INTRODUCTION:
Carpal tunnel syndrome (CTS) is pathologically a disorder with segmental demyelination whereas secondary axonal degeneration, which is indicative of surgical decompression, may co-exist as the disease progresses 1. However, current diagnostic and severity gradation scale (from mild to very severe grade) for CTS cannot clearly indicate co-existence of axonal degeneration, resulting in ambivalence of appropriate treatment prescription. Besides, there are also many technical pitfalls in conventional methods for detecting axonal degeneration. Although nerve conduction studies (NCS) can help to discriminate axonal degeneration, the practical standard practice varies and some physicians usually ignore the palmar distal record for comparison of evoked responses 1. In addition, conduction block and temporal dispersion may also influence its accuracy because both may also cause amplitude drop. On the other hand, needle electromyography (EMG) and nerve biopsy can be more sensitive and accurate in determining axonal degeneration, but they are both restricted in the clinical practice due to invasiveness.
To overcome these shortcomings, ultrasound has been introduced as a complimentary assessment tool to diagnose and grade severity of CTS. Our previous studies has successfully expanded its use to determine potential axonal degeneration associated in CTS by identifying cut-off values of ultrasound measurements, with overall satisfactory, sensitivity and specificity2. This study aims at introducing this innovative protocol in detail. In terms of its non-invasiveness, low cost and efficiency, this approach can be easily applied in routine clinical practice, assisting physicians to prescribe more precise treatment options.
PROTOCOL:
1. Measurement of nerve conduction studies
1.1. Wash hand and keep skin temperature above 32°C.
1.2. Measurement of the sensory evoked potential of the median nerve (Figure 1A): place the E1 electrode on the mid-point of the second digit while E2 attached 4 cm distal to the E1 electrode (Figure 1A-I). Stimulate at 14cm proximal to the E1 at the wrist between tendons of the flexor carpal radialis and palmaris longus and ideally proximal to the distal wrist crease. Supramaximal stimulus is applied to the median nerve.
1.3. Measurement of sensory evoked potential of the ulnar nerve (Figure 1A): place the E1 ring electrode halfway on the proximal phalanx of the fifth digit while E2 4-cm away from the E1 (Figure 1A-II). Stimulate at the wrist (14 cm proximal to the E1 electrode, near the tendon of the flexor carpi ulnaris).
1.4. Measurement of motor evoked potential of the median nerve (Figure 1C): Place the recording electrode (E1) on the most prominent eminence of the thenar area at the motor point of the abductor pollicis brevis (APB) while the recording electrode (E2) is attached on the proximal phalanx of the thumb; stimulation is applied at the midplam (3 to 4 cm distal to the distal wrist crease, Figure 1C-I), 8 or 7 centimeters (cm) proximal to the E1 at the wrist (Between the tendons of the flexor carpi radialis and palmaris longus, Figure 1C-II) and the elbow (medial aspect of the antecubital space, just lateral to the brachial artery, Figure 1C-III).
1.5. Measurement of the motor evoked potential of the ulnar nerve (Figure 1D): Place the E1 over the belly of the abductor digiti minimi while E2 attached on the proximal phalanx of the fifth digit. Stimulate at the wrist (8cm proximal to the E1 electrode, just lateral or medial to the flexor carpi radialis tendon, Figure 2-BI), below and above the elbow (3 to 4 cm distal and proximal to the medial epicondyle respectively, Figure 2-BII and 2-BIII).
2. Measurement of ultrasound
2.1. The client is seated on a plinth facing the examiner, with hands resting in a horizontal supination position and fingers semi-extended (Figure 2A).
2.2. Use a 4-13 MHz linear array transducer to perform transverse scans of the median nerve from distal segment of the forearm to the inlet of carpal tunnel (Figure 2B-I and 2C-I).
2.3. Freeze the real-time imaging and outline the hyperechoic epineurium of the median nerve continuously at the distal (carpal tunnel or level of radial head, Figure 2B-II) and proximal (one third distal forearm, Figure 2C-II) levels.
