Keywords carpal tunnel syndrome - median nerve - surgical procedures - prevalence
Introduction
Carpal tunnel syndrome (CTS) is characterized by median nerve compression at the wrist
level. It is the most common compressive neuropathy of the upper limb.[1 ] Surgical treatment with the classical open carpal tunnel release (OCTR) is one of
the elective procedures most performed in hand surgery.[1 ]
[2 ] Proper preoperative planning, a reliable anatomical knowledge, and an understanding
of unexpected anatomical differences are indispensable requirements to prevent negative
surgical outcomes. This is especially true in a region with frequent intra- and extra-carpal
tunnel anatomical variations, with widely varying prevalence values according to the
study design.[3 ]
[4 ] In addition, combination of these anatomical variations and their relationship with
CTS remain inconsistently and poorly described by the literature. Well-designed primary
studies with good methodological quality on the prevalence of anatomical variations
in patients with CTS are uncommon.[3 ]
[4 ] As such, a study design was proposed based on the routine clinical practice of CTS
experts in an attempt to evaluate the prevalence of anatomical variations in patients
undergoing classical OCTR.
Materials and Methods
This is a primary, single-center, transversal prevalence study approved by the ethics
and research committee of our institution under the number 11/19. A total of 115 patients
were included (n = 115), all females aged from 40 to 80 years old who presented unilateral
or bilateral involvement. All subjects agreed to participate in the present study
and signed the informed consent form. The patients underwent a previous conservative
treatment with no effective clinical improvement and presented a probable CTS clinical
diagnosis equal to or greater than 12 points, according to the list of six clinical
criteria for the diagnosis of CTS (CTS-6) of Graham et al.[5 ] The CTS-6 was used to clinically evaluate potentially eligible patients, and their
respective scores were the following: paresthesia (3.5), nocturnal paresthesia (4.0),
thenar muscles hypotrophy and/or atrophy (5.0), Tinel test (4.0), Phalen test (5.0),
and 2-point discrimination static test (5.0). Subjects with cervical radiculopathy,
other compressive syndromes of the upper limbs, polyneuropathy, history of previous
surgical carpal tunnel release, and sequelae from wrist fracture were excluded.
After clinical evaluation, the eligible patients were referred for further tests.
A cross-sectional area ≥ 10.0 mm2 at the proximal anatomical limit of the carpal tunnel, directly measured using wrist
ultrasound, and a median nerve sensory conduction rate lower than 50 m/s with a median
nerve distal motor latency of 4.2 milliseconds or more at upper limb electroneuromyography
confirmed CTS diagnosis.[6 ]
[7 ]
[8 ]
[9 ]
[10 ]
[11 ] Next, the patients were referred and submitted to surgical treatment at an outpatient
facility. The procedure was performed consecutively at the operating room by the same
hand surgeon. At the operating room, the patients were placed on the table in horizontal
supine position with the hand and wrist supported by an auxiliary table.[12 ] The Bier intravenous regional anesthesia technique was performed on the affected
limb.[13 ] The surgical technique, classical OCTR, started with a palmar longitudinal incision
of about 2 cm in length, which was aligned with the third interdigital space of the
hand but with no proximal extension beyond the distal flexion fold of the wrist or
distal extension beyond the Kaplan line ([Figure 1 ]). The surgical approach was performed by anatomical planes: skin, subcutaneous tissue,
palmar fascia, and transverse carpal ligament, which was opened completely distally
and proximally to the carpal tunnel, largely releasing the median nerve ([Figure 2 ]).[12 ]
[13 ]
[14 ]
Fig. 1 Surgical incision planning using the classical open carpal tunnel release technique.
Fig. 2 Surgical approach showing the transverse carpal ligament (arrow) completely open.
During surgery, a complete inventory of the surgical wound was performed to detect
intra- and extra-carpal tunnel anatomical variations ([Figures 3 ] and [4 ]).
Fig. 3 Surgical finding of the transverse carpal muscle.
Fig. 4 Surgical finding of the bifid median nerve associated to a persistent median artery.
After surgical treatment, the patients were discharged on the same day and followed
up at an outpatient facility for 6 months. The total number of recruited eligible
patients was 10% more than the required amount to cover possible losses or exclusions
during the study. [Figure 5 ] shows each step of the study.
Fig. 5 Study flowchart.
Statistical Analysis
Categorical variables were presented as relative and absolute frequencies, whereas
numerical variables were shown as summary measures.[15 ] The prevalence of intra- and extra-carpal tunnel anatomical variations was presented
as relative and absolute frequencies, as well as their respective 95% confidence intervals,
which were calculated using the statistical software STATA 12 (Stata Corp. LLC, College
Station, TX, USA).
Results
Patients Features
A total of 115 patients were evaluated. Their mean age was 52.9 years old (standard
deviation, 9.1 years), ranging from 40 to 79 years old. Median age was 52 years old.
Mean disease duration was 4.0 years (standard deviation, 3.2 years), ranging from
1 year to 20 years. Median disease duration was 3 years. As for laterality, 109 patients
(90.8%) had bilateral CTS and 6 patients (5.2%) had unilateral CTS. The right side
was affected in 59 patients (51.3%), and the left side was affected in 56 patients
(48.7%). Of the 73 patients (63.5%) with intra- and extra-carpal tunnel anatomical
variations, 38 patients (52.0%) were operated on the right hand, and 35 patients (48.0%)
were operated on the left hand. Sixty-five subjects (89.0%) were right-handed, whereas
8 patients (11.0%) were left-handed.
