Primitive vertebrobasilar system associated with a ruptured aneurysm.

Persistent primitive lateral vertebrobasilar (PPLVB) anastomosis is a rare remnant of early fetal development whose occurrence, to our knowledge, has been reported only once. We now report a new variant in which the basilar artery was fed primarily by a PPLVB anastomosis because on one side the vertebrobasilar junction was incompletely developed and on the other it was aplastic. Aneurysmal rupture in this case may be attributable to excessive hemodynamic stress.