All methods described here have been approved by the University of Hong Kong/Hospital authorities Hong Kong West Institutional Review Board (HKU/HA HKW IRB, Ref. Number: UW17-129).
REPRESENTATIVE RESULTS:
Potential co-existence of axonal degeneration will be determined according to the criteria of NCS and ultrasound measurements indicated in the Table 1. Should the subject fulfill the criteria of NCS and/or above any cut-off values of the ultrasound parameters, it will be considered as potential co-existence of axonal degeneration associated in CTS.
FIGURE AND TABLE LEGENDS:
Figure 1: Measurement of nerve conduction studies. A. Measurement of motor evoked potential of the median nerve by stimulation at (AI) mid-palm, (AII) wrist and (AIII)elbow; B. Measurement of motor evoked potential of the ulnar nerve by stimulation at (BI) wrist, (BII) below and (BIII) above elbow; C. Measurement of the sensory evoked potential of the median nerve; D. Measurement of sensory evoked potential of the ulnar nerve.
Figure 2: Measurement of ultrasound. A. Position of testing hand; B. Measurement of the inlet of carpal tunnel (BI)and its ultrasound imaging (BII); C. Measurement of the distal one third forearm (CI) and its ultrasound imaging (CII).
Table 1: Criteria for confirmation of axonal degeneration associated in carpal tunnel syndrome by nerve conduction studies and ultrasound
DISCUSSION:
• Critical steps in the protocol:
NCS measurement
It is challenging to set up an universal standard for NCS measurement due to the influence of demographic and physical factors such as age, gender, ethnicities and body weight etc. 3It relies on examiner’s experience to cope with individual factors which may influence the accuracy of the outcome measurement.
Ultrasound measurement
On the other hand, as for the measurement of ultrasound, we took the inlet of carpal tunnel, distal one third forearm etc. as the bony marks for measurement. These measurement sites were consistent with a variety of previous studies, bearing comparatively higher accuracy for diagnosing the disease4-9, which may also consequently increase accuracy for discrimination.
• Modifications and troubleshooting of the method:
Regarding SNAP amplitude, it should be noted the difference between orthodromic and antidromic technique as the SNAP amplitude measured by orthodromic method is smaller than antidromic method 10. In our laboratory, orthodromic stimulation has been generally applied clinically such that we set 10 μV as the cut-off value of determing axonal damage. A higher SNAP amplitude is required should antidromic technique is applied.
Should the subject fail to fulfill the NCS criteria for confirming axonal degeneration in the table 1, cut-off values of ultrasound parameters can take a complimentary role for differentiation as they displayed overall satisfactory sensitivity and specificity in our previous study2. It relies on experienced physicians subspecialized in electrodiagnostic test and ultrasound assessment to take a holistic view for diagnosis when coping with client’s specific conditions.
• Limitations of the method:
There is no universal agreement upon NCS criteria for discriminating axonal degeneration2.
Ultrasound imaging cannot directly display axonal degeneration according to its measured structural and anatomical information. For further confirmation, it still relies on conventional invasive method such as EMG etc.
• The significance of the method with respect to existing/alternative methods:
A convenient, low cost and efficient prescreening approach with overall satisfactory accuracy, sensitivity and specificity;
Combinative use of ultrasound and NCS can compensate technical pitfalls of NCS (absent signals etc.) used for diagnosis alone by providing anatomical and structural information;
Improve diagnostic accuracy beyond current severity gradation scale for more appropriate treatment prescription;
• Future applications or directions of the method
This method can be easily applied in routine clinical practice for initial screening of potential co-existence of axonal degeneration associated in CTS. More appropriate treatment prescription can be possibly obtainable based on this method.
ACKNOWLEDGMENTS:
We would like to express our gratitude to Eric, who helped for demonstration of measurement in the figures.
DISCLOSURES:
There are no conflicts of interest declared.