Thoracic Outlet Syndrome (TOS) is a condition in which the neurovascular bundle passing through the anterior and middle scalenes is compressed. This can lead to pain in the neck/shoulder and upper extremity, tingling or numbness and, occasionally, coldness or decoloration of the hand. Compression can affect the nerves only (neurogenic TOS), the subclavian vein (venous TOS) or the subclavian artery (arterial TOS). Neurogenic TOS is, by far, the most common form.
When thoracic outlet syndrome is diagnosed, there is a continuum of options regarding appropriate treatment. Conservative care, in the form of chiropractic, massage and physical therapy, is well accepted as the appropriate course of care in the majority of cases. However, when symptoms persist despite these efforts, surgery is looked upon as an option of last resort.
After surgery, a patient outcomes following are problematic, particularly with neurogenic TOS (NTOS). Diagnosis is one of exclusion made primarily through patient reports of symptoms, history, and, more recently, response to scalene muscle blocks. With neurogenic TOS, negative EMG or nerve conduction studies do not rule out NTOS as a diagnosis. In vascular TOS, diagnosis can be confirmed through objective findings of blood flow compromise. Patient selection and operative techniques have been cited as explanations for the differences in patient outcomes. Some studies suggest that a highly selective process for screening surgical candidates is required and improves the rate of successful outcomes.1
Anesthetic block of the anterior scalene muscles has become a dual-purpose procedure, providing diagnostic confirmation of TOS and as a reliable indicator of which patients may respond favorably to surgery. An anterior scalene muscle (ASM) block is an injection of anesthetic, such as lidocaine, directly into the scalene muscles. Relaxation of the anterior scalene muscles via blocks may partially simulate the results of surgical decompression. Additionally, an effective block, where surgery is not an option, can give an indication of the potential use of Botox injections for temporary (3 to 4 months) relief.