Correlation between clinical outcome and nerve conduction studies before and after surgery in patients with carpal tunnel syndrome

INTRODUCTION

Carpal tunnel syndrome (CTS), first described by Paget in 1854, is the most prevalent focal peripheral neuropathy caused by excessive pressure on the median nerve at the wrist.^[1] Although clinical manifestations (paresthesia, numbness, tingling, wrist pain, thenar muscle atrophy and weakness along the median nerve distribution of hand) and physical findings (positive Phalen’s test and Tinel’s sign) are used to diagnose CTS, nerve conduction study (NCS) is the most accurate method for diagnosing median nerve abnormalities which help to decide the treatment plan (conservative or surgery) and to predict the prognosis after carpal tunnel release (CTR) operation.^[2]

Reviewing the available literature,^[3-9] it appears that to date, no proper correlation is delineated between clinical improvement and electrophysiological findings. The conventional NCS parameters such as distal motor latency (DML), distal sensory latency (DSL) and sensory nerve conduction velocity (SNCV) used in the past did not reflect the nerve impairment distal to the block. We undertook this study to evaluate the correlation between subjective symptoms and function indices compared to electrophysiological measures by comparing the self-administered Boston carpal tunnel questionnaire (BCTQ) consisting of symptom severity scale (SSS) and functional status scale (FSS)^[10] against NCSs before and after CTR operation using conventional NCS and short-segment transcarpal techniques.

In our previous study, we have already shown that short-segment transcarpal motor conduction velocity (TMCV) is the most effective way to evaluate the condition of the median nerve after CTR^[11] as neurological impairment of the median nerve in the form of demyelination or axonal degeneration can be detected more precisely by the short segment rather than the long segment technique which is evident from different studies.^[12-14]

MATERIALS AND METHODS

NCS

NCS was performed using RMS NCV EMG EP MARK II with Aleron 201 electromyograph with RMS stimulator using standard protocol in all the patients during pre-surgical and 6—and 9-month post-operation. Room and skin temperature were maintained over the nerve in the range of 25°C–36°C and 34°C–38°C, respectively, to avoid the effect of lower temperature on prolonging latency, increasing amplitude, and reducing conduction velocity.

The conventional median NCSs were performed by recording compound muscle action potential (CMAP) from abductor pollicis brevis (ABP) by placing the recording electrode on the ABP muscle belly and a reference electrode on distal tendinous insertion (at the distal metacarpophalangeal joint of the thumb). Bipolar stimulation was applied at the wrist between the tendons of the flexor carpi radialis and palmaris longus and at the elbow just medial to the brachial artery.

While performing the wrist-palm motor conduction study (short segment trans carpal technique) – the conventional method was used to stimulate the median nerve at the wrist, while to stimulate the nerve in the palm, the cathode was kept on a line from the wrist to 1^st metacarpal space toward wrist^[4,5] while the anode was placed toward the base of 5^th digit distally to avoid short latency recording which could result in inaccuracy of wrist palm median motor conduction velocity (W-P MCV) as described in our previous study.^[11] Necessary precautions were taken while recording W-P motor conduction. Although TMCV is the most effective parameter in evaluating median nerve condition, it is technically difficult as there is a possibility of stimulation of the deep branch of the ulnar nerve while stimulating the recurrent motor branch of the median nerve in the palm.^[11-14] We have to be extremely cautious about the change in thenar twitch (abduction to adduction of thumb) or any modification of CMAP waveform configuration.

NCS data

Analysation of the following parameters.

1. DML: DML of the median nerve was recorded by stimulating the nerve 3 cm proximal to the distal crease of the wrist.

2. Forearm motor conduction velocity (FMCV): FMCV was calculated by dividing the distance between two points of stimulation (wrist and elbow) by the latency difference between the two stimulation points.

3. TMCV: TMCV was calculated by dividing the distance between two points of stimulation (wrist and palm) by the latency difference between the two stimulation sites.

The placement of the electrodes and stimulation sites is shown in [Figure 1].

