Value and pitfalls of imaging and electrodiagnosis in neurogenic thoracic outlet syndrome: Lessons learned from present and past

The early descriptions of thoracic outlet syndrome (TOS) sparked the interest of a potentially treatable plexopathy, and development of various surgical strategies aimed at resolving suspected causal impingement (eg, first cervical rib, elongated transverse process of C7, anomalous fibromuscular band). In addition to affected brachial plexus (mainly lower plexus) vascular lesions have also been described, now representing distinct entities: neurogenic TOS (nTOS), arterial TOS (ATOS), and venous TOS (VTOS). The presenting features in nTOS frequently result from compression of the lower brachial plexus and may initially be limited to intermittent paresthesia in several nerve territories (mainly ulnar and medial cutaneous) followed by poorly localizable pain that may be provoked by lifting the affected limb or carrying heavy objects, and in more advanced cases weakness and even atrophy of thenar muscles. Weakness and/or sensory signs localizable to the upper brachial plexus (eg, due to compression of hypertrophic scalene muscles), are less common. However, in clinical practice the true prevalence of nTOS is likely very low and other focal neuropathies and musculoskeletal disorders are far more prevalent. This highlights the need for a thorough clinical review, with detailed history and appropriate clinical evaluation to distinguish these entities accurately. Over-diagnosis has been rather common and resulted in avoidable surgical risks that still resonate in the controversy that surrounds nTOS. The recommended ancillary tests are often aimed at excluding potential mimics whereas their results that support nTOS are variable. Common diagnostic strategies include combined electrodiagnostic and imaging evaluation. In this issue of the Journal, Mul et al. retrospectively reviewed the diagnostic data of 14 patients with nTOS from two Dutch centers. They found that in a considerable portion of their patients the electrodiagnostic (EDX) abnormalities did not adhere to the previously described classic patterns, with apparently preserved compound muscle action potentials in thenar muscles and sensory nerve action potentials of the medial cutaneous antebrachial nerves, abnormal median sensory responses, or even abnormal needle electromyography (EMG) proximal to the thenar muscles. There are several important aspects reflected in these findings that warrant careful consideration when using EDX and interpreting its results in the setting of suspected nTOS. The published studies on nTOS documenting EDX findings have used diverse protocols and cutoffs for abnormality, sampling different sets of muscles and nerves/nerve segments, often in small cohorts. Unfortunately, large systematic studies with extensive nerve conduction studies and EMG, evaluating the diagnostic yield of large sets of arm muscles and nerves/nerve segments, as well as accepted gold standards, are lacking in nTOS. Also, segmental localization with electrophysiology may be imprecise due to anatomic variability, overlapping dermatomes, and innervation of individual muscles by more than two spinal nerves. Consequently, the diagnostic accuracy of EDX is variable and warrants cautious interpretation in patients with suspected nTOS. Normal nerve conduction findings and non-classic patterns may still be compatible with the final diagnosis of nTOS. Additional electrophysiological tests that have been proposed (eg, triple stimulation technique, direct nerve root stimulation with needle, somatosensory evoked potentials) have even more methodological caveats and are, therefore, not recommended for routine diagnostic testing in nTOS. Mul et al. report that imaging was able to capture a wide range of nTOS severity, detecting structural abnormalities as mentioned above. They also note that it was complementary to EDX, favoring a diagnosis of nTOS in half of their cases. This is in agreement with previous studies on the utility of MRI in patients with suspected nTOS. However, the lack of formal objective criteria for abnormal MRI limit operational reliability in practice. This, combined with the observation that healthy controls may exhibit similar structural findings without symptoms, compound the complexity and likely underly false negative and positive results. More recently described advanced MRI protocols may help to improve diagnostic performance in nTOS. This includes multi-sequence assessment in several planes, dynamic testing, objective measurements, and additional criteria such as requiring visible plexus impingement and abnormal signal in (parts of) the plexus or surrounding fat at this site. Importantly, MRI allows exclusion of relevant mimics such as local tumor and cervical radiculopathy. Nerve ultrasound is an emerging diagnostic tool, complementing electrophysiology in a broad spectrum of neuromuscular disorders. At present the number of studies in nTOS is limited, but does indicate that nerve ultrasound can be a useful addition (e.g., using the sonographic Tinel sign and measuring differences in cross-sectional area). Others have also suggested that a ABBREVIATIONS: ATOS, arterial thoracic outlet syndrome; EDX, electrodiagnostic; EMG, electromyography; MRI, magnetic resonance imaging; nTOS, neurogenic thoracic outlet syndrome; TOS, thoracic outlet syndrome; VTOS, venous thoracic outlet syndrome.