To assess the effect of Botulinum Toxin Type A (BTX-A) injections into the scalene muscles on pain, paresthesias and function in subjects with TOS. Hypothesis: BTX-A injected into the anterior and middle scalene muscles will reduce the irritation on the neurovascular structures at the interscalene triangle in subjects with TOS. This will lead to reductions in pain and paresthesias, and improvements in function when compared with injection of placebo. Study design: Double-blind, randomized, placebo-controlled parallel groups effectiveness trial evaluating changes in pain, paresthesias and function before, at six weeks and four months following injection. Study population: Sixty subjects at least eighteen years of age with a clinical diagnosis of TOS of at least three months duration but less than one year, referred to our practice for management of TOS. Intervention: Each subject will receive an injection under EMG guidance into the anterior and middle scalene muscles of either 100 units of BTX-A (experimental group), or normal saline (control group). Outcome measures: The primary outcome measure will be pain as measured on a ten point Numeric Rating Scale with a two point reduction considered significant. Secondary outcomes will be paresthesias as measured on a Numeric Rating Scale, function measured on the Disabilities of the arm, shoulder and hand (DASH) questionnaire.

Botulinum toxin type A injected into the anterior and middle scalene muscles will reduce the irritation on the neurovascular structures at the interscalene triangle in subjects with TOS. This will lead to reductions in pain and paresthesias, and improvements in function when compared with injection of placebo.