An ipsilateral upper neck trigger point was found in 21 of 24 patients with unilateral headache. During the prodromic period this trigger point was detected as a tender protrusion on neck palpation. In 18 out of 24 patients it was also found during the headache-free period. On standard roentgenogram, this protrusion seemed to be a laterally developed C2 spinous process. The EMG study showed latent trapezius hypertonicity on the side of the headache, even during the headache-free period. The association of the painful protrusion and trapezius hypertonicity could create an autoreinforcing nociceptive loop, which in turn could be the cause of lateralization of the pain.
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