Circadian variation of flexor withdrawal and crossed extensor reflexes in patients with restless legs syndrome

An evening state of spinal hyperexcitability has been proposed to be a possible cause of evening increases in restless legs syndrome symptoms. Thus, the objective of the current study was to assess the circadian variation in spinal excitability in patients with restless legs syndrome based on flexor withdrawal reflex and crossed extensor reflex responses. The reflexes were elicited on 12 participants with restless legs syndrome and 12 healthy control participants in the evening (PM) and the morning (AM). Reflex response magnitudes were measured electromyographically and kinematically. Both the reflexes showed a circadian rhythm in participants with restless legs syndrome but not in control participants. Changes in ankle (median flexor withdrawal reflex PM: 16.0 ° versus AM: 2.8 °, P = 0.042; crossed extensor reflex PM: 0.8 ° versus AM: 0.2 °, P = 0.001) angle were significantly larger, and ankle angular velocity (median flexor withdrawal reflex PM: 38.8 ° s⁻¹ versus AM: 13.9 ° s⁻¹, P = 0.049; crossed extensor reflex PM: 2.4 ° s⁻¹ versus AM: 0.5 ° s⁻¹, P = 0.002) was significantly faster in the evening compared with the morning in participants with restless legs syndrome, for both reflexes. For participants with restless legs syndrome, evening change in hallux angle was significantly larger than morning responses (median PM: 5.0 ° versus AM: 1.3 °, P = 0.012). No significant differences for any of the electromyographic or kinematic variables were observed between participants with restless legs syndrome and controls. The flexor withdrawal reflex and the crossed extensor reflex show a circadian rhythm in participants with restless legs syndrome suggesting an evening increase in spinal excitability. We hypothesize the circadian variation in spinal excitability may be due to a possible nocturnal form of afferent circuitry central sensitization in the dorsal horn of the spinal cord in patients with restless legs syndrome.