The Upper Limb Nociceptive Reflex: Methodological Considerations in the Assessment of Central Sensitization in Humans
Central Sensitization (CS) is the underlying neurophysiological adaptation that contributes to the development of chronic pain pathologies. The nociceptive reflex (NR) has been used to objectively measure of CS, however, its predominate use in the lower limb may limit the ability to assess CS within cervical spine pathologies. The aim of this thesis was to investigate technical considerations of the upper limb NR in detecting changes in CS. Study 1 observed a selective reduction of upper limb NR thresholds following the experimental induction of CS, representing functional adaptations of the upper limb NR through synergistic muscle responses to noxious stimuli. This provided the first evidence of cervical CS detection via the upper limb NR threshold. Study 2 verified an automated method of NR response detection, enabling the assessment of NR thresholds in real time. This improved the feasibility of the NR technique and provided a method to assess NR amplitudes in the upper limb. Study 3 demonstrated the synergistic effect of heat and topical capsaicin in the experimental induction of CS within the first 30 minutes of capsaicin application. This provided a non-invasive method to experimentally induce varying levels of CS, and subsequently study this effect on upper limb NR outcomes. Study 4 observed no significant effect of induced CS on NR threshold, NR amplitudes, or NR temporal summation amplitudes. This was most notably due to observed habituation to repeated noxious stimuli. This unexpected finding underscores the need to develop novel methods of noxious stimulus delivery in upper limb NR assessments. Collectively, this thesis provides methodological insight into the use of the upper limb NR for the assessment of CS within cervical spine segments. These findings have implications for future research aiming to elucidate underlying mechanisms of CS and its manifestation across a profile of chronic pain pathologies.