Prognostic Value of Ultrasound Imaging of the Median Nerve in Carpal Tunnel Syndrome

Abstract

Background: Ultrasound has been well established as a diagnostic modality for carpal tunnel syndrome, but its prognostic utility has not been deeply investigated. Few studies, showing contradictory results, exist investigating ultrasound results as a predictor of patient outcomes. Methods: Patients with ultrasound measurement of the cross-sectional area (CSA) of the median nerve who completed the Boston Carpal Tunnel Questionnaire (BCTQ) and followed up after surgery were included in the study. A total of 199 wrists from 172 patients met the inclusion criteria. Preoperative CSA of the median nerve at the wrist was compared with change in BCTQ at various follow-up times postoperatively. Results: The BCTQ score was found on average to decrease for patients after surgery at all 3 follow-up times. There was a larger decrease in the preoperative BCTQ with each progressive follow-up time, with the largest change of 1.43 points coming at 6+ months. The average change in BCTQ at each follow-up time was found to be greater than the minimal clinically important difference. The greatest R² for preoperative CSA compared with change in BCTQ was 0.0552 for the 6+ month visits. No specific CSA value or range above or below which patients have better postoperative outcomes was found. Conclusions: Higher preoperative CSA, signifying worse carpal tunnel severity, showed almost no correlation with better outcomes after carpal tunnel release surgery as measured by improvement in patient-reported outcome scores.

Keywords

carpal tunnel syndrome nerve diagnosis ultrasound prognosis median nerve cross-sectional area

Introduction

Carpal tunnel syndrome (CTS), caused by median nerve entrapment at the wrist, is one of the most common forms of peripheral neuropathy. Symptoms are localized to the median nerve distribution of the hand, including paresthesia, pain, motor deficit or atrophy of the thenar muscles, and sensory deficits. Diagnosis is based on history and physical examination and can be confirmed by other diagnostic modalities such as electrodiagnostic studies or ultrasonography.¹

Ultrasonography of the median nerve at the wrist is a painless and noninvasive technique that has been well established as a valid method for the diagnosis of CTS.^2,3 An enlarged cross-sectional area (CSA) of the median nerve at the inlet of the carpal tunnel is caused by compression of the nerve at the level of the flexor retinaculum.⁴ Although the diagnostic potential of ultrasound for CTS has been extensively investigated, its potential as a prognostic indicator for recovery after surgical management has not been deeply explored. Although there are some studies that have explored whether enlargement of the median nerve returns to normal after surgery, few studies have analyzed how preoperative size of the nerve correlates with patient outcomes. A 2009 study of 112 wrists by Naranjo et al⁵ found that preoperative CSA better predicted outcome and patient satisfaction when compared with nerve conduction studies. A 2008 study of 88 wrists by Smidt and Visser⁶ did not find any correlation between preoperative ultrasonographic findings and postoperative outcome. A 2007 study of 145 wrists by Bland and Rudolfer⁷ found no clear relationship between CSA and patient outcome after surgery.

The purpose of this study was to determine whether there is a correlation between preoperative CSA of the median nerve at the carpal tunnel inlet and patient outcome at various time points. By determining a correlation, and ideally finding a specific cutoff value of preoperative median nerve CSA, a patient would be better able to understand the prognosis for recovery.

Materials and Methods

Study Participants

Patients at our institution with clinical suspicion for median nerve compression were evaluated and prospectively enrolled in this study if offered surgical treatment. The decision to offer surgical treatment was offered at the discretion of the physician and was based on clinical findings, electrophysiologic testing, and ultrasonographic imaging. For patients with dual hand involvement, data for each hand were analyzed individually. Patients were included if they elected to have either open or endoscopic carpal tunnel release (CTR) and were asked to follow up in clinic at 2 weeks, 6 weeks, and 6 months to 1 year (6+ months) postoperatively. Patients were excluded if they had additional surgical treatment in the same setting (concurrent cubital tunnel, trigger finger, etc); if they did not have preoperative ultrasound or electrodiagnostic studies or complete the Boston Carpal Tunnel Questionnaire (BCTQ); or if they failed to follow-up at all 3 standard postoperative visits

From September 2016 to January 2020, 1 of the 3 orthopedic hand surgeons conducted CTR in 722 wrists. After removal of patients with no preoperative data or postoperative visits and patients who had concurrent surgeries, 199 hands of 172 patients were included in the study. Final data consisted of 163 hands followed up at 2 weeks, 85 hands at 6 weeks, and 61 hands at 6 months to 1 year.

