Relationship Between Morphological Change of Median Nerve and Clinical Outcome Before and After Open Carpal Tunnel Release: Ultrasonographic 1-Year Follow-up After Operation

Abstract

Background: The aims of this study were 2-fold: (1) to assess the morphological change of the median nerve in patients with carpal tunnel syndrome (CTS) preoperatively and at 6 and 12 months postoperatively; and (2) to analyze correlation between the changes in ultrasonographic findings and the changes in clinical findings after surgical decompression. Methods: Of the 28 patients with CTS, 34 wrists were treated with open carpal tunnel release. We evaluated them using the Boston questionnaire, Japanese Society for Surgery of the Hand Version of the Quick Disability of the Arm, Shoulder, and Hand questionnaire, nerve conduction study (NCS), and ultrasound preoperatively and at 6 and 12 months postoperatively. We measured the cross-sectional area (CSA) of the median nerve at the level of the proximal inlet of the carpal tunnel (CSAc) and more proximally at the level of the distal radioulnar joint (CSAd). Paired t tests and repeated measures analysis of variance of ranks were used to identify changes over time. The Spearman correlation coefficient by rank test was used for the analysis of the relation between the amount of change of CSA and the patient-rated questionnaire score and NCS findings. Results: Findings for CSAc, CSAd, and NCS and patient-rated outcomes at 6 and 12 months postoperatively were significantly lower than their preoperative values. However, no significant correlation was found between the postoperative changes in CSAc, CSAd, and clinical variables obtained preoperatively and postoperatively. Conclusions: Evaluation of sonographic imaging might not be helpful for assessing clinical conditions in patients with CTS after surgical decompression.

Introduction

Carpal tunnel syndrome (CTS) is the most common peripheral nerve entrapment. The diagnosis of CTS is based on a typical history and clinical examination, a provocative test, and a nerve conduction velocity test. Moreover, the recent development of ultrasonography (US) facilitates diagnosis of CTS. Earlier reports have described US as useful for the diagnosis of CTS to measure enlargement of the median nerve cross-sectional area (CSA) and to assess the ratio of CSA or the difference between several points of CSA.^1-6 In fact, several reports have described significant correlation between the value of CSA of the median nerve, the results of neurophysiologic assessment, and clinical symptom scores evaluated using patient-oriented measures.^1-6 This report describes the accuracy of CTS diagnosis by evaluation of the difference (ΔCSA) between the CSA of the median nerve measured at the level of the proximal inlet of the carpal tunnel (CSAc) and that measured more proximally (CSAd) at the level of the distal radioulnar joint (DRUJ). We also reported significant correlation between sonographic imaging results and clinical findings assessed using the Japanese version of the Boston Carpal Tunnel Questionnaire (BCTQ). ⁶ Moreover, some researchers have examined the morphological change of the median nerve before and after surgical decompression.^7-16 Some reports have explained the usefulness of ultrasound findings of the morphological change of median nerve to assess clinical outcomes after surgery.^7,9-11,14,15 However, other reports have asserted that ultrasound assessment of postoperative treatment effects on patients with CTS has only limited value. Furthermore, in patients with CTS undergoing open carpal tunnel release (OCTR), and especially during long-term follow-up, correlations of sonographic changes of CSA with changes in neurologic outcomes and with patient-oriented measures remain unclear. To evaluate image tools for evaluating outcomes of surgical treatment for CTS, this study assessed the morphological change of CSAc and CSAd of the median nerve preoperatively and at 6 and 12 months postoperatively. This study also assessed the correlation between the degree of change of US findings and the degree of change of clinical and electrophysiological findings.

