Assessment of Motor Function Using Ultrasound Quantification of Abductor Pollicis Brevis in Carpal Tunnel Syndrome

Abstract

Background: Besides characterizing pain and sensory disturbance along the distribution of the median nerve, carpal tunnel syndrome (CTS) presents with motor disturbance that can range from weakness of the thenar muscles to complete paralysis and atrophy which is hardly recovered regardless of surgical procedure. Quantification of abductor pollicis brevis (APB) strength can be used as a functional measure of motor function of the thenar muscle, but correlation between the muscle strength and clinical, electrophysiological, and radiological data was not clearly proved. Purpose: The primary purpose of this study was to assess that strength and muscle thickness (MT) of APB were associated with clinical presentation, electrophysiological testing, and sonographic parameters for CTS. The secondary purpose was to determine the most adequate cutoff point of APB strength and MT for APB weakness. Patients and Methods: Fifty-two wrists from consecutive 38 CTS patients, who required carpal tunnel release, and 56 wrists from 38 asymptomatic volunteers were prospectively evaluated. The mean age was 53.6 years (range = 37-70) in the patient and 49.4 years (range = 24-68) in the control group. APB strength, grip and tip pinch strength, and sonographic parameters including cross-sectional area (CSA) and circularity of the median nerve, depth of carpal tunnel inlet, APB MT were evaluated in both groups. Correspondence between clinical and sonographic measures of the APB and the CTS-6, Levine scale, electrophysiological data was determined in the patient group. Results: APB strength and MT in CTS group were decreased compared with the control group (2.6 vs. 6.7 kg, P < .001; 3.56 vs. 5.1 mm, P = .003, respectively). APB strength was significantly correlated with symptom severity and functional status score of the Levine scale ( r = −.454, P = .004; r = −.361, P = .024, respectively). In addition, APB strength were significantly correlated with MT and distal motor latency ( r = .330, P = .040; r = −.289, P = .047, respectively). MT of the APB of 4.44 mm had 83% sensitivity, 81% specificity, 80% positive predictive value, 83% negative predictive value, and 82% accuracy for CTS. There was no correlation between grip and pinch strength and clinical, electrophysiological, and sonographic measures in either CTS or control group. Conclusion: APB strength favorably represents clinical presentation, electrophysiology of the median nerve, and MT of the APB in CTS. The data of this study suggest that MT of the APB cutoff point of 4.4 mm is considered as thenar atrophy.