Patient-Reported Symptom Scales

The primary outcome measure used in this study was change in the BCTQ from initial visit to follow-up visit. The BCTQ is a validated^8,9 patient-reported symptom scale consisting of the 11-item Symptom Severity Scale (SSS) and the 8-item Functional Status Scale (FSS). The FSS and SSS were each scored as an average of responses, with 1 being the minimum score for each and 5 being the maximum score. The total BCTQ score was calculated by averaging the FSS average and SSS average so that the minimum BCTQ score was 1 for a completely asymptomatic and functional hand and 5 for a maximally symptomatic and dysfunctional hand. ∆BCTQ was calculated as the BCTQ at the initial clinic visit minus the BCTQ at the corresponding follow-up visit so that a positive value of ∆BCTQ indicated an improvement in hand function. A 2006 study by Leite et al determined the minimal clinically important difference (MCID) for total BCTQ (FSS and SSS combined) to be 0.74.¹⁰ Patients in this study were asked to fill out the BCTQ themselves in the office while waiting to meet with the physician.

Electrodiagnostic Studies

Electrodiagnostic studies (sensory and motor) were performed and interpreted according to the standards of the American Association of Neuromuscular and Electrodiagnostic Medicine by a certified physician. Specific cutoff values were not used for a positive diagnosis; rather, interpretation by the certified physician was used as the reference standard.

Ultrasonographic Assessment

Ultrasonography was performed with the patient seated across from the ultrasound examiner. The elbow of the affected arm was placed in approximately 70° of flexion with the dorsum of the hand resting comfortably against the table and fingers placed in a normal resting position. A 15-6 MHz linear transducer was used to measure the CSA of the median nerve, reported in square millimeter, at the level of the pisiform using the trace function.

Statistical Analysis

∆BCTQ was calculated for each patient visit and plotted against preoperative median nerve CSA using Microsoft Excel 2016 linear regression analysis to determine correlation coefficients (R²) for each of the 3 follow-up visit times. Data were also separately organized by initial CSA into groups of <10, 10 to 12, 13 to 15, 16 to 19, and 20+ mm², and one-way analysis of variance (ANOVA) was used to compare data between multiple groups and determine statistical significance for each of the 4 follow-up visit times. Statistical analysis was performed using GraphPad Prism 8 software. A value of P < .05 was considered to be statistically significant.

Results

Patient demographic and baseline information can be seen in Table 1. The average ∆BCTQ was 0.97 (n = 163), 1.33 (n = 85), and 1.43 (n = 61) at the 2-week, 6-week, and 6-month to 1-year visits, respectively, increasing with each progressive visit. The greatest R² value of the 3 follow-up visit times was 0.0552 for the 6-month to 1-year follow-up visit (Table 2). The average change in CSA (∆CSA) of the median nerve (mm²) from the preoperative visit to the follow-up visit was 1.19 (n = 163), 1.54 (n = 85), and 1.68 (n = 61) at the 2-week, 6-week, 6-month to 1-year visits, respectively, also increasing with each progressive visit.

Table 1.

Baseline Patient Characteristics.

Patients	172
Hands	199
Age, y	54.6
Male:Female	54:145
Hand L:R	73:126
CSA, mm²	12.73
BCTQ	2.86

Note. CSA = cross-sectional area; BCTQ = Boston Carpal Tunnel Questionnaire; L = left; R = right.

Table 2.

Change in BCTQ.

Follow-up visit	2 weeks	6 weeks	6+ months
Hands	163	85	61
Initial visit BCTQ	2.86	2.96	2.87
Follow-up BCTQ	1.89	1.63	1.44
∆BCTQ	0.97	1.33	1.43
R ²	0.0038	0.0524	0.0552
∆CSA, mm²	1.19	1.54	1.68

Note. BCTQ = Boston Carpal Tunnel Questionnaire; CSA = cross-sectional area.

