Does chronic arthritic pain influence motor cortex excitability?
Parker, Rosalind Sarah
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Background: Painful hand arthritis is a major cause of disability worldwide including New Zealand, causing loss of function and reduced motor control. Arthritis is associated with changes in central nociceptive processing and cortical reorganisation. Previous studies have shown an association between chronic pain and changes in corticomotor excitability. However, few studies have assessed corticomotor excitability in people with arthritis. Aims of the investigation: 1. To examine corticomotor and intracortical excitability in people with chronic hand pain due to arthritis. 2. To explore possible relationships between corticomotor excitability and pain variables. Methods: Transcranial magnetic stimulation (TMS) was used to measure the excitability of the corticomotor pathway to the first dorsal interosseus muscle in 23 people with chronic painful hand arthritis (median age 71; 17 female; median pain duration 9 years) and 20 pain-free control participants (median age 70.5; 14 female). Single-pulse TMS was used to establish the resting motor threshold (RTh), stimulus-response curves, and the cortical silent period duration (CSP). Paired-pulse TMS was used to examine short- and long-interval intracortical inhibition (SICI, LICI) and short-interval intracortical facilitation (SICF). SICI was assessed using conditioning stimulus intensities of 70% and 80% RTh (SICI70, SICI80) and an interstimulus interval (ISI) of 2 ms. The test stimulus was set to an intensity that elicited a motor evoked potential (MEP) of 1 mV (TS1mV). SICF was assessed using a conditioning stimulus of TS1mV, a test stimulus of 90% RTh, and ISIs of 1.4 and 2.8 ms (SICF1.4, SICF2.8). Outcome measures were compared between the two groups using Mann Whitney U tests due to several of the measures being non-normally distributed. Spearman’s rank correlations were used to explore the potential relationships between the corticomotor excitability measures and pain variables (pain duration, pain intensity and a measure of hand-related pain, stiffness and function). Results: The arthritis group demonstrated significantly enhanced SICF1.4 (p = 0.045) compared to the control group. RTh, stimulus-response curves, CSP duration, SICI70, SICI80, SICF2.8, and LICI were not significantly different between the two groups (all p>0.05). Moderate strength correlations were observed between the duration of hand pain and both measures of intracortical inhibition (SICI70 ρ = 0.38; SICI80 ρ = 0.434). There was a moderate strength correlation between the duration of hand pain and SICF1.4 (ρ = 0.346). Conclusions: This study provides evidence of enhanced facilitation in people with hand pain due to arthritis. No significant alterations in overall corticomotor excitability or inhibition were found. Relationships were observed between pain duration and intracortical excitability, with increased pain duration being associated with reduced inhibition and enhanced facilitation. This suggests that with increased hand pain duration, there is greater intracortical excitability. Similarly to other studies in assessing corticomotor excitability in chronic pain conditions, arthritic pain is associated with disinhibition of the motor cortex. Cortical disinhibition may contribute to the deficits in strength, motor control and function, which are known to impact on people with arthritic hand pain. These findings have important implications for motor learning and rehabilitation for people with hand arthritis.