Dermatomal and mixed nerve somatosensory evoked potentials in the diagnosis of neurogenic thoracic outlet syndrome

To evaluate the diagnostic utility of dermatomal and mixed nerve somatosensory evoked potentials (SEPs) in patients with thoracic outlet syndrome (TOS) and to compare their value with routine electrodiagnostic methods, we studied a group of 44 patients with neurogenic TOS and 30 healthy controls. In addition to bilateral median and ulnar SEPs, evoked potentials were recorded after stimulation of C6 and C8 dermatomes from the first and fifth digits, respectively. The patients were classified into 3 groups according to the nature of their clinical condition. The abnormality rate for both ulnar and C8 dermatomal SEPs was 100% in a small group of patients with severe neurological signs like atrophy. In groups of patients with lesser degrees of neurogenic damage, abnormality rates for ulnar and C8 dermatomal SEPs on affected limb(s) were 67 and 50%, respectively. Same abnormality rates were 25 and 18% in patients with only subjective symptoms. In patients with objective neurological signs, the major increase in sensitivity was with electromyography (EMG). Abnormalities of routine nerve conduction studies and F-wave latency were observed in patients with severe neurogenic damage. We concluded that the most useful tests in the diagnosis of neurogenic TOS are needle EMG and ulnar SEPs.