Materials and Methods

This study examined 34 wrists of 28 patients (18 women, 10 men) with a mean age of 57.9 years (range, 30-85 years). Patients who had been evaluated consecutively between January 2009 and March 2016 were recruited. The CTS diagnosis was made according to clinical signs in addition to meeting criteria of nerve conduction study (NCS) findings by the American Association of Neuromuscular and Electrodiagnostic Medicine. Patients underwent OCTR performed by a hand surgeon. First, 15 wrists of 13 consecutive patients (9 women and 4 men; mean age, 54.0 years; range, 30-79 years) were evaluated preoperatively and at 6 and 12 months postoperatively using NCSs, US examination, and 2 self-administered questionnaires. We adapted the Carpal Tunnel Syndrome Instrument of the Japanese Society for Surgery of the Hand (CTSI-JSSH) and the Japanese Society for Surgery of the Hand Version of the Quick Disability of the Arm, Shoulder, and Hand (QuickDASH-JSSH) questionnaire to assess the severity of subjective clinical findings of CTS. The CTSI-JSSH was a Japanese version of BCTQ proposed by Levine et al. ¹⁷ The CTSI-JSSH was validated by Imaeda et al ¹⁸ as having evaluation capacities equivalent to those of the original version. The QuickDASH-JSSH was also a version translated to Japanese. The Japanese version of QuickDASH has evaluation capacities equivalent to those of the original version. ¹⁹ Patients were excluded from this study if they had bifid median nerves or any systemic disorder such as rheumatoid arthritis, diabetes mellitus, connective tissue disorder, chronic renal failure, polyneuropathy, thoracic outlet syndrome, cervical radiculopathy, cervical myelopathy, wrist fractures, or anamnesis of operation at the wrist. All procedures were followed in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Declaration of Helsinki of 1975, as revised in 2008. Informed consent to inclusion in the study was obtained from each patient.

Ultrasonographic Technique

Sonographic measurements were taken by a hand surgeon with more than 12 years of experience in conducting US and surgery. A B-mode real-time apparatus (EUB-7500; Hitachi Medical Corp, Tokyo, Japan) with a 6- to 14-MHz linear array transducer was used. The affected wrist was examined using US before surgery and at 6 and 12 months after OCTR. At sonographic follow-up, another examiner without prior information of clinical and electrophysiological results or sonographic data evaluated the results. Patients were seated facing the examiner to maintain a position of elbows in slight flexion, wrists in slight extension, and forearms in supination with fingers semi-extended. The angle of the linear array probe was kept perpendicular to the median nerve surface. We obtained the CSAc using this pisiform bone as a reference and obtained the CSAd using DRUJ as a reference (Figures 1a and 1b). The CSA of the median nerve was calculated using the direct technique with tracing of a continuous line around the inner hyperechoic rim of the median nerve using an electronic caliper. Measurements were repeated 3 times. The mean value at each level was calculated and used for statistical analyses.

Figure 1.

(a) CSAc: Transverse sonogram of the median nerve (arrow) at the proximal inlet carpal tunnel. (b) CSAd: Transverse sonogram of the median nerve (arrow) at the distal radioulnar joint.

Electrophysiological Studies

Nerve conduction studies were performed for patients with CTS according to the protocol proposed by the American Association of Electrodiagnostic Medicine. The NCSs were conducted using a standard electromyography system (Neuropack MEB-2200; Nihon Kohden Corp, Tokyo, Japan). The NCSs included measurement of the distal motor latency (DML) of the median nerve from the wrist joint to the thenar muscle and the distal sensory latency (DSL) of the median nerve from the wrist joint to the second finger segment, and compound muscle action potential (CMAP) amplitude and sensory nerve action potential (SNAP) amplitude of the median nerve. The NCSs were performed before surgery and at 6 and 12 months after OCTR.

Intervention

Open carpal tunnel release was performed under conduction anesthesia by 1 senior hand surgeon with an air tourniquet using a conventional open procedure with a 2-cm-long skin incision along the line between the third and fourth ray, ending 1 cm distal of the wrist volar crease. Palmar aponeurosis and transverse carpal ligament were divided on the ulnar side of the median nerve.

