Mixed latency difference for diagnosis of ulnar neuropathy at the elbow

Objective: To provide reference values and to compare this technique with the standard motor conduction velocity (MCV) of the ulnar nerve. Design: Retrospective unmasked study. Setting: Private and institutional practice. Participants: The reference group included 57 healthy volunteers. Patients included 100 subjects with suspected ulnar neuropathy at the elbow (UNE) referred for neurophysiologic evaluation. This group was subdivided into 2 groups: group A was composed of 45 patients with UNE confirmed by MCV of the ulnar nerve, and group B included 55 patients with suspected UNE in whom the diagnosis could not be established by MCV of the ulnar nerve. Interventions: Not applicable. Main Outcome Measures: Differences between peak latencies of ulnar and median mixed nerve action potentials at the arm, after stimulating these nerves at the wrist. This was called mixed latency difference. Results: The upper normative limit of the mixed latency difference was 1.1ms, and there was a significant correlation with height. In group A, the mixed latency difference was abnormal in 80% of the cases and could not be calculated in 18%. In group B, the mixed latency difference was abnormal in 8 (15%) patients. All of these had abnormal "inching" of the ulnar nerve across the elbow. Conclusions: The mixed latency difference was particularly useful in cases of mild UNE.