|Year : 2006 | Volume
| Issue : 3 | Page : 177-182
Electrodiagnostic approach to carpal tunnel syndrome
Ajith Cherian, Abraham Kuruvilla
Section of Electromyography, Department of Neurology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum - 695 011, India
Dept. of Neurology, SCTIMST, Trivandrum - 695 011, Kerala
An algorithm for an electrophysiological approach to carpal tunnel syndrome is proposed. This technical note takes into account the standard tests, comparison tests and needle electromyography. If the standard tests are negative, a comparison study can be done to identify cases of minimal or very mild carpal tunnel syndrome (CTS). If comparison studies are negative, CTS can be ruled out and a search for other causes have to be made. If the standard tests are positive, carpal tunnel syndrome can be divided into extreme, severe,moderate and mild cases. Motor comparison study is useful in extreme CTS cases. Needle electromyography is a must in all cases where the standard tests are positive. This streamlined approach allows accurate diagnosis with minimum essential tests.
Keywords: Comparison study for CTS, electrodiagnosis of carpal tunnel syndrome, electrophysiology of CTS, mild CTS, nerve conduction study
|How to cite this article:|
Cherian A, Kuruvilla A. Electrodiagnostic approach to carpal tunnel syndrome. Ann Indian Acad Neurol 2006;9:177-82
| Introduction|| |
Distal median neuropathy is the most common entrapment neuropathy affecting the upper limb. Entrapment occurs in the wrist as the nerve passes under the flexor retinaculum in the carpal tunnel. Carpal tunnel syndrome (CTS) is defined as "a constellation of clinical symptoms and signs caused by compression and slowing of the median nerve at the wrist."
Median nerve is derived from the brachial plexus with a contribution from both the lateral and medial cord. The medial cord consisting of C8-T1 fibers supplies the majority of motor fibers to the distal median muscles of the forearm and hand as well as sensory fibers to the lateral half of the ring finger. C6 - C7 fibers run in the lateral cord and supply the sensation to the thenar eminence, thumb, index and middle fingers as well as majority of motor fibers to the proximal median forearm muscles.
Just proximal to the wrist and carpal tunnel, the palmar cutaneous sensory branch arises running subcutaneously to supply sensation over the thenar eminence [Figure - 1]. The median nerve then enters the wrist through the carpal tunnel, which is formed by carpal bones on the floor and sides with the thick transverse carpal ligament forming the roof. In addition to the median nerve, nine flexor tendons to the digits and thumb transverse the carpal tunnel [Figure - 2]. In the palm, the median nerve terminates into motor and sensory divisions. Motor division supplies the first and the second lumbricals. The recurrent thenar motor branch supplies the opponens pollicis, abductor pollicis brevis and flexor pollicis brevis. Sensory branch supplies the palmar aspect of lateral 3 ½ fingers.
| Symptoms and Signs|| |
The dominant hand is more severely affected especially in idiopathic cases. The highly suggestive symptoms include (a) pain/paresthesias while driving, holding a phone book or newspaper. (b) Sensory disturbance of digits I, II and III with splitting of the 4th digit. (c) Nocturnal awakening of the patient from sleep. (d) Shaking or ringing the hand (e) weakness /wasting of the thenar eminence (f) Phalen's maneuver reproduces symptoms., Sensory fibers are involved early in most patients. Pain and parasthesias usually bring the patients to medial attention. Some patients describe a diffuse poorly localized ache involving the entire arm. Only in more severe or advanced cases do motor fibers become clinically involved with patients describing impaired dexterity from weakness of thumb abduction and opposition.
Physical examination may reveal hypoesthesia in the median distribution. Testing sensations over the thenar eminence and over the ring finger is especially useful [Figure - 3]. Sensation over the thenar area is normal as this area is innervated by the palmar cutaneous sensory branch that arises proximal to the carpal tunnel. Tinel's sign (tapping over the median nerve at wrist) and phalen's maneuver (holding the wrist passively flexed for 30 seconds to 2 minutes) may provoke increased paresthesias and pain. Tinel's sign may be false positive in the general asymptomatic population and is also positive over many nerves in patients who have polyneuropathy. Phalen's maneuver is more sensitive and usually produces paresthesias over the middle or index fingers. Reverse phalen's test which is performed by having the patient maintain full wrist and finger extension for 2 minutes significantly increases the pressure in the carpal tunnel within 10 seconds of the change in wrist posture and the carpal tunnel pressure has the tendency to increase throughout the test's duration.In contrast, the change in carpal tunnel pressure noted in the standard phalen's test is modest and plateaus after 20-30 seconds. In a study comparing the changes in tunnel pressure with the phalen's and the reverse phalen's test, it was noted that the average pressure change for the phalen's test at one and two minutes was only 4 mm Hg. The average pressure changes in the carpal tunnel for the reverse phalen's test were 34 mm Hg at one minute into the test and 42 mm Hg at the two minute point. Thus the extended posture significantly changes the pressure within the carpal tunnel and may be a more useful provocative examination maneuver. Depending on the degree of entrapment, the motor examination will show atrophy of the thenar eminence.