Statistical Analyses

Paired t tests were used to compare changes in the following variables preoperatively and at 6 months post-operatively: CSAc, CSAd, CTSI-JSSH symptom and functional score, QuickDASH-JSSH score, DML of CMAP, and DSL of SNAP. Repeated measures analysis of variance (ANOVA) was applied to compare differences in the following variables measured before the operation and at 6 and 12 months postoperatively (in 15 wrists): CSAc, CSAd, CTSI-JSSH symptom and functional score, QuickDASH-JSSH score, DML of CMAP, and DSL of SNAP. The Spearman correlation coefficient by rank test was used for analyzing the correlation between the amount of change of CSAc and CSAd, the amount of change of CTSI-JSSH and QuickDASH-JSSH scores, and the amount of change of NCS findings over time (before the operation and at 6 and 12 months postoperatively). Based on an earlier study, ¹⁵ a sample size calculation indicated that we would have 80% power to observe a clinically significant difference at an α of 0.05, with at least 11 wrists in paired t tests and at least 27 wrists in repeated measures ANOVA of ranks. Results for which a value of P < .05 was found were inferred as statistically significant.

Results

We performed OCTR for 34 wrists of all 28 patients. Three patients were affected by bilateral CTS. We evaluated 34 wrists of 28 patients, including 10 men and 18 women with a mean age of 57.9 years (range, 30-85 years), at 6 months postoperatively. Also, 15 wrists in 13 consecutive patients, including 4 men and 9 women with a mean age of 54.0 years (range, 30-79 years), were evaluated for 12 months postoperatively.

Median nerve CSA data are presented in Tables 1 and 2. The CSAc and CSAd at 6 and 12 months postoperatively decreased significantly from their respective preoperative values. However, the CSAc and CSAd at 12 months postoperatively were not significantly lower than those at 6 months postoperatively. The CTSI-JSSH symptom and functional score, QuickDASH score, and DML and DSL at 6 and 12 months were significantly lower than preoperative values. No significant difference was found in CTSI-JSSH symptom and functional score, QuickDASH score, and DML and DSL between those measured at 6 months postoperatively and at 12 months postoperatively. No significant correlation was found for postoperative changes in CSAc, CSAd, and clinical variables preoperatively, at 6 months postoperatively, or 12 months postoperatively (Tables 3 and 4).

Table 1.

Results of Ultrasonographic Measurements and Electromyographic Data and Patient-Reported Outcomes Preoperatively and at 6 Months Postoperatively for 34 Wrists of 28 Patients.

	Before	After 6 mo	P value
CSAc, mm2	14.7 (3.1)	10.6 (2.7)	<.001
CSAd, mm2	8.3 (1.9)	7.3 (1.5)	<.001
DML, ms	8.2 (2.3)	5.2 (0.8)	<.001
DSL, ms	5.4 (2.9)	3.7 (0.9)	.002
CTSI functional score	2.9 (1.1)	1.4 (0.5)	<.001
CTSI symptom score	3.0 (0.9)	1.4 (0.4)	<.001
QuickDASH score	44.0 (23.0)	10.4 (11.3)	<.001

Note. Values are presented as mean (SD). CSAc = cross-sectional area of the median nerve measured at the level of the proximal inlet of the carpal tunnel; CSAd = cross-sectional area of the median nerve measured more proximally at the level of the distal radioulnar joint; DML = distal motor latency; DSL = distal sensory latency; CTSI = Carpal Tunnel Syndrome Instrument; QuickDASH = Quick Disability of the Arm, Shoulder, and Hand.

Table 2.

Results of Ultrasonographic Measurements and Electromyographic Data and Patient-Reported Outcomes Preoperatively and at 6 and 12 Months Postoperatively for 15 Wrists of 13 Patients.

	Before OCTR	6 mo after OCTR	12 mo after OCTR
CSAc, mm2	13.9 (2.6)	9.6 (2.6) a	8.6 (2.6) b
CSAd, mm2	8.2 (1.5)	7.4 (1.3) a	7.0 (1.4) b
DML, ms	7.5 (1.2)	5.2 (0.7) a	4.7 (0.5) b
DSL, ms	5.4 (2.5)	3.4 (0.7) a	3.1 (0.4) b
CTSI functional score	2.8 (1.0)	1.2 (0.3) a	1.1 (0.2) b
CTSI symptom score	2.8 (0.9)	1.3 (0.4) a	1.1 (0.2) b
QuickDASH score	40.3 (23.7)	6.0 (7.4) a	2.7 (3.6) b

Note. Values are presented as mean (SD). OCTR = open carpal tunnel release; CSAc = cross-sectional area of the median nerve measured at the level of the proximal inlet of the carpal tunnel; CSAd = cross-sectional area of the median nerve measured more proximally at the level of the distal radioulnar joint; DML = distal motor latency; DSL = distal sensory latency; CTSI = Carpal Tunnel Syndrome Instrument; QuickDASH = Quick Disability of the Arm, Shoulder, and Hand.

