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

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.

Introduction

Thoracic outlet syndrome (TOS) is a syndrome in which the contents of the thoracic outlet are compressed either by bony structures (such as, the presence of a cervical rib or the first rib) or by muscular structures (scalene muscles or aberrant bands). Three subtypes of TOS—neurogenic (nTOS), arterial (aTOS), and venous (vTOS)—are recognized. The most common subtype is nTOS, which represents 95% of all TOS diagnoses.1, 2 Despite being the most common subtype, nTOS remains the most controversial because of lack of objective criteria for diagnosis.³

The evolution of radiographic imaging studies has allowed for ongoing development of objective measures for the diagnosis of nTOS. However, limitations in specificity and sensitivity of radiographic findings continue to limit the diagnostic use of these studies.4, 5, 6, 7, 8, 9 Yet, imaging modalities such as ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI) can be used to enhance the diagnostic ability of tests such as anterior scalene muscle blocks.¹⁰

Image-guided scalene injection with a local anesthetic (LA) is an important diagnostic tool that has been shown to predict surgical outcomes in patients diagnosed with nTOS, with mostly positive results. Local anesthetic blocks have been shown to be a useful tool in selecting patients who are likely to benefit from surgical decompression2, 11, 12, 13 and can also be therapeutic in patients with nTOS.¹⁴ There are mixed results with the use of longer acting anesthetics such as bupivacaine hydrochloride, triamcinolone, or Botox^® in this role. Some studies suggest improvement, or even resolution, of symptoms with the use of these agents, while others show no difference in outcomes.2, 15, 16

The variability in outcomes with scalene blocks for nTOS highlights the fact that the mechanism of the block is incompletely understood. Certainly there is a component of direct analgesia associated with injection of the LA. However, some studies suggest that the mechanism of action of LA blocks may be more complex. A block is considered accurate when the anesthetic exerts an effect of paralysis on the targeted muscle without an inadvertent spread to the neighboring brachial plexus.¹⁷ It is hypothesized, therefore, a successful LA block simulates the decompressive effects of first rib resection and scalenectomy via muscle paralysis, thus relieving the symptoms associated with brachial plexus compression in nTOS.18, 19 This symptomatic relief is potentially due to antispasmodic effects on the anterior scalene muscle or by relaxation of a hypercontracted muscle.¹⁴ However, direct evidence of muscle relaxation after an LA block is lacking. The aim of our study was to assess the effect of an LA block on anterior scalene muscle anatomy as captured by MRI. We hypothesized that patients who were responders to an LA block would have a change in length of the anterior scalene muscle as compared with patients who were nonresponders.