What is the best way to assess focal slowing of the ulnar nerve?

Objective: The study assessed the influence of the length of the across elbow (AE) segment of the ulnar nerve on the true and false positive rates of velocity measurements of the AE segment. Using a short AE length will increase effects of the measurement error (ME), and using a long distance will ‘dilute’ the slowing due to the focal lesion; it is not known which length is optimal to detect focal slowing. Methods: A simulation was performed to assess diagnostic yield for AE lengths of 50, 100 and 150 mm, taking into account ME, variation in true velocity, and severity of the lesion. ME of latencies and distances were first determined in a healthy subject. Results: ME proved lower than in a published study. Diagnostic yield was consistently better for an AE length of 50 mm than for 100 or 150 mm. The optimum length is therefore near 50 mm. Yield increased with severity of the lesion, smaller ME, and when interindividual variation in true velocity was small. Judging AE on its own had a slightly better yield than comparing AE velocity to forearm velocity, except for populations with a larger than normal spread in true conduction variability. Conclusions: The best balance between effects of ME and ‘dilution’ to detect focal nerve slowing is obtained at nerve lengths of about 50 mm. The need to incorporate all possible compression sites necessitates the use of a suboptimal length of about 80 mm.