P1-latency interroot comparison enhances the validity of scalp-recorded somatosensory-evoked potentials to diagnose nerve root dysfunction in sciatica.

STUDY DESIGN

A prospective validity study was done of scalp-recorded somatosensory-evoked potentials as a diagnostic procedure to show lumbosacral radiculopathy in 100 consecutive patients with unilateral or bilateral sciatica.

OBJECTIVE

To determine the validity gained by the use of P1-latency interroot comparison to show P1-latency prolongation.

SUMMARY OF THE BACKGROUND DATA

The validity of scalp-recorded somatosensory-evoked potentials in diagnosing lumbosacral radiculopathy has been debated and is uncertain.

METHOD

Sensory nerves representing nerve roots L4, L5, and S1 were stimulated bilaterally. Height-corrected P1-latency, two new P1-interroot comparison-based criteria, and absence of P1 were studied. The gold standard was defined as clinically-involved nerve roots with radiologic nerve root compression. The false-positive nerve root compression rate was determined, and the gold standard was corrected accordingly. Clinically relevant cut-off values were defined by multilevel likelihood ratio analysis.

RESULTS

The positive and negative likelihood ratios of P1-latency prolongation were 6.79 and 0.53, respectively, for the gold standard, and 10.57 and 0.21, respectively, for the corrected gold standard. The validity was not reduced when scalp-recorded, somatosensory-evoked potentials were blinded to the radiologic results. Absence of P1 was associated to the gold standard and the corrected gold standard. Compared with the combined use of P1-latency interside difference and height-corrected latency, the combination of P1-interroot comparison and height-corrected P1-latency increased the sensitivity by 20% for the gold standard and 32% for the corrected gold standard, and when absent P1 was added, the overall sensitivity was 53% for the gold standard and 81% for the corrected gold standard. The corresponding specificity was 92%, and in asymptomatic nerve roots it was 98%.

CONCLUSION

The P1-interroot comparison permits the use of scalp-recorded somatosensory-evoked potentials as a contributory "rule in" procedure in patients with sciatica.