[Rheological characteristics of the aneurysm at the middle cerebral artery bifurcation]

We performed a retrospective clinical study to estimate the morphological index such as aspect ratio (aneurysm depth/neck) and area ratio (ratio of the cross-sectional area of bifurcated arteries) of 64 cases with saccular aneurysms (ruptured aneurysms: 41, unruptured aneurysms: 23) at the bifurcation of the middle cerebral artery (MCA) and made a flow visualization study using a flat three dimensional acrylic aneurysm model to define the flow characteristics in conditions similar to those obtained from the clinical study. The mean aspect ratio and area ratio of the ruptured and unruptured aneurysm cases were 2.81 +/- 1.45, 2.52 +/- 0.91 and 1.56 +/- 0.59, 1.73 +/- 0.38. These respective values differ statistically. The area ratio of the MCA bifurcation with no aneurysm was 1.25 +/- 0.35. This value was also statistically different from the ruptured aneurysm case. The visualization study was performed changing the neck size and flow ratio into the peripheries to simulate various aspect ratio and flow ratio into peripheries. The results showed that intraanerysmal flow was definitely influenced by aspect ratio and area ratio. The aspect ratio over 2.0 and area ratio below 2.0 failed to drive the inflow inside the aneurysms and developed as a stagnant area in the dome side. The morphological index of aspect ratio and area ratio reflected the flow characteristics in the aneurysm and was statistically different in ruptured and unruptured aneurysm cases. The unruptured aneurysms with an aspect ratio of more than 1.6 and an area ratio of more than 2.0 must be followed up, because of the risk that they might rupture in the near future.