Introduction: Sensory hypersensitivity, central hyper-excitability (lowered nociceptive flexion reflex (NFR) thresholds) and psychological distress are features of chronic whiplash. Relationships between these substrates are not clear. The aim of this study was to investigate relationships between psychological factors (distress, catastrophization) and pain threshold responses to sensory stimuli and spinal cord excitability as assessed by the NFR. The former assessments are considered as global pain responses to sensory stimuli as reported by the patient, whereas the latter, an objective measurement for spinal cord excitability to peripheral stimulation.

Methods: 30 individuals with chronic (> 3 months) whiplash (Grade II or III; Grade IV were excluded) and 30 asymptomatic controls participated. Pressure pain thresholds (PPTs) and thermal pain thresholds (Thermotest, Somedic AB, Sweden) were measured at the cervical spine, upper and lower limbs. The NFR (intensity of electrical stimulation at the sural nerve required to elicit reflex EMG activity of biceps femoris) was measured as per previous protocols (1). Pain and disability levels (NDI), psychological distress (GHQ-28) and catastrophisation (PCS) were also measured in the whiplash group. Ethical clearance for this study was granted by the Medical Research Ethics Committee of the University of Qld. A MANCOVA was used to determine differences between the whiplash group and controls for sensory measures and the NFR. GHQ-28 and PCS scores were used as covariates in the analysis. Group differences for questionnaire data (GHQ-28 and PCS) were analysed using one way ANOVA. Pearson’s correlation coefficients were used to determine the relationship between the psychological measures (PCS and GHQ-28), pain and disability levels (NDI) and the pain threshold measures (mechanical and thermal) and to determine relationships between the psychological measures, pain and disability measures (NDI) and NFR responses (pain intensity at threshold, threshold). p< 0.05.

Results: Whiplash injured participants (23 females, mean (SD) age: 37.7 (11.5) years, NDI: 46.2 (17.6) and VAS scores of pain: 4.2 (2.4)) demonstrated lowered pain thresholds to pressure and cold (p< 0.05); lowered NFR thresholds (p=0.003) and above threshold levels of psychological distress (GHQ-28) compared to controls and levels of catastrophisation comparable to other musculoskeletal conditions. There were no group differences for heat pain thresholds or pain at NFR threshold. In the whiplash group, PCS scores correlated moderately with cold pain threshold (r =0.51, p=0.01). In contrast there were no significant correlations between GHQ-28 scores and pain threshold measures or between psychological factors and NFR responses in whiplash participants. There were no significant correlations between psychological factors and pain thresholds or NFR responses in controls.

Discussion: We have demonstrated that psychological factors have some association with sensory hypersensitivity (cold pain threshold measures) in chronic whiplash but do not seem to influence spinal cord excitability. This suggests that psychological disorders are important, but not the only, determinants of central hypersensitivity in whiplash patients. These findings suggest that both physical and psychological factors will need to be addressed in the management of whiplash.