Neurogenic Thoracic Outlet Syndrome
After covering vascular thoracic outlet syndrome a few weeks ago, we will move on to cover neurogenic thoracic outlet syndrome. There are some similarities and differences among these conditions, but both require a high index of suspicion. Neurogenic thoracic outlet syndrome, sometimes called nTOS, account for up to 95 percent of cases of TOS (1). It is the second most discussed nerve compression entrapment syndrome following carpal tunnel syndrome (2). This is a clinical diagnosis that describes symptomatic manifestation of compression of the brachial plexus as it traverses through the thoracic outlet. It continues to remain somewhat controversial due to it being largely subjective in nature. There have been recent developments in diagnostic studies that have allowed for more clearly-defined guidelines (1).
The brachial plexus trunks or cords, which originate from nerve roots C5 to T1, are the areas that are compressed during TOS. The areas of compression are the same as vascular TOS covered previously. Neurogenic TOS affects around 1-2 % of the population, is more common in women (about 3:1 ratio) and tends to be unilateral (1). The age range most affected is 20-60, although cases have been reported in children at age 10 (3). The lower brachial plexus is affected in about 80 percent of patients. Some will divide neurogenic TOS into true and disputed nTOS with the “true” TOS with nerve conduction slowing or needle electromyography changes.
Etiologies are similar in all variations of thoracic outlet syndrome as well. Anatomical abnormalities such as cervical or rudimentary first ribs have been shown to be more common in patients that suffer with symptoms of TOS (29 percent vs 1.1 percent) (4). Another review stated up to 20% of cases of nTOS are caused solely by a cervical first rib (5). Supernumerary scalene muscles, variant subclavius muscles and prolonged transverse processes can also cause neurogenic TOS. Fibrous bands or fibromuscular abnormalities are either congenital or acquired and are the most common cause of symptoms. Repetitive actions during labor or athletic activities make individuals more prone to nTOS symptoms. Trauma such as high impact accidents or callus from clavicle fractures can cause compression along the thoracic outlet. Malignancy, such as Pancoast tumors or osteochondromas, may also cause compression is another well-documented etiology of TOS.
Symptoms of neurogenic TOS correspond to the level of nerve compression. A systematic review by Sanders et al. described the following symptom distribution: upper extremity paresthesia (98%), neck pain (88%), trapezius pain (92%), shoulder and/or arm pain (88%), supraclavicular pain (76%), chest pain (72%), occipital headache (76%), and paresthesias in all five fingers (58%), the fourth and fifth fingers only (26%), or the first, second, and third fingers (6). Compression of the C5-7 nerves is most often distributed in the lateral neck with radiation towards the ear and occiput. The lower plexus corresponds with the C8-T1 nerves with pain distributed along the posterior shoulder with radiation down the arm in a medio-brachial distribution with paresthesias affecting mainly the ring and little fingers. Even with these typical distributions, it is still difficult to differentiate clinically (6).
Recent proposals have been made in regards to diagnostic criteria during the Consortium for Outcomes Research and Education of Thoracic Outlet Syndrome. This is a group of physicians and scientists from multiple disciplines to accurately identify and employ management strategies (7). The findings must be present for a minimum of three months and cannot be attributable to any other neurologic cause (Table 1). The Society for Vascular Surgery published reporting standards for TOS with the purpose of a clear and consistent understanding and definition nTOS. This more simplistic definition consists of the following four criteria: signs and symptoms of pathology occurring at the thoracic outlet (pain and/or tenderness), signs and symptoms of nerve compression (distal neurologic changes, often worse with arms overhead or dangling), absence of other pathology potentially explaining the symptoms and a positive response to a properly-performed scalene muscle test injection (8). The subjective nature of diagnostic finding and complaints contributes to the controversy surrounding the diagnosis.
Figure 1. Criteria for neurogenic TOS (Weaver and Lum, 2018)
Imaging modalities are utilized in practice for some sports medicine physicians and providers caring for TOS symptoms. Ultrasound is typically used as the first line for vascular TOS and this may be done to ensure there is no vascular involvement. One study with 143 patients with symptoms of nTOS that demonstrated ipsilateral compression of vessels in 31% compared to 8% of asymptomatic or control patients (9). This suggests it is more common to have compression of vessels in the setting of nTOS symptoms. The first line imaging for nTOS is less clear. CT and MRI may be used to identify patency of the thoracic outlet and can identify bony abnormalities, fibromuscular abnormalities and anatomic variants that may predispose patients to the development of nTOS. Dynamic changes and position changes are more difficult with MRI and CT than with ultrasound. The sensitivity and specificity was 41% and 33 %, respectively, in one study with 42 cases of nTOS that underwent surgical decompression. Due to poor correlation of MRI and intraoperative findings, other modalities such as MR Neurography, Short Tau Inversion Recovery (STIR) and Spectral Adiabatic Inversion Recovery (SPAIR) and being used by some providers (7,10). One study with MR Neurography had a 100% positive predictive value in 30 patients with nerve lesions, although these were also identified on ultrasound (11). A barrier to these modalities may be lack of availability or funds, depending on the academic institution or system.
