Title: Distal motor latency in hand intrinsic muscles: effects of less commonly considered sources of variance. Study Design: A repeated measures within subjects design. Objectives: This study evaluated the effects of six extrinsic examiner or technique-related factors on distal motor latency (DML) of hand intrinsic muscles. Summary of Background Data: DML is an important component in the electrodiagnosis of peripheral neuropathic changes, particularly compression neuropathy. Both extrinsic and intrinsic factors can potentially cause variability in DML. Methods: The study population was compromised of 30 adults, 16 men and 14 women, representing 60 hands. The mean age was 46.7 (plus or minus) 15.9 years (range 24 to 84 years); 96.5% (28/30) of subjects were right-hand dominant. A rapid test-retest protocol was used to study effects of intra-rater reliability, recording electrode replacement, hand posture, recording sensitivity, reference recording electrode site, and presence of premotor waves or initial positive deflections on DML in the abductor pollicis brevis (APB), abductor digiti quinti (ADQ), second lumbrical (SL), second interossei (SI), and the SL – SI Differential. Repeated measures ANOVA and paired t Tests with Bonferroni Correction were used to compare findings and detect significant differences. Descriptive statistics were reported for the premotor waves and initial positive deflections. Results: Intra-class Correlation Coefficients showed excellent intra-rater reliability across two DML measurement trials for the SL, SI, and SL – SI Differential. Removal and replacement of electrodes caused no significant effect on DML or the SL-SI Differential. Significant effects were detected for changes related to hand posture in the SLand SL-SI Differential, for recording sensitivities in all muscles and the SL-SI Differential, and for different reference recording sites in the SI and SL-SI Differential. The incidence of premotor waves and initial positive deflections was greater in the SL and SI than in the APB and ADQ, and their presence appeared to shorten DML using the particular measurement protocol described. Conclusions: In this study, DML in hand intrinsic muscles could be reliabily measured and replicated. Hand posture, recording sensitivities, and reference recording sites, were demonstrated to significantly affect DML in specific muscles. These effects were clinically relevant and should be carefully controlled during DML testing. The origins of disruptions of the baseline preceding the main motor response need to be identified and controlled.
