Diminished Corticomotor Excitability in Gulf War I Veterans with Chronic Pain Symptoms

Abstract Background: Diminished motor cortical excitability, as a measurement of increased resting motor threshold with transcranial magnetic stimulation (TMS), is known to be associated with chronic pain conditions. Over 175,000 veterans who were deployed to the Persian Gulf in 1990-91 suffer from an unexplained multisymptom illness termed Gulf War Illness (GWI) with chronic headaches and diffuse body pain as the most commonly reported symptoms. This study hypothesized that veterans with Gulf War Illness related pain exhibit diminished corticomotor excitability associated with chronic pain states and aimed to assess the resting motor threshold (RMT) at the primary motor cortex (M1) in veterans with GWI. Methods: Single pulse TMS was administered over the left motor cortex, using anatomical scans of each subject to guide the TMS coil, starting at 25% of maximum stimulator output (MSO) and increasing in steps of 2% until a motor response with a 50 µV peak to peak amplitude, defined as the RMT, was evoked at the flexor pollicis brevis muscle. RMT was then analyzed using Repeated Measures Analysis of Variance (RM-ANOVA). Results: Gulf War Veterans (GWV) with chronic headaches and body pain (N=20) had a significantly (P<0.001) higher average resting motor threshold (%± SD) of 77.2%±16.7% compared to age and gender matched GWV Controls (N=20), whose average was 55.6%±8.8%. Conclusion: This study demonstrated that a higher level of stimulation was required to evoke a motor response in veterans with GWI related pain. This diminished corticomotor excitability that suggests a reduced state of supraspinal pain modulatory function. Though further studies will be needed, TMS may provide a means of effectively rectifying this modulatory deficit.