Changes in the median nerve include focal demyelination and conduction block. In advanced cases, Wallerian degeneration More Details and axonal loss occurs.
The aims of the nerve conduction study (NCS) and electromyography (EMG) are:,
- To demonstrate a distal lesion of the median nerve.
- To exclude other peripheral conditions that can result in similar symptoms like high median neuropathy, C6-C7 radiculopathy, lesions of the brachial plexus or even polyneuropathy.,
- To assess severity of CTS and for therapeutic decisions.
- Baseline to assess the outcome after intervention.
The degree of electrophysiological abnormalities may not always be proportional to the duration or severity of symptoms.
Electrodiagnostic grading of CTS (modified from Padua et al. ),
Grade 1. Very mild CTS -normal standard tests, abnormal comparative tests. Grade 2. Mild CTS - abnormal sensory with a normal motor response. Grade 3. Moderate CTS - abnormal median sensory and motor response. Grade 4. Severe CTS -absence of sensory response, abnormal distal motor latency. Grade 5. Extreme CTS - absence of median motor and sensory responses.
1. Distal median motor latency : Recording is done from the abductor pollicis brevis while the median nerve is stimulated 3 cm proximal to the distal crease of the wrist. A 6.5 cm distance between the stimulating and the recording electrodes is ideal [Figure - 4]. Onset motor latency > 4.2 msec is considered abnormal,so is a compound muscle action potential (CMAP) amplitude <5 mV. Right - left latency difference is normally < 0.7 millisecond. Specificity of abnormal values of >4.2 msec was 99% for normal population., Prolongation of distal latency is seen in those cases with demyelination present at the site of lesion. In cases with either conduction block at the wrist or secondary axonal loss, median motor and sensory amplitudes are reduced. Palmar stimulation of the median nerve distal to carpal tunnel will elicit a larger CMAP in CTS. Elbow stimulation may show slightly slowed motor conduction in the forearm if some of the largest and fastest median fibers have been blocked and have undergone wallerian degeneration. Patients who have an underlying polyneuropathy are often at risk for developing superimposed entrapment neuropathies. Therefore, it is always essential to study at least one additional motor and sensory nerve to ensure that the abnormalities seen in the median nerve do not represent a brachial plexopathy or more widespread polyneuropathy. Hence, an ulnar distal motor latency is done with recording from abductor digiti quinti and a distance of 7 cm is kept between the stimulating and the recording electrode. Onset motor latency >3.0 msec and CMAP amplitude <5 mV are considered abnormal. The Martin-Gruber anastamosis is an anomalous communication in the forearm of nerve fibres from the median nerve to the ulnar nerve. The Martin Gruber anomaly does not affect the measurement of the median motor distal latency with stimulation of the median nerve at the wrist. Occurrence of a CTS in a patient with an underlying Martin Gruber anastamosis results in a change in the wave form of the thenar muscle action potential with proximal median nerve stimulation (initial positive deflection and increased amplitude) compared to distal median nerve stimulation (initial negative deflection). In addition, a spuriously fast median nerve conduction velocity will be noted in the forearm segment.
2 Antidromic sensory recording from median nerve [Figure - 5]
The recording ring electrode (R1) is placed near the proximal interphalangeal (PIP) joint of the second digit and R2 is placed over the distal IP joint. The median nerve is stimulated near the proximal crease with the cathode 12-14 cm proximal to the R1 ring electrode. A peak latency prolongation >3.5 msec and a peak to peak amplitude of sensory nerve action potential (SNAP) <10 micro V are considered abnormal. The cut off value for sensory peak latency varied from >3.0 ms between 16-39 years to >3.5 ms in elderly age group >60 years. These values had a specificity of 100% for the normal population. Right - left latency difference is usually less than 0.5 msec.
An ulnar nerve SNAP is recorded in a similar fashion from the 5th digit. A peak latency >3.2 millisec is considered abnormal and is used for comparison and to rule out polyneuropathy.
| Internal Comparison Tests for CTS|| |
Indications for these tests are
1. To identify patients with minimal / very mild CTS.
2. To rule out CTS as a cause for patients symptoms.
3. Identify CTS in cases of polyneuropathy.
Ulnar or radial nerves are used for comparison with median nerves.
