Ultrasonographic changes in the anterior scalene muscle in neurogenic thoracic outlet syndrome

Thoracic outlet syndrome (TOS) encompasses multiple symptoms produced by compression of the neurovascular bundle within the thoracic outlet. The subtypes of thoracic outlet are termed for the major affected structure including neurogenic TOS (nTOS), venous TOS, and arterial TOS. nTOS accounts for >95% of TOS cases and occurs from compression of the brachial plexus. Patients present with numbness, tingling, and upper arm weakness that is reproduced by activities requiring arm elevation or sustained hand use. A diagnosis of nTOS is based on physical examination, electrodiagnostic testing, scalene muscle injection testing, and imaging. Ultrasonographically identifiable changes in the anterior scalene muscle (ASM) in symptomatic patients undergoing treatment for nTOS have not been reported previously. We sought to describe a consistently seen change in the ASM seen on ultrasound imaging in many patients treated for nTOS at our institution.

Symptomatic patients undergoing ASM block for nTOS were imaged using ultrasound examination and compared with a control group of asymptomatic patients. Patients excluded were those who had previously undergone surgical intervention in the neck. Images were randomized and evaluated by four separate observers who regularly treat nTOS to determine if images of symptomatic patients differed from asymptomatic patients. Identifiable abnormalities in the ASM were compared between the groups (n = 50 and n = 50). Twenty separate observers who do not regularly treat nTOS were instructed to delineate the difference between normal and abnormal ASMs after being trained with three normal and three abnormal images. This group of observers was then given a 100-question examination of randomized ultrasound images of ASMs from symptomatic and asymptomatic patients to determine the reproducibility of our findings.

Forty-eight 50 ultrasound images of symptomatic patients' ASMs and 48 of 50 ultrasound images of asymptomatic patients were identified correctly as such by four expert observers (P = .6171). Sensitivity was 96% and specificity was 96%. Twenty nonexpert observers were able to identify muscular abnormalities accurately at a mean rate of 90.55%, with a false-positive rate of 3.65% and false-negative rate of 5.80%.

Patients with symptomatic nTOS demonstrate specific pathological abnormalities in the ASM that are identifiable on ultrasound. Nonexperts can be taught to characterize changes accurately after a short period of instruction.