Background: Electrophysiologic tests are utilized in the evaluation of patients reporting pain and/or sensory changes in peripheral nerve distributions. Nerve conduction studies reporting latency, compound muscle action potential (CMAP) maximal amplitude, and conduction velocity are useful components of that evaluative process. Latency and CMAP maximal amplitude values are reported in the literature as revealing no significant side-to-side differences. Changes in the CMAP maximal amplitude are associated with either partial or complete axonal loss (conduction block). The side-to-side differences in CMAP maximal amplitude, when reported, are usually reported as a percentage difference. A percentage difference greater than 50% has been reported as significant. The need to attain such a large percentage difference may lead to an increased risk of failing to identify a conduction block when in fact one exists. The ability to identify a conduction black of the Spinal Accessory Nerve (SAN) is useful in evaluating patients with shoulder girdle, neck, and craniofacial pain. In order to identify a conduction block involving the SAN, it is necessary to know the normal numeric range (mV) of the side-to-side difference of the CMAP maximal amplitude of the SAN. Purpose: The purposes of this research project were to (1) analyze the intr-subject side-to-side CMAP maximal amplitudes of response recorded from the upper trapezius muscles for significant difference and (2) perform a simple linear regression analysis in order to determine if there was a linear relationship between the left and right side responses and if the ipsilateral response could be used to predict the contralateral response with both a 95% confidence interval and 95% prediction interval plotted for the population mean. Methods: The study design was descriptive research, reporting on normative data from a sample of 25 asymptomatic volunteer subjects, ranging in age from 24 to 69 years of age. Measurements of the CMAP maximal amplitudes for the ipsilateral and contralateral upper trapezius muscles were recorded. Informed consent was obtained from all subjects. The Internal Review Board of Rocky Mountain University of Health Professions approved this study. Data Analysis: Data analysis was performed using a two-tailed, dependent Student’s t test with an a=.05 and a 95% confidence interval for the population mean. A linear regression analysis was performed in order to determine if there was a linear relationship between the left and right side responses with both a 95% confidence interval and 95% prediction interval for the population mean. Results: Paired, dependent Student’s t-test for the mean of the absolute difference for the left and right CMAP maximal amplitudes revealed no significant difference. Linear regression analysis revealed a linear relationship between the variables with the points clustered about the fitted lines and a strong positive correlation for predicting the contralateral response based upon the ipsilateral response. Conclusion: In a asymptomatic adult population, the mean of the absolute difference between the ipsilateral and contralateral CMAP maximal amplitudes of response for the upper trapezius muscle is not significant. The side-to-side mean of the absolute difference, carried to 2 standard deviations, should not exceed 2.07 mV with 95% confidence. Linear regression analysis revealed a strong positive correlation for predicting the contralateral response based upon the ipsilateral response. These results may be useful in evaluating a patient with a suspected SAN neuropathy.
