Asymmetrical reduction of the nociceptive flexion reflex threshold in cluster headache

The nociceptive flexion reflex (NFR) of the lower limbs (RIII reflex) was e xamined bilaterally in 54 cluster headache (CH) patients suffering from epi sodic CH (ECH) and chronic CH (CCH). Fifteen ECH patients were examined in both remission and active phases. The RIII reflex threshold (Tr) and the th reshold of pain sensation (Tp) were significantly reduced on the symptomati c side in patients with episodic CH during the bout. During the active phas e of episodic CH an inverse correlation was found between the severity of C H (ratio: number of cluster periods/years of illness duration) and the Tp, which may suggest a role for secondary central sensitization in pain pathwa ys. The lower Tr and Tp on the symptomatic side is in keeping with previous observations exploring pain mechanisms using different methods (i.e. corne al reflex, pain pressure threshold). On the whole, these data tie in with t he view of an impairment of the pain control system, which parallels the pe riodicity of the disorder in the episodic form.