Ultrasound of the Median Nerve in the Surgical Treatment of Severe Carpal Tunnel Syndrome

Abstract

Background

High-resolution ultrasound (HRU) has demonstrated utility in the diagnosis and treatment of carpal tunnel syndrome (CTS) by measuring the cross-sectional area (CSA) of the median nerve. We investigated whether HRU could be helpful in evaluating outcomes of carpal tunnel release in patients with severe CTS.

Methods

Patients greater than 18 years of age with severe CTS on electrodiagnostic (EDX) studies and scheduled to have carpal tunnel release were enrolled. At baseline visit within 6 weeks preoperatively, HRU was used to measure median nerve CSA at the carpal tunnel inlet and forearm, and the wrist/forearm ratio (WFR) was calculated. Patients also completed the Boston Carpal Tunnel Questionnaire (BCTQ). Ultrasound and BCTQ were repeated at 6 weeks and 6 months postoperatively.

Results

Twelve patients completed the study (average age, 69 years; range, 52-80 years). The WFR improved significantly at 6 weeks and reached normal levels at 6 months. The CSA at the wrist also improved at 6 months, although this did not reach statistical significance (P = .059). Boston Carpal Tunnel Questionnaire symptoms and function scores improved significantly at 6 weeks and 6 months.

Conclusions

High-resolution ultrasound provides an objective assessment of surgical outcomes in cases of severe CTS, demonstrating normalization of WFR in our series of successful cases. Future study of poor outcomes may help determine whether improvement in WFR and CSA can provide reassurance and support for observation rather than reoperation. Ultrasound also provides anatomical evaluation and may be helpful in cases with medicolegal or psychosocial issues while potentially being less costly and better tolerated than EDX or magnetic resonance imaging.

Introduction

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy with an estimated prevalence of 6% in men and 9.2% in women. ¹ Electrodiagnostic (EDX) studies are frequently used to diagnose CTS, but recent literature has shown ultrasound to be comparable and possibly superior regarding sensitivity, specificity, and cost-effectiveness.^2-4 Enlargement of the median nerve at the carpal tunnel inlet on ultrasound with a cross-sectional area (CSA) of ≥10 mm² is considered positive for CTS, with EDX-confirmed severe cases exceeding 12 mm².^2,5,6 Furthermore, a ratio of CSA measurements at the wrist compared with the forearm allows patients to serve as their own control, and a wrist/forearm ratio (WFR) greater than 1.4 is supportive of CTS.^7,8

Although carpal tunnel release is a reliably successful surgery for most patients who have exhausted nonoperative treatment, some patients have persistent postoperative symptoms, particularly those with severe CTS as demonstrated by preoperative EDX. ⁹ Others may have poor outcomes due to factors beyond carpal tunnel pathophysiology, such as medicolegal or psychosocial issues. A study measuring the Pain Anxiety Symptom Scale (PASS) and the Pain Catastrophizing Scale (PCS) found that dissatisfaction and perceived disability after carpal tunnel release are predicted primarily by depression and ineffective coping skills. ¹⁰

The ability to objectively and anatomically evaluate patients with persistent symptoms after surgery would be highly desirable to help determine the need for reoperation or observation. The goal of our study is therefore to determine the clinical and sonographic outcomes of carpal tunnel release in patients with severe CTS.

Materials and Methods

Patients greater than 18 years of age presenting to our group of 5 orthopedic surgeons with hand surgery subspecialty certification at a single tertiary medical center were considered for this study. Patients were invited to participate if they were scheduled for carpal tunnel release after being diagnosed with severe CTS by EDX performed by a board-certified neurologist using the American Association of Electrodiagnostic Medicine criteria (prolonged or absent median sensory and motor distal latencies with absent sensory nerve action potential and low amplitude or absent thenar compound muscle action potential, abductor pollicis brevis fibrillation potentials, reduced recruitment, and motor unit action potential morphological changes). ¹¹ Exclusions included other causes of neuropathy (such as diabetes), revision cases, or concurrent surgical procedures. Patients were prospectively enrolled between June 2016 and April 2020 before carpal tunnel release.

At the baseline visit within 6 weeks before surgery, ultrasound was performed by a board-certified neurologist with training in nerve ultrasound using an Esaote (Fishers, Indiana) MyLab 25 ultrasound machine. The transducer was placed perpendicular to the long axis of the forearm, which was positioned in supination on a table. The median nerve CSA was measured by digitally tracing the nerve on the image at the carpal tunnel inlet (distal wrist crease) and forearm (12 cm proximal to the distal wrist crease), and the WFR was calculated. Patients also completed the Boston Carpal Tunnel Questionnaire (BCTQ). ¹² Ultrasound measurements and BCTQ were repeated at 6 weeks and 6 months postoperatively. Each surgery was performed by 1 of the 5 orthopedic hand surgeons with a mini-open technique under local anesthesia and tourniquet, with or without sedation.

Demographic, clinical, and ultrasound data were analyzed. T test was used to compare means. Power analysis demonstrated that 10 patients would be necessary to detect a 3-mm² improvement in median nerve CSA at the wrist (α < 0.05, β < 0.20). This study was approved by the institutional review board, and informed consent was obtained from all participants.

