Bow hunter's stroke associated with atlantooccipital assimilation--case report.

A 39-year-old male presented with bow hunter's stroke manifesting as repeated vertebrobasilar ischemic attacks induced by head rotation 45 degrees to the left. Three-dimensional computed tomography angiography clearly showed the occluded right vertebral artery (VA) between the axis and atlas. Single photon emission computed tomography study showed diffuse hypoperfusion of the brain stem and bilateral cerebellar hemispheres, suggesting hemodynamic compromise of these regions. He refused surgery and was treated conservatively. The most likely mechanism is that the affected VA was fixed by the ossification of the atlantooccipital membrane, vascular groove, and transverse foramen of the atlas, and therefore became elongated and compressed by head-turning.