Assessment of central sensitization in chronic pain patients: The influence of crosstalk on reflex receptive field mapping

M.B. JENSEN; J.A. BIURRUN MANRESA; O.K. ANDERSEN

San Diego

Conferencia; Neuroscience 2013; 2013

Recent studies indicate that evaluation of the human withdrawal reflex is useful for assessing central sensitization. Enlarged reflex receptive fields (RRF) have been detected in groups of chronic pain patients compared to control groups, most likely reflecting expansion of spinal neuronal receptive fields. The RRF of muscles in the lower extremities can be quantified by randomised, distributed electrical stimulation, typically on the sole of the foot. Each muscle or group of synergistic muscles has a unique cutaneous RRF where noxious stimulation may elicit a reflex in that specific muscle. Interestingly, the RRFs of the two antagonistic muscles tibialis anterior (TA) and soleus (SOL) exhibit substantial overlap, in contrast to what is predicted by the modular organization theory. Most likely, this observation reflects limited specificity of the single differential (SD) EMG recordings used to quantify the reflexes, meaning that crosstalk from relatively strong reflexes in TA dominates the EMG recorded over SOL.To evaluate this assertion, this study mapped RRFs for both TA and SOL using three different approaches to detect reflexes; evaluation of SD EMG, evaluation of double differential (DD) EMG and a novel conduction velocity analysis (CVA) specifically developed to reject crosstalk during reflex detection (Jensen et al. 2013). Reflexes were elicited by five sweeps of noxious electrical stimulation of 10 sites on the sole of the foot of 17 chronic pain patients lying in supine position, a position rendering genuine reflex activity in SOL unlikely. The area of the RRF was quantified as the fraction of the foot where a reflex was elicited by at least 25% of the stimulations. Results are expressed as median (quartiles).In 15 of the 17 subjects, reflexes were identified from the SD EMG recorded over SOL. Moreover, the RRF mappings for SOL based on evaluation of SD EMG resemble to a great extent the corresponding mappings for TA. The RRF area for SOL was 0.17(0.00,0.51) when derived from SD EMG, whereas DD EMG (0.00(0.00,0.06)) and CVA (0.00(0.00,0.00)) entailed significantly smaller and nearly non-existing RRF areas. A markedly smaller but significant reduction in RRF area was observed for TA when reflex detection was based on CVA, from 0.34(0.17,0.53) for SD EMG and 0.37(0.20,0.53) for DD EMG to 0.28(0.17,0.45) for CVA.These results support the notion that crosstalk may significantly influence mappings of RRFs based on evaluation of SD EMG. However, this issue can readily be reduced by the use of DD EMG or CVA, both of which constitute valuable measures to deal with crosstalk and enable improved mapping of RRFs for specific muscles.Jensen et al. BMC Neuroscience 2013, 14:39