Electrophysiologic Measures of Ulnar Sensory Nerve Function: Reference Values and Reliability

1. Electrophysiologic Measures of Ulnar Sensory Nerve Function: Reference Values and Reliability Thomas M. Ruediger, University of Michigan – Flint; Stephen C. Allison, Rocky Mountain University of Health Professions; Josef H. Moore, U.S. Army – Baylor University, U.S. Army Medical Department; Robert S. Wainner, Texas State University ABSTRACT PURPOSE: Determine reliability and clinical reference values for electrophysiologic measures of ulnar sensory nerve. SUBJECTS: 100 disease free volunteers. METHODS: Surface antidromiculnar sensory nerve conduction studies were performed; sensory nerve evoked potentials (SNAP) were elicited from stimulation at wrist, below-elbow, and above-elbow sites. Thirty subjects were tested a second time for intrarater reliability analyses. ANALYSES: Sensory distal latency (SDL) and sensory nerve conduction velocity (SNCV) were calculated to the onset and to the peak of the evoked potentials; SNAP amplitude measured from onset to negative peak. Intrarater reliability analyses were performed. Multiple linear regression models were constructed to examine the use of age, body mass index, gender, and finger girth to predict forearm and across-elbow SNCV and SNAP amplitude. RESULTS:Intraclass correlation coefficients (ICC [3,1]) were greater than 0.85 for all paired measures of SDL, SNCV, and SNAP amplitude. Reference values computed from data optimally transformed to minimize skew were: SDL measured to onset and peak 2.34 msec and 3.11 msec; forearm SNCV measured to onset and peak 54 m/sec and 53 m/sec; across-elbow SNCV measured to onset and peak 47 m/sec and 52 m/sec; SNAP amplitude from wrist, below elbow, and above elbow stimulation 10.79µV, 4.66µV, and 3.33µV; SNAP amplitude decrement across the elbow 46%. No independent variable accounted for more than 5% of the variance in across-elbow SNCV; finger girth accounted for approximately 28% of the variance in above-elbow SNAP amplitude. CONCLUSION: No strong statistical models for prediction of SNCV or SNAP amplitude could be derived from the limited set of predictor variables.The reliability analyses in the current study suggest that these ulnar sensory reference may be used with confidence. Measures: Antidromic sensory nerve conduction studies of the ulnar nerve using surface electrodes were performed on 100 disease free volunteers. Sensory nerve evoked potentials were elicited from stimulation at the wrist, below-elbow, and above-elbow sites in all subjects. Sensory distal latency (SDL) and sensory nerve conduction velocity (SNCV) were calculated to both the onset and to the peak of the evoked potentials; sensory nerve action potential (SNAP) amplitude was measured from onset to negative peak of the evoked potential. Thirty subjects were tested a second time to collect data for intrarater reliability analyses. Multiple linear regression models were constructed to examine the use of age, body mass index, gender, and finger girth to predict forearm and across-elbow SNCV and SNAP amplitude. Intraclass Correlation Coefficients (3,1) Descriptive statistics – untransformed electrophysiologic variables • ANALYSES • Intraclass correlation coefficients (ICC) were calculated to establish intra-reliability for single measurements. (ICC 3,1) • Seven separate multiple linear regression models were constructed to examine predictive relationships among the four independent variables and the seven dependent variables. • The effect of sample size on descriptive statistics of the dependent variables was analyzed. • Reference values were calculated from the optimally transformed data. • The mean difference of the SNCV across the elbow to the SNCV of the forearm and the 95% CI around this difference were computed. BACKGROUND Seven previous studies reporting reference values for the ulnar sensory nerve at the elbow; none met current reporting standards. One of these studies addressed the statistical limitations of using descriptive statistics designed for normative data when the data are not normally distributed by reporting data transformation; the other six did not address the data distribution. No previous published work has reported the reliability of ulnar sensory nerve conduction measurements. Various patient attributes may be predictive for electrophysiologic measurements. For ulnar sensory values at the elbow, only one of the reviewed studies explored of association and was for a single potential predictor: age. The previous reference studies used sample sizes of between 20 and 53 subjects. A minimum sample size of 100 has been suggested to allow reporting the percentile values using statistical guidelines for reference values. RESULTS Responses were obtained from all subjects. Intraclass correlation coefficients (ICC [3,1]) were greater than 0.85 for all paired measures of SDL, SNCV, and SNAP amplitude. Reference values (mean ± 2SD) were computed from data optimally transformed to minimize skew. No independent variable accounted for more than 5% of the variance in across-elbow SNCV; finger girth accounted for approximately 28% of the variance in above-elbow SNAP amplitude. Reference values for electrophysiologic measures SUMMARY Reference values for SNCV and SNAP amplitude were computed using recommended statistical methods and elbow position. No strong statistical models for prediction of SNCV or SNAP amplitude could be derived from the limited set of predictor variables. Statistical transformation methods to correct for minor skewness in the data distributions improved reference value determination for most measurements, even though raw data distributions were reasonably normal. PURPOSE To determine reliability and clinical reference values computed with appropriate statistical methods for electrophysiologic measures of ulnar sensory nerve function using data from healthy subjects obtained with the recommended elbow testing position. CONCLUSION Ulnar SDL, SNCV, and SNAP amplitude are reliable measures. The reliability results from the current study, combined with relatively low levels of measurement error, suggest that clinical electrophysiologists can be confident using ulnar sensory reference values computed using the recommended elbow position and appropriate statistical methods. METHODS This study was approved by the Institutional Review Boards of both Walter Reed Army Medical Center and Rocky Mountain University of Health Professions. All volunteers read and signed an informed consent document. Reference values calculated from data : mean + 2 SD for the SDL , mean – 2SD for SNCV and SNAP amplitude. The 97.5th percentile values were used for SDL, and 2.5th percentile values for NCV and SNAP amplitude.