^aResults of the comparison between preoperatively obtained data and those obtained 6 months after OCTR were statistically significant at P <.05.

^bResults of the comparison between preoperatively obtained data and those obtained 12 months after OCTR were statistically significant at P <.05.

Table 3.

Correlation of the Change of CSAc, CSAd, and Clinical Variables Before and 6 Months After Surgical Decompression.

	ΔDML	ΔDSL	ΔCTSI symptom score	ΔCTSI functional score	ΔQuickDASH score
ΔCSAc	−0.12	0.03	0.09	0.19	0.29
ΔCSAd	−0.08	−0.04	0.17	−0.11	0.09

Note. CSAc = cross-sectional area of the median nerve measured at the level of the proximal inlet of the carpal tunnel; CSAd = cross-sectional area of the median nerve measured more proximally at the level of the distal radioulnar joint; DML = distal motor latency; DSL = distal sensory latency; CTSI = Carpal Tunnel Syndrome Instrument; QuickDASH = Quick Disability of the Arm, Shoulder, and Hand.

Table 4.

Correlation of the Change of CSAc, CSAd, and Clinical Variables at 6 and 12 Months After Surgical Decompression.

	ΔDML	ΔDSL	ΔCTSI symptom score	ΔCTSI functional score	ΔQuickDASH score
ΔCSAc	0.18	−0.01	−0.19	0.05	0.14
ΔCSAd	−0.13	−0.13	−0.08	−0.48	−0.38

Note. CSAc = cross-sectional area of the median nerve measured at the level of the proximal inlet of the carpal tunnel; CSAd = cross-sectional area of the median nerve measured more proximally at the level of the distal radioulnar joint; DML = distal motor latency; DSL = distal sensory latency; CTSI = Carpal Tunnel Syndrome Instrument; QuickDASH = Disability of the Arm, Shoulder, and Hand.

Discussion

This study has 2 distinguishing characteristics. First, CSAc and CSAd at 6 and 12 months postoperatively were significantly lower than the respective preoperative measurements. However, the CSAc and CSAd measured at 12 months postoperatively were not significantly lower than those measured at 6 months postoperatively. Earlier studies have demonstrated reduction in CSA after conventional open and endoscopic release of carpal tunnel.^7-16 Abicalaf et al evaluated the CSA of the proximal level before and at 4, 8, and 12 weeks after endoscopic release of the transverse ligament. Their findings show that the CSA decreased significantly with the passage of time. ⁸ Chappell et al ¹⁶ reported significant reduction in the CSA of the median nerve of the proximal carpal tunnel at 2 to 4 weeks and 6 to 10 weeks postoperatively in 23 consecutive patients treated with US-guided carpal tunnel release. Most of the earlier studies evaluated the CSA of the median nerve by sonographic short-term and mid-term follow-up after surgical decompression. By contrast, our study followed up CSAc and CSAd in 34 wrists of 28 consecutive patients at 6 months and in 15 wrists of 13 consecutive patients at 12 months after OCTR. This study found no significant difference in morphological change of CSAc and CSAd between measurements taken at 6 and 12 months postoperatively. Yayama et al ²⁰ demonstrated in experimental animal models that mechanical compression and local ischemia of the peripheral nerve induced morphological change because of intraneural edema. Our results suggest that a marked reduction in intraneural edema of the median nerve occurred within 6 months after surgical decompression.

