INTRODUCTION

Carpal tunnel syndrome (CTS) is the most common compressive neuropathy in the upper extremity and it is responsible for sustained annual coasts to society  in both terms of lost productivity and the coasts of treatment, accurate diagnostic criteria, the selection of treatment strategies based on high level evidence have been inconsistent despite of the prevalence of the condition.¹    

Nerve conduction studies have the ability to detect variations in the median nerve conduction secondary to compression of the nerve.² However in 16%-34% in patients with classic symptoms of CTS neurophysiology is negative.³

To ensure higher sensitivity of electrodiagnosis in CTS different comparative techniques have been developed^4,5, that are based on the concept of comparing the values of an electrodiagnostic test obtained by median stimulation and those obtained by either ulnar or radial stimulation.⁴ Comparative techniques are advantageous in the prospect of having

stricter control of variables that might act as a source of errors in data such as; temperature and distances⁴ or variations that are due to personal differences such as; age and genetic differences.⁶ 

Distal median latency to the lumbrical muscle has been considered helpful in detecting CTS patients who remain normal in conventional recording technique from abductor pollicis brevis muscle (APB).⁷

The main objective of the current work is to determine the diagnostic yield of measuring median motor distal latency (MMDL) to the second lumbrical muscle as compared to that of APB muscle, with regard to the simplicity and time saving of measuring distal latency to the second lumbrical muscle.

MATERIAL AND METHODS

This study was conducted in the Clinical Neurophysiology Unit, Faculty of Medicine and the Faculty of Physical Therapy, Cairo University. 

Recruited subjects were assigned to either normal group or patient group (after giving their informed consent prior their inclusion in the study) based on history taking and positive or negative physical tests (Phalen's test). All candidates with peripheral neuropathy, cervical radiculopathy, ulnar neuropathy, diabetes mellitus and previous injuries or surgeries in the forearm (e.g. carpal ligament release) were excluded from this study. In the normal group we recruited 30 wrists of 17 apparently healthy subjects (21 males and 9 females). In the patient group 30 wrists of 18 patients (15 males and 15 females) were tested.

Two 5 mm surface electrodes were used to obtain the compound motor action potential (CMAP) from the APB and 2^nd lumbrical muscle, with flexible bracelet ground electrode placed around the wrist joint between the stimulating and recording electrodes at the level of the wrist crease. The recording electrodes were positioned following the motor point-tendon placement rule. In order to record the distal motor latency from APB muscle, the active recording electrode was placed on the motor point of the muscle midway between the 1^st carpometacarpal joint and metacarpophalangeal joint, while the reference electrode was placed on the lateral surface of the metacarpophalangeal joint. For acquisition of the distal motor latency from 2^nd lumbrical muscle the active electrode was placed 3 cm proximal to the proximal digital crease of digit 3, while the reference electrode on the lateral surface of the proximal interphalangeal joint of the same digit. The median nerve was stimulated using bipolar surface electrode just proximal to the wrist crease on the nerve trunk between the tendons of the flexor carpi radialis and palmaris longus muscles and recording the distal motor delay to 2^nd lumbrical and APB muscles for both groups.

Statistical Analysis

Descriptive statistical analysis was used to determine means, standard deviations, ranges and confidence intervals of the collected data. Unpaired t-test was applied on the data to compare between groups. Sensitivity and specificity were calculated. Sensitivity as percentage was calculated by dividing the number of subject being positive in the test in the patient group by the total number of subjects in the patient group then the result is multiplied by 100. Specificity as percentage was calculated by dividing the number of subject being negative in the test in normal group by the total number of subjects in the normal group then the result is multiplied by 100.

RESULTS

The mean age of the normal group was 28.4±7.7 years and in the patient group the mean age was 34.03±11.3 years.

As shown in Table (1), the MMDL to the abductor pollicis brevis muscle in the normal group has shown a mean latency of 3.4±0.3 msec. and 95% confidence interval of 3.3 to 3.5 msec., while the patient group has shown a mean latency of 4.4±1.02 msec., (3.1 to 7.3 msec) and 95% confidence interval of 3.9 to 4.6 msec. There was significant difference of the MMDL to the APB muscle between normal and patient groups where t and p values were -4.63 and 0.000 respectively.

As shown in Table (2) , the MMDL to the 2nd lumbrical muscle in the normal group has shown a mean latency of 3.6±0.4 msec. and 95% confidence interval of 3.4 to 3.6 msec., while the patient group has shown a mean latency of 4.5±0.8 msec. and 95% confidence interval of 4.2 to 4.8 msec. There was significant difference of the MMDL to the 2nd lumbrical muscle between normal and patient groups where t and p values were -5.965 and 0.000 respectively. 

Sensitivity and specificity of both tests at different cutoff points are shown in Table (3). When the cutoff point for the median motor distal latency to the abductor pollicis brevis muscle was set to be 3.7 msec. (the mean + 1 SD) sensitivity and specificity were 63.33% and 90% respectively, and when it was set to be 4.0 msec. (the mean + 2 SD) sensitivity and specificity were 50% and 100% respectively. The median motor distal latency to the 2nd lumbrical muscle has shown a sensitivity and specificity of 66.66% and 96.66% respectively when cutoff point was set to be 4.0 msec. (the mean + 1 SD), and 36.66% and 100% respectively when cutoff point was set to be 4.4 msec. (the mean + 2 SD).  Figure (1) shows sensitivity and specificity of both tests at the lower cutoff point, while Figure (2) shows their sensitivity and specificity at the higher cutoff points.

