Chronological Relationships between the Irreversible Vasoconstriction by the Vasodilator and the Electron Microscopic Changes of the Arterial Wall in the Rabbit Chronic Vasospasm Model

ABSTRACT

One of the characteristics of the cerebral vasospasm is its irreversibility with the vasodilators. Under the hypothesis that the irreversibility with vasodilators might be caused by the structural change in the arterial wall, authors examined the chronological relationships between the irreversibility and the electron microscopic findings of the arterial wall in the rabbit chronic vasospasm model. The development of the vasospasm and the irreversibility of the vasospasm with the intra-arterial papaverine were defined angiographically. After the second angiography done in ont to 30 days after subarachnoid hemorrhage(SAH), eighteen rabbits were sacrificed, and the basilar artery was examined with electron microscope. Arterial narrowing was the severest one day after SAH(54.1% of the pre-SAH diameter), and was maintained up to 30 days after SAH. The irreversibility of the arterial constriction with the papaverine developed 5 days after SAH, which had a tendency to recover 6 to 9 days after SAH. However the irreversibility was noted again 16 days after SAH. Electron microscopy revealed the endothelial wrinkling, disorganization of muscle fiber, myonecrosis, thickening of smooth muscle fibers, and increase of connective tissue in the tunica media. These structural changes were severest one day after SAH, and gradually diminished up to 30 days after SAH. These data show that there are no chronological relationships between the irreversibility and the structural change per se. However the fact that the irreversibility developed during the reparative phase of the arterial wall injury by SAH suggests that the chronic vasospasm is not a primary event but a secondary phenomenon following an injury to the cerebral arterial wall.