Asymmetrical reduction of the nociceptive flexion reflex threshold in cluster headache.

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