Hiroshi Ujiie

Effects of size and shape (aspect ratio) on the hemodynamics of saccular aneurysms: A possible index for surgical treatment of intracranial aneurysms

OBJECTIVEnThe present study was undertaken to explore the relationship between the characteristic geometry of aneurysms prone to rupture and the blood flow patterns therein, using microsurgically produced aneurysms that simulated human middle cerebral artery aneurysms in scale and shape.nnnMETHODSnWe measured in vivo velocity profiles using our 20-MHz, 80-channel, Doppler ultrasound velocimeter. We produced small (< or =5 mm, 5 cases) and large (6-13 mm, 12 cases) aneurysms with round, dumbbell, or multilobular shapes.nnnRESULTSnThe fundamental patterns of intra-aneurysmal flow were composed of inflow, circulating flow, and outflow. The inflow, which entered the aneurysm only during the systolic phase, was strongly influenced by the position and size of the neck and the flow ratio into the distal branches. The outflow was usually nonpulsatile and of low velocity. The circulating flow depended on the aspect ratio (depth/neck width). A single recirculation zone was observed in aneurysms with aspect ratios of less than 1.6. This circulation did not seem to extend to areas with aspect ratios greater than this value; in aneurysms with aspect ratios of more than 1.6, a much slower circulation was observed near the dome. Furthermore, in the dome of dumbbell-shaped aneurysms and daughter aneurysms, no flow was detected. Intra-aneurysmal flow was determined by the aspect ratio, rather than the aneurysm size.nnnCONCLUSIONnThe localized, extremely low-flow condition that was observed in the dome of aneurysms with aspect ratios of more than 1.6 is a common flow characteristic in the geometry of ruptured aneurysms, so great care should be taken for patients with unruptured intracranial aneurysms with aspect ratios of more than 1.6.

Clinical analysis of incidentally discovered unruptured aneurysms.

Background and Purpose We analyzed the risk factors for rupture of an intracranial aneurysm based on a retrospective angiographic study of ruptured and unruptured aneurysms. Methods The 44 cases of asymptomatic aneurysms were selected from 1612 patients whose lesions had been discovered fortuitously by angiography (2.7%) during the period from 1980 to 1989. All these patients were free from any sign of intracranial aneurysm. The variations in age, sex, and location of the aneurysms were analyzed compared with 638 ruptured aneurysms that had been treated in our institute during the same period. The size, shape, and arterial geometry of the unruptured aneurysms were examined angiographically. Results Unruptured aneurysm was discovered fortuitously in 44 (2.7%) of 1612 patients, with greater incidence in women aged older than 60 years. Unruptured aneurysms were less likely to occur in the anterior communicating artery (12.8%) and the middle cerebral artery (6.4%). However, they were frequently found in the internal carotid artery, with an incidence of 10.6% in the cavernous portion of the internal carotid artery, 19.1% in the internal cartoid-ophthalmic artery, 19.1% in the internal carotid-posterior communicating artery, and 12.8% in the internal carotid-anterior choroidal artery. Seven of the nine internal carotid-posterior communicating artery aneurysms showed a hypoplastic or aplastic posterior communicating artery. The mean diameter of the unruptured aneurysms was 4.8 mm, and 80% were smaller than 6 mm. Conclusions Intracranial aneurysms are formed not only at the bifurcation of an artery but also at its branching and bending points. However, an aneurysm located at the bifurcation, such as the anterior communicating artery and the middle cerebral artery, bleeds easily in contrast with lateral aneurysms such as those found at the branching and bending points on the internal carotid artery.

