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Neurogenic Thoracic Outlet Syndrome: The Role of Imaging and Clinical Tests

By Carlos Camelo, MD and Charles Berde, MD, PhD
Department of Anesthesiology, Critical Care and Pain Medicine
Boston Children’s Hospital
Department of Anaesthesia
Harvard Medical School
Boston, Massachusetts

INTRODUCTION
TOS (Thoracic outlet syndrome) is defined as a group of potentially disabling conditions thought to be caused by compression of neurovascular structures serving the upper extremity1 at the level of the thoracic outlet, which is the area between the clavicle above and the bony circle below, consisting of the sternum in front, the first rib laterally, and its attachment to the vertebra posteriorly.2

Diagnosis of TOS (Thoracic outlet syndrome) is challenging given the variety of clinical presentations and the lack of a gold standard for definitive diagnosis,3 so different diagnostic tools (clinical tests, imaging) have varying importance.

CLASSIFICATION
There are three distinct types of TOS, depending on the principal anatomic structures involved and the clinical syndromes that result: neurogenic (NTOS, compression of brachial plexus), venous (VTOS, compression of subclavian vein), and arterial (ATOS, compression of subclavian artery).1 Neurovascular compression, either primary or recurrent, may occur at any or all the following anatomic areas of the thoracic outlet: the interscalene triangle, the costoclavicular space, or the pectoralis minor space.2

Neurogenic Thoracic Outlet Syndrome (NTOS) is the most common subtype representing 95% of overall TOS cases.4 Some classification systems include another subcategory of “disputed TOS”, where a diagnosis of NTOS is uncertain (due to the lack of supporting nerve conduction studies) but where the symptomatology is consistent with it.5 This subdivision is losing its basis in some cases from the point of view of both diagnostic criteria and a lack of evidence-based treatment after findings from a Cochrane Collaboration review.6

EPIDEMIOLOGY
It has been difficult to determine the actual incidence (and prevalence) of thoracic outlet syndrome, especially neurogenic due to lack of uniform diagnostic criteria mostly in disputed TOS cases.  In terms of actual incidence in the United States population by some authors, estimates of cases of TOS range from 3 to 80 per 1,000 population.7 TOS generally affects young patients, with a male preponderance, although women are also affected.

ETIOLOGY
Etiologies of neurogenic TOS include neck trauma, usually a hyperextension neck injury; repetitive work injury including sitting at a keyboard for prolonged periods; anatomical predisposing abnormalities including cervical or anomalous first rib, cervical bands, and congenital narrow scalene triangle; or it may be unknown.

CLINICAL PRESENTATION
The common symptom to all presentations of TOS is the presence of pain. Symptoms depends upon what structure is compressed causing either neurologic symptoms and/or signs of vascular dysfunction. Symptoms attributable to brachial plexus nerve compression include some combination of pain in the neck and upper extremity, along with numbness, tingling (paresthesia) and weakness (with muscle wasting in advanced cases) in the neck, shoulder, arm, and hand.  NTOS may present with a combination of upper and lower plexus involvement in 85-90% of cases.5

History should also focus on potential differential diagnoses. If radicular pain is present, cervical radiculopathy should be suspected. Peripheral nerve entrapment neuropathies like carpal tunnel syndrome and cubital tunnel syndrome could be present and coexist with the diagnosis of TOS.

DIAGNOSTIC CRITERIA
Society for Vascular Surgery published reporting standards for diagnosis of NTOS,1 consisting 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. At least three of four criteria should be positive for the diagnosis to be made.

