Physical Medicine and Rehabilitation for Thoracic Outlet Syndrome Workup

If there is a strong clinical suspicion of neurogenic TOS, the diagnosis should not be excluded even if the electrical findings do not reveal the classic T1 > C8 pattern, because some authors have found variations. These include equal or greater involvement of C8 (or even C7), a normal amplitude for the sensory response (ie, for the sensory nerve action potential [SNAP]) of the medial antebrachial cutaneous nerve, and an abnormal median SNAP from the third digit. ^[18]

Imaging with MRI or ultrasonography may aid diagnosis in some cases, and ultrasonography may have advantages, detecting abnormalities earlier (when findings are milder) through higher resolution or revealing more severe cases that do not demonstrate the classic pattern of damage electrically. ^[18]

Laboratory studies are used to rule out other systemic problems.

In cases of nonspecific thoracic outlet syndrome (TOS), all studies have normal findings; however, testing to evaluate for other treatable problems or to establish true neurogenic or true vascular TOS usually is indicated.

Radiographs

Radiographs of the chest (with apical lordotic views) and cervical spine should be obtained.

Rule out significant cervical spine degenerative changes that may be causing neck/shoulder pain or impingement of the spinal nerve roots; also eliminate the possibility of upper lobe lung masses.

Radiographs can also help to identify cervical ribs, which may be responsible for plexus injury or have associated fibrous bands.

Radiographs may help to identify elevated first ribs, caused by tight, contracted anterior/middle scalene muscles. Displaced clavicle fractures, nonunion, and exuberant callus also can be identified.

Magnetic resonance imaging (MRI) and computed tomography (CT) scanning

An MRI of the cervical spine and supraclavicular/brachial plexus area is useful to identify other causes, or a CT scan of the brachial plexus area and apical lung may be indicated.

MRI and CT scans can identify cervical root injury from degenerative spurs, disc herniation, or other causes. Some authors have reported that MRI can identify distortion or displacement of the plexus in the thoracic outlet or supraclavicular space, usually from fibrous bands but also from clavicular abnormality (callus, fracture displacement). In addition, MRI has been reported to demonstrate differences in the costoclavicular measurements during provocative stress maneuvers, in patients with thoracic outlet compression. ^[19]

In the aforementioned study by Kim et al evaluating the characteristics of true neurogenic TOS, the most common radiologic finding was CT angiography–determined focal stenosis of the subclavian artery. ^[10]

Diagnostic ultrasonographic imaging

Ultrasonography has been used to measure the pectoral bowing ratio (PBR) in TOS patients. The PBR objectively measures deformation of the pectoralis minor muscle during arm abduction and is considered abnormal if greater than 10%. This ratio reflects the amount of indentation by the neurovascular bundle as it is tethered under the shortened muscle as the arm abducts above 90°. Repeat imaging following successful myofascial release treatment can demonstrate that the PBR reverts to a normal value of less than 10%. ^{[6, 8]}

Doppler and noninvasive blood flow studies

Doppler and plethysmography studies can identify interruption of blood flow to the involved extremity. A near-complete cut-off of flow during the stress maneuver with reproduction of the primary symptoms should be noted.

Hyperabduction is somewhat controversial, since most normal individuals lose flow with the position and also may develop symptoms.

Angiography/venography

Angiography and venography can identify blockage of the vessels from thrombi or emboli, which result in vascular symptoms.

Angiography can detect aneurysms that may be compressing the plexus and causing neurologic symptoms. This finding documents true vascular TOS.

Autonomic assessment (thermography)

Autonomic assessment has demonstrated thermal/temperature asymmetry consistent with nerve irritation or some interference with blood flow.

Following successful treatment, thermal symmetry usually is restored and should correlate well with reduction or elimination of symptoms. This technique remains controversial, and, although the sensitivity has been considered high, the specificity is low and has not been well established.

EMG and nerve conduction studies, also known as electrodiagnostic studies, are necessary to identify true neurogenic thoracic outlet syndrome (TOS) and to rule out other sites of compression neuropathy. With true neurogenic TOS, reported abnormalities include the following:

• Low-amplitude ulnar sensory response. This finding may be variable, since the fifth digit is believed to be supplied primarily by C8 nerve fibers, whereas the T1 fibers are more deflected, stretched, or angulated by a cervical band (or rib) and thereby more likely to sustain injury in TOS.

• Low-amplitude median motor response. The thenar muscles are supplied more from the T1 root, whereas the hypothenar muscles receive primarily C8 innervation.

• Normal median sensory response.

• Neurogenic motor units within the C7-T1 or lower trunk distribution on needle EMG.

• Abnormal medial antebrachial cutaneous response (low-amplitude or unobtainable). Some authors believe that this finding can aid in the early diagnosis of neurogenic TOS. This test primarily assesses the T1 fibers. In the aforementioned study by Kim et al, 100% of the patients, all of whom had true neurogenic TOS, demonstrated reduced action potential amplitude in the medial antebrachial cutaneous sensory nerve, as measured via nerve conduction study. ^[10]

• Abnormal C7-T1 root stimulation (prolonged latency).

• Abnormal F-wave study (prolonged latency, with either full F-wave or axillary Floop)

• Absent or attenuated N13 response in ulnar-derived somatosensory evoked potentials

The degree of nerve injury determines the abnormalities described above. In severe injuries with massive axon loss all of the above findings are seen. In cases where part of the lower plexus is spared, the findings are less dramatic. In these cases, the needle EMG is more likely to show abnormalities.

In most patients with nonspecific thoracic outlet syndrome (TOS), there are no indicated procedures. An anterior scalene block can be performed to anesthetize the anterior scalene muscle and to relieve pressure from the brachial plexus. Thus, this type of block is diagnostic but also may be therapeutic.

The anterior scalene block is a potentially dangerous procedure because the plexus can be injured directly in the process; however, its reliability, accuracy, and safety may be improved with the use of electrophysiologic guidance to verify needle tip placement.

Jordan and Machleder believe that a negative response to this block excludes a surgical treatment option, while a positive response predicts which patients should benefit from surgical decompression. ^[20]

In a study of patients being tested for neurogenic TOS, Torriani et al concluded that diagnostic injection of anesthetic into the anterior scalene muscle can be safely performed with ultrasonographic guidance. ^[21] The investigators administered injections to 26 patients with suspected neurogenic TOS, reporting technical success (ie, anterior scalene muscle identification, intramuscular needle placement, and intramuscular medication delivery) in all procedures, with no major complications occurring. They concluded that this was a well-tolerated diagnostic test.

Biopsy results have not been reported systematically, and the procedure is not indicated unless some other condition is being considered.