A common structural variation of the atlas vertebra is called ponticulus posticus (also known as foramen arcuale or "Kimmerle's anomaly"). Investigators studied the relationship between this condition and headache symptoms in 895 first-time chiropractic patients. The patients' complaints included migraine with aura (classical migraine), migraine without aura (common migraine), cervicogenic headache, neck pain only, and other problems. They were examined for the presence or absence of partial or complete ponticulus posticus. The authors found a significant correlation of ponticulus posticus with migraine without aura. They explain that because the ponticulus posticus is intimately attached to the atlanto-occipital membrane (where the spine and skull meet) and this membrane, in turn, is attached to the dura (the outermost covering of the brain and spinal cord), small tensions exerted on the dura may result in excruciating head pain of a type experienced in migraine. The beneficial results of chiropractic for migraine and cervicogenic headache are possibly related to the nature of the structures connecting the upper spine to the skull.
Wight S, Osborne N, Breen AC. J Manipulative Physiol Ther. 1999 (Jan); 22 (1): 15-20