PROVOCATIVE MANEUVERS
Many of the original tests include (A) the Adson's test, (B) the Halstead maneuver, and (C) the Wright's maneuver that assess vascular integrity by the monitoring of the radial pulse.6 Because most patients with TOS have complaints that relate to brachial plexus nerve compression and not to the compression of the subclavian artery, the tests that monitor the radial artery will be inaccurate in many patients with suspected NTOS.1 Peripheral symptoms are often exacerbated by maneuvers that either: narrow the thoracic outlet (lifting the arms overhead) or stretch the brachial plexus (dangling; often driving or walking/running).  Symptoms may worsen by provocative maneuvers that are believed to narrow the scalene triangle (EAST: Elevated arm stress test) or to stretch the brachial plexus (ULTT: Upper limb tension test).1

DIAGNOSTIC STUDIES
Although imaging modalities are generally able to provide conclusive evidence of the presence of vascular forms of TOS, the efficacy of diagnostic imaging modalities in the evaluation of NTOS is less clear.5 In cases of suspected TOS presumed not to be secondary to venous or arterial compression, classically electrodiagnostic studies were used to stratify cases of “true” NTOS from “disputed” NTOS cases of similar symptomatology but lacking such conduction defects.4 There are objective radiologic diagnostic findings in some, but not all, patients who fit the criteria for diagnosis of NTOS.5 For instance, CT, chest radiography, and cervical spine films may show the presence of a cervical rib or elongated C7 transverse process.  MRI on the other hand can evaluate soft tissue structures that might contribute to compression and can exclude cervical root compression as a differential diagnosis.4 Although, MRI has satisfactory specificity, its sensitivity is too low to be used as a screening tool.  One study demonstrated that MRI performed poorly for identifying fibrous bands of the anterior and middle scalene muscles or determining that the coracoclavicular ligament caused compression.8

ROLE OF ULTRASOUND
David Ross was the first to focus attention on anomalous fibromuscular bands with or without a cervical rib in the thoracic outlet region as cause of neurogenic TOS.9 Ultrasound could potentially detect other sources of compression including fibrous bands and scalene muscle abnormalities.10 Ultrasound is also a more easily accessible modality that allows for dynamic evaluation to identify brachial plexus branching anatomic variants in which susceptibility to compression by the scalene muscle is increased.11  Another diagnostic modality that is important in the evaluation of NTOS is scalene injection which can be a qualitative diagnostic tool that is additionally predictive of surgical outcomes in those patients under consideration for surgical management.

TREATMENT
In cases of arterial or venous TOS, surgery is typically the only option due to the severity of the injury but for other forms of TOS specially in disputed neurogenic cases, surgery is typically not the first option for treatment, due to the conflicting evidence regarding procedures and recovery outcomes.6 Exercise has shown to be a useful approach in 50 to 90% of all TOS cases.12 Although there is much variation within and between individuals, symptoms of TOS generally improve with exercise and other physical therapy techniques. Management is usually conservative, employing physiotherapy and postural exercises, but pain or muscle wasting may be indications for surgery. Thoracic outlet surgery should be the last resort in intervention for NTOS.13

CONCLUSIONS
The Diagnosis of TOS (particularly neurogenic) is subjective, controversial, and highly variable, most difficult in the absence of complications (vascular thrombosis or neurological deficit). The provocative tests can add weight to a suspected diagnosis of TOS, but alone their utility is variable. Imaging is used to evaluate the anatomy of the thoracic outlet, regarding potential sources of compression to guide management approach but in the absence of gross abnormalities, the exact mechanism of compression may not be fully understood. MRI studies are useful for identifying cases with static compression, but sensitivity remains low for some causes. Ultrasonography of the thoracic outlet can be used to dynamically evaluate brachial plexus compression and can be of value in a very select NTOS patient group. According to the current evidence there is no single clinical test has been accepted as diagnostic of TOS. The diagnostic challenge remains in how to address pain and dysfunction in those patients with NTOS who have pathology, but do not present with objective documentable evidence. We must remember that early diagnosis is important because the neurologic deficit associated with thoracic outlet syndrome may be irreversible. Conservative treatment with physical therapy and posture hygiene should be tried first and surgical options should be considered only in cases of significant anatomic abnormalities or individualized specific cases refractory to other interventions.

REFERENCES

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