Does pain sensitivity increase during ictal period? Evidence from absence epileptic WAG/Rij rats.

Objective: The hyperexcitable brain provides a common ground for comorbidity of pain syndromes and epilepsy. There are controversial reports about pain sensitivity during the ictal period. We analyzed the pain sensitivity during the ictal period in the genetic absence epilepsy animal model, Wistar Albino Glaxo/Rijswijk (WAG/Rij) rats.

Methods: The ictal and interictal pain sensitivities of symptomatic WAG/Rij rats (8 months old, n = 19) were determined and compared with those of age-matched control Wistar rats (n = 19). Pain sensitivity was assessed by applying heat stimulation to hind paws and measuring the paw-withdrawal latency using a thermal plantar analgesia meter in awake and freely moving animals. All measurements were made during the interictal and ictal periods and confirmed by simultaneous electroencephalography (EEG) through intracranially implanted electrodes.

Results: The nociceptive stimulus-induced withdrawal latency during the ictal period in absence epilepsy WAG/Rij rats was significantly shorter when compared with that during the interictal period (p = 0.007) and when compared with that in the control Wistar rats (p = 0.001).

Conclusion: Our data indicate higher pain sensitivity during the ictal period in absence epilepsy rats. Considering the fact that subjects are less responsive during spike-wave discharges, there is a decrease in the level of consciousness and/or responsiveness ictally during all generalized genetic seizures, this increased pain sensitivity is rather surprising during the ictal period. Although the mechanism remains unknown, this novel finding deserves further investigation.