A new objective method for acquisition and quantification of reflex receptive fields

The nociceptive withdrawal reflex (NWR) is a polysynaptic spinal reflex correlated with pain perception. Assessment of this objective physiological measure constitutes the core of existing methods for quantification of reflex receptive fields (RRFs), which however still suffer from a certain degree of subjective involvement. This article proposes a strictly objective methodology for RRF quantification based on automated identification of NWR thresholds (NWR-Ts). Nociceptive withdrawal reflex thresholds were determined for 10 individual stimulation sites using an interleaved up-down staircase method. Reflexes were detected from electromyography by evaluation of interval peak z scores and application of conduction velocity analysis. Reflex receptive field areas were quantified from interpolated mappings of NWR-Ts and compared with existing RRF quantifications. A total of 3 repeated measures were performed in 2 different sessions to evaluate the test-retest reliability of the various quantifications, using coefficients of repeatability (CRs) and hypothetical sample sizes. The novel quantifications based on identification of NWR-Ts showed a similar level of reliability within and between sessions, whereas existing quantifications all demonstrated worse between-session than within-session reliability. The NWR-T-based quantifications required a smaller sample size than any of the existing RRF measures to detect a clinically relevant effect in a crossover study design involving more than 1 session. Of all measures, quantification from mapping of inversed NWR-Ts demonstrated superior reliability both within (CR, 0.25) and between sessions (CR, 0.28). The study presents a more reliable and robust quantification of the RRF to be used as biomarker of pain hypersensitivity in clinical and experimental research.