What is the normal latency difference for ulnar motor nerve conduction velocity (NCV) between the abductor digiti minimi (ADM) muscle and the first dorsal interosseous (1st DI) muscle?

Normal Latency Difference of Ulnar Motor NCV to ADM and 1st DI Muscles

The normal latency difference between the abductor digiti minimi (ADM) muscle and first dorsal interosseous (1st DI) muscle for ulnar motor nerve conduction velocity is 0.6 ± 0.4 ms, with an upper limit of normal being 1.3 ms. 1

Normal Values and Interpretation

• The mean latency to the first dorsal interosseous (FDI) muscle is 3.8 ± 0.5 ms, with upper limits of normal being 4.7 ms for males, 4.4 ms for females, and 4.6 ms overall 1
• The mean latency to the abductor digiti minimi (ADM) muscle is typically shorter than to the FDI 1, 2
• When comparing same-limb measurements, the latency to the ADM should not exceed the latency to the FDI by more than 1.3 ms 1
• For side-to-side comparisons of the same muscle (FDI to FDI), normal difference in latency should not exceed 0.8 ms 1

Clinical Significance

• Recording from both ADM and FDI provides more complete assessment of ulnar nerve function, as the FDI is innervated by the deep terminal branch of the ulnar nerve 1, 3
• The ADM recording alone may miss lesions affecting only the deep branch of the ulnar nerve 2
• When evaluating for ulnar neuropathy at the elbow, absolute across-elbow motor nerve conduction velocity measurements are superior to velocity difference calculations between elbow and forearm segments 4

Technical Considerations

• For reliable measurements, a standardized recording technique should be used, with the active electrode placed over the dorsal first web space for FDI recording 1
• The reference electrode for FDI recording is typically placed at the fifth metacarpophalangeal joint 1
• Surface temperature should be controlled and consistent between measurements 5
• Joint position should be standardized during testing to ensure reproducibility 5

Pitfalls to Avoid

• Using normal values derived from ADM recordings when interpreting FDI recordings can lead to false-positive diagnoses 5
• Each recording site (ADM vs FDI) should be evaluated using its own specific normal values 5
• Low CMAP amplitudes can affect the reliability of latency measurements and should be taken into consideration when interpreting results 4
• Side-to-side amplitude differences up to 52% can be normal and should not be misinterpreted as pathological 1