EXPLORING THE NOCICEPTIVE WITHDRAWAL REFLEX AS A PHYSIOLOGICAL CORRELATE OF CHRONIC LOW BACK PAIN

Low back pain (LBP) is a widespread and debilitating health problem affecting millions worldwide. While prognosis is generally favourable, a considerable proportion (~30%) of patients develop chronic LBP. Traditional approaches such as quantitative sensory testing (QST) has yielded inconsistent associations with chronic LBP outcomes. An alternative neurophysiological measure is the nociceptive withdrawal reflex (NWR), involving an involuntary muscle contraction response to a noxious stimulus. The aim of the present study was to evaluate the relationship between NWR parameters and clinical features of chronic LBP. We recruited adults (n=21, 8 females) with chronic LBP after ethics approval and informed consent. Participants received electrical stimulation (5 pulses, 200 Hz) on their foot and had their leg muscle activity recorded using electromyography (EMG), in addition to completing clinical questionnaires. We measured the minimum current threshold needed to evoke a reflex. NWR response magnitude was evaluated as area under the curve of the root mean square EMG in response to four stimulation amplitudes between pain and tolerance thresholds. Slopes were computed from participants’ stimulus-response curves. No correlations were found between participants’ ratings of their LBP with either the slope of the curves or NWR threshold. However, we found a significant negative correlation (r=−0.50, p=0.022) between the NWR threshold and fatigue severity. The preliminary findings show that higher fatigue severity was linked to lower reflex thresholds, suggesting increased nociceptive sensitivity. The NWR could provide a more objective assessment of the responsiveness of spinal nociceptive pathways to pain, compared with subjective pain reports.