Principles of comparison tests
1. Identical distances employed between stimulating and recording electrode.
2. Factors affecting conduction time are constant: age, distance, temperature, muscle size.
3. Any mild slowing can be easily appreciated.
4. Meticulous attention to distance measurement, supramaximal stimulation and electrode placement.
5. Avoid over stimulation and spread of stimulus to adjacent nerves.
Common internal comparison studies [Figure - 6]
1. Median - Ulnar palm - wrist mixed nerve studies:
This takes advantage of measuring the mixed nerve potential that contains both motor and sensory fibers including the largest sensory fibers, the Ia afferents from the muscle spindles. Because these fibers are the largest and contain the most myelin, they are theoretically the most susceptible to demyelination, the primary pathology in distal median neuropathy. The mixed nerve study also has the advantage of being conducted over a very short distance, thereby minimizing the length of normal nerve that dilutes the slowing. Across a distance of 8cm, a latency difference of > 0.4 msec is significant.,
2. Median versus the ulnar wrist to digit IV sensory latency study:
This study can be done antidromically or orthodromically. With identical distances, the median sensory latency to the ring finger can be directly compared with the ulnar sensory latency. Method of recording the median and the ulnar SNAP simultaneously on the median nerve at the wrist with stimulation of the ring finger shows a double peak response in CTS [Figure - 7]. Across a distance of 12-14 cm a latency difference of > 0.5 msec is significant.
3 Median versus radial digit I sensory latency:
This is the only internal comparison study that utilizes the radial nerve., This is a better comparative study for CTS as radial nerve is less likely to be involved in entrapment neuropathies compared to ulnar nerve. This increases the diagnostic sensitivity by 9-11%. Across a distance of 8-10 cm, a latency difference of > 0.5 msec is significant.
4 Median second lumbrical versus ulnar interossei distal motor latency:
This is the only internal comparison technique where motor studies are used. In the case of a superimposed polyneuropathy, the study can still be easily performed even if all the sensory responses are absent. Also, in extreme CTS this study will show latency difference as fibres to lumbricals are most resistant and the last motor fibres to be affected. Active electrode is placed 1cm lateral and proximal to midpoint of 3rd metacarpal bone and reference point in the PIP joint of the second digit. A latency difference of > 0.5 msec is considered significant [Table - 1].
None of the comparative studies or routine median sensory studies are informative about the presence of conduction block. Low median motor or sensory amplitudes on stimulating at the wrist may signify secondary axonal loss, conduction block or a combination. Secondary axonal loss signifies greater injury to the distal median nerve and a much longer or incomplete recovery even after appropriate therapy. Conduction block which signifies demyelination is readily reversible provided that the initial inciting event is no longer present. Palmar motor stimulation of the median nerve is a very useful technique to demonstrate conduction block across the carpal tunnel. In normal median nerves the ratio of the palm/wrist CMAP amplitude does not exceed 1.2. Ratios larger than this suggest some element of conduction block.
It is important that both stimulations are supramaximal, that there is no co-stimulation of adjacent nerves and that the baseline is not obscured by shock artifact or noise that precludes an amplitude measurement.
| Other Electrodiagnostic Tests in CTS|| |
The value of following tests for electrodiagnostic evaluation of patients with carpal tunnel syndrome remains to be established.
a) Inching technique:
In this technique, the median motor nerve fibres are difficult to activate sequentially in steps of 1 cm because of the recurrent course of the motor branch of median nerve to the thenar muscle and the proximity of the stimulating electrodes to the same muscle. This technique is time consuming because it is often difficult to eliminate the stimulus artifact from the recording. Also inching technique has an unacceptably high rate of false positive results.,,
b) Terminal latency index (TLI):
The TLI adjusts the distal motor latency (DL) for the terminal distance and the proximal nerve conduction velocity. The formula for calculating the TLI is as follows:
Normal value for TLI is 0.34 (median and ulnar nerves) and in CTS, it falls below normal. The value of this test for the electrodiagnosis of CTS remains to be established.
| Role of Needle EMG (nEMG)|| |
The primary goal of nEMG in patients who have suspected CTS is to assess any ongoing or past axonal loss and to exclude a high median neuropathy, brachial plexopathy or cervical radiculopathy. Abductor pollicis brevis muscle is chosen for EMG. Flexor pollicis brevis is best avoided being dually innervated by the median and the ulnar nerves.
The presence of spontaneous activity or large reinnervated motor units in the abductor pollicis brevis signifies active or chronic denervation, respectively., These signify an advanced CTS. Proximal median muscles eg flexor carpi radialis and C6 - 7 innervated muscles eg pronator teres, triceps are sampled when two peripheral nervous system conditions are suspected to be present in same person. (e.g. distal median neuropathy and polyneuropathy or CTS and C6-7 radiculopathy). In these situations EMG helps to assess the relative severity of each of these lesions.,,
| Proposed Algorithm|| |
| Conclusion|| |
Electrophysiological examination should always be considered as an extension of the clinical neurological examination. It is not a mere laboratory test, rather, it should be considered as an electrodiagnostic consultation. Hence, each patient needs to be approached with a clear-cut strategy after gathering the clinical information so that appropriate tests could be performed within a stipulated time frame in a busy EMG laboratory. We believe that the above algorithm will help to sort out the electrical diagnosis of CTS in a systematic way.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7]
[Table - 1]