Introduction
The basic treatment of carpal tunnel syndrome (CTS) is an operation which consists in the division of the flexor retinaculum and release of pressure on the entrapped median nerve(1). Sometimes, however, patients do not experience any improvement after the procedure and even report exacerbation of symptoms.
Compared with numerous publications on the usefulness of ultrasound imaging in diagnosing CTS and selecting patients eligible for a surgery, there are relatively few papers on its value in the evaluation of postoperative CTS treatment efficacy(2–6). That is why the authors attempted to evaluate certain parameters that are essential in postoperative median nerve assessment.
The publication was prepared on the basis of a doctoral dissertation entitled: High-frequency ultrasound in carpal tunnel syndrome: assessment of patient eligibility for surgical treatment and postoperative treatment efficacy by Katarzyna Kapuścińska, MD, PhD (defended on October, 8 2014 in Kraków, Poland).
Aim
The aim of the paper was to determine the usefulness of high frequency ultrasound in postoperative evaluation of CTS treatment efficacy.
Material and methods
The study involved 62 patients (50 women and 12 men aged 28–70, mean age 55.2) referred to the Ultrasound Laboratory of the Department of Medical Imaging of the University Hospital in Krakow from the Neurosurgery Outpatient Clinic of the University Hospital in Krakow, Poland. Three months post-treatment, all patients under-went a follow-up ultrasound examination.
US scans were performed with the use of a broadband linear array transducer (6–18 MHz, using 18 MHz band) of MyLab 70/Esaote.
The following parameters were assessed 3 months after surgery:
cross-sectional area of the median nerve in the widest place at the level of the wrist (in mm2);
nerve echogenicity and echotexture;
evidence of hypervascularization of the median nerve at the wrist level using the Power Doppler (PD) technique;
evidence of compression, deformation or absence of compression of the median nerve at the wrist level;
movability of the median nerve at the wrist level.
Based on information delivered by US examinations, a correlation analysis was conducted involving signs of median nerve hypervascularization in a PD scan and nerve compression in a US scan.
US presentation of the median nerve and the carpal tunnel with incompletely divided flexor retinaculum
In a situation when no improvement is reported due to incomplete division of the flexor retinaculum, a US examination presents preserved flexor retinaculum fibers as well as persisting “notch sign” or “inverted notch sign.”
Early examinations enable incompletely divided fibers to be easily differentiated from scars since they present different echogenicity. Scars are initially hypoechoic and amorphous whilst flexor retinaculum fibers are hyperechoic and present a typical course in the ligament line (Fig. 1 and 2). The differentiation becomes more difficult, and sometimes even impossible, as fibrous remodeling of scar tissue progresses. No improvement (symptoms do not subside) after surgery can therefore be an indication for conducting a US examination in the first 1–3 months post-treatment.
Fig. 1
Transverse view of the carpal tunnel 3 months after surgical median nerve decompression: asterisks mark visible preserved fibers of the flexor retinaculum and an ellipse encircles the entrapped median nerve
Fig. 2
Longitudinal view of the median nerve 3 months after surgical median nerve decompression: red asterisks mark preserved fibers of the flexor retinaculum, yellow asterisks indicate scar tissue between divided parts of the flexor retinaculum and blue arrows point to the “notch” and “inverted notch” sign
US imaging enables the visualization of fixation or damage to the nerve or its branches.
Based on information delivered by a follow-up US examination and based on symptoms reported by patients, a correlation analysis was conducted involving the presence of CTS symptoms typically reported after median nerve decompression and:
the difference in cross-sectional median nerve areas in the widest place pre- and post-treatment;
evidence of hypervascularization of the median nerve in a PD examination;
presence of median nerve compression in a follow-up US examination post-treatment;
movability of the median nerve at the wrist level after surgery.
The following tests were applied in the statistical analysis: Student's t test for independent samples, single-factor analysis of variance (ANOVA), a post-hoc multiple comparison Tukey test and the χ2 test of independence.
Results with the significance level lower than or equal to 0.05 were deemed statistically significant (p ≤ 0.05). The statistical calculations were conducted with the use of a Statistica 10 PL and StatsDirect 2.7.9.
Results
Of 62 patients examined after surgery, symptoms subsided completely in 48 cases (77.4%), incomplete symptom resolution was reported by 5 patients (8.1%) and no improvement or exacerbation of complaints – by 9 patients (14.5%).
