Motor Unit Number Estimate and Isometric Hand Grip Strength in Military Veterans with or Without Muscular Complaints: Reference Values for Longitudinal Follow-up

Introduction: It remains unclear if Gulf War (GW) veterans have a higher risk of developing motor neuron disorder. We intended to establish baseline neurophysiological values, including thenar motor unit number estimate (MUNE) and isometric hand grip (IHG) strength, to compare future follow-ups of deployed GW veterans with or without muscular complaints. Materials and Methods: We evaluated 19 GW veterans with self-reported weakness, cramps, or excessive muscle fatigue (Ill-19) and compared them with 18 controls without such muscular complaints (C-18). We performed MUNE on hand thenar muscles using adapted multipoint stimulation method for Ill-19 and 15 controls (C-15). We measured IHG strength (maximum force, endurance, and fatigue level) on Ill-19 and C-18 with a hand dynamometer. We performed nerve conduction studies on all study participants to determine which subjects had mild carpal tunnel syndrome (CTS). We compared the MUNE and RIG strength measures between Ill group and controls and between those with CTS and those without CTS. Results: We obtained thenar MUNE of Ill-19 (95% CI of mean: 143-215; mean age: 46 yr) and compared it with that of C-15 (95% CI of mean: 161-230; mean age: 45 yr), and 95% of CI of mean among RIG strength variables (maximum force: 324-381 Newton; endurance: 32-42 s; fatigue level: 24%-33%) compared with C-18 (maximum force: 349-408 Newton; endurance: 35-46 s; fatigue level: 21%-27%). There was no significant difference in either MUNE or RIG strength between Ill-19 group and controls. The MUNE and IHG maximum forces were significantly lower in those with CTS compared with those without CTS. As a surrogate of mild CTS, the median versus ulnar distal sensory latency on nerve conduction study was only weakly associated with MUNE, maximum force, and fatigue level, respectively. Conclusion: To our knowledge, no published study on MUNE reference values of military veteran population has been available. The quantifiable values of both thenar MUNE and IHG strength of military veterans serve as baselines for our longitudinal follow-up of motor neuron function of deployed troops. These reference values are also useful for other laboratories to study veterans' motor system with or without mild CTS.