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Figure 1:

Median nerve – motor nerve conduction study.

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The Graphical representation of the temporal changes in NCV recordings in pre-operative, 6 months post-operative and 9 months post-operative follow up of all the 3 groups of patients - moderate (Group I), severe (Group II) and extreme (Group III) CTS are illustrated in Figures 2-4 respectively.

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Figure 2:

(a) Pre-operative NCS in Group I moderate CTS patients. (b) Post-operative 6 months NCS in Group I moderate CTS patients. (c) Post-operative 9 months NCS in Group I moderate CTS patients. NCS: Nerve conduction study, CTS: Carpal tunnel syndrome.

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Figure 3:

(a) Pre-operative NCS in Group II severe CTS patients. (b) Post-operative 6 months NCS in Group II severe CTS patients. (c) Post-operative 9 months NCS in Group II severe CTS patients. NCS: Nerve conduction study, CTS: Carpal tunnel syndrome.

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Figure 4:

(a) Post-operative 6 months NCS in Group III extreme CTS patients. (b) Post-operative 9 months NCS in Group III extreme CTS patients. NCS: Nerve conduction study, CTS: Carpal tunnel syndrome. NCS: Nerve conduction study, CTS: Carpal tunnel syndrome.

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RESULTS

In our study, all the patients were female and aged between 45 and 55 years, having a mean weight of 55.14 ± 5.76 kg and a mean height of 150.7 ± 1.52 cm.

Table 1 shows statistically significant (P < 0.001) improvement in SSS and FSS of BCTQ in all three groups of patients after 6 and 9 months of follow-up.

In all three groups, there is a statistically significant improvement of the conventional NCS parameter DML as well as short-segment technique TMCV in a 6-month follow-up period, which consistently improved in the 9-month follow-up period (P < 0.001) when compared to preoperatively as evident from the Table 2. In all 16 hands of extreme CTS patients of Group III, both FMCV and TMCV were non-recordable before the operation. FMCV became recordable in eight hands after 6-month follow-up, but TMCV became recordable in all 16 hands. Hence, in extreme CTS patients (Group III), we failed to show statistically significant improvement in FMCV, but in Group I and Group II patients, FMCV showed significant improvement in comparison to pre-operative values.

Table 3 shows the Spearman correlation between BCTQ parameters (SSS and FSS) and NCS parameters (DML, FMCV and TMCV) in all three groups of patients preoperatively and 6 and 9 months after the operation. Spearman correlation coefficients preoperatively were statistically significant in most of the cases but after CTR, it became non-significant.

Table 4 shows the Pearson correlation coefficient between the changes in BCTQ and changes in NCS parameters (calculated as the difference between the basal values and those at 9 months post-operative follow-up values), which is statistically non-significant in all three groups of patients.

Figures 5-7 show the correlation between BCTQ parameters (SSS and FSS) and TMCV preoperatively and 6 and 9 months after CTR in all three groups of patients.

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Figure 5:

Correlation between BCTQ (SSS and FSS) and TMCV preoperative and 6 and 9 months postoperative in moderate (Group I) CTS patients. BCTQ: Boston carpal tunnel questionnaire, SSS: Symptom severity scale, FSS: Functional status scale, TMCV: Transcarpal motor conduction velocity, CTS: Carpal tunnel syndrome.

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Figure 6:

Correlation between BCTQ (SSS and FSS) and TMCV preoperative and 6 and 9 months postoperative in severe (Group II) CTS patients. BCTQ: Boston carpal tunnel questionnaire, SSS: Symptom severity scale, FSS: Functional status scale, TMCV: Transcarpal motor conduction velocity, CTS: Carpal tunnel syndrome.

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Figure 7:

Correlation between BCTQ (SSS and FSS) and TMCV 6 and 9 months postoperative in extreme (Group III) CTS patients. TMCV was non recordable preoperatively in this group so the correlation could not be calculated. BCTQ: Boston carpal tunnel questionnaire, SSS: Symptom severity scale, FSS: Functional status scale, TMCV: Transcarpal motor conduction velocity, CTS: Carpal tunnel syndrome.