The average cross-sectional area of the median nerve in the widest place was 16.6 mm2 (9–29 mm2).
All 62 patients (100%) presented lower echogenicity of the median nerve at the wrist level.
In 14 patients (22.58%), the echotexture of the median nerve was preserved whereas in 48 patients (77.42%), it was blurred.
Of 62 patients, 10 (16.1%) presented evidence of hypervascularization of the median nerve at the carpal level. No increased vascularity was observed in the remaining 52 patients (83.9%).
Ultrasound showed median nerve entrapment in 9 of 62 patents (14.52%).
In 53 of 62 patients (85.48%), no signs of median nerve entrapment at the carpal tunnel were observed.
In 44 of 62 patients (71%), the movability of the median nerve at the wrist level was normal.
In 9 of 62 patients (14.5%), the movability of the median nerve at the wrist level was reduced.
In 9 of 62 patients (14.5%), there was no movability, or there was minimum movability of the median nerve against scars or flexor digitorum tendons.
Figure 3 presents a division of the main group into subgroups based on the manner of patient conduction after surgical median nerve release.
Fig. 3
Division of the main group into subgroups based on the manner of patient conduction after surgical median nerve release
It was found that there is a statistically significant correlation (p < 0.001) between median nerve compression visible in a follow-up US examination and median nerve hypervascularization. All patients with evidence of nerve entrapment after surgery also presented enhanced vascularity, whereas 100% of patients without evidence of entrapment did not demonstrate median nerve hypervascularization.
All patients with persisting or exacerbated symptoms presented with enhanced nerve vascularization, whilst none of the patients in whom symptoms subsided completely showed any signs of vessels in a PD scan (p < 0.001).
The patients who presented nerve entrapment in a US examination reported persisting or exacerbated symptoms. However, nearly all patients in whom no compression was observed upon a US examination, reported complete resolution of symptoms (92.16%; p < 0.001).
Moreover, another analysis tested the existence of correlations between the movability of the median nerve at the level of the wrist, seen in a US examination, and the presence of symptoms typical of CTS (p < 0.001). The symptoms subsided in 97.73% of patients with restored movability and persisted in 77.78% of patients who presented no nerve movability.
Discussion
Compared with numerous publications on the usefulness of ultrasound imaging in diagnosing CTS and selecting patients eligible for a surgery, there are relatively few papers on its value in the evaluation of postoperative CTS treatment efficacy(7–14).
According to various reports published so far(7–14), follow-up US examinations are performed at various times after surgical CTS treatment (from 3 week to 8 months). They are conducted with transducers of the frequently not greater than 12 MHz. Currently, ultrasound probes of even higher frequencies are available (up to 22 MHz). They enable more accurate assessment of slight nerve branches, such as the palmar branch of the median nerve(15).
Parameters that are most commonly evaluated in postoperative ultrasound examinations of the carpal tunnel are the cross-sectional area of the median nerve at its widest site and the comparison between these values obtained pre- and post-operatively. Most authors(7–9, 11–14) have confirmed that the separation of the flexor retinaculum and median nerve release results in a reduction of median nerve edema. In most cases, a US examination does not show a return of a completely normal image of the median nerve. The cross-sectional values are usually greater than 10 mm2 (which is the most commonly applied cut-off point for median nerve edema and the diagnosis of median nerve pathology at the carpal tunnel).
Only Lee et al.(10), in a postoperative follow-up US examination of the carpal tunnel, observed an increase in the median nerve cross-sectional area at the level of the hook of the hamate bone and pisiform bone by approximately 2 mm2 (0.9–3 mm2; p = 0.01).
In the author's own material, the values of the cross-sectional area decreased and correlated with the resolution of symptoms (p = 0.026). In the group with complete resolution of symptoms, the author found that the cross-sectional area of the median nerve decreased by approximately 1.48 mm2 (p = 0.002). These results are similar to those presented in most previous studies.
In the group of patients who reported no improvement (or even exacerbation of symptoms) and the preserved flexor retinaculum fibers were observed, the cross-sectional area of the median nerve increased by approximately 2.22 mm2 (p = 0.002). This study has also demonstrated that the greatest reduction in the cross-sectional area occurred in patients with complete resolution of symptoms.
