Ultrasonographic Changes in the Anterior Scalene Muscle in Neurogenic Thoracic Outlet Syndrome.

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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 (nTOS), venous (vTOS), and arterial (aTOS) thoracic outlet syndrome. Neurogenic thoracic outlet syndrome accounts for over 95 percent of thoracic outlet syndrome 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. Diagnosis of neurogenic thoracic outlet syndrome is based on physical exam, electrodiagnostic testing, scalene muscle injection testing, and imaging. Ultrasonographically identifiable changes in the anterior scalene muscle in symptomatic patients undergoing treatment for neurogenic thoracic outlet syndrome (nTOS) have not previously been reported. We sought to describe a consistently seen change in the anterior scalene muscle seen on ultrasound in many patients treated for nTOS at our institution. Symptomatic patients undergoing anterior scalene muscle block for nTOS were imaged using ultrasound and compared to 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 anterior scalene muscle 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 anterior scalene muscles after being trained with three normal and three abnormal images. This group of observers was then given a one hundred question exam of randomized ultrasound images of anterior scalene muscles from symptomatic and asymptomatic patients to determine the reproducibility of our findings. 48/50 ultrasound images of symptomatic patients' anterior scalene muscles and 48/50 ultrasound images of asymptomatic patients were correctly identified as such by four expert observers (p=0.6171). Sensitivity was 96% and specificity was 96%. Twenty non-expert observers were able to accurately identify muscular abnormalities at a mean rate of 90.55% with a false positive rate of 3.65% and false negative rate of 5.8% CONCLUSION: Patients with symptomatic nTOS demonstrate specific pathological abnormalities in the anterior scalene muscle that are identifiable on ultrasound. Non-experts can be taught to accurately characterize changes after a short period of instruction.