Median-to-ulnar sensory nerve action potential amplitude ratio as an electrodiagnostic adjunct for carpal tunnel syndrome

OBJECTIVES: (1) To obtain normative data for the median-to-ulnar sensory nerve action potential (SNAP) amplitude ratio (MUSAR) and (2) to discuss the adjunctive use of MUSAR in electrodiagnosis of carpal tunnel syndrome. DESIGN: Analysis of previously collected normative data. SETTING: Veterans Administration electromyography laboratory. SUBJECTS: Forty-six apparently healthy male volunteers without neurologic complaints. MAIN OUTCOME MEASURES: Antidromic median and ulnar SNAPs were recorded and the respective MUSAR ratios were calculated. Descriptive statistical analysis was completed, with the assumption that SNAP values are not Gaussian in distribution. RESULTS: The normal MUSAR ranged from .74 (5th percentile) to 2.5 (95th percentile). CONCLUSION: The MUSAR, being an intrapersonal ratio, may decrease the false positives and false negatives that would otherwise arise when using absolute values of SNAP amplitudes. Hence, it may increase the diagnostic specificity for carpal tunnel syndrome. Further studies are needed to confirm the clinical and presurgical application of the MUSAR value.