Most quoted papers emphasize the value and usefulness of US imaging in the postoperative evaluation of the median nerve in CTS(7–11, 14). Only Naranjo has found that ultrasound imaging is of limited value(12), but in the opinion of the author of the present study, the final conclusions raise doubts. Naranjo et al. analyzed the median nerve cross-sectional area at the widest site (i.e. at tunnel inlet) and at tunnel outlet as well as the nerve flattening ratio. In patients with noted improvement, the cross-sectional area of the nerve at the widest site decreased from 14.2 to 13.3 mm2. In those with no or little improvement, however, a decrease of this value was also noted (from 12.5 to 11.6 mm2). There were no significant changes in the cross-sectional value at the tunnel outlet or flattening ratio.
The limitations of the study carried out by Naranjo et al. include the lack of assessment of: nerve movability prior to and after surgery, nerve vascularity and possible postoperative scar, fiber continuity, restoration of bundle echotexture and no correlation analysis between individual parameters. In the author's own opinion, the assessment of retinaculum division and nerve movability against the scar are very significant data. If the symptoms of median nerve damage persist, US findings direct the further diagnostic process and indicate the necessity of median nerve assessment along its entire course as well as brachial plexus evaluation (including spinal nerve roots at the level of the spine).
Smidt et al., based on a study in which decreased cross-sectional area was observed in six of ten patients with no improvement after surgery, concluded that median nerve US may serve as a tool for selecting patients eligible for re-operation(13). The author's own research has shown that the lack of median nerve compression signs and its normal movability indicate that re-operation is not necessary. No clinical improvement or exacerbation of changes in a nerve image as well as the identification of persistent compression at the level of preserved fibers is an indication for re-operation.
High usefulness of ultrasound imaging (both in selecting patients for surgery and in postoperative evaluation) is emphasized by Toros(16). Postoperative US can be helpful in detecting the cause of the lack of improvement and facilitate the decision-making regarding re-operation. One of the most common causes of surgical treatment inefficacy in CTS is incomplete separation of the retinaculum(17). It is well-visible in a US image and, therefore, some surgeons use intraoperative ultrasonography to ensure that the flexor retinaculum has been divided completely(18).
Another parameter assessed in certain studies is the cross-sectional area of the median nerve at the tunnel outlet (10, 11). The results of these studies have confirmed the limited value of this criterion.
In some studies, groups of patients with no reported improvement in spite of surgical treatment were distinguished(9, 12–14). It is surprising that the presence of preserved flexor retinaculum fibers, which could be a potential cause of ineffective treatment, was not assessed in any of these studies. Their authors were mostly focused on the image of the median nerve at the wrist level. Only one study(14) presents a detained analysis of the causes of this inefficacy.
The author, in her own study, noted the lack of improvement or exacerbation of CTS symptoms in nine patients. In a postoperative follow-up US examination, all of them presented preserved retinaculum fibers which compressed the median nerve. A surgical procedure confirmed the diagnosis.
The ability of US to assess median nerve movability against adjacent structures in dynamic examination is an advantage of this modality in postoperative assessment. In a study of Yoshii et al., conducted with the use of cadaver dmaterial, it was found that nerve and tendon movability increases after flexor retinaculum separation(19).
None of the quoted studies presents a degree of improvement of median nerve movability at the carpal tunnel after CTS surgical treatment. In the author's own material, median nerve movability was decreased or absent prior to the procedure in all patients. After surgery, it improved in all patients in whom clinical signs subsided.
The assessment of median nerve vascularity with the Power Doppler before surgery is the most sensitive parameter that indicates entrapment neuropathy. None of the quoted studies presents the assessment of median nerve vascularity after surgery.
In the author's own study, nerve hypervascularization was observed in a PD scan in all patients presenting with persisting or exacerbated symptoms after CTS surgery (9 of 62 patients). All patients with complete resolution of symptoms presented normal nerve vascularity (i.e. no hypervascularity was detected in a PD scan). These results suggest that the presence of hypervascularity may indicate that treatment has been ineffective. This, however, requires further investigation since, at least in theory, hypervascularity can result from an ongoing repair process.
Conclusions
Ultrasound imaging with the use of a high frequency transducer is a valuable imaging modality for postoperative assessment of CTS treatment efficacy.