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DISCUSSION

In the present study, we have shown statistically significant improvement of Boston carpal tunnel questionnaire parameters – SSS and FSS and also motor NCS parameters – DML, FMCV and TMCV in all the three groups of patients (Group I n = 9 hands, Group II n = 14 hands and Group III n = 16 hands) after CTR operation which is well documented in previous studies.^[3-5,15-22]

Although there was a significant correlation between BCTQ parameters – SSS and FSS and motor NCS parameters – DML, FMCV and TMCV in all three groups of CTS patients preoperatively, we found that the Spearman correlation coefficient value decreased and became statistically non-significant after 6 and 9 months of CTR operation. This leads us to the conclusion that in the pre-operative period, clinical outcome and NCS motor parameters were proportional; however, after CTR, there was more improvement in clinical parameters than motor NCS parameters. Several studies documented significant improvement in BCTQ and NCS parameters, but no correlation was found between clinical outcome and NCS.^[5-9,23-29] In India, a study showed a significant correlation between BCTQ parameters and motor and sensory parameters in CTS patients^[30] which corroborates with our pre-operative findings but no study has been conducted comparing pre- and post-operative cases. Our study was attempted to find the correlation between symptomatic improvement and electrophysiological parameters before and after CTR.

Dhong et al.^[23] found SSS correlated with motor latency (Pearson’s correlation coefficient 0.35; P = 0.004) than with sensory latency (Pearson’s correlation coefficient 0.25; P = 0.05). Although Rotman et al.^[3] found a significant correlation between DML before CTR and 1 h after operation; no significant correlation was recorded between clinical outcome and 3 and 6-month post-surgical DML. Schrijver et al.^[7] found a modest correlation between two clinical symptoms (night awakening and night paresthesia) and median–unnar latency differences and not between the

BQ scores and absolute sensory latency or motor conduction measures. Similarly, Green et al.^[4] observed no significant correlation between SSS and median conduction measures and a weak correlation between FSS and median sensory conduction preoperatively, but following surgery, both SSS and FSS showed no significant correlation with conventional NCS. BCTQ and NCS measure different aspects of outcome after CTR operation. Self-administered questionnaires are more susceptible to confounding by subjective information by the patients, whereas NCS demonstrates objective evidence of median nerve function.^[7] Therefore, patient satisfaction after a successful operation plays a key role in the BCTQ score; hence, the correlation between BCTQ parameters and NCS motor parameters becomes non-significant after CTR. In our study, there was no significant correlation between the difference of the baseline value from 9 months post-operative of BCTQ parameters with that of NCS motor parameters in all three groups, which are also supported by Mondelli et al.^[5] and Schrijver et al.^[7]

In spite of the fact that there is no correlation between BCTQ and NCS, pre-operative NCS is necessary to confirm the diagnosis, evaluate the severity, predict the prognosis, determine pathophysiology (axonal loss versus demyelination) and also to exclude other pathologies like cervical radiculopathy, brachial plexopathy, spondylotic myelopathy or polyneuropathy.^[7,11,31]

Pre-operative NCS provides a baseline value to differentiate between the recurrence of nerve compression and inadequate release of transverse ligament after CTR.^[7,31] In patients who have undergone CTR but symptoms have not been resolved, post-operative NCS provides the only way to determine the cause of failure, which can be due to the presence of other pathology or surgical complications such as neuroma formation, pisohamate pain, hypertrophic scar, pillar pain, scar tenderness, infection, reflex sympathetic dystrophy, lesions of palmer and motor branch.^[5-7,31]

CONCLUSION

Relief of symptoms, improvement of function scale and patient satisfaction are important determinants of the success of CTR operation. It may appear that NCS is not essential in assessing the outcome after the operation, but it cannot be completely abandoned as an unneeded luxury because it provides additional information about the different aspects of the median nerve. Hence, clinicians should use BCTQ and NCS in combination to evaluate the status of the median nerve after a CTR operation.