaccurate when the RMB has a
varying course. As an example, in middle finger flexion tests,
the transligamentous course showed a deviation of 10–25 mm
from where it was expected [27, 28]. RMB evaluation with
HRUS overcomes all these limitations. It allows visualization
of the nerve along its entire course, which may help surgeons
to plan their approach for CT release. In the case of a
‘dangerous’ variation, preoperative skin-marking could be provided
to facilitate exploration of the nerve. Although we did not
observe an iatrogenic injury prior to re-operation during our
short observation period, we saw one patient postoperatively
who was treated for a complete transection of the RMB during
carpal tunnel release. Therefore, the results of this study
suggest that HRUS evaluation of the RMB should be included as
part of the conventional sonographic examination for CTS to
minimize iatrogenic injury during CT release.
In 1982, Bennet and Crouch [23] reported two cases of
isolated compression of the RMB, characterized by
selective involvement of thenar motor fibres. In these cases,
the surgical observation showed compression of the
branch due to a transligamentous course or an excessive
angle of the thenar branch at the distal edge of the
transverse ligament with neuroma formation proximal to the
entrapment sites. Four subsequent electrophysiological
and clinical studies underlined the theory that motor
fascicles alone may be involved in CTS, or represent a
separate entity without the classic CTS [24–26, 38]. Our case
2 and case 3 may provide a hint that these conditions may
be detectable by HRUS in the future. Nevertheless, further
comparative HRUS studies between normal and
pathological RMB conditions are needed to reliably answer this
question.
This study has several strengths and limitations. Its
strengths include the first-time use of HRUS for specific
assessment of the RMB and confirmation of the
findings by the gold standard of anatomical dissection. Its
limitations include the fact that findings in vivo were
uncontrolled. However, accuracy in anatomical
specimens was 100% and RMB in volunteers could be
followed into the abductor muscle. A further limitation
is the fact that pathological findings in this manuscript
are case reports and do not provide reliable information
about the future role of HRUS in RMB pathology
detection. For this reason, further comparative studies
between normal and pathological conditions are needed.
In conclusion, this study confirms the reliable ability to
visualize the RMB and its variations with HRUS, in
anatomical specimens and in healthy volunteers. We therefore
encourage the use of HRUS, especially for preoperative
evaluation for carpal tunnel release or if thenar muscle weakening is
present. Further studies are needed to assess the value of
HRUS in diagnosing RMB pathologies.
Acknowledgements Open access funding provided by Medical
University of Vienna. The authors thank Mary McAllister for her
comments on the manuscript. This work was orally presented at the ECR 2016
in Vienna. The scientific guarantor of this publication is Gerd Bodner.
The authors of this manuscript declare no relationships with any
companies whose products or services may be related to the subject matter of the
article. The authors state that this work has not received any funding. No
complex statistical methods were necessary for this paper. Institutional
Review Board approval was obtained. Written informed consent was
obtained from all subjects (patients) in this study. Methodology:
prospective, experimental, performed at one institution.
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