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Abstract
Carpal tunnel syndrome is the most common nerve entrapment syndromes, resulting from compression of the median nerve between the flexor retinaculum superiorly and the flexor tendons and carpal bones inferiorly. So any pathological process that reduces capacity or increases the volume of the contents tends to increase interstitial pressure within the carpal canal(average pressure 2.5 mm Hg. This results in mechanical distortion of the myelin sheath or ischemia of the median nerve.
CTS is a condition of middle aged people, more common in women than in men. Most cases are idiopathic. CTS occurred at a rate of 3.5 cases per 1000 person per year
Diagnosis of CTS is based on a combination of clinical signs, symptoms and abnormal electrophysiological studies.
Provocative tests (Tinel test, Phalen test and reverse Phalen test) are commonly used to diagnose CTS during the physical examination.
Electrophysiological studies are objective and quantitative, and have been recognized to be highly sensitive and specific for the assessment of nerve function in CTS patients. Electrophysiological classifications act as a guide for surgical and non-surgical treatments of CTS.
The majority of electrodiagnostic tests yield high specificity, but variable sensitivity. The sensitivity of the motor and sensory conduction studies may be improved by a number of methods. These include internal comparison tests between the median nerve and other nerves in the same hand , in which the median nerve across the carpal tunnel is compared to adjacent nerves as the ulnar or radial nerve, allowing detection of relatively mild median abnormalities.
The aim of our study was to assess the significance of the second-lumbrical interosseous latency difference test and compare its sensitivity with the other conventional tests in the diagnosis of carpal tunnel syndrome.
This study included thirty one patients, diagnosed as idiopathic CTS. Nine patients were males and twenty two were females. In addition to twenty normal individuals who served as a control group. All patients were subjected to full history taking, clinical examination with special emphasis to provocative tests for CTS (Tinel test, Phalen test and reverse Phalen test), sensory examination and motor examination with particular attention to weakness or atrophy of thenar muscles. We have applied an internal comparison test between median and ulnar nerves using the second lumbrical-interosseous latency difference technique. By this technique, the median and ulnar nerves were stimulated at the wrist at an equal distance (9-10cm) form the active electrode on the palm (lateral to the mid-point of the third metacarpal). In our study the sensitivity of the second lumbrical-interosseous latency difference technique was compared to the sensitivities of the following conventional electrodiagnostic techniques : median sensory latency from second digit, median (2nd digit)-ulnar(5th digit) sensory latency difference, abductor pollicis brevis distal motor latency, median (abductor pollicis brevis)-ulnar (abductor digiti minimi) distal motor latency difference.
In our study we found that thirty hands out of thirty one hands had abnormal second lumbrical-interosseous latency difference (greater than 0.4ms) where the mean value of the second lumbrical-interosseous latency difference was 1.5 ms (SD ± 1.04). This result was statistically highly significant by the student t-test when compared to the control mean (0.3 ms with SD ± 0.1).
Our results showed that the sensitivity of the second lumbrical-interosseous latency difference test 96.8% was the highest among other tests applied. The sensitivities of other tests applied in our study were as follows : 43.3% for median sensory latency, 76.7% for median-ulnar (M-U)sensory latency difference, 61.3% for abductor pollicis brevis distal motor latency, 77.4% for abductor pollicis brevis-abductor digiti minimi distal motor latency difference.
In our study, the second lumbrical-interossseous latency difference test was the only abnormal test in two of our patients, whereas the other conventional electrodiagnostic tests revealed normal values, thereby exhibiting an appreciable disparity between the sensitivity of second lumbrical-interossseous latency difference and other tests done. Furthermore, one of our cases which showed severe wasting in the thenar muscles, absent sensory potential, marked prolonged distal motor latency to the abductor pollicis brevis (10.5ms), marked diminution in median compound motor action potential amplitude (2.5uv); the response from the second lumbrical was very well obtained with a markedly prolonged latency of the second lumbrical-interossseous latency difference (5.1ms).