To the Editor:-The article by Riley et al. regarding sensory changes after intrathecal sufentanil was well written, detailed, and informative. The authors stated that the basis for the neuroselectivity of the different stimulus frequencies used in the CPT evaluation performed by the Neurometer [registered sign] CPT device (Neurotron, Inc., Baltimore, MD) was “theoretical and unsubstantiated.” Unfortunately, the authors must have been unaware of the significant number of peer-reviewed studies that have been published during the past 10 years, establishing the neuroselectivity of the CPT stimuli. [1,2]These studies include, but are not limited to, comparison with other neurodiagnostic tests, peripheral nerve demonstrations of neuroselectivity, and spinal cord demonstrations of neuroselectivity. In fact, there have been more than 190 articles published in peer-reviewed journals using and validating the clinical use, reproducibility, and sensitivity of the CPT evaluation.
Apparently the only statistically significant change detected in CPTs before and after intrathecal administration of sufentanil was at 250 Hz at the knee. I agree with their point in the discussion section that there should have been a greater effect at 5 Hz. The reason for this discrepancy could be the way the data were analyzed. CPT values before and after intervention should always be expressed as a percent change as opposed to change in intensity (mA) because the amount of charge delivered is different for a 5-Hz versus 2,000-Hz sine wave stimulus. For instance, a 1-mA, 5-Hz sine wave stimulus delivers approximately X400 the charge (coulombs) as a 1-mA, 2,000-Hz sine wave stimulus. Therefore, a 10-CPT unit (100 [micro sign]A) change at 5 Hz results in approximately X400 greater difference in charge delivery than a 10-CPT unit change at 2,000 Hz. Perhaps looking at the data as a percent change before and after sufentanil administration would reveal a significant effect at 5 Hz.
Herbert N. Chado, M.D.
Senior Medical Consultant; Neurotron, Inc.; Evergreen Medical Consultants; Evergreen, Colorado
(Accepted for publication November 24, 1998.)