Hemodynamic Study of the Anterior Communicating Artery

BACKGROUND AND PURPOSEnThe anterior communicating artery (ACoA) is a site of predilection for intracranial saccular aneurysms causing subarachnoid hemorrhage. ACoA aneurysms are frequently associated with an asymmetrical circle of Willis. In such cases, the ACoA is probably exposed to high hemodynamic stress caused by a considerable shunt flow across the ACoA to the distal segment of the contralateral anterior cerebral artery (ACA). In the present study, the flow pattern and flow-induced shear stress in the ACoA complex that may initiate aneurysmal lesions were studied under steady and pulsatile flow conditions.nnnMETHODSnFlow visualization was studied with dye injection and birefringent flow visualization in symmetrical and asymmetrical models of various sizes of ACoA. The distribution of wall shear stress was measured using an electrochemical method based on a diffusion-controlled reaction of ferricyanide ion to ferrocyanide ion at a platinum electrode embedded in the wall of the ACoA model.nnnRESULTSnWith equal flow rate (Reynolds number 150 to 600), vortical flow was formed in the mouth of the ACoA, and no cross flow through the ACoA was observed. The wall shear stress on the mid-wall of the ACoA was almost zero. However, as soon as the flow rate became unequal, a cross flow through the ACoA was observed. The stagnation point also appeared at the medial junction of the ACoA and ACA. The wall shear stress increased to a very high level at the wall of the ACoA and around the stagnation point.nnnCONCLUSIONSnGeometric changes from the symmetrical to the asymmetrical ACoA develop higher shear stress on the ACoA than critical values and the stagnation point at the ACoA junction. A combination of these hemodynamic factors is considered to play an important role in initiation of aneurysm.

Repression of wall shear stress inside cerebral aneurysm at bifurcation of anterior cerebral artery by stents

The effect of a simple bare metal stent on repression of wall shear stress inside a model cerebral aneurysm was experimentally investigated by two-dimensional particle image velocimetry in vitro. The flow model simulated a cerebral aneurysm induced at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. Stent placement effectively repressed the temporal and spatial variations and the magnitude of wall shear stress. Hence, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Study for Flow Structure within Real Aneurysm Induced in Anterior Communicating Artery

The mechanism involved in the formation and subsequent rupture of cerebral aneurysm has not been clarified adequately. The aneurysm frequently appears at the flow divider where the flow bifurcates to the anterior communicating artery and the anterior cerebral artery. In the present study, the flow pattern and the wall shear stress (WSS) within the real aneurysm model having bleb are measured using PIV. On the basis of the MRI data of human cerebral artery with aneurysm, the aneurysm model is reconstructed by the optical 3D design with CAD. WSS abruptly changes along the aneurysmal wall and minimum WSS appears at bottom wall of the bleb. Particularly, WSS is small along the bleb wall. It would be associated with the degeneration of bleb wall such as the formation of thrombosis.

Analysis of Intraoperative Prognostic Factors in Patients with Ruptured Intracranial Aneurysms

Induced hypotension has commonly been used during intracranial aneurysm surgery in order to facilitate dissection and reduce the risk of intraoperative rupture. It is not known for certain, however, whether this procedure is appropriate or not. We analyzed intraoperative prognostic factors, including induced hypotension below 60mmHg mean arterial blood pressure (MABP) using trimetaphan camsylate, in 90 patients who underwent clipping of intracranial aneurysms. The intraoperative factors were temporary clipping, low density areas and brain swelling on postoperative CT scan, and intraoperative rupture. In addition, we investigated the Hunt & Kosnik grade, CT grade, and past history of hypertension as preoperative factors, and delayed ischemic neurological deficits (DIND) as a postoperative factor. The data were evaluated using methods of multivariate statistical analysis (quantification theory 2nd family). These factors affected the prognosis in the following order according to their relative influence strengths: preoperative factors, postoperative factor, and intraoperative factors. In all cases induced hypotension below 60mmHg MABP had a little influence on prognosis, but in patients with a past history of hypertension or in Hunt & Kosnik grade III and IV patients, induced hypotension had an adverse effect on outcome. These patients have a high probability of developing reduced cerebral blood flow and impaired autoregulation after subarachnoid hemorrhage. Especially when extended for over 30 minutes, induced hypotension below 60 mg MABP was more harmful. It is concluded that, in order to facilitate dissection during intracranial aneurysm surgery, temporary clipping is superior to prolonged induced hypotension.