The diagnostic value of dermatomal somatosensory evoked potentials in lumbosacral disc herniations: a critical approach.

In this study, we examined scalp recorded dermatomal somatosensory evoked potentials (DSEPs) to electrical stimulation of L4, L5 and S1 dermatomes in 55 patients with proven lumbosacral radiculopathy and in 22 healthy subjects. We did not observe any significant ipsilateral intertrial variations and arithmetic mean side-to-side differences in normal subjects. As for segmental latencies, the dermatomal P40 latencies were directly correlated with height. The regression equations for L4, L5 and S1 dermatomes were as follows: (L4) y = 5.8 + 21.3 (height), (L5) y = -2.09 + 28.04 (height), (S1) y = -13.9 + 36.4 (height). Maximum side-to-side latency differences and maximum P 40 latencies were obtained by adding 2 standard deviations to the arithmetic means. The abnormal latencies for L4, L5 and S1 dermatomes were 48 msec., 52 msec., and 55 msec., respectively. The abnormal side-to-side latency difference were 3 msec. for all dermatomes. DSEPs accurately predicted the lesion only in four patients (7.2%). In 20 patients (36.3%), DSEPs correctly identified the lesion but also gave misleading information on the other levels. DSEP abnormality were on the correct level but opposite side in 5 patients (9%). The findings were normal in 11 patients (20%), and misleading in 15 patients (27.2%). We conclude that the ultimate diagnostic utility of DSEPs in lumbosacral radiculopathies is doubtful and controversial needs further investigations.