Local Anesthetic Block of the Anterior Scalene Muscle Increases Muscle Height in Patients With Neurogenic Thoracic Outlet Syndrome

Abstract

Background: Local anesthetic (LA) blocks of the anterior scalene muscle are used to predict which patients with neurogenic thoracic outlet syndrome (TOS) may benefit from surgical decompression. The block is thought to work through both analgesic and muscle relaxation effects, but evidence of the latter is lacking. The aim of our study was to assess the effects of LA blocks on anterior scalene muscle anatomy as captured by magnetic resonance imaging (MRI).

Methods: Over a two-year period, a series of patients with neurogenic TOS underwent MRI-guided anterior scalene blocks with an LA. Patients underwent MRI both before injection and 30 minutes after injection. Anterior scalene muscle heights (measured from the superior border of the first rib to the top of C3 vertebrae) before and after injection were compared for the injected side and the noninjected (control) side, both overall and stratified by subjective patient response to injection.

Results: A total of 54 patients with neurogenic TOS were included. The median age was 39 years (interquartile range, 27-45), 61% were women, and 46% had a history of neck trauma. Forty-five patients (83%) had a favorable response to injection. Overall, there was no significant change in scalene muscle height for either the injected side (preinjection: 90.0 ± 1.2 mm vs. postinjection: 90.7 ± 1.2; P = 0.12) or the control side (preinjection: 89.3 ± 1.4 mm vs. postinjection: 88.9 ± 1.3 mm; P = 0.83). However, when stratified by patient response, those with a positive response had a larger increase in muscle height for the injected side than for the control side (change in baseline: 0.65 ± 0.58 mm vs. -0.53 ± 0.48 mm; P = 0.05). In contrast, nonresponders had no significant change in scalene height for either the injected or control side (change in baseline: 0.59 ± 1.30 mm vs. 0.37 ± 1.07; P = 1.00). Notably, responders had significantly longer anterior scalene muscles at baseline than nonresponders (92.2 ± 1.1 mm vs. 79.5 ± 2.5; P < 0.001).

Conclusions: LA blocks of the anterior scalene muscle may provide symptomatic relief in patients with neurogenic TOS by increasing muscle height, although the clinical significance of this small change is unclear. Patients who do not have a response to the block tend to have significantly shorter anterior scalene muscle heights than patients who respond, suggesting an anatomic difference in responders versus nonresponders.