In addition, patients were separated into 5 groups for analysis based on preoperative CSA: CSA <10, 10 to 12, 13 to 15, 16 to 19, and 20+ mm² (Table 3). For preoperative CSA <10 mm², ∆BCTQ was 1.00 (n = 26), 1.06 (n = 16), and 1.08 (n = 3) at the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively. For preoperative CSA 10 to 12 mm², ∆BCTQ was 0.97 (n = 64), 1.28 (n = 30), and 1.26 (n = 28) at the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively. For preoperative CSA 13 to 15 mm², ∆BCTQ was 0.83 (n = 40), 1.26 (n = 21), and 1.53 (n = 12) at the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively. For preoperative CSA 16 to 19 mm², ∆BCTQ was 1.13 (n = 25), 1.78 (n = 13), and 1.72 (n = 16) at the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively. For preoperative CSA 20+ mm², ∆BCTQ was 1.20 (n = 8), 1.62 (n = 5), and 1.73 (n = 2) at the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively. One-way ANOVA was used to compare data between the 5 CSA groups, yielding P values of .649, .251, and .363 for the 2-week, 6-week, and 6-month to 1-year follow-up visits, respectively.

Table 3.

CSA Grouped Analysis of Change in BCTQ.

Visit time	2 weeks		6 weeks		6+ months
Visit time	n	∆BCTQ	n	∆BCTQ	n	∆BCTQ
CSA <10	26	1.00	16	1.06	3	1.08
CSA 10-12	64	0.97	30	1.28	28	1.26
CSA 13-15	40	0.83	21	1.26	12	1.53
CSA 16-19	25	1.13	13	1.78	16	1.72
CSA 20+	8	1.20	5	1.62	2	1.73
P value		.6494		.2510		.3625

Note. CSA = cross-sectional area; BCTQ = Boston Carpal Tunnel Questionnaire.

Discussion

Preoperative CSA of the median nerve at the wrist showed no correlation with change in the BCTQ score postoperatively at follow-up times of 2 weeks, 6 weeks, and 6 months to 1 year. Linear regression models at all 3 follow-up times yielded slightly positive slopes, suggesting that higher preoperative CSA results in a slightly greater decrease in the BCTQ score and thus improved patient outcome; however, the R² value for all 3 plots was very small, with the highest being 0.0552. We hypothesize that this is due to patients with higher preoperative CSA having more severe disease and noting greater improvements from their preoperative state when filling out the BCTQ compared with patients with less severe disease. This hypothesis is supported by the positive slope of the linear regression of preoperative CSA and preoperative BCTQ score, which suggests patients with higher initial CSA have more severe disease with greater impairment (Figure 1). When hands were separated into 5 groups based on preoperative CSA, ANOVA showed no significant difference in ∆BCTQ across groups at 2 weeks, 6 weeks, and 6+ months. None of the groups consistently had a change in BCTQ at the level of 1 MCID or higher compared with the other groups across the aforementioned follow-up times. All groups had a change in BCTQ higher than the MCID of 0.74 at all 3 follow-up times (Figures 2 -4).

The change in CSA in square millimeter from the preoperative value increased in a stepwise fashion with time after CTR. The average ΔCSA was 1.19 mm² at the 2-week visit, and CSA shrunk further with time as evidenced by an average ΔCSA of 1.68 mm² by the 6+ month follow-up. These data are likely affected by a large number of patients lost to follow-up with each progressive visit but warrant further investigation into how median nerve CSA changes over time after CTR surgery.

This study has several limitations. One of the main limitations of this study was loss of patients to follow-up, particularly at later follow-up times. At the 2-week visit, 82% (163 of 199) of hands presented for follow-up. This percentage further declined across each follow-up visit, dwindling to 31% (61 of 199) by the time of the final follow-up at 6+ months. Second, the patients included in this study were drawn from a single surgeon at a tertiary referral center. Therefore, these results may not be generalizable to the general population. Third, age, other medical conditions or physique, and duration of patient symptoms prior to seeking care were not included in this study. While many patients may have sought care quickly with presentation of symptoms, others may have had chronic and more severe and irreversible symptoms and nerve damage. Finally, ultrasound examinations of patients in this study were performed by a surgeon with extensive experience in ultrasonographic imaging, and it is unclear whether inexperienced examiners would replicate these results. However, Fowler et al¹¹ noted a reasonable degree of agreement between inexperienced examiners and an expert examiner.

Although ultrasound measurement of median nerve CSA remains a valuable diagnostic modality for CTS, its prognostic utility warrants further investigation with higher rates of patient follow-up than we were able to attain in this study. Further investigation with larger, more generalizable patient populations is needed to provide the most accurate prognostic data and give patients higher quality information when contemplating surgical intervention for CTS.

Footnotes

Ethical Approval

This study was approved by our institutional review board.

Statement of Human and Animal Rights

All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008.

Statement of Informed Consent

Informed consent was obtained from all individual participants included in the study.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Cory Demino

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