Results

Seventeen patients met inclusion criteria and consented to participate. Five patients did not complete follow-up, leaving 12 patients in the study group. The average age was 69 years (range, 52-80 years). Eight were men, and 8 had surgery on the right side. Ultrasound results are shown in Table 1. There was significant improvement in WFR at both 6 weeks and 6 months compared with preoperative values, reaching normal levels at 6 months (1.38). Cross-sectional area at the wrist improved at 6 months postoperatively, although this did not reach statistical significance (from 14.9 to 12.6 mm², P = .059). Boston Carpal Tunnel Questionnaire symptoms and function scores are shown in Table 2. Both scores demonstrated significant improvements at 6 weeks and 6 months compared with preoperative values.

Table 1.

Ultrasound Measurements of Median Nerve CSA.

	Preoperatively	6 wk postoperatively	6 wk postoperatively vs preoperatively	6 mo postoperatively	6 mo postoperatively vs preoperatively
CSA wrist, mm2	14.9	13.3	P = .125	12.6	P = .059
CSA forearm, mm2	8.3	9.0	P = .398	9.1	P = .181
CSA WFR	1.85	1.51	P = .036	1.38	P = .010

Note. CSA = cross-sectional area; WFR = wrist/forearm ratio.

Table 2.

Boston Carpal Tunnel Questionnaire Results.

	Preoperatively	6 wk postoperatively	6 wk postoperatively vs preoperatively	6 mo postoperatively	6 mo postoperatively vs preoperatively
BCTQ symptoms	3.4	1.8	P < .001	1.5	P < .00001
BCTQ function	3.3	1.9	P < .01	1.5	P < .0001

Note. BCTQ = Boston Carpal Tunnel Questionnaire.

Discussion

Our investigation of the clinical and sonographic outcomes of carpal tunnel release in patients with severe CTS demonstrated significant improvements in median nerve WFR at 6 weeks and 6 months, as well as significant improvements in BCTQ scores. Improvements in median nerve CSA at the wrist did not reach statistical significance (P = .059) in our series. Although some prior studies have found significant improvements in median nerve CSA at the wrist, they did not focus on severe cases as in our study, nor did they use a validated patient-reported disease-specific outcome measure such as the BCTQ (Table 3).^13-15

Table 3.

Studies Measuring Median Nerve CSA Before and After Carpal Tunnel Release.

	Year	Preoperative CSA, mm2	Postoperative CSA, mm2	Follow-up interval
Abicalaf et al 13	2007	15	8.6	12 wk
Smidt and Visser 14	2008	14	11.5	6 mo
Pimentel et al 15	2013	19.7	13.5	96 wk
Current study	2021	14.9	12.6	6 mo

Note. CSA = cross-sectional area.

Interestingly, although median nerve CSA at the wrist in our patients decreased after release as expected, demonstrating resolution of nerve swelling near the site of compression, the median nerve CSA at the forearm actually increased, although this was not statistically significant. This latter phenomenon may reflect restoration of antegrade and retrograde axoplasmic transport, as well as resolution of retrograde axonal atrophy, which is known to occur with CTS. ¹⁶

Based on our findings, ultrasound may be useful as an objective assessment of surgical outcomes in severe cases of CTS, such that improvement in WFR may be a reassuring indication that reoperation can be deferred and persistent postoperative symptoms may improve with observation. Our study is thus complementary to a prior investigation which showed conversely that WFR did not improve in 16 of 18 cases with poor outcomes, although that study did not focus on severe CTS. ¹⁷ Ultrasound has the additional benefit of anatomical evaluation of the transverse carpal ligament to rule out incomplete release, ¹⁸ as well as to evaluate possible additional compressive structures, such as ganglion cysts or forearm lesions, and other anomalies, such as persistent median artery, bifid median nerve, and palmaris profundus. ⁸

Another potential benefit of ultrasound evaluation after carpal tunnel release is objective demonstration of improvement in cases that may be confounded by issues beyond carpal tunnel pathophysiology. These may include cases involving medicolegal disputes, such as malpractice and secondary gain, as well as psychosocial factors, such as depression and ineffective coping skills.

Other methods of postoperative assessment include repeat EDX and magnetic resonance imaging, although these are unfavorable due to cost and patient discomfort. Electrodiagnostic outcomes have been shown to correlate poorly with clinical outcomes and may not improve at all in the most severe cases.^19,20 Magnetic resonance imaging is likely not superior to ultrasound, as the 2 modalities have demonstrated nearly perfect correlation in a study of median nerve CSA at the carpal tunnel inlet. ²¹

Our study was limited by sample size, which diminished our power to detect small differences in ultrasound measurements. Enrollment was limited by our strict inclusion criteria for electrodiagnostically severe cases, as well as patient resistance to additional preoperative and postoperative visits in narrow time windows, and ultimately suspension due to COVID-19 concerns. Besides the issue of sample size, long-term follow-up may have shown further CSA improvements beyond 6 months, possibly reaching statistical significance and achieving normal values, as did WFR at earlier time points. Finally, we did not evaluate psychosocial issues by using the PASS and PCS mentioned earlier.

Future studies should investigate ultrasound findings of patients with poor outcomes compared with satisfactory outcomes in the same cohort, although considering the low rate of poor outcomes, a large number of patients would likely be needed for statistically robust comparison. Alternatively, as our study has already demonstrated significant WFR improvement in successful severe cases, future studies could focus resources on enrolling only failed severe CTS cases for comparison.

Persistent symptoms after surgical release in cases of severe CTS present a difficult clinical problem, and ultrasound may provide valuable information in the evaluation and treatment of these patients.