Second, no significant correlation was found between the amount of change of patient-reported outcomes (CTSI-JSSH and QuickDASH) and electrophysiological findings and the amount of change of CSAc and CSAd before and after operation (preoperatively, at 6 months postoperatively, and at 12 months postoperatively). Our earlier study demonstrated that the degree of CSAc found preoperatively was positively correlated with DML of the median nerve from the wrist joint to the thenar muscle for the nerve conduction velocity test and CTSI-JSSH symptom severity score. ⁶ These results indicate that the extent of nerve swelling has been associated with CTS severity. Therefore, we hypothesized that the degree of reduction in the CSA after surgical treatment is correlated with the degree of change of clinical symptom in patients with CTS. However, no association was found with these findings in our study. A few reports have described a limited value of ultrasound assessment of patients with CTS after carpal tunnel release.^12,13,16 Naranjo et al performed OCTR for 104 hands of 88 consecutive patients with CTS. The CSA at the carpal tunnel inlet was measured using ultrasound and was assessed according to the patient’s rating of satisfaction using a 5-point Likert scale preoperatively and at 3 months postoperatively. No significant difference in reduction of the CSA of the median nerve at the tunnel was found between patients reporting a cure or great improvement and patients with poor outcome after carpal tunnel release. ¹² Kim et al ¹³ measured the CSA at the carpal tunnel inlet preoperatively and at 2 and 12 weeks after OCTR. They also investigated the BCTQ at 2 and 12 weeks after OCTR in 32 patients with CTS. They reported that postoperative reduction in CSA does not reflect postoperative reductions in clinical symptoms and functional disability. Some earlier studies have demonstrated that evaluation of sonographic imaging might be helpful for assessing subjective and objective symptom severity after surgical decompression in patients with CTS.^13,15 Kim et al investigated the relation between morphological changes of the median nerve and the CTS clinical changes with BCTQ and electrophysiological changes with NCSs in 44 wrists of 24 consecutive patients before CTS surgery and at 3 weeks and 3 months postoperatively. The results of their study demonstrated that the amount of change of CSA at the distal wrist crease level and the amount of change of CSA ratio at the distal wrist crease level and 2 cm proximal to this level correlated positively and significantly with the amount of change of the symptom severity score of BCTQ and the amount of change of motor latency and sensory latency. ¹³ Oh et al ¹⁵ found that the improvement in symptom severity of BCTQ was correlated significantly with the decrease in CSA at the carpal tunnel inlet in 67 CTS wrists by open and endoscopic carpal tunnel release at 24 weeks postoperatively. The relation between the degree of CSA reduction and the degree of improvement in clinical findings after surgical decompression has remained controversial. The degree of CTS severity in enrolled patients, the extent of sample size, the period of evaluation of CSA and clinical findings, and the difference in evaluation tools of clinical findings apparently influence the research results. Further studies must be conducted to elucidate these relations through compensation of these factors.

Our research includes several limitations. First, our small sample size influenced the statistical capability to assess relations between the amount of CSA change and the amount of change recorded in clinical findings. We must perform additional research to reevaluate our conclusions using the appropriate sample size. Second, we did not assess the CSA and clinical findings of the longer follow-up period. Further clinical trials must be conducted to reevaluate our conclusions for a longer follow-up period. Third, we did not evaluate quantitative sensory and motor tests (eg, 2-point discrimination, grip strength). Fourth, we only enrolled patients with mild and moderate idiopathic CTS. Additional studies must be conducted to confirm the association between the amount of change of CSA and the results of change of the qualitative and quantitative clinical findings for a large sample of patients with various CTS severity.

Our results demonstrate that CSA at the proximal inlet carpal tunnel and CSA at the DRUJ and clinical findings at 6 months and 1 year postoperatively were significantly lower than those measured preoperatively. However, no significant difference was found in CSAc, CSAd, CTSI-JSSH symptom and functional score, QuickDASH score, and DML and DSL between those measured at 6 and 12 months postoperatively. The CSA of the median nerve and clinical findings at least 6 months after OCTR might show no significant reduction over time. No significant correlation was found between the amount of change of patient-reported outcomes (CTSI-JSSH, QuickDASH) and electrophysiological findings and the difference in CSAc and CSAd measurements taken before and after OCTR. Evaluation of sonographic imaging might not be helpful for assessing subjective and objective symptoms or the functional severity after surgical decompression in patients with CTS.