On the contrary when we considered isolated cases evaluation we found that at lower cutoff point four cases were taken as positive in reference to the MMDL to APB muscle while being negative on the MMDL to the 2nd lumbrical muscle and other five cases who were taken as positive in reference to MMDL to the 2^nd lumbrical muscle and negative on MMDL to APB muscle. At the higher cutoff point five cases were positive on MMDL to the APB muscle while being negative on the other test and only one case positive on MMDL to the 2^nd lumbrical muscle and negative on the other test.

Table 1. Mean, standard deviation, 95% confidence interval, t value and p value for the median motor distal latency to the APB muscle of normal and carpal tunnel syndrome patients’ groups.

APB abductor pollicis brevis muscle, CI confidence interval, SD standard deviation

* Significant at p<0.05

Table 2. Mean, standard deviation, 95% confidence interval, t value and p value for the median motor distal latency to the 2^nd lumbrical  muscle of normal and carpal tunnel syndrome patients’ groups.

CI confidence interval, SD standard deviation

* Significant at p<0.05

Table 3. Sensitivity and specificity of MMDL to APB muscle and to the 2^nd lumbrical  muscle in patients with carpal tunnel syndrome

APB Abductor Pollicis Brevis, MMDL Median Motor Distal Latency

Figure 1. Represents sensitivity and specificity of MMDL (Median Motor Distal Latency) to APB (Abductor Pollicis Brevis) and 2^nd lumbrical muscles at lower cutoff points (mean + 1 SD)

in patients with carpal tunnel syndrome

Figure 2. Represents sensitivity and specificity of MMDL (Median Motor Distal Latency) to APB (Abductor Pollicis Brevis) and 2^nd lumbrical muscles at higher cutoff points (mean + 2 SD)

in patients with carpal tunnel syndrome

DISCUSSION

Diagnosis of CTS is made  by the suggestive history and the positive physical tests⁸, but yet confirmative electrodiagnostic tests that exclusively evaluate the function of the median nerve  has to be done to exclude other causes of parasthesia and atrophy that might be misdiagnosed as CTS.^8,9

In the current study we measure the median nerve distal latency to APB and 2^nd lumbrical muscles in a group of CTS patients and another equal control group and we find that both tests show a statistically significant difference between patient and control groups.

When we set a low cut off point (mean + 1SD) the sensitivities of both latencies were relatively close to each other as well as their specificities. While at the higher cutoff point (mean + 2 SD) the sensitivity of the latency to the APB was relatively higher than that of the latency to 2nd lumbrical, but their specificities were the same, so we can say that as both tests showed high specificity either at lower or higher cutoff points which ranged from 90% to 100%. This means that both tests are effective screening tools to exclude CTS in normal subjects.

Uncini et al. (1993) and Sheu et al. (2006) reported that the topographic arrangement of the median nerve fascicles has made the 2nd lumbrical MMDL relatively unaffected in CTS when compared to APB MMDL^4,10,  which was opposed by Wilder et al. (2006), who reported consistent involvement of the motor fascicle supplying the second lumbrical muscle in all stages and degrees of CTS.²

Brannegan and Bratt had a study to determine how often the 2^nd lumbrical motor potential is present when the APB motor potential is absent in severe CTS and they found that the 2^nd lumbrical potential was present in 77% of cases and the distal latency was prolonged in all, so they conclude that recording the 2^nd lumbrical potential improves the localization of severe median nerve entrapment in CTS when routine median sensory and motor conduction studies produce no potentials.¹¹

In mild CTS we can’t find (as far we know) a study that perform 2^nd lumbrical as a separate test where it is included as a part of 2^nd lumbrical-interosseous latency difference  in CTS diagnosis in many studies^4,12,3,13, where it shows a high sensitivity in diagnosing CTS even in mild cases in which standard tests fail to detect abnormalities

The findings of the isolated evaluation of the cases could be explained  by the fact that any peripheral nerve runs an undulatory course inside the nerve trunk¹⁴, and undergoes repeated ungrouping and regrouping throughout its course^15-17, so the topographic funicular arrangement of the median nerve fascicles may not be the same at the distal end of the carpal tunnel where the nerve is maximally compressed  and hence any patient might be diagnosed positive on one test and negative on the ⁽⁴⁾other owing to the funicular nerve topography of the median nerve at the carpal tunnel and which fascicle is maximally compressed for this patient.

For these we conclude that latency to 2^nd lumbrical is a sensitive, simple, rapid and non invasive technique and it should take a wider role in CTS diagnosis than currently accepted and to be done in all cases especially in cases with normal APB latency studies, which raise the recommendation to apply more in depth studies to focus highlights with large group of patients over the effect of compression on different median nerve fascicles and provide an answer to which fascicles are more affected and which test is better and simpler as well to diagnose CTS in all degrees and stages.

[Disclosure: Authors report no conflict of interest]

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