Figure 2. Provocative testing for TOS (Jones, 2019)
Electrodiagnostic testing is another tool used to aid in diagnosis, although many studies are negative. EMG studies can rule out other neurologic etiologies and can also show slowing of the C5 through T1 nerve roots. The most sensitive finding is the demonstration of a diminished amplitude in median antebrachial cutaneous nerve (MABC) action potential. One study in particular revealed abnormal MABC action potential in 85.7% of patients diagnosed with nTOS compared to ulnar action potential (77.8%) and median and ulnar compound muscle action potentials (55.6% and 33.3%, respectively) (12). Although less sensitive, reduced action potential of the ulnar nerve or decreased thenar M-wave voltage are associated with impingement of the brachial plexus (8).
Management strategies depend on multiple factors and most will undergo a trial conservative management. A consensus on the appropriate conservative regimen for nTOS remains controversial. However, a multimodal treatment approach including patient education, TOS-specific rehabilitation, and pharmacologic therapies have shown positive results (13). Physical therapy should include patient education (active stretching, targeted muscle strengthening, avoidance of provocative maneuvers, etc.) and TOS-focused therapies including soft tissue modalities. Kuhn, Lebus and Bible (2011) showed symptomatic relief in 60% of individuals with nTOS following 6 months of physical therapy (14). Many providers will also utilize pharmacologic management and often provide symptomatic relief. NSAIDs and corticosteroids are most frequently used. Other options include muscle relaxants, anticonvulsants such as gabapentin, and select antidepressants. Local anesthetic, botulinum toxin A and steroid injections into the scalenes has been used with varying levels of success in observational studies (15).
If patients fail conservative measures (usually 4-6 months), some may elect for surgical management. Appropriate patient selection is a key determining factor in surgical success. It is important to exclude other peripheral nerve compression syndromes. The ideal factors for surgical management include patients less than 40 years of age, shorter symptom duration, less narcotic use and non-smokers. One long-term study noted better surgical outcomes in patients with 35 months of symptoms versus 52 months and better outcomes in patients with less pre-operative narcotic use (16). It is also worth noting that only about one third of around 900 patients underwent first rib resection, showing improvement in many with conservative measures. It was also shown that patients with symptoms resolution with lidocaine or botox had a higher success with surgery (90%) than those with a negative block (80%) (16). Another study showed more success in individuals older than 40 with a positive block (87%) than those with a negative block (67%) (17). It is also worth noting that another study showed worse outcomes in patients with work-associated injuries including workers compensation, with 60% of patients remaining disabled or unable to continue work at one year (18).
Traditionally, first rib resection along with or without scalenectomy has been performed for nTOS. Additional procedures such as cervical first rib resection and debridement of fibromuscular structures is also performed in individuals depending on imaging and findings once in surgery. Various approaches including supraclavicular, infraclavicular and transaxillary approaches are all employed with equivalent excellent outcomes achieved at high volume centers (>90 percent satisfaction) (7). One recent report on 221 competitive athletes with the majority being college and high school baseball or softball athletes showed a 73% return to at least the same or better level within 1 year. However, 37% required another procedure, making the results less successful than the general population (19). The transaxillary approach requires a smaller incision and this is typically done in the axilla, making this more cosmetically attractive. More recent minimally-invasive approaches have developed with Video-Assisted Thoracoscopic Surgery (VATS) possibly having better visualization of the operative field. Another minimally-invasive technique described is robotic first rib resection, though the reports have had a limited amount of patients. The minimally-invasive techniques to this point have had longer average hospital stays (3 days) compared to traditional approaches (one day) (7,20). Complications include pneumothorax, wound infection, hematoma and hemothorax. There is also potential for arterial, venous or nerve root injuries, though these are rare.
In conclusion, there are many similarities between the vascular and neurogenic TOS, but neurogenic TOS accounts for more than 90% of cases of thoracic outlet syndrome. The most common presenting symptoms include paresthesia, trapezius pain, neck and arm pain, and occipital headaches. The diagnosis has historically been controversial, but recent proposals have been made to help diagnose nTOS. Imaging modalities such as MRI are commonly used, although standard MRI imaging has poor sensitivity and specificity. Electrodiagnostic studies can rule out other peripheral neuropathies and many times are normal, but there are characteristic changes that occur with nTOS. Conservative or nonoperative management is often used for 4-6 months before first rib resection to decompress the thoracic outlet is offered. Surgical management overall has historically very good results with proper patient selection and newer minimally invasive techniques are becoming more available.
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