Prevalence of Anatomical Variations
The total prevalence of anatomical variations in patients from this study was 63.5%
(95% confidence interval [CI], 54.5–72.4%). Regarding extra-carpal tunnel anatomical
variations, the prevalence of a transverse carpal muscle was 57.4% (95% CI, 47.8–66.6%).
As for intra-carpal tunnel anatomical variations, the prevalence of bifid median nerve
with a persistent median artery was 1.7% (95% CI, 0.0–4.2%). Regarding combined intra-
and extra-carpal tunnel variations, the prevalence of a bifid median nerve associated
with a persistent median artery and transverse carpal muscle was 1.7% (95% CI, 0.0–4.2%).
Other observed anatomical variations were a transverse carpal muscle associated with
a transligamentous recurrent branch of the medial nerve, a transverse carpal muscle
associated with the muscular belly of the flexor and longus digitorum superficialis
tendon and a subligamentous recurrent branch of the median nerve, each with a 0.9%
prevalence (95% CI, 0.0–2.6%). [Table 1 ] summarizes the results from this study.
Table 1
Anatomical variations
N (%)
95% CI (%)
TCM
66 (57.4)
47.8–66.6
BMN + PMA
2 (1.7)
0.0–4.2
BMN + PMA + TCM
2 (1.7)
0.0–4.2
TCM + TRB
1 (0.9)
0.0–2.6
TCM + MB
1 (0.9)
0.0–2.6
SRB
1 (0.9)
0.0–2.6
TOTAL
73 (63.5)
54.5–72.4
Discussion
This study follows the research line on CTS from our institution and originated from
a clinical accuracy trial based on ultrasound and nerve conduction studies effectiveness
for CTS diagnosis.[16 ] Carpal tunnel anatomical variations were classified by Singer and Asworth[17 ] as type I, intrinsic, or intra-carpal tunnel, when found inside the carpal tunnel,
and type II, extrinsic, or extra-carpal tunnel, when lying over the transverse carpal
ligament. In our study, both intra- and extra-carpal tunnel anatomy, whether associated
or not, were directly related to a probable clinical diagnosis of CTS.[18 ]
[19 ] According to Green and Morgan,[20 ] in the presence of an extra-carpal tunnel anatomical variation, such as a transverse
carpal muscle, there is a greater than 90% probability that the recurrent branch of
the median nerve is anomalous, like the transligamentous branch found in our study.
The younger age group and the dominant hand of CTS patients from our study were not
determining factors correlated to a higher prevalence of intra- and extra-carpal tunnel
anatomical variations as reported by Singer and Asworth.[17 ] The bifid median nerve is an intra-carpal tunnel anatomical variation represented
by a high bifurcation at the median nerve, proximal to the carpal tunnel.[21 ]
[22 ] This anatomical variation can coexist with a persistent median artery in 45% of
the cases.[19 ]
[22 ]
[23 ] The prevalence of a bifid median nerve associated with a persistent median artery
and the total prevalence of intra-carpal tunnel anatomical variations found in our
study were similar to those reported by Lindley and Kleinert in their surgical study.[3 ] The transverse carpal muscle is an extra-carpal tunnel anatomical variation described
by Rangoowansi et al.[24 ] as transverse muscle fibers that cover the transverse carpal ligament or interfere
with it. In our study, the prevalence of the transverse carpal muscle was higher than
expected, especially when compared with the prevalence reported by the surgical literature.[20 ]
[21 ]
[24 ]
[25 ]
[26 ]
[27 ] Tuncalli et al.[26 ] warned us about it, stating that the transverse carpal muscle was described in a
similar way in most studies, but it was interpreted differently by several authors.
Tuncalli believed that the frequency of the transverse carpal muscle was underestimated
in the literature, with a higher prevalence than previously thought, because when
it was found during OCTR it was erroneously interpreted as a hypertrophic thenar muscles,
as a short palmar muscle or it was not aligned with the third interdigital space of
the hand due to a more radial surgical incision, not complying with the surgical technique.[26 ] The main limitation of this study was the absence of a study group in cadavers of
the intra- and extra-carpal tunnel anatomical variations found in our surgical study.
The strength of this study was its design and consistency using a sample of patients
with a high probability of CTS diagnosis and who underwent a good preoperative planning
for a universal, reproducible surgical technique. As a future perspective, it would
be interesting to conduct a study on how clinical and complementary findings in CTS
patients behave in the presence of these intra- and extra-carpal tunnel anatomical
variations.
Conclusions
In patients with carpal tunnel syndrome who underwent classical OCTR, the most prevalent
extra-carpal tunnel anatomical variation was the transverse carpal muscle, whereas
the most prevalent intra-carpal tunnel anatomical variation was the bifid median nerve
associated with a persistent median artery.
The surgical finding of an extra-carpal tunnel anatomical variation, such as a transverse
carpal muscle, may reveal other intra-carpal tunnel anatomical variations, including
bifid median nerve, persistent median artery, and anatomical differences at the recurrent
branch of the median nerve.
